Kinetic Isotope Effects for Cl + CH4 ⇌ HCl + CH3 Calculated Using ab Initio...

Barker, J., T. L. Nguyen, and J. F. Stanton (2012), Kinetic Isotope Effects for Cl + CH4 ⇌ HCl + CH3 Calculated Using ab Initio Semiclassical Transition State Theory, J. Phys. Chem. A, 116, 6408-6419, doi:10.1021/jp212383u.

Calculations were carried out for 25 isotopologues of the title reaction for various combinations of 35Cl, 37Cl, 12C, 13C, 14C, 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 Xij vibrational anharmonicity coefficients calculated at the CCSD(T) /aug-ccpVDZ 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 CH3D, CH2D2, CHD3, and CD4 are in very good agreement with literature experimental data. The ab initio 12C/13C KIE is in error by ∼2% at 298 K for calculations using Xij coefficients computed with the aug-cc-pVDZ basis set, but the error is reduced to ∼1% when Xij 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.

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Upper Atmosphere Research Program (UARP)