A Direct ab Initio Trajectory Study on the Gas-Phase SN2 Reaction OH- + CH3Cl → CH3OH + Cl-

Abstract

Direct ab initio trajectory calculations have been applied to a SN2 reaction, OH- + CH3Cl → CH3OH + Cl-. First, static ab initio molecular orbital (MO) calculations with several basis sets were examined to select the most convenient and best fit basis set to that of high-quality calculations. As a result of the static ab initio calculations, it was found that the Hartree−Fock (HF)/3-21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6-311++G(2df,2pd) level. Next, direct ab initio dynamics calculations using the 3-21+G(d) basis set were carried out for the SN2 reaction. A full dimensional potential energy surface including all degrees of freedom was used in the dynamics calculation. The collision energies chosen were Ecoll = 5 and 25 kcal/mol. In the collisions at Ecoll = 5 and 25 kcal/mol, 48% and 63% of the total available energies were, respectively, partitioned into the relative translational mode between CH3OH and Cl-. Also, it was predicted that the C−H stretching mode of the product CH3OH is excited after the SN2 reaction, which is not detected in the case of the halogen-atom exchange SN2 reaction. The reaction mechanism was discussed on the basis of theoretical results.