Formation of a pre-reaction hydrogen-bonded complex and its significance in the potential energy surface of the OH + SO₂ → HOSO₂ reaction: A computational study

Using computational calculations, we have revisited the potential energy surface (PES) of the reaction between OH and SO₂, which is believed as the rate-limiting step in the atmospheric formation of H₂SO₄. In this work, we report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO₂ in the reaction PES. Based on this finding, it has been shown that the reaction can be considered as a two-step process in which the first step is the formation of the pre-reaction complex and the second step is the transformation of this complex to the product. It was observed that due to the presence of this pre-reaction complex as a potential well in the reaction PES, the barrier height got increased by around two-fold for the second step. Based on this observation, it has been proposed that the kinetics of the reaction is going to be affected. Also based on the analysis of the geometries of this pre-reaction complex and the transition state, it has been argued that the step involving the transformation of this pre-reaction complex to the product via the transition state is going to be the slowest step as this transformation involves large structural changes of the stationary points involved.