Microcanonical variational transition state theory: quasi-collinear configurations. Application to the Cl + CH3I → ClI + CH3 excitation function

Abstract

Microcanonical transition state theory (µCTST) is used to derive analytical expressions for the excitation functions σ(E) of atom–diatom exchange reactions which are constrained by the potential to near-collinear collisions. To minimize the product flux, the location of the transition state is found variationally (µVTST), and its properties are obtained according to the method of Garrett and Truhlar. The theory is used to analyse the recently measured excitation function for the reaction Cl + ICH₃→ ClI + CH₃. LEPS surfaces were constructed from the properties of the quasi-diatomic fragments (CH₃ was taken as an atom) and by adjusting the Sato parameters to reproduce the experimental threshold energy. Over the post-threshold energy range the variational transition state did not coincide with the barrier, nor did its position change noticeably with energy. The variational microcanonical calculation was done for different subsets of the normal modes of the transition state. Good agreement with experiment is obtained when one considers only the bending modes ν_B actives in addition to the symmetric stretching ν_S mode.