Predicting the Formation of Sulfur-Based Brønsted Acids from the Reactions of SOx with H2O and H2S

We have performed an extensive computational investigation of the potential energy surfaces for the reactions of SOx (x = 2 or 3) with H2S and H2O in the gas phase and in aqueous solution at the CCSD(T)/CBS level of theory plus a self-consistent reaction field approach. Formation of a gas-phase H2SO4 from the hydrolysis of SO3 at lower temperatures requires the presence of additional water molecules. When additional waters are introduced, the barrier for H2SO4 formation is significantly reduced, and a barrierless transition occurs with only three excess waters as well as in aqueous solution. In a mixture of H2O, H2S, and SO3, formation of H2S2O3 has a lower barrier in the gas phase than does the formation of H2SO4. In aqueous solution, no barrier is predicted, and both are likely to be formed. Introduction of an additional water to the SO3 + H2S reaction results in a decrease in barrier height, nearly identical to the ∼18 kcal/mol catalytic effect of the first additional water in SO3 hydrolysis, with the barrier for H2S2O3 formation disappearing altogether in aqueous solution. Formation of H2SO3 by the way of SO2 hydrolysis is unlikely. Excess waters reduce the barrier for SO2 hydrolysis; however, the overall endothermicity is increased as waters are added. The formation of H2S2O2 from SO2 and H2S via an isostructural pathway to SO2 hydrolysis is unlikely, with additional water molecules resulting in a small increase in the overall endothermicity and a catalytic effect smaller than that observed for the SO3 reactions. The results of this work have implications pertaining to the formation of H2SO4, H2S2O3, H2SO3, and H2S2O2 in the atmospheres of Earth and Venus. These results also question the existence of H2S2O2 as an intermediate in the Claus process.