Gas-phase and water-mediated mechanisms for the OCS plus OH reaction

We report a computational study of the gas-phase and water-mediated mechanisms for the oxidation of carbonyl sulfide (OCS) by the hydroxyl radical. To achieve reliable results, we employ a dual-level strategy within interpolated single-point energies (VTST-ISPE) at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. In the gas-phase mechanism, we have determined the rate constants by kinetic Monte Carlo simulation in the interval of temperatures of 250-550 K. The calculated rate constant, at room temperature, is 4.86 x 10-16 cm3 molecule-1 s-1, in agreement with experimental measurement: 6.00 +/- 4.00 x 10-16 cm3 molecule-1 s-1 [M. T. Leu and R. H. Smith, J. Phys. Chem., 1981, 85, 2570-2575]. The water-mediated mechanism, a more complex process than the gas-phase, revealed six reaction pathways. The application of the pre-equilibrium model allowed us to determine termolecular thermal rate constants. Considering the concentrations of water as a function of the relative humidity at 0 km altitude, we estimated effective rate constants. The magnitude of the rate coefficients for this mechanism suggested a negligible effect of the water in the OCS + OH reaction.