- Branche: Chemistry
- Number of terms: 1965
- Number of blossaries: 0
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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 characterizes conditions that lead to reaction products in a proportion governed by the equilibrium constant for their interconversion and/or for the interconversion of reaction intermediates formed in or after the rate-limiting step. (Some workers prefer to describe this phenomenon as "equilibrium control".)
Industry:Chemistry
The uncatalyzed cleavage of one or more covalent bonds resulting from exposure of a compound to a raised temperature, or a process in which such cleavage is an essential part.
Industry:Chemistry
The term refers to the preference for "inward" or "outward" rotation of substituents in conrotatory or disrotatory electrocyclic ring opening reactions.
Industry:Chemistry
Transferability assumes invariance of properties, associated conceptually with an atom or a fragment present in a variety of molecules. The property, such as electronegativity, nucleophilicity, NMR chemical shift, etc. is held as retaining a similar value in all these occurrences.
Industry:Chemistry
The conversion of a substrate into a particular product, irrespective of reagents or mechanisms involved. For example, the transformation of aniline (C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>) into N-phenylacetamide (C<sub>6</sub>H<sub>5</sub>NHCOCH<sub>3</sub>) may be effected by use of acetyl chloride or acetic anhydride or ketene. A transformation is distinct from a reaction, the full description of which would state or imply all the reactants and all the products.
Industry:Chemistry
Relating to a short-lived reaction intermediate. It can be defined only in relation to a time scale fixed by the experimental conditions and the limitations of the technique employed in the detection of the intermediate. The term is a relative one.
Transient species are sometimes also said to be "metastable". However, this latter term should be avoided, because it relates a thermodynamic term to a kinetic property, although most transients are also thermodynamically unstable with respect to reactants and products.
Industry:Chemistry
The reaction coordinate at the transition state corresponding to a vibration with an imaginary frequency. Motion along it in the two opposite senses leads towards the reactants or towards the products.
Industry:Chemistry
In theories describing elementary reactions it is usually assumed that there is a transition state of more positive molar Gibbs energy between the reactants and the products through which an assembly of atoms (initially composing the molecular entities of the reactants) must pass on going from reactants to products in either direction. In the formalism of "transition state theory" the transition state of an elementary reaction is that set of states (each characterized by its own geometry and energy) which an assembly of atoms, when randomly placed there, would have an equal probability of forming the reactants or of forming the products of that elementary reaction. The transition state is characterized by one and only one imaginary frequency. The assembly of atoms at the transition state has been called an activated complex. (It is not a complex according to the definition in this Glossary.)
It may be noted that the calculations of reaction rates by the transition state method and based on calculated potential-energy surfaces refer to the potential energy maximum at the saddle point, as this is the only point for which the requisite separability of transition state coordinates may be assumed. The ratio of the number of assemblies of atoms that pass through to the products to the number of those that reach the saddle point from the reactants can be less than unity, and this fraction is the "transmission coefficient" κ. (There are also reactions, such as the gas-phase colligation of simple radicals, that do not require "activation" and which therefore do not involve a transition state.)
Industry:Chemistry
A substrate designed to mimic the properties or the geometry of the transition state of reaction.
Industry:Chemistry
A saddle point on a potential-energy surface. It has one negative force constant in the harmonic force constant matrix.
Industry:Chemistry
