Characterization of Reaction Pathways on the Potential Energy Surfaces for H + SO₂ and HS + O₂

Unimolecular pathways for the isomerization and/or dissociation of HSOO, HOSO, HSO 2, and HOOS to H + SO 2 and OH + SO have been investigated computationally, as well as HSO formation via an HSOO intermediate. The atmospheric lifetime of HSO 2 is discussed. Some pathways have no barrier, including OH + SO → HOSO and H + SOO → HSOO and SOOH, while structures and vibrational frequencies of transition states for HOSO → H + SO 2, HOOS → OH + SO, HOSO → HSO 2, HSOO → HS + O 2, and HSO 2 → H + SO 2 have been characterized at the MP2=FULL/6-31G(d) level. Some geometries were further refined at the QCISD/6-311G(d,p) level. Gaussian-2 theory was employed to calculate approximate QCISD(T)/6-311+G(3df,2p) energy barriers, and the kinetics were analyzed by RRKM theory. Rate constant expressions at the high and low-pressure limits and thermochemical properties for transient intermediates are tabulated, and the results are discussed in the context of atmospheric and combustion chemistry. A revised theoretical H−OSO bond strength is compatible with the flame data for SO 2 -catalyzed recombination of H atoms.