Separable unimolecular reaction rate theory

A new unimolecular reaction rate theory is derived on the basis of a classical model of the reaction. The fundamental assumptions are (i) instantaneous activation-deactivation events represented by a statistical transition probability, (ii) irreversible dissociation, and (iii) separability of internal and external dynamics. The third assumption is found to be satisfied when the statistical transition probability satisfies a generalized strong collision assumption and it leads to a rate theory which depends on internal molecular dynamics only through lifetime information for the isolated molecule. The new theory shows that the rate coefficient is non-Markoffian although the memory effects will not be apparent on the macroscopic time scales of traditional experiments. Thus Slater's “new approach to rate theory” is a special case of the separable rate theory when applied to slow reactions. New experimental techniques are predicted to have the capacity to resolve the memory effects in the rate coefficient and provide lifetime information which would then allow greatly improved accuracy in the prediction of rates for a wide range of unimolecular reactions.