Basis set effects in ab initio studies of sulfur trioxide–water complexes

Abstract

Sulfur trioxide–water complexes containing one to three water molecules have been investigated using ab initio methods with basis sets up to 6-311++G(2df,2pd). The transition states for conversion of the sulfur trioxide–water complexes to sulfuric acid and water molecules have been also studied. Binding and activation energies have been determined for the systems at the MP2 and QCISD levels, with single point energy corrections included at the MP4 and QCISD(T) levels of theory. Significant basis set effects are observed in the equilibrium geometries of the sulfur trioxide–water complexes and transition states. While the binding energies of the sulfur trioxide–water complexes are relatively insensitive to basis set and level of theory, the activation energies for conversion of the sulfur trioxide–water complexes to sulfuric acid and water exhibit a large drop as the basis set is increased. The large lowering of the activation barriers as a function of basis set leads to significantly increased rates of reaction compared to previous predictions. Furthermore, for reactions of sulfur trioxide with two water molecules, the SO 3·H 2O+H 2O reaction is predicted to be favored over the SO 3+(H 2O) 2 reaction.