Rate Coefficients of the Cl + CH3C(O)OCH3 → HCl + CH3C(O)OCH2 Reaction at Different Temperatures Calculated by Transition-State Theory with ab Initio and Density Functional Theory Reaction Paths

Abstract

The complex relationship of computed rate coefficients (k's) with different ab initio/DFT and TST levels was studied. The MEPs, gradients, and Hessians of the title reaction were computed using the MP2 and DFT methods. Electronic energies were improved to the UCCSD(T)-F12x/CBS level, and k's were calculated at the TST, CVT, and ICVT levels with various tunnelling corrections. Although computed microcanonical and tunnelling effects are small, computed k(TST) values are larger than computed k(TST/ZCT) and k(TST/SCT) values by 3 orders of magnitude at low temperatures, because computed κ((TST/CAG)) values are as small as 6 × 10(-4). In some cases, the maximum of the ΔG/s curves at a certain T is far away from the MEP maximum. This raises the question of the range of s to be considered in a VTST calculation and, of a possible scenario, where no maximum on the ΔG curve can be located and hence a breakdown of VTST occurs. For dual-level direct dynamics calculations, different entropic contributions from different lower levels can lead to computed k's, which differ by more than 1 order of magnitude. Matching computed and experimental k values leads to an empirical barrier of 1.34 kcal mol(-1) for the title reaction.