Ab Initio Study of SO₂+ H₂O

We present quantum chemical calculations of several stationary points in the SO 2 + H 2 O system. Final calculations at the QCISD(T)/ 6-311++G(d,p)//MP2/6-31G(d) level predict the following gas-phase thermochemistry at 270 K. (1) SO 2 + H 2 O → H 2 SO 3 (Δ H ° = +44.9 kJ mol -1; Δ G ° = +92.7 kJ mol -1; E a ≥ 130 kJ mol -1 ). (2) SO 2 + H 2 O → (SO 2 −H 2 O) (Δ H ° = −7.6 kJ mol -1; Δ G ° = +20.1 kJ mol -1; K p = 1.29 × 10 -4 ). The high activation barrier for H 2 SO 3 formation and its large positive Δ G ° are consistent with this species having never been detected. Lower level calculations on reaction 1 in the aqueous phase indicate a positive Δ G ° and high activation barrier in solution as well. The calculated 1:1 SO 2 −water complex geometry, vibrational frequencies, and rotational constants are in good agreement with experimental results. Its low binding energy suggests that it is unimportant in the atmosphere.