A quantum chemical calculation of the potential energy surface in the formation of HOSO₂ from OH + SO₂

The formation of HOSO₂ from OH and SO₂ has been thoroughly investigated using several different methods (MP2=Full, MP2=FC, B3LYP, HF and composite G* methods) and basis sets (6-31G(d,p), 6-31++G(d,p), 6-31++G(2d,2p), 6-31++G(2df,2p) and aug-cc-pVnZ). We have found two different possible transition state structures, one of which is a true transition state since it has a higher energy than the reactants and products (MP2=Full, MP2=FC and HF), while the other is not a true transition state since it has an energy which lies between that of the reactants and products (B3LYP and B3LYP based methods). The transition state structure (from MP2) has a twist angle of the OH fragment relative to the SO bond of the SO₂ fragment of -50.0°, whereas this angle is 26.7° in the product molecule. Examination of the displacement vectors confirms that this is a true transition state structure. The MP2=Full method with a larger basis set (MP2=Full/6-31++G(2df,2p)) predicts the enthalpy of reaction to be -112.8 kJ mol^-1 which is close to the experimental value of -113.3 ± 6 kJ mol^-1, and predicts a rather high barrier of 20.0 kJ mol^-1. When the TS structure obtained by the MP2 method is used as the input for calculating the energetics using the QCISD/6-31++G(2df,2p) method, a barrier of 4.1 kJ mol^-1 is obtained (ZPE corrected). The rate constant calculated from this barrier is 1.3 × 10^-13 cm³ molecule^-1 s^-1. We conclude that while the MP2 methods correctly predict the TS from a structural point of view, higher level energy corrections are needed for estimation of exact barrier height.