Simplified molecular-input line-entry system
The simplified molecular-input line-entry system is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules; the original SMILES specification was initiated in the 1980s. It has since been extended. In 2007, an open standard called. Other linear notations include the Wiswesser line notation, ROSDAL, SYBYL Line Notation; the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. Acknowledged for their parts in the early development were "Gilman Veith and Rose Russo and Albert Leo and Corwin Hansch for supporting the work, Arthur Weininger and Jeremy Scofield for assistance in programming the system." The Environmental Protection Agency funded the initial project to develop SMILES. It has since been modified and extended by others, most notably by Daylight Chemical Information Systems.
In 2007, an open standard called "OpenSMILES" was developed by the Blue Obelisk open-source chemistry community. Other'linear' notations include the Wiswesser Line Notation, ROSDAL and SLN. In July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is considered to have the advantage of being more human-readable than InChI; the term SMILES refers to a line notation for encoding molecular structures and specific instances should be called SMILES strings. However, the term SMILES is commonly used to refer to both a single SMILES string and a number of SMILES strings; the terms "canonical" and "isomeric" can lead to some confusion when applied to SMILES. The terms are not mutually exclusive. A number of valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol. Algorithms have been developed to generate the same SMILES string for a given molecule; this SMILES is unique for each structure, although dependent on the canonicalization algorithm used to generate it, is termed the canonical SMILES.
These algorithms first convert the SMILES to an internal representation of the molecular structure. Various algorithms for generating canonical SMILES have been developed and include those by Daylight Chemical Information Systems, OpenEye Scientific Software, MEDIT, Chemical Computing Group, MolSoft LLC, the Chemistry Development Kit. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database; the original paper that described the CANGEN algorithm claimed to generate unique SMILES strings for graphs representing molecules, but the algorithm fails for a number of simple cases and cannot be considered a correct method for representing a graph canonically. There is no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, double bond geometry; these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES.
A notable feature of these rules is. The term isomeric SMILES is applied to SMILES in which isotopes are specified. In terms of a graph-based computational procedure, SMILES is a string obtained by printing the symbol nodes encountered in a depth-first tree traversal of a chemical graph; the chemical graph is first trimmed to remove hydrogen atoms and cycles are broken to turn it into a spanning tree. Where cycles have been broken, numeric suffix labels are included to indicate the connected nodes. Parentheses are used to indicate points of branching on the tree; the resultant SMILES form depends on the choices: of the bonds chosen to break cycles, of the starting atom used for the depth-first traversal, of the order in which branches are listed when encountered. Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, have no formal charge, have the number of hydrogens attached implied by the SMILES valence model, are the normal isotopes, are not chiral centers.
All other elements must be enclosed in brackets, have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or. Hydrogen may be written as a separate atom; when brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1 by the sign + for a positive charge or by - for a negative charge. For example, for ammonium. If there is more than one charge, it is written as digit.
Rocket propellant is a material used either directly by a rocket as the reaction mass, or indirectly to produce the reaction mass in a chemical reaction. The reaction mass is that, ejected with high speed, from a rocket engine to produce thrust. There are a number of different kinds of propellant: chemical rockets use propellants capable of undergoing exothermic chemical reactions, which provide the energy to accelerate the resulting gases through the nozzle. Thermal rockets instead use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures, while cold gas thrusters use pressurized stored inert gases. Electric propulsion requires propellants that are ionized or made into plasma. In the extreme case of nuclear pulse propulsion, the propellant consists of many small non-weapon nuclear explosives. Rocket propellant is either a high oxygen-containing fuel or a mixture of fuel plus oxidant, whose combustion takes place, in a definite and controlled manner with the evolution of a huge volume of gas.
In the rocket engine, the propellant is burnt in the combustion chamber and the hot jet of gases escapes through the nozzle at high velocity. Rockets create thrust by expelling mass backward in a high-speed jet. Chemical rockets, the subject of this article, create thrust by reacting propellants within a combustion chamber into a hot gas at high pressure, expanded and accelerated by passage through a nozzle at the rear of the rocket; the amount of the resulting forward force, known as thrust, produced is the mass flow rate of the propellants multiplied by their exhaust velocity, as specified by Newton's third law of motion. Thrust is, the equal and opposite reaction that moves the rocket, not by the interaction of the exhaust stream with air around the rocket. Equivalently, one can think of a rocket being accelerated upwards by the pressure of the combusting gases against the combustion chamber and nozzle; this operational principle stands in contrast to the commonly-held assumption that a rocket "pushes" against the air behind or below it.
Rockets in fact perform better in outer space, because there is a reduction in air pressure on the outside of the engine, because it is possible to fit a longer nozzle without suffering from flow separation, in addition to the lack of air drag. The maximum velocity that a rocket can attain in the absence of any external forces is a function of its mass ratio and its exhaust velocity; the relationship is described by the rocket equation: V f = V e ln , where V f is the final velocity, V e is the exhaust velocity relative to the rocket, M 0 is the initial total mass, M f is the mass after the propellant is burned. The mass ratio expresses. A single-stage rocket might have a mass fraction of 90% propellant, 10% structure, hence a mass ratio of 9:1; the impulse delivered by the motor to the rocket vehicle per weight of propellant consumed is the rocket propellant's specific impulse. A propellant with a higher specific impulse is said to be more efficient as more thrust is produced per unit of propellant.
Lower rocket stages use high-density propellants due to their lighter tankage to propellant weight ratios and because higher performance propellants require higher expansion ratios for maximum performance than can be attained when operated in the atmosphere. Thus, the Saturn V first stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on its upper stages; the Space Shuttle used high-thrust, high-density solid rocket boosters for its lift-off with the liquid hydrogen-liquid oxygen Space Shuttle Main Engines used for lift-off but for orbital insertion. Solid rocket propellant was first developed during the 13th century under the Chinese Song dynasty; the Song Chinese first used solid propellant in 1232 during the military siege of Kaifeng. There are four main types of chemical rocket propellants: solid, storable liquid, cryogenic liquid, a liquid monopropellant. Hybrid solid/liquid. Solid propellants are either "composites" composed of large, distinct macroscopic particles or single-, double-, or triple-bases, which are homogeneous mixtures of one or more primary ingredients.
Composites consist of a mixture of granules of solid oxidizer in a polymer binder with flakes or powders of energetic compounds, metallic additives, stabilizers, and/or burn rate modifiers. Single-, double-, or triple-bases are mixtures of the fuel, oxidizer and plasticizers that are macroscopically indistinguishable and blended as liquids and cured in a single batch; the ingredi
Specific impulse is a measure of how a rocket uses propellant or a jet engine uses fuel. By definition, it is the total impulse delivered per unit of propellant consumed and is dimensionally equivalent to the generated thrust divided by the propellant mass flow rate or weight flow rate. If mass is used as the unit of propellant specific impulse has units of velocity. If weight is used instead specific impulse has units of time. Multiplying flow rate by the standard gravity converts specific impulse from the mass basis to the weight basis. A propulsion system with a higher specific impulse uses the mass of the propellant more in creating forward thrust and, in the case of a rocket, less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation. In rockets, this means the engine is more effective at gaining altitude and velocity; this effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.
Specific impulse includes the contribution to impulse provided by external air, used for combustion and is exhausted with the spent propellant. Jet engines use outside air, therefore have a much higher specific impulse than rocket engines; the specific impulse in terms of propellant mass spent has units of distance per time, a notional velocity called the effective exhaust velocity. This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air or other intake propellant such as water. Specific impulse is inversely proportional to specific fuel consumption by the relationship Isp = 1/ for SFC in kg/ and Isp = 3600/SFC for SFC in lb/; the amount of propellant is measured either in units of mass or weight. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to have units of speed, so specific impulses are measured in meters per second and are termed effective exhaust velocity.
However, if propellant weight is used, an impulse divided by a force turns out to be a unit of time, so specific impulses are measured in seconds. These two formulations are both used and differ from each other by a factor of g0, the dimensioned constant of gravitational acceleration at the surface of the Earth. Note that the rate of change of momentum of a rocket per unit time is equal to the thrust; the higher the specific impulse, the less propellant is needed to produce a given thrust for a given time. In this regard a propellant is more efficient the greater its specific impulse; this should not be confused with energy efficiency, which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so. Thrust and specific impulse should not be confused; the specific impulse is the impulse produced per unit of propellant expended, while thrust is the momentary or peak force supplied by a particular engine. In many cases, propulsion systems with high specific impulse—some ion thrusters reach 10,000 seconds—produce low thrust.
When calculating specific impulse, only propellant carried with the vehicle. For a chemical rocket, the propellant mass therefore would include both oxidizer. For air-breathing engines, only the mass of the fuel is counted, not the mass of air passing through the engine. Air resistance and the engine's inability to keep a high specific impulse at a fast burn rate are why all the propellant is not used as fast as possible. A heavier engine with a higher specific impulse may not be as effective in gaining altitude, distance, or velocity as a lighter engine with a lower specific impulse. If it were not for air resistance and the reduction of propellant during flight, specific impulse would be a direct measure of the engine's effectiveness in converting propellant weight or mass into forward momentum; the most common unit for specific impulse is the second, both in SI contexts as well as where imperial or customary units are used. The advantage of seconds is that the unit and numerical value are identical across systems of measurements, universal.
Nearly all manufacturers quote their engine performance in seconds, the unit is useful for specifying aircraft engine performance. The use of metres per second to specify effective exhaust velocity is reasonably common; the unit is intuitive when describing rocket engines, although the effective exhaust speed of the engines may be different from the actual exhaust speed, which may be due to the fuel and oxidizer, dumped overboard after powering turbopumps. For airbreathing jet engines, the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes; the values expressed in N·s/kg are not uncommon and are numerically equal to the effective exhaust velocity in m/s. Specific fuel consumption is inversely proportional to specific impulse and has units of g/ or lb/. Specific fuel consumption is used extensively for describing the performance of air-breathing jet engines; the curious unit of seconds to measure the'goodness' of a fuel/engine combination can be thought of as "How many seconds this propellant can accelerate its own initial mass at 1 gee".
The more seconds it can accelerate its own mass, the more delta-V it delivers to the whole system. For all vehicles, specific impulse (impulse per unit weight-on-Earth of prope
European Space Agency
The European Space Agency is an intergovernmental organisation of 22 member states dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a worldwide staff of about 2,200 in 2018 and an annual budget of about €5.72 billion in 2019. ESA's space flight programme includes human spaceflight; the main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching and further developing this launch vehicle. The agency is working with NASA to manufacture the Orion Spacecraft service module, that will fly on the Space Launch System; the agency's facilities are distributed among the following centres: ESA science missions are based at ESTEC in Noordwijk, Netherlands. After World War II, many European scientists left Western Europe in order to work with the United States. Although the 1950s boom made it possible for Western European countries to invest in research and in space-related activities, Western European scientists realised national projects would not be able to compete with the two main superpowers.
In 1958, only months after the Sputnik shock, Edoardo Amaldi and Pierre Auger, two prominent members of the Western European scientific community, met to discuss the foundation of a common Western European space agency. The meeting was attended by scientific representatives from eight countries, including Harrie Massey; the Western European nations decided to have two agencies: one concerned with developing a launch system, ELDO, the other the precursor of the European Space Agency, ESRO. The latter was established on 20 March 1964 by an agreement signed on 14 June 1962. From 1968 to 1972, ESRO launched seven research satellites. ESA in its current form was founded with the ESA Convention in 1975, when ESRO was merged with ELDO. ESA had ten founding member states: Belgium, France, West Germany, the Netherlands, Sweden and the United Kingdom; these signed the ESA Convention in 1975 and deposited the instruments of ratification by 1980, when the convention came into force. During this interval the agency functioned in a de facto fashion.
ESA launched its first major scientific mission in 1975, Cos-B, a space probe monitoring gamma-ray emissions in the universe, first worked on by ESRO. The ESA collaborated with NASA on the International Ultraviolet Explorer, the world's first high-orbit telescope, launched in 1978 and operated for 18 years. A number of successful Earth-orbit projects followed, in 1986 ESA began Giotto, its first deep-space mission, to study the comets Halley and Grigg–Skjellerup. Hipparcos, a star-mapping mission, was launched in 1989 and in the 1990s SOHO, Ulysses and the Hubble Space Telescope were all jointly carried out with NASA. Scientific missions in cooperation with NASA include the Cassini–Huygens space probe, to which ESA contributed by building the Titan landing module Huygens; as the successor of ELDO, ESA has constructed rockets for scientific and commercial payloads. Ariane 1, launched in 1979, carried commercial payloads into orbit from 1984 onward; the next two versions of the Ariane rocket were intermediate stages in the development of a more advanced launch system, the Ariane 4, which operated between 1988 and 2003 and established ESA as the world leader in commercial space launches in the 1990s.
Although the succeeding Ariane 5 experienced a failure on its first flight, it has since established itself within the competitive commercial space launch market with 82 successful launches until 2018. The successor launch vehicle of Ariane 5, the Ariane 6, is under development and is envisioned to enter service in the 2020s; the beginning of the new millennium saw ESA become, along with agencies like NASA, JAXA, ISRO, CSA and Roscosmos, one of the major participants in scientific space research. Although ESA had relied on co-operation with NASA in previous decades the 1990s, changed circumstances led to decisions to rely more on itself and on co-operation with Russia. A 2011 press issue thus stated: Russia is ESA's first partner in its efforts to ensure long-term access to space. There is a framework agreement between ESA and the government of the Russian Federation on cooperation and partnership in the exploration and use of outer space for peaceful purposes, cooperation is underway in two different areas of launcher activity that will bring benefits to both partners.
Notable outcomes are ESA's include SMART-1, a probe testing cutting-edge new space propulsion technology, the Mars Express and Venus Express missions, as well as the development of the Ariane 5 rocket and its role in the ISS partnership. ESA maintain
In chemistry, a salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral; these component ions can be inorganic, such as organic, such as acetate. Salts can be classified in a variety of ways. Salts that produce hydroxide ions when dissolved in water are called alkali salts. Salts that produce acidic solutions are acidic salts. Neutral salts are those salts that are neither basic. Zwitterions contain an anionic and a cationic centres in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites and proteins. Solid salts tend to be transparent. In many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries, larger crystals tend to be transparent, while the polycrystalline aggregates look like white powders.
Salts exist in many different colors, which arise either from the cations. For example: sodium chromate is yellow by virtue of the chromate ion potassium dichromate is orange by virtue of the dichromate ion cobalt nitrate is red owing to the chromophore of hydrated cobalt. copper sulfate is blue because of the copper chromophore potassium permanganate has the violet color of permanganate anion. Nickel chloride is green of sodium chloride, magnesium sulfate heptahydrate are colorless or white because the constituent cations and anions do not absorb in the visible part of the spectrumFew minerals are salts because they would be solubilized by water. Inorganic pigments tend not to be salts, because insolubility is required for fastness; some organic dyes are salts, but they are insoluble in water. Different salts can elicit all five basic tastes, e.g. salty, sour and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless, whereas salts of either weak acids or weak bases may smell like the conjugate acid or the conjugate base of the component ions.
That slow, partial decomposition is accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds exhibit significant solubility in water or other polar solvents. Unlike molecular compounds, salts dissociate in solution into cationic components; the lattice energy, the cohesive forces between these ions within a solid, determines the solubility. The solubility is dependent on how well each ion interacts with the solvent, so certain patterns become apparent. For example, salts of sodium and ammonium are soluble in water. Notable exceptions include potassium cobaltinitrite. Most nitrates and many sulfates are water-soluble. Exceptions include barium sulfate, calcium sulfate, lead sulfate, where the 2+/2− pairing leads to high lattice energies. For similar reasons, most alkali metal carbonates are not soluble in water; some soluble carbonate salts are: potassium carbonate and ammonium carbonate. Salts are characteristically insulators.
Molten salts or solutions of salts conduct electricity. For this reason, liquified salts and solutions containing dissolved salts are called electrolytes. Salts characteristically have high melting points. For example, sodium chloride melts at 801 °C; some salts with low lattice energies are liquid near room temperature. These include molten salts, which are mixtures of salts, ionic liquids, which contain organic cations; these liquids exhibit unusual properties as solvents. The name of a salt starts with the name of the cation followed by the name of the anion. Salts are referred to only by the name of the cation or by the name of the anion. Common salt-forming cations include: Ammonium NH+4 Calcium Ca2+ Iron Fe2+ and Fe3+ Magnesium Mg2+ Potassium K+ Pyridinium C5H5NH+ Quaternary ammonium NR+4, R being an alkyl group or an aryl group Sodium Na+ Copper Cu2+Common salt-forming anions include: Acetate CH3COO− Carbonate CO2−3 Chloride Cl− Citrate HOC2 Cyanide C≡N− Fluoride F− Nitrate NO−3 Nitrite NO−2 Oxide O2− Phosphate PO3−4 Sulfate SO2−4 Salts with varying number of hydrogen atoms, with respect to the parent acid, replaced by cations can be referred to as monobasic, dibasic or tribasic salts: Sodium phosphate monobasic Sodium phosphate dibasic Sodium phosphate tribasic Salts are formed by a chemical reaction between: A base and an acid, e.g. NH3 + HCl → NH4Cl A metal and an acid, e.g. Mg + H2SO4 → MgSO4 + H2 A metal and a non-metal, e.g. Ca + Cl2 → CaCl2 A base and an a
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