1.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
2.
Adamantane
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Adamantane is a colorless, crystalline chemical compound with a camphor-like odor. With a formula C10H16, it is a cycloalkane and also the simplest diamondoid, Adamantane molecules consists of four connected cyclohexane rings arranged in the armchair configuration. It is unique in that it is rigid and virtually stress-free. A boat-shaped configuration can also exist, Adamantane is the most stable among all the isomers with formula C10H16, which include the somewhat similar twistane. The spatial arrangement of atoms in the adamantane molecule is the same as in the diamond crystal. This motivates the name adamantane, which is derived from the Greek adamantinos, the discovery of adamantane in petroleum in 1933 launched a new field of chemistry dedicated to studying the synthesis and properties of polyhedral organic compounds. Adamantane derivatives have found application as drugs, polymeric materials. The possibility of the existence of a hydrocarbon with the C10H16 formula, Decker called this molecule decaterpene and was surprised that it had not been synthesized yet. The first attempted laboratory synthesis was made in 1924 by German chemist Hans Meerwein using the reaction of formaldehyde with diethyl malonate in the presence of piperidine. Instead of adamantane, Meerwein obtained 1,3,5, 7-tetracarbomethoxybicyclononane-2, 6-dione, later, another German chemist D. Bottger tried to obtain adamantane using Meerweins ester as precursor. However, the product, tricyclo- decane ring system, was again an adamantane derivative, other researchers attempted to synthesize adamantane using phloroglucinol and derivatives of cyclohexanone, but also with failure. Adamantane was first synthesized by Vladimir Prelog in 1941 from Meerweins ester, the process was impractical, as it contained five stages and had a yield of about 0. 16%. However, it was used to synthesize certain derivatives of adamantane. Prelogs method was refined in 1956, the decarboxylation yield was increased by the addition of the Heinsdecker pathway and the Hoffman reaction that raised the total yield to 6. 5%. This method increased the yield to 30–40% and provided a source of adamantane. The adamantane synthesis yield was increased to 60% and 98% by ultrasound. Today, adamantane is an chemical compound with a cost of about $1 a gram. All the above methods yield adamantane as a polycrystalline powder, using this powder, single crystals can be grown from the melt, solution, or vapor phase
3.
Diamond
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Diamond is a metastable allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the rate from diamond to graphite is negligible at standard conditions. Diamond is renowned as a material with superlative physical qualities, most of which originate from the covalent bonding between its atoms. In particular, diamond has the highest hardness and thermal conductivity of any bulk material and those properties determine the major industrial application of diamond in cutting and polishing tools and the scientific applications in diamond knives and diamond anvil cells. Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as boron, small amounts of defects or impurities color diamond blue, yellow, brown, green, purple, pink, orange or red. Diamond also has relatively high optical dispersion, most natural diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers in the Earths mantle. Carbon-containing minerals provide the source, and the growth occurs over periods from 1 billion to 3.3 billion years. Diamonds are brought close to the Earths surface through deep volcanic eruptions by magma, Diamonds can also be produced synthetically in a HPHT method which approximately simulates the conditions in the Earths mantle. An alternative, and completely different growth technique is chemical vapor deposition, several non-diamond materials, which include cubic zirconia and silicon carbide and are often called diamond simulants, resemble diamond in appearance and many properties. Special gemological techniques have developed to distinguish natural diamonds, synthetic diamonds. The word is from the ancient Greek ἀδάμας – adámas unbreakable, the name diamond is derived from the ancient Greek αδάμας, proper, unalterable, unbreakable, untamed, from ἀ-, un- + δαμάω, I overpower, I tame. Diamonds have been known in India for at least 3,000 years, Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history, later in 1797, the English chemist Smithson Tennant repeated and expanded that experiment. By demonstrating that burning diamond and graphite releases the same amount of gas, the most familiar uses of diamonds today are as gemstones used for adornment, a use which dates back into antiquity, and as industrial abrasives for cutting hard materials. The dispersion of light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology developed methods of grading diamonds, four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds, these are carat, cut, color, and clarity. A large, flawless diamond is known as a paragon and these conditions are met in two places on Earth, in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike. The conditions for diamond formation to happen in the mantle occur at considerable depth corresponding to the requirements of temperature and pressure
4.
Bravais lattice
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This discrete set of vectors must be closed under vector addition and subtraction. For any choice of position vector R, the lattice looks exactly the same, when the discrete points are atoms, ions, or polymer strings of solid matter, the Bravais lattice concept is used to formally define a crystalline arrangement and its frontiers. A crystal is made up of an arrangement of one or more atoms repeated at each lattice point. Consequently, the crystal looks the same when viewed from any equivalent lattice point, two Bravais lattices are often considered equivalent if they have isomorphic symmetry groups. In this sense, there are 14 possible Bravais lattices in three-dimensional space, the 14 possible symmetry groups of Bravais lattices are 14 of the 230 space groups. In two-dimensional space, there are 5 Bravais lattices, grouped into four crystal families, the unit cells are specified according to the relative lengths of the cell edges and the angle between them. The area of the cell can be calculated by evaluating the norm || a × b ||. The properties of the families are given below, In three-dimensional space. These are obtained by combining one of the six families with one of the centering types. For example, the monoclinic I lattice can be described by a monoclinic C lattice by different choice of crystal axes, similarly, all A- or B-centred lattices can be described either by a C- or P-centering. This reduces the number of combinations to 14 conventional Bravais lattices, the unit cells are specified according to the relative lengths of the cell edges and the angles between them. The volume of the cell can be calculated by evaluating the triple product a ·, where a, b. The properties of the families are given below, In four dimensions. Of these,23 are primitive and 41 are centered, ten Bravais lattices split into enantiomorphic pairs. Bravais, A. Mémoire sur les systèmes formés par les points distribués régulièrement sur un plan ou dans lespace, hahn, Theo, ed. International Tables for Crystallography, Volume A, Space Group Symmetry. Catalogue of Lattices Smith, Walter Fox
5.
Diamantane
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Diamantane is an organic compound that is a member of the diamondoids. These are a cage hydrocarbons with similar to a subunit of the diamond lattice. It is a solid that has been a topic of research since its discovery in oil. Diamondoids such as diamantane exhibit unusual properties, including low surface energies, high densities, high hydrophobicities, diamantane occurs naturally in crude petroleum. It is currently assumed that adamantanes and diamantanes were formed via the catalytic rearrangements of polycyclic naphthenic hydrocarbons. S, gulf of Mexico, and the Western Canada Basin. Diamantane was chosen as the Congress Emblem of the 1963 London IUPAC meeting, and was featured as a decoration on the cover of abstracts, program, Congress participants were challenged to synthesize diamantane. The first preparation of this chemical was achieved in 1965 in 1% yield by aluminum halide-catalyzed isomerization of a mixture of norbornene photodimers. Adamantane was the first, and Congressane, as came to be known, was only the second member of an entire family of compounds known as the diamandoids. The synthesis of the member of the series in 1969 emphasized the need for a more general scheme of semitrivial nomenclature. The compound was renamed “diamantane” and the member designated triamantane. The year 1966 also marked the isolation of diamantane from the high-boiling fractions of the oil of Hodonin. While this permitted a start to be made in the exploration of the chemistry of diamantane, diamantane then became as readily available as adamantane and its chemistry could be studied more easily. Diamantane can be prepared by Lewis acid catalyzed rearrangements of various pentacyclic tetradecanes, the best yield can obtained from trans-tetrahydro-Binor-S. A convenient synthetic procedure involves rearrangement of a hydrogenated Binor-S compound, other more highly strained precursors give diamantane in lower yield owing to disproportionation. The synthetic route begins with the dimerization of norbornadiene catalyzed by a mixture of CoBr22, the resulting dimer is hydrogenated to give tetrahydro-binor-S. The rearrangement occurs in a hot solution of cyclohexane or carbon disulfide with aluminum bromide, diamantane can be produced by thermal cracking of long chained n-alkanes. The mechanism for this conversion is thought to be a free-radical addition, although this method does produce diamantane that has been alkylated, adamantane derivatives are also produced in greater amounts due to its greater thermodynamic stability. This method also produces a series of n-alkanes of up to 35 carbons and coke
6.
Diamondoid
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In chemistry, diamondoids are variants of the carbon cage molecule known as adamantane, the smallest unit cage structure of the diamond crystal lattice. Diamondoids also known as nanodiamonds or condensed adamantanes may include one or more cages as well as numerous isomeric and these diamondoids occur naturally in petroleum deposits and have been extracted and purified into large pure crystals of polymantane molecules having more than a dozen adamantane cages per molecule. These species are of interest as molecular approximations of the cubic diamond framework, cyclohexamantane may be thought of as a nanometer-sized diamond of approximately 5.6 * 10−22 grams. Examples include, Adamantane Iceane BC-8 Diamantane also diadamantane, two face-fused cages Triamantane, also triadamantane, Diamantane has 4 identical faces available for anchoring a new C4H4 unit. Triamantane has 8 faces on to which a new C4H4 unit can be added resulting in 4 isomers, one of these isomers displays a helical twist and is therefore prochiral. The P and M enantiomers have been separated, longer diamondoids have been formed from diamantane dicarboxylic acid. In one study a tetramantane compound is fitted with thiol groups at the bridgehead positions and this allows their anchorage to a gold surface and formation of self-assembled monolayers. Additionally, functionalized diamondoids have been proposed as molecular building blocks for self-assembled molecular crystals, organic chemistry of diamondoids even extends to pentamantane. This alcohol can react with thionyl bromide to the bromide 6, pentamantane can also react with tetrabromomethane and tetra-n-butylammonium Bromide in a free radical reaction to the bromide but without selectivity. Diamondoids are found in mature high-temperature petroleum fluids and these fluids can have up to a spoonful of diamondoids per gallon. A review by Mello and Moldowan in 2005 showed that although the carbon in diamonds is not biological in origin and this was determined by comparing the ratios of carbon isotopes present. The optical absorption for all diamondoids lies deep in the UV spectral region with optical band gaps around 6 eV, the spectrum of each diamondoid is found to reflect its individual size, shape and symmetry. Due to their well defined size and structure diamondoids also serve as a system for electronic structure calculations. As a result, the energy of the lowest unoccupied molecular orbital is roughly independent of the size of the diamondoid, diamondoids have been found to exhibit a negative electron affinity, making them potentially useful in electron-emission devices. In the context of building materials for nanotechnology components, diamondoid was used by K. Examples would include crystalline diamond, sapphire, and other stiff structures similar to diamond but with various atom substitutions which might include N, O, Si, S, Adamantane derivatives has been proposed as a functionalizing molecule for enhancing electron tunneling based DNA sequencing technologies
7.
Petroleum
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Petroleum is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earths surface, which is commonly refined into various types of fuels. Components of petroleum are separated using a technique called fractional distillation and it consists of hydrocarbons of various molecular weights and other organic compounds. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that are made up of refined crude oil. A fossil fuel, petroleum is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock, Petroleum has mostly been recovered by oil drilling. Drilling is carried out studies of structural geology, sedimentary basin analysis. Petroleum is used in manufacturing a variety of materials. Concern over the depletion of the earths finite reserves of oil, the burning of fossil fuels plays the major role in the current episode of global warming. The word petroleum comes from Greek, πέτρα for rocks and Greek, the term was found in 10th-century Old English sources. It was used in the treatise De Natura Fossilium, published in 1546 by the German mineralogist Georg Bauer, Petroleum, in one form or another, has been used since ancient times, and is now important across society, including in economy, politics and technology. Great quantities of it were found on the banks of the river Issus, ancient Persian tablets indicate the medicinal and lighting uses of petroleum in the upper levels of their society. By 347 AD, oil was produced from bamboo-drilled wells in China, early British explorers to Myanmar documented a flourishing oil extraction industry based in Yenangyaung that, in 1795, had hundreds of hand-dug wells under production. The mythological origins of the oil fields at Yenangyaung, and its hereditary monopoly control by 24 families, Pechelbronn is said to be the first European site where petroleum has been explored and used. The still active Erdpechquelle, a spring where petroleum appears mixed with water has been used since 1498, Oil sands have been mined since the 18th century. In Wietze in lower Saxony, natural asphalt/bitumen has been explored since the 18th century, both in Pechelbronn as in Wietze, the coal industry dominated the petroleum technologies. In 1848 Young set up a small business refining the crude oil, Young eventually succeeded, by distilling cannel coal at a low heat, in creating a fluid resembling petroleum, which when treated in the same way as the seep oil gave similar products. The production of oils and solid paraffin wax from coal formed the subject of his patent dated 17 October 1850. In 1850 Young & Meldrum and Edward William Binney entered into partnership under the title of E. W. Binney & Co. at Bathgate in West Lothian, the worlds first oil refinery was built in 1856 by Ignacy Łukasiewicz. The demand for petroleum as a fuel for lighting in North America, edwin Drakes 1859 well near Titusville, Pennsylvania, is popularly considered the first modern well
8.
Diamond cubic
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The diamond cubic crystal structure is a repeating pattern of 8 atoms that certain materials may adopt as they solidify. Diamond cubic is in the Fd3m space group, which follows the face-centered cubic Bravais lattice, the diamond lattice can be viewed as a pair of intersecting face-centered cubic lattices, with each separated by 1/4 of the width of the unit cell in each dimension. Many compound semiconductors such as gallium arsenide, β-silicon carbide, and indium antimonide adopt the analogous zincblende structure, zincblendes space group is F43m, but many of its structural properties are quite similar to the diamond structure. The atomic packing factor of the cubic structure is π√3/16 ≈0.34. Zincblende structures have higher packing factors than 0.34 depending on the sizes of their two component atoms. The first-, second-, third-, fourth- and fifth-nearest-neighbor distances in units of the lattice constant are √3/4, √2/2, √11/4,1 and √19/4. Mathematically, the points of the cubic structure can be given coordinates as a subset of a three-dimensional integer lattice by using a cubic unit cell four units across. With these coordinates, the points of the structure have coordinates satisfying the equations x = y = z, and x + y + z =0 or 1. There are eight points that satisfy these conditions, All of the points in the structure may be obtained by adding multiples of four to the x, y. Adjacent points in this structure are at distance √3 apart in the integer lattice and this structure may be scaled to a cubical unit cell that is some number a of units across by multiplying all coordinates by a/4. Alternatively, each point of the cubic structure may be given by four-dimensional integer coordinates whose sum is either zero or one. Two points are adjacent in the structure if and only if their four-dimensional coordinates differ by one in a single coordinate. The total difference in values between any two points gives the number of edges in the shortest path between them in the diamond structure. These four-dimensional coordinates may be transformed into three-dimensional coordinates by the formula →, because the diamond structure forms a distance-preserving subset of the four-dimensional integer lattice, it is a partial cube. Yet another coordinatization of the diamond cubic involves the removal of some of the edges from a grid graph. The diamond cubic is sometimes called the lattice but it is not, mathematically. However, it is still a highly symmetric structure, any incident pair of a vertex, moreover the diamond crystal as a network in space has a strong isotropic property. Another crystal with this property is the Laves graph, the compressive strength and hardness of diamond and various other materials, such as boron nitride, is attributed to the diamond cubic structure
9.
Isomer
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An isomer is a molecule with the same molecular formula as another molecule, but with a different chemical structure. That is, isomers contain the number of atoms of each element. Isomers do not necessarily share similar properties, unless they also have the functional groups. There are two forms of isomerism, structural isomerism and stereoisomerism. In structural isomers, sometimes referred to as constitutional isomers, the atoms, Structural isomers have different IUPAC names and may or may not belong to the same functional group. For example, two position isomers would be 2-fluoropropane and 1-fluoropropane, illustrated on the side of the diagram above. In skeletal isomers the main chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers, tautomers are structural isomers that spontaneously interconvert with each other, even when pure. They have different chemical properties and, as a consequence, distinct reactions characteristic to each form are observed, if the interconversion reaction is fast enough, tautomers cannot be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, there is, however, another isomer of C3H8O that has significantly different properties, methoxyethane. Unlike the isomers of propanol, methoxyethane has an oxygen connected to two carbons rather than to one carbon and one hydrogen. Methoxyethane is an ether, not an alcohol, because it lacks a hydroxyl group, propadiene and propyne are examples of isomers containing different bond types. Propadiene contains two double bonds, whereas propyne contains one triple bond, in stereoisomers the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers which are non-superposable mirror-images of each other, and diastereomers, enantiomers always contain chiral centers and diastereomers often do, but there are some diastereomers that neither are chiral nor contain chiral centers. Another type of isomer, conformational isomers, may be rotamers, diastereomers, for example, ortho- position-locked biphenyl systems have enantiomers. E/Z isomers, which have restricted rotation at a bond, are configurational isomers. They are classified as diastereomers, whether or not they contain any chiral centers, e/Z notation depicts absolute stereochemistry, which is an unambiguous descriptor based on CIP priorities. Cis–trans isomers are used to describe any molecules with restricted rotation in the molecule, for molecules with C=C double bonds, these descriptors describe relative stereochemistry only based on group bulkiness or principal carbon chain, and so can be ambiguous
10.
Axial chirality
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Certain allene compounds and spirans also display axial chirality. The enantiomers of chiral compounds are usually given the stereochemical labels Ra. The designations are based on the same Cahn-Ingold-Prelog priority rules used for tetrahedral stereocenters and this property can also be called helicity, since the axis of the structure has a helical, propeller, or screw-shaped geometry. P or Δ is a helix, where M or Λ is a left-handed helix. The P/M or Δ/Λ terminology is used particularly for molecules that resemble a helix. It can also be applied to other structures having axial chirality by considering the helical orientation of the front vs back Cahn–Ingold–Prelog rankings, axial Chirality in 6, 6-Dinitrobiphenyl-2, 2-dicarboxylic acid 3D representation
11.
Enantiomer
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A single chiral atom or similar structural feature in a compound causes that compound to have two possible structures which are non-superposable, each a mirror image of the other. Each member of the pair is termed an enantiomorph, the property is termed enantiomerism. The presence of multiple features in a given compound increases the number of geometric forms possible. Enantiopure compounds refer to samples having, within the limits of detection and they are sometimes called optical isomers for this reason. Enantiomer members often have different chemical reactions with other enantiomer substances, since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. Owing to this discovery, drugs composed of one enantiomer can be developed to enhance the pharmacological efficacy. An example is eszopiclone, which is enantiopure and therefore administered in doses that are exactly 1/2 of the older, in the case of eszopiclone, the S enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive. A dose of 2 mg of zopiclone must be administered to produce the therapeutic effect as 1 mg of eszopiclone. In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures, in the absence of an effective enantiomeric environment, separation of a racemic mixture into its enantiomeric components is impossible. The R/S system is an important nomenclature system for denoting distinct enantiomers, another system is based on prefix notation for optical activity, - and - or d- and l-. The Latin for left and right is laevus and dexter, respectively, left and right have always had moral connotations, and the Latin words for these are sinister and rectus. The English word right is a cognate of rectus and this is the origin of the D, L and S, R notations, and the employment of prefixes levo- and dextro- in common names. Most compounds that one or more asymmetric carbon atoms show enantiomerism. There are a few compounds that do have asymmetric carbon atoms. An example of such an enantiomer is the sedative thalidomide, which was sold in a number of countries across the world from 1957 until 1961 and it was withdrawn from the market when it was found to cause of birth defects. One enantiomer caused the desirable effects, while the other, unavoidably present in equal quantities. The herbicide mecoprop is a mixture, with the --enantiomer possessing the herbicidal activity. Another example is the antidepressant drugs escitalopram and citalopram, citalopram is a racemate, escitalopram is a pure enantiomer
12.
Carbenoid
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In chemistry a carbenoid is a reactive intermediate that shares reaction characteristics with a carbene. Carbenoids appear as intermediates in many other reactions, in one system a carbenoid chloroalkyllithium reagent is prepared in situ from a sulfoxide and t-BuLi which reacts the boronic ester to give an ate complex. The ate complex undergoes a 1, 2-metallate rearrangement to give the homologated product, the enantiopurity of the chiral sulfoxide is preserved in the ultimate product after oxidation of the boronic ester to the alcohol indicating that a true carbene was never involved in the sequence
13.
Vacuum distillation
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This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure. Vacuum distillation is used with or without heating the mixture, laboratory-scale vacuum distillation is used when liquids to be distilled have high atmospheric boiling points or chemically change at temperatures near their atmospheric boiling points. Temperature sensitive materials also require vacuum distillation to remove solvents from the mixture without damaging the product, another reason vacuum distillation is used is that compared to steam distillation there is a lower level of residue build up. This is important in applications where heat transfer is produced using heat exchangers. There are many applications for vacuum distillation as well as many types of distillation set-ups. Safety is an important consideration when using glassware as part of the set-up, all of the glass components should be carefully examined for scratches and cracks which could result in implosions when the vacuum is applied. Wrapping as much of the glassware with tape as is practical helps to prevent dangerous scattering of glass shards in the event of an implosion, rotary evaporation is a type of vacuum distillation apparatus used to remove bulk solvents from the liquid being distilled. It is also used by regulatory agencies for determining the amount of solvents in paint, coatings. Rotary evaporation set-ups include an apparatus referred to as a Rotovap which rotates the distillation flask to enhance the distillation, rotating the flask throws up liquid on the walls of the flask and thus increases the surface area for evaporation. Heat is often applied to the distillation flask by partially immersing it in a heated bath of water or oil. Typically, the vacuum in such systems is generated by a water aspirator or a pump of some type. Some compounds have high boiling point temperatures as well as being air sensitive, a simple laboratory vacuum distillation glassware set-up can be used, in which the vacuum can be replaced with an inert gas after the distillation is complete. However, this is not a completely satisfactory system if it is desired to collect fractions under a reduced pressure, for better results or for very air sensitive compounds, either a Perkin triangle distillation set-up or a short-path distillation set-up can be used. To do this, the distillate receiver vessel is first isolated from the vacuum by means of the Teflon valves, the vacuum over the sample is then replaced with an inert gas and the distillate receiver can then be stoppered and removed from the system. Vacuum distillation of moderately air/water-sensitive liquid can be done using standard Schlenk-line techniques, when assembling the set-up apparatus, all of the connecting lines are clamped so that they cannot pop off. Care is taken to prevent potential bumping as the liquid in the still pot degases, while establishing the vacuum, the flow of coolant is started through the short-path distillation head. Once the desired vacuum is established, heat is applied to the still pot, if needed, the first portion of distillate can be discarded by purging with inert gas and changing out the distillate receiver. When the distillation is complete, the heat is removed, the connection is closed, and inert gas is purged through the distillation head
14.
Boiling point
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The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the environmental pressure. A liquid in a vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure has a boiling point than when that liquid is at atmospheric pressure. For a given pressure, different liquids boil at different temperatures, for example, water boils at 100 °C at sea level, but at 93.4 °C at 2,000 metres altitude. The normal boiling point of a liquid is the case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level,1 atmosphere. At that temperature, the pressure of the liquid becomes sufficient to overcome atmospheric pressure. The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar, the heat of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure. Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation, evaporation is a surface phenomenon in which molecules located near the liquids edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, a saturated liquid contains as much thermal energy as it can without boiling. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase, the liquid can be said to be saturated with thermal energy. Any addition of energy results in a phase transition. If the pressure in a system remains constant, a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy is removed, similarly, a liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied. The boiling point corresponds to the temperature at which the pressure of the liquid equals the surrounding environmental pressure. Thus, the point is dependent on the pressure. Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa, at higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased pressure up to the critical point, the boiling point cannot be increased beyond the critical point. Likewise, the point decreases with decreasing pressure until the triple point is reached
15.
Pyrolysis
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Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. It involves the change of chemical composition and physical phase. The word is coined from the Greek-derived elements pyro fire and lysis separating, Pyrolysis is a type of thermolysis, and is most commonly observed in organic materials exposed to high temperatures. It is one of the involved in charring wood, starting at 200–300 °C. It also occurs in fires where solid fuels are burning or when vegetation comes into contact with lava in volcanic eruptions, in general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization and these specialized uses of pyrolysis may be called various names, such as dry distillation, destructive distillation, or cracking. Pyrolysis is also used in the creation of nanoparticles, zirconia, Pyrolysis also plays an important role in several cooking procedures, such as baking, frying, grilling, and caramelizing. It is a tool of analysis, for example, in mass spectrometry. Indeed, many important chemical substances, such as phosphorus and sulfuric acid, were first obtained by this process, Pyrolysis has been assumed to take place during catagenesis, the conversion of buried organic matter to fossil fuels. It is also the basis of pyrography, in their embalming process, the ancient Egyptians used a mixture of substances, including methanol, which they obtained from the pyrolysis of wood. Pyrolysis differs from other processes like combustion and hydrolysis in that it usually does not involve reactions with oxygen, water, in practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any system, a small amount of oxidation occurs. The term has also applied to the decomposition of organic material in the presence of superheated water or steam, for example. Pyrolysis is usually the first chemical reaction occurs in the burning of many solid organic fuels, like wood, cloth, and paper. Thus, the pyrolysis of common materials like wood, plastic, in pyrolysis there is a gas phase present. Pyrolysis occurs whenever food is exposed to high temperatures in a dry environment, such as roasting, baking, toasting. It is the process responsible for the formation of the golden-brown crust in foods prepared by those methods. In normal cooking, the food components that undergo pyrolysis are carbohydrates
16.
High-performance liquid chromatography
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High-performance liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pass a pressurized liquid solvent containing the mixture through a column filled with a solid adsorbent material. HPLC has been used for manufacturing, legal, research, Chromatography can be described as a mass transfer process involving adsorption. HPLC relies on pumps to pass a pressurized liquid and a sample mixture through a column filled with adsorbent, the active component of the column, the adsorbent, is typically a granular material made of solid particles, 2–50 micrometers in size. The components of the mixture are separated from each other due to their different degrees of interaction with the adsorbent particles. The pressurized liquid is typically a mixture of solvents and is referred to as a mobile phase and its composition and temperature play a major role in the separation process by influencing the interactions taking place between sample components and adsorbent. These interactions are physical in nature, such as hydrophobic, dipole–dipole and ionic, due to the small sample amount separated in analytical HPLC, typical column dimensions are 2. 1–4.6 mm diameter, and 30–250 mm length. Also HPLC columns are made with smaller sorbent particles and this gives HPLC superior resolving power when separating mixtures, which makes it a popular chromatographic technique. The schematic of an HPLC instrument typically includes a sampler, pumps, the sampler brings the sample mixture into the mobile phase stream which carries it into the column. The pumps deliver the flow and composition of the mobile phase through the column. The detector generates a signal proportional to the amount of sample component emerging from the column, a digital microprocessor and user software control the HPLC instrument and provide data analysis. Some models of pumps in a HPLC instrument can mix multiple solvents together in ratios changing in time. Various detectors are in use, such as UV/Vis, photodiode array or based on mass spectrometry. Most HPLC instruments also have an oven that allows for adjusting the temperature at which the separation is performed. The sample mixture to be separated and analyzed is introduced, in a small volume. The components of the move through the column at different velocities. The velocity of each component depends on its nature, on the nature of the stationary phase. The time at which an analyte elutes is called its retention time
17.
Thiol
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In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl or sulphydryl group. Thiols are the analogue of alcohols, and the word is a portmanteau of thion + alcohol. The –SH functional group itself is referred to as either a group or a sulfhydryl group. Many thiols have strong odors resembling that of garlic or rotten eggs, thiols are used as odorants to assist in the detection of natural gas, and the smell of natural gas is due to the smell of the thiol used as the odorant. Thiols are sometimes referred to as mercaptans, thiols and alcohols have similar connectivity. Because sulfur is a larger element than oxygen, the C–S bond lengths, the C–S–H angles approach 90° whereas the angle for the C-O-H group are more open. In the solid or liquids, the hydrogen-bonding between individual groups is weak, the main cohesive force being van der Waals interactions between the highly polarizable divalent sulfur centers. Due to the lesser electronegativity difference between sulfur and hydrogen compared to oxygen and hydrogen, an S–H bond is less polar than the hydroxyl group, thiols have a lower dipole moment relative to the corresponding alcohol. There are several ways to name the alkylthiols, The suffix -thiol is added to the name of the alkane and this method is nearly identical to naming an alcohol and is used by the IUPAC. The word mercaptan replaces alcohol in the name of the equivalent alcohol compound, example, CH3SH would be methyl mercaptan, just as CH3OH is called methyl alcohol. The term sulfanyl or mercapto is used as a prefix, many thiols have strong odors resembling that of garlic. The odors of thiols, particularly those of low weight, are often strong. The spray of skunks consists mainly of low-molecular-weight thiols and derivatives and these compounds are detectable by the human nose at concentrations of only 10 parts per billion. Human sweat contains /-3-methyl-3-sulfanylhexan-1-ol, detectable at 2 parts per billion and having a fruity, methanethiol is a strong-smelling volatile thiol, also detectable at parts per billion levels, found in male mouse urine. Lawrence C. Katz and co-workers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice. Copper has been shown to be required by a specific mouse olfactory receptor, MOR244-3, thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast and the skunky odor of beer that has been exposed to ultraviolet light. Not all thiols have unpleasant odors, for example, furan-2-ylmethanethiol contributes to the aroma of roasted coffee, whereas grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit. The effect of the compound is present only at low concentrations
18.
Gold
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Gold is a chemical element with symbol Au and atomic number 79. In its purest form, it is a bright, slightly yellow, dense, soft, malleable. Chemically, gold is a metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions, Gold often occurs in free elemental form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the element silver and also naturally alloyed with copper. Less commonly, it occurs in minerals as gold compounds, often with tellurium, golds atomic number of 79 makes it one of the higher numbered, naturally occurring elements. It is thought to have produced in supernova nucleosynthesis, from the collision of neutron stars. Because the Earth was molten when it was formed, almost all of the present in the early Earth probably sank into the planetary core. Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of acid and hydrochloric acid. Gold also dissolves in solutions of cyanide, which are used in mining and electroplating. Gold dissolves in mercury, forming amalgam alloys, but this is not a chemical reaction, as a precious metal, gold has been used for coinage, jewelry, and other arts throughout recorded history. A total of 186,700 tonnes of gold is in existence above ground, the world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry. Gold is also used in infrared shielding, colored-glass production, gold leafing, certain gold salts are still used as anti-inflammatories in medicine. As of 2014, the worlds largest gold producer by far was China with 450 tonnes, Gold is cognate with similar words in many Germanic languages, deriving via Proto-Germanic *gulþą from Proto-Indo-European *ǵʰelh₃-. The symbol Au is from the Latin, aurum, the Latin word for gold, the Proto-Indo-European ancestor of aurum was *h₂é-h₂us-o-, meaning glow. This word is derived from the root as *h₂éu̯sōs, the ancestor of the Latin word Aurora. This etymological relationship is presumably behind the frequent claim in scientific publications that aurum meant shining dawn, Gold is the most malleable of all metals, a single gram can be beaten into a sheet of 1 square meter, and an avoirdupois ounce into 300 square feet. Gold leaf can be thin enough to become semi-transparent
19.
Self-assembled monolayer
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Self-assembled monolayers of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate and this is the case for instance of the two-dimensional supramolecular networks of e. g. Perylenetetracarboxylic dianhydride on gold or of e. g. porphyrins on highly oriented pyrolitic graphite. In other cases the molecules possess a head group that has an affinity to the substrate. Such a SAM consisting of a group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc, SAMs are created by the chemisorption of head groups onto a substrate from either the vapor or liquid phase followed by a slow organization of tail groups. The head groups assemble together on the substrate, while the tail groups assemble far from the substrate, areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer. Adsorbate molecules adsorb readily because they lower the surface free-energy of the substrate and are due to the strong chemisorption of the head groups. These bonds create monolayers that are more stable than the bonds of Langmuir–Blodgett films. A Trichlorosilane based head group, for example in a FDTS molecule, reacts with a group on a substrate. Thiol-metal bonds are on the order of 100 kJ/mol, making them fairly stable in a variety of temperatures, solvents, the monolayer packs tightly due to van der Waals interactions, thereby reducing its own free energy. The adsorption can be described by the Langmuir adsorption isotherm if lateral interactions are neglected, if they cannot be neglected, the adsorption is better described by the Frumkin isotherm. Selecting the type of head group depends on the application of the SAM, typically, head groups are connected to a molecular chain in which the terminal end can be functionalized to vary the wetting and interfacial properties. An appropriate substrate is chosen to react with the head group, Substrates can be planar surfaces, such as silicon and metals, or curved surfaces, such as nanoparticles. Alkanethiols are the most commonly used molecules for SAMs, alkanethiols are molecules with an alkyl chain, ⁿ chain, as the back bone, a tail group, and a S-H head group. Other types of interesting molecules include aromatic thiols, of interest in molecular electronics, an example is the dithiol 1, 4-Benzenedimethanethiol ). Interest in such dithiols stems from the possibility of linking the two ends to metallic contacts, which was first used in molecular conduction measurements. Thiols are frequently used on metal substrates because of the strong affinity of sulfur for these metals. The sulfur gold interaction is semi-covalent and has a strength of approximately 45kcal/mol, in addition, gold is an inert and biocompatible material that is easy to acquire
20.
Molecular self-assembly
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Molecular self-assembly is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. There are two types of self-assembly and these are intramolecular self-assembly and intermolecular self-assembly. Commonly, the term molecular self-assembly refers to intermolecular self-assembly, while the analog is more commonly called folding. Molecular self-assembly is a key concept in supramolecular chemistry and this is because assembly of molecules in such systems is directed through noncovalent interactions as well as electromagnetic interactions. Common examples include the formation of micelles, vesicles, liquid crystal phases, further examples of supramolecular assemblies demonstrate that a variety of different shapes and sizes can be obtained using molecular self-assembly. Molecular self-assembly allows the construction of challenging molecular topologies, one example is Borromean rings, interlocking rings wherein removal of one ring unlocks each of the other rings. DNA has been used to prepare a molecular analog of Borromean rings, more recently, a similar structure has been prepared using non-biological building blocks. While a mechanistic understanding of how supramolecular self-assembly occurs remains largely unknown, molecular self-assembly underlies the construction of biologic macromolecular assemblies in living organisms, and so is crucial to the function of cells. Molecular self-assembly of incorrectly folded proteins into insoluble amyloid fibers is responsible for infectious prion-related neurodegenerative diseases, molecular self-assembly is an important aspect of bottom-up approaches to nanotechnology. Using molecular self-assembly the final structure is programmed in the shape, Self-assembly is referred to as a bottom-up manufacturing technique in contrast to a top-down technique such as lithography where the desired final structure is carved from a larger block of matter. In the speculative vision of nanotechnology, microchips of the future might be made by molecular self-assembly. An advantage to constructing nanostructure using molecular self-assembly for biological materials is that they will degrade back into individual molecules that can be broken down by the body, DNA nanotechnology is an area of current research that uses the bottom-up, self-assembly approach for nanotechnological goals. DNA nanotechnology uses the unique molecular recognition properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties and these DNA structures have also been used as templates in the assembly of other molecules such as gold nanoparticles and streptavidin proteins. The spontaneous assembly of a layer of molecules at interfaces is usually referred to as two-dimensional self-assembly. Early direct proofs showing that molecules can assembly into higher-order architectures at solid interfaces came with the development of scanning tunneling microscopy, the design of molecules and conditions leading to the formation of highly-crystalline architectures is considered today a form of 2D crystal engineering at the nanoscopic scale. Supramolecular assembly Foldamer Macromolecular assembly Ice-nine Self-assembly of nanoparticles
21.
Carbocation
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A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are methenium CH+3, methanium CH+5, some carbocations may have two or more positive charges, on the same carbon atom or on different atoms, such as the ethylene dication C 2H2+4. Until the early 1970s, all carbocations were called carbonium ions and this nomenclature was proposed by G. A. Olah. One textbook retains the name of carbonium ion for carbenium ion to this day. The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene and then heated the product to obtain a crystalline, water-soluble material, C 7H 7Br. He did not suggest a structure for it, however, Doering and this ion is predicted to be aromatic by Hückels rule. In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep-yellow solutions in concentrated sulfuric acid, triphenylmethyl chloride similarly formed orange complexes with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the character of the compounds formed. He dubbed the relationship between color and salt formation halochromy, of which green is a prime example. Carbocations are reactive intermediates in organic reactions. This idea, first proposed by Julius Stieglitz in 1899, was developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, the E1 reaction, the chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them. The first NMR spectrum of a carbocation in solution was published by Doering et al. in 1958. It was the ion, made by treating hexamethylbenzene with methyl chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C, the NMR spectrum established that it was non-classically bridged. In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a species on dissolving tert-butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al. the charged carbon atom in a carbocation is a sextet, i. e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability. Therefore, carbocations are often reactive, seeking to fill the octet of electrons as well as regain a neutral charge
22.
Bromination
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Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. Dehalogenation is the reverse of halogenation and results in the removal of a halogen from a molecule, the pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation, several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The structure of the substrate is one factor that determines the pathway, saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. The regiochemistry of the halogenation of alkanes is usually determined by the weakness of the available C–H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the free radicals. Free radical halogenation is used for the production of chlorinated methanes. Unsaturated compounds, especially alkenes and alkynes, add halogens, RCH=CHR′ + X2 → RCHX–CHXR′ The addition of halogens to alkenes proceeds via intermediate halonium ions, in special cases, such intermediates have been isolated. Aromatic compounds are subject to electrophilic halogenation, RC6H5 + X2 → HX + RC6H4X The facility of halogenation is influenced by the halogen, fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than fluorine and chlorine, while iodine is least reactive of them all. The facility of hydrogenolysis follows the trend, iodine is most easily removed from organic compounds. In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide, in the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated. Fluorination with elemental fluorine requires highly specialised conditions and apparatus, many commercially important organic compounds are fluorinated electrochemically using hydrogen fluoride as the source of fluorine. The method is called electrochemical fluorination, aside from F2 and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt fluoride. Both saturated and unsaturated compounds react directly with chlorine, the former usually requiring UV light to initiate homolysis of chlorine, chlorination is conducted on a large scale industrially, major processes include routes to 1, 2-dichloroethane, as well as various chlorinated ethanes, as solvents. Bromination is more selective than chlorination because the reaction is less exothermic, most commonly bromination is conducted by the addition of Br2 to alkenes. An example of bromination is the synthesis of the anesthetic halothane from trichloroethylene, Organobromine compounds are the most common organohalides in nature. Their formation is catalyzed by the enzyme bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant, the oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually
23.
Bromine
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Bromine is a chemical element with symbol Br and atomic number 35. It is the third-lightest halogen, and is a fuming red-brown liquid at room temperature that readily to form a similarly coloured gas. Its properties are intermediate between those of chlorine and iodine. Isolated independently by two chemists, Carl Jacob Löwig and Antoine Jérôme Balard, its name was derived from the Ancient Greek βρῶμος stench, referencing its sharp and disagreeable smell. Elemental bromine is very reactive and thus does not occur free in nature, while it is rather rare in the Earths crust, the high solubility of the bromide ion has caused its accumulation in the oceans. Commercially the element is easily extracted from brine pools, mostly in the United States, Israel, the mass of bromine in the oceans is about one three-hundredth of that of chlorine. At high temperatures, organobromine compounds readily convert to free bromine atoms and this effect makes organobromine compounds useful as fire retardants and more than half the bromine produced worldwide each year is put to this purpose. Unfortunately, the same property causes sunlight to convert volatile organobromine compounds to free bromine atoms in the atmosphere, as a result, many organobromide compounds—such as the pesticide methyl bromide—are no longer used. Bromine compounds are used in well drilling fluids, in photographic film. Bromine has sometimes been considered to be essential in humans, but with the support of only limited circumstantial evidence. As a pharmaceutical, the bromide ion has inhibitory effects on the central nervous system. They retain niche uses as antiepileptics, bromine was discovered independently by two chemists, Carl Jacob Löwig and Antoine Balard, in 1825 and 1826, respectively. Löwig isolated bromine from a water spring from his hometown Bad Kreuznach in 1825. Löwig used a solution of the mineral salt saturated with chlorine, after evaporation of the ether a brown liquid remained. With this liquid as a sample for his work he applied for a position in the laboratory of Leopold Gmelin in Heidelberg, the publication of the results was delayed and Balard published his results first. Balard found bromine chemicals in the ash of seaweed from the marshes of Montpellier. The seaweed was used to produce iodine, but also contained bromine, Balard distilled the bromine from a solution of seaweed ash saturated with chlorine. In his publication, Balard states that he changed the name from muride to brôme on the proposal of M. Anglada, brôme derives from the Greek βρωμος
24.
Alcohol
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In chemistry, an alcohol is any organic compound in which the hydroxyl functional group is bound to a saturated carbon atom. The term alcohol originally referred to the alcohol ethanol, the predominant alcohol in alcoholic beverages. The suffix -ol in non-systematic names also typically indicates that the substance includes a functional group and, so. But many substances, particularly sugars contain hydroxyl functional groups without using the suffix, an important class of alcohols, of which methanol and ethanol are the simplest members is the saturated straight chain alcohols, the general formula for which is CnH2n+1OH. The word alcohol is from the Arabic kohl, a used as an eyeliner. Al- is the Arabic definite article, equivalent to the in English, alcohol was originally used for the very fine powder produced by the sublimation of the natural mineral stibnite to form antimony trisulfide Sb 2S3, hence the essence or spirit of this substance. It was used as an antiseptic, eyeliner, and cosmetic, the meaning of alcohol was extended to distilled substances in general, and then narrowed to ethanol, when spirits as a synonym for hard liquor. Bartholomew Traheron, in his 1543 translation of John of Vigo, Vigo wrote, the barbarous auctours use alcohol, or alcofoll, for moost fine poudre. The 1657 Lexicon Chymicum, by William Johnson glosses the word as antimonium sive stibium, by extension, the word came to refer to any fluid obtained by distillation, including alcohol of wine, the distilled essence of wine. Libavius in Alchymia refers to vini alcohol vel vinum alcalisatum, Johnson glosses alcohol vini as quando omnis superfluitas vini a vino separatur, ita ut accensum ardeat donec totum consumatur, nihilque fæcum aut phlegmatis in fundo remaneat. The words meaning became restricted to spirit of wine in the 18th century and was extended to the class of substances so-called as alcohols in modern chemistry after 1850, the term ethanol was invented 1892, based on combining the word ethane with ol the last part of alcohol. In the IUPAC system, in naming simple alcohols, the name of the alkane chain loses the terminal e and adds ol, e. g. as in methanol and ethanol. When necessary, the position of the group is indicated by a number between the alkane name and the ol, propan-1-ol for CH 3CH 2CH 2OH, propan-2-ol for CH 3CHCH3. If a higher priority group is present, then the prefix hydroxy is used, in other less formal contexts, an alcohol is often called with the name of the corresponding alkyl group followed by the word alcohol, e. g. methyl alcohol, ethyl alcohol. Propyl alcohol may be n-propyl alcohol or isopropyl alcohol, depending on whether the group is bonded to the end or middle carbon on the straight propane chain. As described under systematic naming, if another group on the molecule takes priority, Alcohols are then classified into primary, secondary, and tertiary, based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl functional group. The primary alcohols have general formulas RCH2OH, the simplest primary alcohol is methanol, for which R=H, and the next is ethanol, for which R=CH3, the methyl group. Secondary alcohols are those of the form RRCHOH, the simplest of which is 2-propanol, for the tertiary alcohols the general form is RRRCOH
25.
Nitrate
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Nitrate is a polyatomic ion with the molecular formula NO−3 and a molecular mass of 62.0049 g/mol. Nitrates also describe the functional group RONO2. These nitrate esters are a class of explosives. The anion is the base of nitric acid, consisting of one central nitrogen atom surrounded by three identically bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a charge of −1. This arrangement is used as an example of resonance. Like the isoelectronic carbonate ion, the ion can be represented by resonance structures. A common example of a nitrate salt is potassium nitrate. A rich source of nitrate in the human body comes from diets rich in leafy green foods, such as spinach. NO3- is the active component within beetroot juice and other vegetables. Nitrite and water are converted in the body to nitric oxide, nitrate salts are found naturally on earth as large deposits, particularly of nitratine, a major source of sodium nitrate. Nitrates are found in man-made fertilizers, as a byproduct of lightning strikes in earths nitrogen-oxygen rich atmosphere, nitric acid is produced when nitrogen dioxide reacts with water vapor. Nitrates are mainly produced for use as fertilizers in agriculture because of their solubility and biodegradability. The main nitrate fertilizers are ammonium, sodium, potassium, several million kilograms are produced annually for this purpose. The second major application of nitrates is as oxidizing agents, most notably in explosives where the oxidation of carbon compounds liberates large volumes of gases. Sodium nitrate is used to air bubbles from molten glass. Mixtures of the salt are used to harden some metals. Explosives and table tennis balls are made from celluloid, although nitrites are the nitrogen compound chiefly used in meat curing, nitrates are used in certain specialty curing processes where a long release of nitrite from parent nitrate stores is needed
26.
Steric effects
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See also, intramolecular forces Steric effects arise from a fact that each atom within a molecule occupies a certain amount of space. If atoms are too close together, there is an associated cost in energy due to overlapping electron clouds. Steric hindrance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related molecules with smaller groups. Steric hindrance between adjacent groups can also restrict torsional bond angles, however, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small. This is the responsible for the observed shape of rotaxanes. When a Lewis acid and Lewis base cannot combine due to steric hindrance, Steric shielding occurs when a charged group on a molecule is seemingly weakened or spatially shielded by less charged atoms, including counterions in solution. Steric attraction occurs when molecules have shapes or geometries that are optimized for interaction with one another, in these cases molecules will react with each other most often in specific arrangements. Chain crossing, A chain, ring, or a set of rings cannot change from one conformation to another if it would require a chain to pass through itself or another chain and this effect, or lack of, is responsible for the shape of catenanes and molecular knots. Steric repulsions between different parts of a system were found to be of key importance in governing the direction of transition-metal-mediated transformations. Steric effects can even induce a switch in the catalytic reaction. Steric inhibition of resonance is present only in benzene rings, the presence of any group at the ortho position in benzoic acid will throw the carboxylic acid group out of the plane, and thus its mesomeric connection with the benzene ring vanishes. This means that ortho-substituted benzoic acids are stronger than meta- and para-substituted benzoic acids, Steric inhibition of protonation Consider 2,2,6, 6-Tetramethylpiperidine and in, in-diphosphine. The structure, properties, and reactivity of a molecule is dependent on straightforward bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and this bonding supplies a basic molecular skeleton that is modified by repulsive forces. These repulsive forces include the interactions described above. Basic bonding and steric are at times insufficient to explain many structures, properties, there are more esoteric electronic effects but these are among the most important when considering structure and chemical reactivity. A special computational procedure was developed to separate electronic and steric effects of an group in the molecule and to reveal their influence on structure. Understanding steric effects is critical to chemistry, biochemistry and pharmacology, in organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees. Steric effects often dictate reaction pathways in organic synthesis because there are configurations in which molecules can collide
27.
Electrophile
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In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to a rich centre. It participates in a reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids, most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons. The electrophiles are attacked by the most electron-populated part of one nucleophile and these occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of water to titrate against a sample to deduce the number of double bonds present. Forming of a bromonium ion The alkene is working as an electron donor. The three-membered bromonium ion 2 consisted with two atoms and a bromine atom forms with a release of Br−. Attacking of bromide ion The bromonium ion is opened by the attack of Br− from the back side and this yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether and this process is called AdE2 mechanism. Iodine, chlorine, sulfenyl ion, mercury cation, and dichlorocarbene also react through similar pathways, the direct conversion of 1 to 3 will appear when the Br− is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group, there is an example of the isolation of the bromonium ion 2. Hydrogen halides such as hydrogen chloride adds to alkenes to give alkyl halides in hydrohalogenation, for example, the reaction of Hcl with ethylene furnishes chloroethane. The reaction proceeds with an intermediate, being different from the above halogen addition. An example is shown below, Proton adds to one of the atoms on the alkene to form cation 1. Chloride ion combines with the cation 1 to form the adducts 2 and 3, in this manner, the stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikovs rule, thus, H+ attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation will form. This process is called A-SE2 mechanism, hydrogen fluoride and hydrogen iodide react with alkenes in a similar manner, and Markovnikov-type products will be given
28.
Thionyl bromide
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Thionyl bromide is the chemical compound SOBr2. It is less stable and less used than its chloride analogue. Otherwise it hydrolyzes readily to release sulfur dioxide, SOBr2 + H2O → SO2 + 2HBr SOBr2 hydrolyzes readily to release dangerous HBr, in Paquette, E. Encyclopedia of Reagents for Organic Synthesis. New York, J. Wiley & Sons
29.
Quaternary ammonium cation
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Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR+4, R being an alkyl group or an aryl group. Unlike the ammonium ion and the primary, secondary, or tertiary ammonium cations, Quaternary ammonium salts or quaternary ammonium compounds are salts of quaternary ammonium cations. Quaternary ammonium compounds are prepared by the alkylation of amines with a halocarbon. In older literature this is called a Menshutkin reaction, however modern chemists usually refer to it simply as quaternization. The reaction can be used to produce a compound with unequal alkyl chain lengths and they also are stable toward most nucleophiles. The latter is indicated by the stability of the hydroxide salts such as tetramethylammonium hydroxide, because of their resilience, many unusual anions have been isolated as the quaternary ammonium salts. Examples include tetramethylammonium pentafluoroxenate, containing the highly reactive pentafluoroxenate ion, permanganate can be solubilized in organic solvents, when deployed as its NBu+4 salt. With exceptionally strong bases, quat cations degrade and they undergo Sommelet–Hauser rearrangement and Stevens rearrangement, as well as dealkylation under harsh conditions. Quaternary ammonium cations containing N–C–C–H units can also undergo the Hofmann elimination, Quaternary ammonium salts are used as disinfectants, surfactants, fabric softeners, and as antistatic agents. In liquid fabric softeners, the salts are often used. In dryer anticling strips, the salts are often used. Spermicidal jellies also contain quaternary ammonium salts, Quaternary ammonium compounds have also been shown to have antimicrobial activity. Certain quaternary ammonium compounds, especially those containing long alkyl chains, are used as antimicrobials, also good against fungi, amoebas, and enveloped viruses, quaternary ammonium compounds are believed to act by disrupting the cell membrane. Quaternary ammonium compounds are lethal to a variety of organisms except endospores, Mycobacterium tuberculosis. Quaternary ammonium compounds are cationic detergents, as well as disinfectants and they are very effective in combination with phenols. Quaternary ammonium compounds are deactivated by anionic detergents, also, they work best in soft waters. Effective levels are at 200 ppm and they are effective at temperatures up to 100 °C. Quaternary ammonium salts are used in the foodservice industry as sanitizing agents
30.
Isotope
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Isotopes are variants of a particular chemical element which differ in neutron number. All isotopes of an element have the same number of protons in each atom. The number of protons within the nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope, the number of nucleons in the nucleus is the atoms mass number, and each isotope of a given element has a different mass number. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12,13 and 14 respectively. The atomic number of carbon is 6, which means that carbon atom has 6 protons. Nuclide refers to a rather than to an atom. Identical nuclei belong to one nuclide, for each nucleus of the carbon-13 nuclide is composed of 6 protons and 7 neutrons. The nuclide concept emphasizes nuclear properties over chemical properties, whereas the isotope concept emphasizes chemical over nuclear, the neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. Because isotope is the term, it is better known than nuclide. An isotope and/or nuclide is specified by the name of the particular element followed by a hyphen, when a chemical symbol is used, e. g. C for carbon, standard notation is to indicate the number with a superscript at the upper left of the chemical symbol. Because the atomic number is given by the element symbol, it is common to only the mass number in the superscript. The letter m is sometimes appended after the number to indicate a nuclear isomer. For example, 14C is a form of carbon, whereas 12C. There are about 339 naturally occurring nuclides on Earth, of which 286 are primordial nuclides, primordial nuclides include 32 nuclides with very long half-lives and 254 that are formally considered as stable nuclides, because they have not been observed to decay. In most cases, for reasons, if an element has stable isotopes. Theory predicts that many apparently stable isotopes/nuclides are radioactive, with extremely long half-lives, of the 254 nuclides never observed to decay, only 90 of these are theoretically stable to all known forms of decay
31.
Absorption (electromagnetic radiation)
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In physics, absorption of electromagnetic radiation is the way in which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the energy is transformed into internal energy of the absorber. The reduction in intensity of a wave propagating through a medium by absorption of a part of its photons is often called attenuation. The mass attenuation coefficient, also called mass extinction coefficient, which is the absorption coefficient divided by density, the absorption cross section and scattering cross-section are closely related to the absorption and attenuation coefficients, respectively. Extinction in astronomy is equivalent to the attenuation coefficient, absorbance and optical depth are two related measures All these quantities measure, at least to some extent, how well a medium absorbs radiation. However, practitioners of different fields and techniques tend to use different quantities drawn from the list above. The absorbance of an object quantifies how much of the incident light is absorbed by it and this may be related to other properties of the object through the Beer–Lambert law. A few examples of absorption are ultraviolet–visible spectroscopy, infrared spectroscopy, understanding and measuring the absorption of electromagnetic radiation has a variety of applications. Here are a few examples, In meteorology and climatology, global and local temperatures depend in part on the absorption of radiation by gases and land. In medicine, X-rays are absorbed to different extents by different tissues, for example, see computation of radiowave attenuation in the atmosphere used in satellite link design. In chemistry and materials science, because different materials and molecules will absorb radiation to different extents at different frequencies, in optics, sunglasses, colored filters, dyes, and other such materials are designed specifically with respect to which visible wavelengths they absorb, and in what proportions. In physics, the D-region of Earths ionosphere is known to significantly absorb radio signals that fall within the electromagnetic spectrum. Optical Propagation in Linear Media, Atmospheric Gases and Particles, Solid-State Components, physics Archive - Ask a scientist
32.
Ultraviolet
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Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
33.
Band gap
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In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. It is closely related to the HOMO/LUMO gap in chemistry, therefore, the band gap is a major factor determining the electrical conductivity of a solid. Every solid has its own characteristic energy-band structure and this variation in band structure is responsible for the wide range of electrical characteristics observed in various materials. In semiconductors and insulators, electrons are confined to a number of bands of energy, the term band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another, however, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials, electrons can gain enough energy to jump to the conduction band by absorbing either a phonon or a photon. In contrast, a material with a band gap is an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap, the conductivity of intrinsic semiconductors is strongly dependent on the band gap. Band-gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs and it is also possible to construct layered materials with alternating compositions by techniques like molecular-beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors, laser diodes, the distinction between semiconductors and insulators is a matter of convention. One approach is to think of semiconductors as a type of insulator with a band gap. Insulators with a band gap, usually greater than 4 eV, are not considered semiconductors. Electron mobility also plays a role in determining a materials informal classification, the band-gap energy of semiconductors tends to decrease with increasing temperature. When temperature increases, the amplitude of atomic vibrations increase, leading to larger interatomic spacing, the interaction between the lattice phonons and the free electrons and holes will also affect the band gap to a smaller extent. The relationship between band gap energy and temperature can be described by Varshnis empirical expression, E g = E g − α T2 T + β, in a regular semiconductor crystal, the band gap is fixed owing to continuous energy states. In a quantum dot crystal, the gap is size dependent. It is also known as quantum confinement effect, Band gaps also depend on pressure. Band gaps can be direct or indirect, depending on the electronic band structure
34.
Electronvolt
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In physics, the electronvolt is a unit of energy equal to approximately 1. 6×10−19 joules. By definition, it is the amount of energy gained by the charge of an electron moving across an electric potential difference of one volt. Thus it is 1 volt multiplied by the elementary charge, therefore, one electronvolt is equal to 6981160217662079999♠1. 6021766208×10−19 J. The electronvolt is not a SI unit, and its definition is empirical, like the elementary charge on which it is based, it is not an independent quantity but is equal to 1 J/C √2hα / μ0c0. It is a unit of energy within physics, widely used in solid state, atomic, nuclear. It is commonly used with the metric prefixes milli-, kilo-, in some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion electronvolts, it is equivalent to the GeV. By mass–energy equivalence, the electronvolt is also a unit of mass and it is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c2, where c is the speed of light in vacuum. It is common to express mass in terms of eV as a unit of mass. The mass equivalent of 1 eV/c2 is 1 eV / c 2 = ⋅1 V2 =1.783 ×10 −36 kg. For example, an electron and a positron, each with a mass of 0.511 MeV/c2, the proton has a mass of 0.938 GeV/c2. In general, the masses of all hadrons are of the order of 1 GeV/c2, the unified atomic mass unit,1 gram divided by Avogadros number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula,1 u =931.4941 MeV/c2 =0.9314941 GeV/c2, in high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy and this gives rise to usage of eV as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of units are LMT−1. The dimensions of units are L2MT−2. Then, dividing the units of energy by a constant that has units of velocity. In the field of particle physics, the fundamental velocity unit is the speed of light in vacuum c. Thus, dividing energy in eV by the speed of light, the fundamental velocity constant c is often dropped from the units of momentum by way of defining units of length such that the value of c is unity
35.
Molecular symmetry
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Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a concept in chemistry, as it can predict or explain many of a molecules chemical properties, such as its dipole moment. Many university level textbooks on chemistry, quantum chemistry. While various frameworks for the study of symmetry exist, group theory is the predominant one. This framework is useful in studying the symmetry of molecular orbitals, with applications such as the Hückel method, ligand field theory. Another framework on a scale is the use of crystal systems to describe crystallographic symmetry in bulk materials. Many techniques for the assessment of molecular symmetry exist, including X-ray crystallography and various forms of spectroscopy. Spectroscopic notation is based on symmetry considerations, the study of symmetry in molecules is an adaptation of mathematical group theory. The symmetry of a molecule can be described by 5 types of symmetry elements, symmetry axis, an axis around which a rotation by 360 ∘ n results in a molecule indistinguishable from the original. This is also called a rotational axis and abbreviated Cn. Examples are the C2 axis in water and the C3 axis in ammonia, a molecule can have more than one symmetry axis, the one with the highest n is called the principal axis, and by convention is aligned with the z-axis in a Cartesian coordinate system. Plane of symmetry, a plane of reflection through which a copy of the original molecule is generated. This is also called a plane and abbreviated σ. Water has two of them, one in the plane of the molecule itself and one perpendicular to it, a symmetry plane parallel with the principal axis is dubbed vertical and one perpendicular to it horizontal. A third type of symmetry plane exists, If a vertical symmetry plane additionally bisects the angle between two 2-fold rotation axes perpendicular to the axis, the plane is dubbed dihedral. A symmetry plane can also be identified by its Cartesian orientation, center of symmetry or inversion center, abbreviated i. A molecule has a center of symmetry when, for any atom in the molecule, in other words, a molecule has a center of symmetry when the points and correspond to identical objects. For example, if there is an atom in some point
36.
HOMO/LUMO
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In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, the energy difference between the HOMO and LUMO is termed the HOMO–LUMO gap. HOMO and LUMO are sometimes called frontier orbitals in frontier molecular orbital theory, the difference in energy between these two frontier orbitals can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution. Roughly, the HOMO level is to organic semiconductors what the valence band maximum is to inorganic semiconductors, the same analogy exists between the LUMO level and the conduction band minimum. In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur, a SOMO is a singly occupied molecular orbital such as half-filled HOMO of a radical. This abbreviation may also be extended to semi occupied molecular orbital, frontier molecular orbital theory Diels–Alder reaction Electron configuration Koopmans theorem Ligand Organic semiconductor OrbiMol Molecular orbital database
37.
Surface states
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Surface states are electronic states found at the surface of materials. They are formed due to the transition from solid material that ends with a surface and are found only at the atom layers closest to the surface. The termination of a material with a surface leads to a change of the band structure from the bulk material to the vacuum. In the weakened potential at the surface, new states can be formed. As stated by Blochs theorem, eigenstates of the single-electron Schrödinger equation with a periodic potential. Here u n k is a function with the periodicity as the crystal, n is the band index. The allowed wave numbers for a potential are found by applying the usual Born–von Karman cyclic boundary conditions. The termination of a crystal, i. e. the formation of a surface, a simplified model of the crystal potential in one dimension can be sketched as shown in figure 1. In the crystal, the potential has the periodicity, a, the step potential shown in figure 1 is an oversimplification which is mostly convenient for simple model calculations. At a real surface the potential is influenced by image charges, given the potential in figure 1, it can be shown that the one-dimensional single-electron Schrödinger equation gives two qualitatively different types of solutions. The first type of states extends into the crystal and has Bloch character there and these type of solutions correspond to bulk states which terminate in an exponentially decaying tail reaching into the vacuum. The second type of states decays exponentially both into the vacuum and the bulk crystal and these type of solutions correspond to surface states, with wave functions localized close to the crystal surface. The first type of solution can be obtained for both metals and semiconductors, in semiconductors though, the associated eigenenergies have to belong to one of the allowed energy bands. The second type of solution exists in forbidden energy gap of semiconductors as well as in local gaps of the band structure of metals. It can be shown that the energies of these states all lie within the band gap, as a consequence, in the crystal these states are characterized by an imaginary wavenumber leading to an exponential decay into the bulk. In the discussion of states, one generally distinguishes between Shockley states and Tamm states, named after the American physicist William Shockley and the Russian physicist Igor Tamm. However, there is no real distinction between the two terms, only the mathematical approach in describing surface states is different. Historically, surface states that arise as solutions to the Schrödinger equation in the framework of the free electron approximation for clean
38.
Electron affinity
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X + e− → X− + energy In solid state physics, the electron affinity for a surface is defined somewhat differently. This property is measured for atoms and molecules in the state only. A list of the electron affinities was used by Robert S. Mulliken to develop an electronegativity scale for atoms, equal to the average of the electron affinity, other theoretical concepts that use electron affinity include electronic chemical potential and chemical hardness. Another example, a molecule or atom that has a positive value of electron affinity than another is often called an electron acceptor. Together they may undergo charge-transfer reactions, to use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change ΔE in total energy has a negative value, electron capture for almost all non-noble gas atoms involves the release of energy and thus are exothermic. The positive values that are listed in tables of Eea are amounts or magnitudes and it is the word, released within the definition energy released that supplies the negative sign to ΔE. Confusion arises in mistaking Eea for a change in energy, ΔE, the relation between the two is Eea = −ΔE. However, if the assigned to Eea is negative, the negative sign implies a reversal of direction. In this case, the capture is an endothermic process. Negative values typically arise for the capture of a second electron, since almost all detachments an amount of energy listed on the table, those detachment reactions are endothermic, or ΔE >0. Although Eea varies greatly across the table, some patterns emerge. Generally, nonmetals have more positive Eea than metals, atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons, mercury most weakly attracts an extra electron, the electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values. Eea generally increases across a period in the periodic table, a trend of decreasing Eea going down the groups in the periodic table might be expected. The additional electron will be entering an orbital farther away from the nucleus, since this electron is farther from the nucleus it is less attracted to the nucleus and would release less energy when added. However, a counterexample to this trend can be found in Group 2. Thus, electron affinity follows the trend of electronegativity but not the up-down trend
39.
Field electron emission
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Field electron emission is emission of electrons induced by an electrostatic field. The most common context is field emission from a surface into vacuum. However, field emission can take place from solid or liquid surfaces, into vacuum, air, the field-induced promotion of electrons from the valence to conduction band of semiconductors can also be regarded as a form of field emission. When field emission is used without qualifiers it typically means cold emission, Field emission in pure metals occurs in high electric fields, the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon the work function. Examples of applications for surface field emission include construction of bright electron sources for electron microscopes or the discharge of induced charges from spacecraft. Devices which eliminate induced charges are termed charge-neutralizers, Field emission was explained by quantum tunneling of electrons in the late 1920s. This was one of the triumphs of the nascent quantum mechanics, the theory of field emission from bulk metals was proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim. A family of approximate equations, Fowler–Nordheim equations, is named after them, in some respects, field electron emission is a paradigm example of what physicists mean by tunneling. Unfortunately, it is also an example of the intense mathematical difficulties that can arise. Simple solvable models of the tunneling barrier lead to equations that get predictions of current density too low by a factor of 100 or more. To get even an approximate solution, it is necessary to use special approximate methods known in physics as semi-classical or quasi-classical methods, a consequence of these difficulties has been a heritage of misunderstanding and disinformation that still persists in some current field emission research literature. This article tries to present an account of field emission for the 21st century. Field electron emission, field-induced electron emission, field emission and electron field emission are general names for this experimental phenomenon, the first name is used here. Fowler–Nordheim tunneling is the tunneling of electrons through a rounded triangular barrier created at the surface of an electron conductor by applying a very high electric field. Individual electrons can escape by Fowler-Nordheim tunneling from many materials in different circumstances. Many solid and liquid materials can emit electrons in a CFE regime if a field of an appropriate size is applied. Fowler–Nordheim-type equations are a family of approximate equations derived to describe CFE from the electron states in bulk metals. The different members of the family represent different degrees of approximation to reality, there is no theoretical reason to believe that Fowler-Nordheim-type equations validly describe field emission from materials other than bulk crystalline solids
40.
Nanotechnology
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Nanotechnology is manipulation of matter on an atomic, molecular, and supramolecular scale. It is therefore common to see the plural form nanotechnologies as well as nanoscale technologies to refer to the range of research. Because of the variety of applications, governments have invested billions of dollars in nanotechnology research. Until 2012, through its National Nanotechnology Initiative, the USA has invested 3.7 billion dollars, scientists currently debate the future implications of nanotechnology. Nanotechnology may be able to create new materials and devices with a vast range of applications, such as in nanomedicine, nanoelectronics, biomaterials energy production. These concerns have led to a debate among advocacy groups and governments on whether regulation of nanotechnology is warranted. The term nano-technology was first used by Norio Taniguchi in 1974, also in 1986, Drexler co-founded The Foresight Institute to help increase public awareness and understanding of nanotechnology concepts and implications. In the 1980s, two major breakthroughs sparked the growth of nanotechnology in modern era, the microscopes developers Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory received a Nobel Prize in Physics in 1986. Binnig, Quate and Gerber also invented the atomic force microscope that year. Second, Fullerenes were discovered in 1985 by Harry Kroto, Richard Smalley, and Robert Curl, in the early 2000s, the field garnered increased scientific, political, and commercial attention that led to both controversy and progress. Controversies emerged regarding the definitions and potential implications of nanotechnologies, exemplified by the Royal Societys report on nanotechnology, challenges were raised regarding the feasibility of applications envisioned by advocates of molecular nanotechnology, which culminated in a public debate between Drexler and Smalley in 2001 and 2003. Meanwhile, commercialization of products based on advancements in nanoscale technologies began emerging and these products are limited to bulk applications of nanomaterials and do not involve atomic control of matter. Governments moved to promote and fund research into nanotechnology, such as in the U. S, by the mid-2000s new and serious scientific attention began to flourish. Projects emerged to produce nanotechnology roadmaps which center on atomically precise manipulation of matter and discuss existing and projected capabilities, goals, Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced, one nanometer is one billionth, or 10−9, of a meter. By comparison, typical carbon-carbon bond lengths, or the spacing between atoms in a molecule, are in the range 0. 12–0.15 nm. On the other hand, the smallest cellular life-forms, the bacteria of the genus Mycoplasma, are around 200 nm in length, by convention, nanotechnology is taken as the scale range 1 to 100 nm following the definition used by the National Nanotechnology Initiative in the US. The lower limit is set by the size of atoms since nanotechnology must build its devices from atoms and molecules
41.
K. Eric Drexler
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Kim Eric Drexler is an American engineer best known for popularizing the potential of molecular nanotechnology, from the 1970s and 1980s. K. Eric Drexler was strongly influenced by ideas on Limits to Growth in the early 1970s, during his first year at Massachusetts Institute of Technology, he sought out someone who was working on extraterrestrial resources. He found Gerard K. ONeill of Princeton University, a physicist famous for his work on storage rings for particle accelerators, Drexler participated in NASA summer studies on space colonies in 1975 and 1976. He fabricated metal films a few tens of nanometers thick on a wax support to demonstrate the potentials of high performance solar sails and he was active in space politics, helping the L5 Society defeat the Moon Treaty in 1980. Besides working summers for ONeill, building mass driver prototypes, Drexler delivered papers at the first three Space Manufacturing conferences at Princeton, the 1977 and 1979 papers were co-authored with Keith Henson, and patents were issued on both subjects, vapor phase fabrication and space radiators. During the late 1970s, Drexler began to develop ideas about molecular nanotechnology, in 1979, he encountered Richard Feynmans provocative 1959 talk Theres Plenty of Room at the Bottom. In 1981, Drexler wrote a research article, published by PNAS, Molecular engineering. This article has continued to be cited, more than 620 times, in that book, he proposed the idea of a nanoscale assembler which would be able to build a copy of itself and of other items of arbitrary complexity. He also first published the term grey goo to describe what might happen if a hypothetical self-replicating molecular nanotechnology went out of control and he has subsequently tried to clarify his concerns about out-of-control self-replicators, and make the case that molecular manufacturing does not require such devices. Drexler and Christine Peterson, at that time husband and wife, in March 2004, Drexler signed scientists open letter in support of cryonics. In August 2005 Drexler joined Nanorex, an engineering software company based in Bloomfield Hills, Michigan. Nanorexs nanoENGINEER-1 software was able to simulate a hypothetical differential gear design in a snap. Drexler holds three degrees from MIT and he received his B. S. in Interdisciplinary Sciences in 1977 and his M. S. in 1979 in Astro/Aerospace Engineering with a Masters thesis titled Design of a High Performance Solar Sail System. In 1991, he earned a Ph. D. through the MIT Media Lab after the departments of electrical engineering, Drexler was married to Christine Peterson for 21 years. In 2006, Drexler married Rosa Wang, an investment banker who works with Ashoka. Drexlers work on nanotechnology was criticized as naive by Nobel Prize winner Richard Smalley in a 2001 Scientific American article, Smalley first argued that fat fingers made MNT impossible. He later argued that nanomachines would have to resemble chemical enzymes more than Drexlers assemblers, Drexler maintained that both were straw man arguments, and in the case of enzymes, wrote that Prof. Klibanov wrote in 1994. Using an enzyme in organic solvents eliminates several obstacles, Drexler had difficulty in getting Smalley to respond, but in December 2003, Chemical and Engineering news carried a four-part debate
42.
Covalent bond
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A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, for many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, bent bonds, the term covalent bond dates from 1939. In the molecule H2, the atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities, thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity. Covalent bonding that entails sharing of electrons more than two atoms is said to be delocalized. The term covalence in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled The Arrangement of Electrons in Atoms and Molecules. Langmuir wrote that we shall denote by the term covalence the number of pairs of electrons that an atom shares with its neighbors. The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis and he introduced the Lewis notation or electron dot notation or Lewis dot structure, in which valence electrons are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds, multiple pairs represent multiple bonds, such as double bonds and triple bonds. An alternative form of representation, not shown here, has bond-forming electron pairs represented as solid lines, Lewis proposed that an atom forms enough covalent bonds to form a full outer electron shell. In the diagram of methane shown here, the atom has a valence of four and is, therefore, surrounded by eight electrons, four from the carbon itself. Each hydrogen has a valence of one and is surrounded by two electrons – its own one electron plus one from the carbon, walter Heitler and Fritz London are credited with the first successful quantum mechanical explanation of a chemical bond in 1927. Their work was based on the valence bond model, which assumes that a bond is formed when there is good overlap between the atomic orbitals of participating atoms. Atomic orbitals have specific directional properties leading to different types of covalent bonds, sigma bonds are the strongest covalent bonds and are due to head-on overlapping of orbitals on two different atoms. A single bond is usually a σ bond, pi bonds are weaker and are due to lateral overlap between p orbitals. A double bond between two given atoms consists of one σ and one π bond, and a bond is one σ. Covalent bonds are also affected by the electronegativity of the atoms which determines the chemical polarity of the bond
43.
Period (periodic table)
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A period in the periodic table is one of the horizontal rows, all of whose elements have the same number of electron shells. Going across a period, each element has one proton and is less metallic than its predecessor. Arranged this way, groups of elements in the column have similar chemical and physical properties. For example, the alkaline metals lie in the first column and share similar properties, such as high reactivity, at the present time, the periodic table of elements has a total of 118 elements which have been discovered to date. Modern quantum mechanics explains these periodic trends in properties in terms of electron shells, as atomic number increases, shells fill with electrons in approximately the order shown at right. The filling of each corresponds to a row in the table. In the s-block and p-block of the table, elements within the same period generally do not exhibit trends. However, in the d-block, trends across periods become significant, seven periods of elements occur naturally on Earth. For period 8, which includes elements which may be synthesized after 2015, a group in chemistry means a family of objects with similarities like different families. There are 7 periods, going horizontally across the periodic table, the first period contains fewer elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule, chemically, helium behaves as a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its structure it belongs to the s block. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element, hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the universes elemental mass. Ionized hydrogen is just a proton, stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is rare on Earth, and is industrially produced from hydrocarbons such as methane. Hydrogen can form compounds with most elements and is present in water, helium exists only as a gas except in extreme conditions. It is the second lightest element and is the second most abundant in the universe, most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars. On Earth, helium is rare, only occurring as a byproduct of the natural decay of some radioactive elements
. Bibcode:2009PhRvB..80l5421G. doi:10.1103/PhysRevB.80.125421.