High-level theoretical study of the hydrogen abstraction reaction H2S + O2 = SH + HO2 and prediction of the rate constants

Hydrogen abstraction reaction of hydrogen sulfide (H2S) by molecule oxygen (O-2) forming hydroperoxyl radical (HO2) and hydrosulfide radical (SH) is very important for both atmospheric and combustion chemistry. This work reports a systematic theoretical study on this reaction using a combination of high-level quantum chemistry methods and chemical kinetic theory. Geometry optimizations for all species are performed at QCISD/cc-pVQZ level of theory. The energies for minima and transition state are computed using both single- and multi reference ab initio methods. It is found that the transition state of this reaction exhibits significant multi-reference nature based on < S-2 > and T1 diagnostic during CCSD(T) calculations. The predicted reaction barrier from high-level multi-reference CASPT2(20e,13o)/aug-cc-pVTZ method is higher than those from CCSD(T) and W1BD methods by 6-7 kcal mol(-1). Reaction energy barriers from a series of DFT methods also show large deviations compared with both single- and multi-reference methods. Thermal rate constants are calculated over a temperature range from 300 to 2500 K by employing canonical transition state theory with Eckart tunneling correction and the treatment of torsional mode. This work provides accurate rate constants for this reaction and fundamental benchmark results for H/O/S reaction systems, which is valuable for theoretical chemistry and kinetic modeling of sulfur chemistry.