The Arrhenius Parameters of Spin-Non-Conservative Reactions

In the standard treatment by transition state theory of a first-order reaction, such as a thermal fragmentation or isomerization [25, p. 97-101], the familiar Eyring equation is manipulated as follows  

The sign and magnitude of the entropy of activation, A5, is taken as a gauge of the probability of forming the transition state. In unimolecular reactions, does not depend on concentration units and is assigned a normal value of zero, so that normal reactions have pre-exponential factors close to, or 10 s at ordinary temperatures, and reactions with improbable transition states are expected to have negative entropies of activation and - as a result -pre-exponential factors significantly lower than 10 s .  

Loosely speaking, one might say that reactive intersystem crossing - depending as it does on the incursion of a weak magnetic interaction as the reactant approaches the singlet-triplet intersection - is an inherently improbable process. Spin-non-conservative reactions should therefore have low values of the pre-exponential factor, which would be expected to become more normal when heavy atoms are present in the reactant or the reaction medium. 

Spin-non-conservative reactions are indeed characterized by abnormally low preexponential factors that are increased by the presence of heavy atoms in the reactant or its immediate surroundings. 

Different authors interpret k more or less broadly [26, pp. 61ff.] we are evidently adopting the broadest definition. 

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