An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes; the parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are called cumulenes. Allenes are much more reactive than most other alkenes. For example, their reactivity with gaseous chlorine is more like the reactivity of alkynes than that of alkenes; the central carbon atom of allene forms two pi bonds. The central carbon is sp-hybridized, the two terminal carbon atoms are sp2-hybridized; the bond angle formed by the three carbon atoms is 180°, indicating linear geometry for the carbon atoms of allene. It can be viewed as an "extended tetrahedral" with a similar shape to methane; the symmetry and isomerism of allenes has long fascinated organic chemists. For allenes with four identical substituents, there exist two twofold axes of rotation through the center carbon, inclined at 45° to the CH2 planes at either end of the molecule.
The molecule can thus be thought of as a two-bladed propeller. A third twofold axis of rotation passes through the C=C=C bonds, there is a mirror plane passing through both CH2 planes, thus this class of molecules belong to the D2d point group. Because of the symmetry, an unsubstituted allene has no net dipole moment. An allene with two different substituents on each of the two carbon atoms will be chiral because there will no longer be any mirror planes. Where A has a greater priority than B according to the Cahn-Ingold-Prelog priority rule, the configuration of the axial chirality can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the bottom, only the group of higher priority need be considered. Chiral allenes have been used as building blocks in the construction of organic materials with exceptional chiroptical properties. Although allenes require specialized syntheses, the parent, propadiene is produced on a large scale as an equilibrium mixture with methylacetylene: H2C=C=CH2 ⇌ CH3C≡CHThis mixture, known as MAPP gas, is commercially available.
Laboratory methods for the formation of allenes include: from geminal dihalocyclopropanes and organolithium compounds in the Skattebøl rearrangement from reaction of certain terminal alkynes with formaldehyde, copper bromide, added base from dehydrohalogenation of certain dihalides from reaction of a triphenylphosphinyl ester with an acid halide, a Wittig reaction accompanied by dehydrohalogenation Allenes function as ligands, not unlike alkenes. A typical complex is Pt2. Ni reagents catalyze the cyclooligomerization of allene. Using a suitable catalyst, it is possible to reduce just one of the double bonds of an allene. Many rings or ring systems are known by semisystematic names that assume a maximum number of noncumulative bonds. To unambiguously specify derivatives that include cumulated bonds, a lowercase delta may be used with a subscript indicating the number of cumulated double bonds from that atom, e.g. 8δ2-Benzocyclononene. This may be combined with the λ-convention for specifying nonstandard valency states, e.g. 2λ4δ2,5λ4δ2-Thienothiophene.
Compounds with three or more adjacent carbon–carbon double bonds are called cumulenes. The allene motif is encountered in carbo-mers. IUPAC, Compendium of Chemical Terminology, 2nd ed.. Online corrected version: "allenes". Doi:10.1351/goldbook. A00238 Allene chemistry Kay M. Brummond Thematic Series in the open-access Beilstein Journal of Organic Chemistry Stereochemistry study guide Synthesis of allenes
Triclinic crystal system
In crystallography, the triclinic crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. In addition, the angles between these vectors must all be different and may include 90°; the triclinic lattice is the least symmetric of the 14 three-dimensional Bravais lattices. It has the minimum symmetry all lattices have: points of inversion at each lattice point and at 7 more points for each lattice point: at the midpoints of the edges and the faces, at the center points, it is the only lattice type. The triclinic crystal system class names, Schönflies notation, Hermann-Mauguin notation, point groups, International Tables for Crystallography space group number, orbifold and space groups are listed in the table below. There are a total 2 space groups. With each only one space group is associated. Pinacoidal is known as triclinic normal. Pedial is triclinic hemihedral Mineral examples include plagioclase, rhodonite, turquoise and amblygonite, all in triclinic normal.
Crystal structure Hurlbut, Cornelius S..
Chemiluminescence is the emission of light, as the result of a chemical reaction. There may be limited emission of heat. Given reactants A and B, with an excited intermediate ◊, + → → + lightFor example, if is luminol and is hydrogen peroxide in the presence of a suitable catalyst we have: C 8 H 7 N 3 O 2 luminol + H 2 O 2 hydrogen peroxide ⟶ 3 − APA ⟶ 3 − APA + light where: 3-APA is 3-aminophthalate 3-APA is the vibronic excited state fluorescing as it decays to a lower energy level; the decay of this excited state to a lower energy level causes light emission. In theory, one photon of light should be given off for each molecule of reactant; this is equivalent to Avogadro's number of photons per mole of reactant. In actual practice, non-enzymatic reactions exceed 1% QC, quantum efficiency. In a chemical reaction, reactants collide to form a transition state, the enthalpic maximum in a reaction coordinate diagram, which proceeds to the product. Reactants form products of lesser chemical energy.
The difference in energy between reactants and products, represented as Δ H r x n, is turned into heat, physically realized as excitations in the vibrational state of the normal modes of the product. Since vibrational energy is much greater than the thermal agitation, it disperses in the solvent through molecular rotation; this is. In a chemiluminescent reaction, the direct product of the reaction is an excited electronic state; this state decays into an electronic ground state and emits light through either an allowed transition or a forbidden transition, depending on the spin state of the electronic excited state formed. Chemiluminescence differs from fluorescence or phosphorescence in that the electronic excited state is the product of a chemical reaction rather than of the absorption of a photon, it is the antithesis of a photochemical reaction, in which light is used to drive an endothermic chemical reaction. Here, light is generated from a chemically exothermic reaction; the chemiluminescence might be induced by an electrochemical stimulus, in this case is called electrochemiluminescence.
A standard example of chemiluminescence in the laboratory setting is the luminol test. Here, blood is indicated by luminescence upon contact with iron in hemoglobin; when chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits light by chemiluminescence. Chemiluminescence in aqueous system is caused by redox reactions. Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper, or an auxiliary oxidant, produces chemiluminescence; the luminol reaction is C 8 H 7 N 3 O 2 luminol + H 2 O 2 hydrogen peroxide ⟶ 3 − APA ⟶ 3 − APA + light One of the oldest known chemiluminescent reactions is that of elemental white phosphorus oxidizing in moist air, producing a green glow. This is a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing the excited states 2 and HPO. Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air-quality testing.
Ozone is combined with nitric oxide to form nitrogen dioxide in an activated state. NO+O3 → NO2+ O2The activated NO2 luminesces broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier and associated electronics counts the photons that are proportional to the amount of NO present. To determine the amount of nitrogen dioxide, NO2, in a sample it must first be converted to nitric oxide, NO, by passing the sample through a converter before the above ozone activation reaction is applied; the ozone reaction produces a photon count proportional to NO, proportional to NO2 before it was converted to NO. In the case of a mixed sample that contains both NO and NO2, the above reaction yields the amount of NO and NO2 combined in the air sample, assuming that the sample is passed through the converter. If the mixed sample is not passed through the converter, the ozone reaction produces activated NO2 only in proportion to the NO in the sample; the NO2 in the sample is not activated by the ozone reaction.
Though unactivated NO2 is present with the activated NO2, photons are emitted only by the activated species, proportional to original NO. Final step: Subtract NO from to yield NO2 In chemical kinetics, infrared chemiluminiscence refers to the e
A dimer is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. The term homodimer is used when the two molecules are heterodimer when they are not; the reverse of dimerisation is called dissociation. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs. Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers. Borane occurs as the dimer diborane, due to the high Lewis acidity of the boron center. Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but do form the excimers Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation. Molecular dimers are formed by the reaction of two identical compounds e.g.: 2A → A-A.
In this example, monomer "A" is said to dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene: 2 C2 → 2C=C2Carbenes are reactive and form bonds. Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" to give identical monomers: C10H12 → 2 C5H6Many nonmetallic elements occur as dimers: hydrogen, oxygen, the halogens, i.e. fluorine, chlorine and iodine. Noble gases can form dimers linked for example dihelium or diargon. Mercury occurs as a mercury cation, formally a dimeric ion. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include Li2, Na2, K2, Rb2 and Cs2. Many small organic molecules, most notably formaldehyde form dimers; the dimer of formaldehyde is dioxetane. In the context of polymers, "dimer" refers to the degree of polymerization 2, regardless of the stoichiometry or condensation reactions.
This is applicable to disaccharides. For example, cellobiose is a dimer of glucose though the formation reaction produces water: 2C6H12O6 → C12H22O11 + H2OHere, the dimer has a stoichiometry different from the pair of monomers. Amino acids can form dimers, which are called dipeptides. An example is glycylglycine. Other examples are carnosine. Pyrimidine dimers are formed by a photochemical reaction from pyrimidine DNA bases; this cross-linking causes DNA mutations, causing skin cancers. Monomer Trimer Polymer Protein dimer "IUPAC "Gold Book" definition". Retrieved 2009-04-30
A glow stick is a self-contained, short-term light-source. It consists of a translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence, so it does not require an external energy source; the light can only be used once. Glow sticks are used for recreation, but may be relied upon for light during military, fire, or EMS operations, they are used by military and police to mark ‘clear’ areas. Bisoxalate, trademarked "Cyalume", was invented in 1969 by Michael M. Rauhut and Laszlo J. Bollyky of American Cyanamid, based on work by Edwin A. Chandross of Bell Labs. Other early work on chemiluminescence was carried out at the same time, by researchers under Herbert Richter at China Lake Naval Weapons Center. Several US patents for "glow stick" type devices were received by various inventors. Bernard Dubrow and Eugene Daniel Guth patented a packaged chemiluminescent material in June 1965. In October 1973, Clarence W. Gilliam, David Iba Sr. and Thomas N. Hall were registered as inventors of the Chemical Lighting Device.
In June 1974, a patent for a Chemiluminescent Device was issued with Herbert P. Richter and Ruth E. Tedrick listed as the inventors. In January 1976, a patent was issued for the Chemiluminescent Signal Device, with Vincent J. Esposito, Steven M. Little, John H. Lyons listed as the inventors; this patent recommended a single glass ampoule, suspended in a second substance, that when broken and mixed together, provide the chemiluminescent light. The design included a stand for the signal device so it could be thrown from a moving vehicle and remain standing in an upright position on the road; the idea was this would replace traditional emergency roadside flares and would be superior, since it was not a fire hazard, would be easier and safer to deploy, would not be made ineffective if struck by passing vehicles. This design, with its single glass ampoule inside a plastic tube filled with a second substance that when bent breaks the glass and is shaken to mix the substances, most resembles the typical glow stick sold today.
In December 1977, a patent was issued for a Chemical Light Device with Richard Taylor Van Zandt as the inventor. This design alteration features a steel ball that shatters the glass ampoule when the glow stick is exposed to a predetermined level of shock. Glow sticks are waterproof, do not use batteries, generate negligible heat, are inexpensive, are reasonably disposable, they can tolerate high pressures, such as those found under water. They are used as light sources and light markers by military forces and recreational divers. Glowsticking is the use of glow sticks in dancing; this is one of their most known uses in popular culture, as they are used for entertainment at parties and dance clubs. They are used by marching band conductors for evening performances. Glow sticks serve multiple functions as toys visible night-time warnings to motorists, luminous markings that enable parents to keep track of their children, yet another use is for balloon-carried light effects. Glow sticks are used to create special effects in low light photography and film.
The Guinness Book of Records says. It was created using Plexiglass by KNIXS GmbH in Darmstadt Weiterstadt, Germany, on 29 June 2009. Glow sticks emit light; the reaction between the two chemicals is catalyzed by a base sodium salicylate. The sticks consist of a brittle container within a flexible outer container; each container holds a different solution. When the outer container is flexed, the inner container breaks, allowing the solutions to combine, causing the necessary chemical reaction. After breaking, the tube is shaken to mix the components; the glow stick contains two chemicals, a base catalyst, a suitable dye. This creates an exothermic reaction; the chemicals inside the plastic tube are a mixture of the dye, the base catalyst, diphenyl oxalate. The chemical in the glass vial is hydrogen peroxide. By mixing the peroxide with the phenyl oxalate ester, a chemical reaction takes place, yielding two moles of phenol and one mole of peroxyacid ester; the peroxyacid decomposes spontaneously to carbon dioxide, releasing energy that excites the dye, which relaxes by releasing a photon.
The wavelength of the photon—the color of the emitted light—depends on the structure of the dye. The reaction releases energy as light, with little heat; the reason for this is that the reverse photocycloadditions of 1,2-dioxetanedione is a forbidden transition and cannot proceed through a regular thermal mechanism. By adjusting the concentrations of the two chemicals and the base, manufacturers can produce glow sticks that either glow brightly for a short amount of time or more dimly for an extended length of time; this allows design of glow sticks that perform satisfactorily in hot or cold climates, by compensating for the temperature dependence of reaction. At maximum concentration, mixing the chemicals results in a furious reaction, producing large amounts of light for only a few seconds; the same effect can be achieved by adding copious amounts of sodium salicyate or other base
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
Organic field-effect transistor
An organic field-effect transistor is a field-effect transistor using an organic semiconductor in its channel. OFETs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate; these devices have been developed to realize low-cost, large-area electronic products and biodegradable electronics. OFETs have been fabricated with various device geometries; the most used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor using thermally grown SiO2 as gate dielectric. Organic polymers, such as poly, can be used as dielectric. In May 2007, Sony reported the first full-color, video-rate, all plastic display, in which both the thin-film transistors and the light-emitting pixels were made of organic materials; the field-effect transistor was first proposed by J. E. Lilienfeld, who received a patent for his idea in 1930.
He proposed that a field-effect transistor behaves as a capacitor with a conducting channel between a source and a drain electrode. Applied voltage on the gate electrode controls the amount of charge carriers flowing through the system; the first field-effect transistor was designed and prepared in 1960 by Kahng and Atalla using a metal–oxide–semiconductor. However, rising costs of materials and manufacturing, as well as public interest in more environmentally friendly electronics materials have supported development of organic based electronics in more recent years. In 1987, Koezuka and co-workers reported the first organic field-effect transistor based on a polymer of thiophene molecules; the thiophene polymer is a type of conjugated polymer, able to conduct charge, eliminating the need to use expensive metal oxide semiconductors. Additionally, other conjugated polymers have been shown to have semiconducting properties. OFET design has improved in the past few decades. Many OFETs are now designed based on the thin-film transistor model, which allows the devices to use less conductive materials in their design.
Improvement on these models in the past few years have been made to field-effect mobility and on–off current ratios. One common feature of OFET materials is the inclusion of an aromatic or otherwise conjugated π-electron system, facilitating the delocalization of orbital wavefunctions. Electron withdrawing groups or donating groups can be attached that facilitate hole or electron transport. OFETs employing many aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, pentacene, perylenediimides, tetracyanoquinodimethane, polymers such as polythiophenes, polydiacetylene, poly; the field is active, with newly synthesized and tested compounds reported weekly in prominent research journals. Many review articles exist documenting the development of these materials. Rubrene-based OFETs show the highest carrier mobility 20–40 cm2/. Another popular OFET material is pentacene, used since the 1980s, but with mobilities 10 to 100 times lower than rubrene.
The major problem with pentacene, as well as many other organic conductors, is its rapid oxidation in air to form pentacene-quinone. However if the pentacene is preoxidized, the thus formed pentacene-quinone is used as the gate insulator the mobility can approach the rubrene values; this pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics. Polycrystalline tetrathiafulvalene and its analogues result in mobilities in the range 0.1–1.4 cm2/. However, the mobility exceeds 10 cm2/ in solution-grown or vapor-transport-grown single crystalline hexamethylene-tetrathiafulvalene; the ON/OFF voltage is different for devices grown by those two techniques due to the higher processing temperatures using in the vapor transport grows. All the above-mentioned devices are based on p-type conductivity. N-type OFETs are yet poorly developed, they are based on perylenediimides or fullerenes or their derivatives, show electron mobilities below 2 cm2/. Three essential components of field-effect transistors are the drain and the gate.
Field-effect transistors operate as a capacitor. They are composed of two plates. One plate works as a conducting channel between two ohmic contacts, which are called the source and the drain contacts; the other plate works to control the charge induced into the channel, it is called the gate. The direction of the movement of the carriers in the channel is from the source to the drain. Hence the relationship between these three components is that the gate controls the carrier movement from the source to the drain; when this capacitor concept is applied to the device design, various devices can be built up based on the difference in the controller – i.e. the gate. This can be the gate material, the location of the gate with respect to the channel, how the gate is isolated from the channel, what type of carrier is induced by the gate voltage into channel. Classified by the properties of the carrier, three types of FETs are shown schematically in Figure 1, they are MOSFET, MESFET and TFT. The most prominent and used FET in modern microelectronics is the