Vibrational relaxation of HOD by collisions with Ar atoms

Vibrational relaxation of HOD(v12, v3) molecules by collisions with Ar was studied at 298 K (v12 denotes coupled bending, v2, and OD stretching, v1, vibrational modes and v3 denotes OH stretching mode). The vibrationally excited HOD molecules were generated by exothermic abstraction reactions of OD radicals with 13 different RH reactants and observed by infrared emission from a fast-flow reactor as a function of Ar pressure and reaction time. State-specific relaxation rate constants were obtained by comparison of the time evolution of the experimental vibrational distributions with numerical kinetic calculations for vibrational populations. The relaxation mechanism was based on the relaxation scheme of H2O studied earlier with the addition of specific channels for HOD(v12, v3). Unlike H2O, energy in stretching and bending vibrations of HOD cannot be separated due to close ν1 and 2ν2 energies, which leads to fast collisional equilibration between these Fermi-resonant levels. For relaxation of the only pure bending state (10), a rate constant of (1.5 ± 0.3) × 10-13 cm3 molecule-1 s-1 was obtained. The relaxation rate of higher v12 states linearly increases with quantum number and very likely includes transfer of population from OD stretch levels, v1, to a lower energy bend level. The average rate constants for the loss of population from (01), (02), and (03) stretching states are (1.1 ± 0.3) × 10-14, (3.2 ± 1.0) × 10-14, and (5.6 ± 1.2) × 10-14 cm3 molecule-1 s-1, respectively.

Atmospheric Fate of the CH3SOO Radical from the CH3S + O2 Equilibrium

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH₃S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH₃S will predominantly react with O₂ to form the peroxy radical CH₃SOO. The formed CH₃SOO has two competing unimolecular reaction pathways: isomerization to CH₃SO₂, which further decomposes into CH₃ and SO₂, or a hydrogen shift followed by HO₂ loss, leading to CH₂S. Previous theoretical calculations have suggested that CH₂S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH₃ and SO₂ is the dominant route and the formation of CH₂S and HO₂ can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH₃SH reaction for CH₃S formation under low NO_x conditions and find a SO₂ yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO₂ yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.