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Bailar Twist
The Bailar twist is a mechanism proposed for the racemization of octahedral complexes containing three bidentate chelate rings. Such complexes typically adopt an octahedral molecular geometry, in which case they possess helical chirality. One pathway by which these compounds can racemize is via the formation of a trigonal prismatic intermediate with D3h point group symmetry. This pathway is named in honor of John C. Bailar, Jr., an inorganic chemist who investigated this process.{{cite journal , author = A. Rodger, B. F. G. Johnson , title = Which is more likely: the Ray–Dutt twist or the Bailar twist? , journal = Inorganic Chemistry , year = 1988 , volume = 27 , issue = 18 , pages = 3061–3062 , doi =10.1021/ic00291a001 An alternative pathway is called the Ray–Dutt twist. See also * Pseudorotation * Bartell mechanism * Berry mechanism * Ray–Dutt twist * Fluxional molecule In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Racemization
In chemistry, racemization is a conversion, by heat or by chemical reaction, of an optically active compound into a racemic (optically inactive) form. This creates a 1:1 molar ratio of enantiomers and is referred too as a racemic mixture (i.e. contain equal amount of (+) and (−) forms). Plus and minus forms are called Dextrorotation and levorotation. The D and L enantiomers are present in equal quantities, the resulting sample is described as a racemic mixture or a racemate. Racemization can proceed through a number of different mechanisms, and it has particular significance in pharmacology as different enantiomers may have different pharmaceutical effects. Stereochemistry Chiral molecules have two forms (at each point of asymmetry), which differ in their optical characteristics: The ''levorotatory form'' (the ''(−)-form'') will rotate counter-clockwise on the plane of polarization of a beam of light, whereas the ''dextrorotatory'' form (the ''(+)-form'') will rotate clockw ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Octahedral Molecular Geometry
In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix ''octa''. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, , which is not octahedral in the mathematical sense due to the orientation of the bonds, is referred to as octahedral. The concept of octahedral coordination geometry was developed by Alfred Wern ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Helical Chirality
Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A threaded insert, also known as a threaded bushing, is a fastener element that is inserted into an object to add a threaded hole. They may be used to repair a stripped threaded hole, provide a durable threaded hole in a soft material, place a thr ..., a mechanical thread repairing insert * H-el-ical//, stage name for Hikaru, Japanese singer {{disambig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Symmetry Group
In group theory, the symmetry group of a geometric object is the group of all transformations under which the object is invariant, endowed with the group operation of composition. Such a transformation is an invertible mapping of the ambient space which takes the object to itself, and which preserves all the relevant structure of the object. A frequent notation for the symmetry group of an object ''X'' is ''G'' = Sym(''X''). For an object in a metric space, its symmetries form a subgroup of the isometry group of the ambient space. This article mainly considers symmetry groups in Euclidean geometry, but the concept may also be studied for more general types of geometric structure. Introduction We consider the "objects" possessing symmetry to be geometric figures, images, and patterns, such as a wallpaper pattern. For symmetry of physical objects, one may also take their physical composition as part of the pattern. (A pattern may be specified formally as a scalar field, a funct ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
John C
John is a common English name and surname: * John (given name) * John (surname) John may also refer to: New Testament Works * Gospel of John, a title often shortened to John * First Epistle of John, often shortened to 1 John * Second Epistle of John, often shortened to 2 John * Third Epistle of John, often shortened to 3 John People * John the Baptist (died c. AD 30), regarded as a prophet and the forerunner of Jesus Christ * John the Apostle (lived c. AD 30), one of the twelve apostles of Jesus * John the Evangelist, assigned author of the Fourth Gospel, once identified with the Apostle * John of Patmos, also known as John the Divine or John the Revelator, the author of the Book of Revelation, once identified with the Apostle * John the Presbyter, a figure either identified with or distinguished from the Apostle, the Evangelist and John of Patmos Other people with the given name Religious figures * John, father of Andrew the Apostle and Saint Peter * Pope Jo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Inorganic Chemistry
Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture. Key concepts Many inorganic compounds are ionic compounds, consisting of cations and anions joined by ionic bonding. Examples of salts (which are ionic compounds) are magnesium chloride MgCl2, which consists of magnesium cations Mg2+ and chloride anions Cl−; or sodium oxide Na2O, which consists of sodium cations Na+ and oxide anions O2−. In any salt, the proportions of the ions are such that the electric charges cancel out, so that the bulk compound is e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ray–Dutt Twist
The Ray–Dutt twist is a mechanism proposed for the racemization of octahedral complexes containing three bidentate chelate rings. Such complexes typically adopt an octahedral molecular geometry in their ground states, in which case they possess helical chirality. The pathway entails formation of an intermediate of C2v symmetry group, point group symmetry. An alternative pathway that also does not break any metal-ligand bonds is called the Bailar twist. Both of these mechanism product complexes wherein the ligating atoms (X in the scheme) are arranged in an approximate trigonal prism. This pathway is called the Ray–Dutt twist in honor of Priyadaranjan Ray (not Prafulla Chandra Ray) and N. K. Dutt, inorganic chemistry, inorganic chemists at the Indian Association for the Cultivation of Science, Indian Association for the Cultivation of Science abbr. ''IACS'' who proposed this process. See also * Pseudorotation * Bailar twist * Bartell mechanism * Berry mechanism * Fluxion ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pseudorotation
In chemistry, a pseudorotation is a set of intramolecular movements of attached groups (i.e., ligands) on a highly symmetric molecule, leading to a molecule indistinguishable from the initial one. The International Union of Pure and Applied Chemistry (IUPAC) defines a pseudorotation as a " ereoisomerization resulting in a structure that ''appears'' to have been produced by rotation of the entire initial molecule", the result of which is a "product" that is "superposable on the initial one, unless different positions are distinguished by substitution, including isotopic substitution." Well-known examples are the intramolecular isomerization of trigonal bipyramidal compounds by the Berry pseudorotation mechanism, and the out-of-plane motions of carbon atoms exhibited by cyclopentane, leading to the interconversions it experiences between its many possible conformers (envelope, twist). Note, no angular momentum is generated by this motion. In these and related examples, a small d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bartell Mechanism
The Bartell mechanism is a pseudorotational mechanism similar to the Berry mechanism. It occurs only in molecules with a pentagonal bipyramidal molecular geometry, such as IF7. This mechanism was first predicted by H. B. Bartell. The mechanism exchanges the axial atoms with one pair of the equatorial atoms with an energy requirement of about 2.7 kcal/mol. Similarly to the Berry mechanism in square planar molecules, the symmetry of the intermediary phase of the vibrational mode is "chimeric"LS Bartell, MJ Rothman & A Gavezzotti, 1982, , ''J. Chem. Phys.'' 76:4136-4413.M Cass, KK Hii & HS Rzepa, 2005, "Mechanisms that interchange axial and equatorial atoms in fluxional processes: Illustration of the Berry pseudorotation, the turnstile and the lever mechanisms via animation of transition state normal vibrational modes", ''J. Chem. Educ.'' (online), 2005; se, accessed 28 May 2014 of other mechanisms; it displays characteristics of the Berry mechanism, a "lever" mechanism seen in pseudor ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Berry Mechanism
The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see Figure at right) for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation and most commonly occurs in trigonal bipyramidal molecules such as PF5, though it can also occur in molecules with a square pyramidal geometry. The Berry mechanism is named after R. Stephen Berry, who first described this mechanism in 1960.RS Berry, 1960, "Correlation of rates of intramolecular tunneling processes, with application to some Group V compounds," ''J. Chem. Phys.'' 32:933-938, DOI 10.1063/1.1730820; seo accessed 28 May 2014M Cass, KK Hii & HS Rzepa, 2005, "Mechanisms that interchange axial and equatorial atoms in fluxional processes: Illustration of the Berry pseudorotation, the turnstile and the lever mechanisms via animation of transition state normal vibrational modes", ''J. Chem. Educ. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fluxional Molecule
In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g. bond rotations in most organic compounds, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening (beyond that dictated by the Heisenberg uncertainty principle) due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling and other methods. Spectroscopic studies Many organometallic compounds exhibit fluxionality. Fluxionality is however pervasive. NMR spectroscopy Temperature dependent changes in the NMR spectra result from dynamics associated with the fluxional molecules when those dynamics proceed at rates comparabl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
