- المجال: Chemistry
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The International Union of Pure and Applied Chemistry (IUPAC) serves to advance the worldwide aspects of the chemical sciences and to contribute to the application of chemistry in the service of people and the environment. As a scientific, international, non-governmental and objective body, IUPAC ...
The term is used to compare molecular fragments with each other and with familiar species from organic chemistry. Two fragments are isolobal if the number, symmetry properties, approximate energy, and shape of the frontier orbitals and the number of electrons in them are similar.
Industry:Chemistry
One of several species (or molecular entities) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.
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A chemical reaction, the principal product of which is isomeric with the principal reactant. An intramolecular isomerization that involves the breaking or making of bonds is a special case of a molecular rearrangement.
Isomerization does not necessarily imply molecular rearrangement (e.g. in the case of the interconversion of conformational isomers).
Industry:Chemistry
This term is usually employed with reference to a set of absorption spectra, plotted on the same chart for a set of solutions in which the sum of the concentrations of two principal absorbing components, A and B, is constant. The curves of absorbance against wavelength (or frequency) for such a set of mixtures often all intersect at one or more points, called isosbestic points.
Isosbestic points are commonly met when electronic spectra are taken (a) on a solution in which a chemical reaction is in progress (in which case the two absorbing components concerned are a reactant and a product, A + B), or (b) on a solution in which the two absorbing components are in equilibrium and their relative proportions are controlled by the concentration of some other component, typically the concentration of hydrogen ions, e.g., an acid-base indicator equilibrium.
<center>A ⇌ B + H<sup>+</sup><sub>aq</sub></center>
The effect may also appear (c) in the spectra of a set of solutions of two unrelated non-interacting components having the same total concentration. In all these examples, A (and/or B) may be either a single chemical species or a mixture of chemical species present in invariant proportion.
If A and B are single chemical species, isosbestic points will appear at all wavelengths at which their molar absorption coefficients (formerly called extinction coefficients) are the same. (A more involved identity applies when A and B are mixtures of constant proportion.)
If absorption spectra of the types considered above intersect not at one or more isosbestic points but over progressively changing wavelength, this is prima facie evidence in case (a) for the formation of a reaction intermediate in substantial concentration (A → C → B), in case (b) for the involvement of a third absorbing species in the equilibrium, e.g.
<center>A ⇌ B + H<sup>+</sup><sub>aq</sub> ⇌ C + 2 H<sup>+</sup><sub>aq</sub></center>
or in case (c) for some interaction of A and B, e.g.,
<center>A + B ⇌ C</center>
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A relationship analogous to the isokinetic relationship, but applied to selectivity data of reactions. At the isoselective temperature, the selectivities of the series of reactions following the relationship are identical.
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The effect on the rate or equilibrium constant of two reactions that differ only in the isotopic composition of one or more of their otherwise chemically identical components is referred to as a kinetic isotope effect (see isotope effect, kinetic) or a thermodynamic (or equilibrium) isotope effect respectively.
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The ratio (x<sub>1</sub>/x<sub>2</sub>)<sub>A</sub>/(x<sub>1</sub>/x<sub>2</sub>)<sub>B</sub>, where x is the abundance, expressed as the atom fraction of the isotope distinguished by the subscript numeral, when the two isotopes are equilibrated between two different chemical species A and B (or between specific sites A and B in the same or different chemical species). The term is most commonly met in connection with deuterium solvent isotope effects, when the fractionation factor φ expresses the ratio
<center>φ = (x<sub>D</sub>/x<sub>H</sub>)<sub>solute</sub>/(x<sub>D</sub>/x<sub>H</sub>)<sub>solvent</sub></center>
for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states.
Industry:Chemistry
NMR shift difference measurement of the isotope effect on a fast (degenerate) equilibrium between two, except for isotopic substitution, species which are equivalent. This can be used to distinguish a rapidly equilibrating mixture with time-averaged symmetry from a single structure with higher symmetry.
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A molecular entity that differs only in isotopic composition (number of isotopic substitutions), e.g. CH<sub>4</sub>, CH<sub>3</sub>D, CH<sub>2</sub>D<sub>2</sub>... .
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Isomers having the same number of each isotopic atom but differing in their positions. The term is a contraction of "isotopic isomer".
Isotopomers can be either constitutional isomers (e.g. CH<sub>2</sub>DCH=O and CH<sub>3</sub>CD=O) or isotopic stereoisomers (e.g. (R)- and (S)-CH<sub>3</sub>CHDOH or (Z)- and (E)-CH<sub>3</sub>CH=CHD).
Industry:Chemistry
