AN ABINITIO STUDY OF THE REACTION OF ATOMIC-HYDROGEN WITH SULFUR-DIOXIDE

The potential energy surface for H (1 2S) + SO2 has been investigated computationally in order to study the catalytic removal of atomic hydrogen in flames by sulfur dioxide. HF/3-21G(*) and MP2/3-21G(*) levels of theory were employed to locate stationary points, which were then characterized by calculation of the vibrational frequencies. Some geometries were also optimized with the 6-31G* basis set. Two adducts HOSO and HSO2, with H bonded to O or S, respectively, were studied. Energies were estimated at the optimized geometries using spin-projected MP4/6-31G* calculations, which show that planar cis HOSO is more stable than C(s) HSO2. An H-OSO bond energy of 109 kJ mol-1 is predicted. By contrast HSO2 is predicted to be 25 kJ mol-1 endothermic with respect to H + SO2, and is insufficiently stable to be significant in combustion chemistry. Transition states were located and the information used to derive the kinetics of H + SO2 + Ar reversible HOSO + Ar from 298 to 2000 K. An unusually large energy barrier to recombination, of about 95 kJ mol-1 relative to H + SO2, is proposed. The results are compared with available kinetic measurements. Other potential decomposition channels for HOSO, to SO + OH and isomerization to HSO2, were also analyzed.