Kinetic isotope effects for Cl + CH _4 ? HCl + CH _3 calculated using ab initio semiclassical transition state theory

摘要

Calculations were carried out for 25 isotopologues of the title reaction for various combinations of ~(35)Cl, ~(37)Cl, ~(12)C, ~(13)C, ~(14)C, H, and D. The computed rate constants are based on harmonic vibrational frequencies calculated at the CCSD(T)/aug-cc-pVTZ level of theory and X _(ij) vibrational anharmonicity coefficients calculated at the CCSD(T) /aug-cc-pVDZ level of theory. For some reactions, anharmonicity coefficients were also computed at the CCSD(T)/aug-cc-pVTZ level of theory. The classical reaction barrier was taken from Eskola et al. J. Phys. Chem. A 2008, 112, 7391-7401, who extrapolated CCSD(T) calculations to the complete basis set limit. Rate constants were calculated for temperatures from ～100 to ～2000 K. The computed ab initio rate constant for the normal isotopologue is in good agreement with experiments over the entire temperature range (～10 lower than the recommended experimental value at 298 K). The ab initio H/D kinetic isotope effects (KIEs) for CH _3D, CH _2D _2, CHD _3, and CD _4 are in very good agreement with literature experimental data. The ab initio ~(12)C/ ~(13)C KIE is in error by ～2 at 298 K for calculations using X _(ij) coefficients computed with the aug-cc-pVDZ basis set, but the error is reduced to ～1 when X _(ij) coefficients computed with the larger aug-cc-pVTZ basis set are used. Systematic improvements appear to be possible. The present SCTST results are found to be more accurate than those from other theoretical calculations. Overall, this is a very promising method for computing ab initio kinetic isotope effects.