An ab Initio Study of the Kinetics of the Reactions of Halomethanes with the Hydroxyl Radical. 1. CH2Br2

Abstract

Ab initio calculations were carried out with Møller−Plesset second- and fourth-order perturbation theory (MP2 and MP4), and the coupled cluster method, CCSD(T), on the H atom abstraction reaction from dibromomethane by hydroxyl radical attack. Geometry optimization and vibrational frequency calculations at the MP2 level were performed on reactants, products, and the transition state using the 6-311G(d,p) and 6-311G(2d,2p) basis sets. The geometry parameters optimized at the MP2/6-311G(2d,2p) level of theory were used in single-point energy calculations with increasing basis set sizes, from 6-311G(2d,2p) to 6-311++G(3df,3pd) at both the MP2 and MP4(SDTQ) levels of theory. Canonical transition-state theory was used to predict the rate constants as a function of the temperature (250−400 K). It was found that the kinetic parameters obtained in this work with the spin-projected method PMP4(SDTQ)/6-311++G(3df,3pd)//MP2/6-311G(2d,2p) are in reasonable agreement with the experimental values. The prospect of using relatively low level ab initio electronic structure calculations aimed at the implementation of inexpensive semiquantitative “screening tools” that could aid scientists in predicting the kinetics of similar processes is also discussed.