Transition State and G2Q Analysis of Hydrocarbon Radical Reactions with Cl2

Abstract

A combination of ab initio quantum mechanics (QM) and transition state (TS) theory has been used to predict the preexponential factors (k 0 's) for reactions of methyl, ethyl, isopropyl, tert-butyl, and propargyl radicals with Cl 2 . The QM UHF/6-31G* and QCISD/6-311G** calculations show that the TS structures and vibrational frequencies associated with the C-Cl-Cl reaction center are similar for all reactions considered. This supports the thermochemical hypothesis of a tight TS that suggests those reaction centers of other R . + Cl 2 → RCl + Cl . reactions should have similar properties. Using the calculated k 0 's, the activation energies at 0 K (E 0 's) for the reactions considered are derived from known data. The predicted k 0 's are in good agreement with the methyl and propargyl radicals' reaction rate data but are in poor agreement with ethyl, isopropyl, and tert-butyl radical data that are indicated by unrealistically negative E 0 's.