Quantitative Kinetics of the Hydrogen Shift Reaction of Methylthiomethyl Peroxy Radical (CHSCHOO) in the Atmosphere.

Hydrogen shift processes of peroxy radicals are paramount to understanding atmospheric chemical oxidations of volatile organic compounds. However, quantifying these processes using experimental and theoretical methods is also very difficult. Here, we chose methylthiomethyl peroxy radical (CHSCHOO) as a typical reaction to investigate the hydrogen shift in CHSCHOO using a dual-level (DL) strategy. In the DL strategy, GMMQ.L3//CCSD(T)-F12a/cc-pVTZ-F12 is used as a high-level method to calculate the rate constant using transition state theory. Here, GMMQ.L3 is a newly developed composite method for single-point energy calculations that approximates CCSDTQ/CBS accuracy (coupled cluster theory with single, double, triple, and connected quadruple excitations at the complete basis set limit). Additionally, MN15/MG3S is used as a low level to do multistructural canonical variational transition state theory with large curvature tunneling (MS-CVT/LCT) calculations. The calculated rate constants of 0.05-0.08 s agree well with the corresponding experimental values and the previous MC-TST results for the hydrogen shift of CHSCHOO at 293-298 K. The calculated results unravel that the zero-point vibrational energies depend strongly on the basis set in the CCSD(T)-F12 calculations. We find the large effects of the enthalpy of activation at 0 K, tunneling, and multistructural torsional anharmonicity on the calculated rate constant of the hydrogen shift of CHSCHOO. The current study provides a valuable reference case for the quantitative kinetic calculations of the peroxy radical isomerization reaction in the atmosphere.