- المجال: Chemistry
- Number of terms: 1965
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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 ...
Historically an electron transfer between two metal centers sharing a ligand or atom in their respective coordination shells. The definition has more recently been extended to any situation in which the interaction between the donor and acceptor centers in the transition state is significant (>20 kJ mol<sup>-1</sup>).
Industry:Chemistry
A chemical reaction or transformation of the general type
<center>X-Z + Y → X-Y-Z</center>
in which the connecting atom or group Y replaces the bond joining the parts X and Z of the reactant XZ. An example is the carbene insertion reaction
<center>R<sub>3</sub>C-H + H<sub>2</sub>C: → R<sub>3</sub>C-CH<sub>3</sub></center>
The reverse of an insertion is called an extrusion.
Industry:Chemistry
A molecular entity with a lifetime appreciably longer than a molecular vibration (corresponding to a local potential energy minimum of depth greater than RT) that is formed (directly or indirectly) from the reactants and reacts further to give (either directly or indirectly) the products of a chemical reaction; also the corresponding chemical species.
Industry:Chemistry
(1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) or interactions between two or more molecular entities.
(2) Relating to a comparison between different molecular entities.
Industry:Chemistry
(1) Descriptive of any process that involves a transfer (of atoms, groups, electrons, etc.) or interactions between different parts of the same molecular entity.
(2) Relating to a comparison between atoms or groups within the same molecular entity.
Industry:Chemistry
The acceleration of a chemical transformation at one site of a molecular entity through the involvement of another functional ("catalytic") group in the same molecular entity, without that group appearing to have undergone change in the reaction product. The use of the term should be restricted to cases for which analogous intermolecular catalysis by chemical species bearing that catalytic group is observable. Intramolecular catalysis can be detected and expressed in quantitative form by a comparison of the reaction rate with that of a comparable model compound in which the catalytic group is absent, or by measurement of the effective molarity of the catalytic group.
Industry:Chemistry
A kinetic isotope effect observed when a single substrate, in which the isotopic atoms occupy equivalent reactive positions, reacts to produce a non-statistical distribution of isotopologue products. In such a case the isotope effect will favor the pathway with lower force constants for displacement of the isotopic nuclei in the transition state.
Industry:Chemistry
A kinetic isotope effect which k<sup>l</sup>/k<sup>h</sup> < 1, i.e. the heavier substrate reacts more rapidly than the lighter one, as opposed to the more usual "normal" isotope effect, in which k<sup>l</sup>/k<sup>h</sup> > 1. The isotope effect will normally be "normal" when the frequency differences between the isotopic transition states are smaller than in the reactants. Conversely, in inverse isotope effect can be taken as evidence for an increase in the corresponding force constants on passing from the reactant to the transition state.
Industry:Chemistry
The reversible formation of association colloids from surfactants in non-polar solvents leads to aggregates termed inverted (or inverse, reverse or reversed) micelles. Such association is often of the type
<center>Monomer ⇌ Dimer ⇌ Trimer ⇌ ... n-mer</center>
and the phenomenon of critical micelle concentration (or an analogous effect) is consequently not observed.
In an inverted micelle the polar groups of the surfactants are concentrated in the interior and the lipophilic groups extend towards and into the non-polar solvent.
Industry:Chemistry
A pair of oppositely charged ions held together by Coulomb attraction without formation of a covalent bond. Experimentally, an ion pair behaves as one unit in determining conductivity, kinetic behavior, osmotic properties, etc.
Following Bjerrum, oppositely charged ions with their centers closer together than a distance
<center>q = 8.36 x 10<sup>6</sup> Z<sup>+</sup>Z<sup>-</sup>/(ε<sub>r</sub>T) pm</center>
are considered to constitute an ion pair ("Bjerrum ion pair"). (Z<sup>+</sup> and Z<sup>-</sup> are the charge numbers of the ions, and ε<sub>r</sub> is the relative permittivity (or dielectric constant) of the medium.)
An ion pair, the constituent ions of which are in direct contact (and not separated by an intervening solvent or other neutral molecule) is designated as a "tight ion pair" (or "intimate" or "contact ion pair"). A tight ion pair of X<sup>+</sup> and Y<sup>-</sup> is symbolically represented as X<sup>+</sup>Y<sup>-</sup>.
By contrast, an ion pair whose constituent ions are separated by one or several solvent or other neutral molecules is described as a "loose ion pair", symbolically represented as X<sup>+</sup>
Industry:Chemistry
