Quantum chemical and dynamical characterisation of the reaction OH + SO2 ⇌ HOSO2 over an extended range of temperature and pressure

Abstract

The recombination reaction of the OH radical with SO₂ has been investigated using current theoretical methods. Quantum chemical calculations were performed to locate the stationary points on the potential energy surface (PES) for the recombination/dissociation process, including structures and vibrational frequencies calculated at the B3LYP/aug-cc-pVTZ(+1) and QCISD/6-311G(d,p) level of theory. Furthermore, the energetics are characterised by the application of G3X-theory which provides all sensitive data such as the critical energy barrier and the enthalpy of reaction. The results obtained suggest a compact transition state with regards to the dissociation of HOSO₂. Combining the calculated enthalpy of formation for the HOSO₂ radical (ΔH_f(298.15 K) = −368.8 kJ mol⁻¹) and the well known enthalpy of formation for OH and SO₂ this allows the calculation of the temperature dependent equilibrium constant K_eq(T). In addition, a very shallow barrier (∼0.5 kJ mol⁻¹) is predicted for the recombination process at T = 0 K. The novel data have been used as input to statistical kinetic calculations. Focussing on the reverse process (e.g. the HOSO₂ dissociation), RRKM-theory in conjunction with a subsequent solution of the master equation has been utilised to derive complete fall-off curves for the thermal decomposition of HOSO₂ as well as for the OH + SO₂ recombination reaction, both in a weak collision scenario ([M] = N₂,He). Furthermore, the temperature dependence of the recombination reaction has been analysed in the temperature range between 150 and 1500 K and Arrhenius expressions have been derived from the “numerically exact” fall-off data in different pressure regimes.