Energetic and structural aspects of the solvation of anions in liquid SO2

The solvation of halogen ions (F-, Cl-) in liquid SO2 was investigated by means of ab initio calculations. Discrete complexes of one to four SO2 ligands with a halide ion were optimized at various levels of theory. In the cases of the SO2 molecule and the primarily formed halosulfite ions, the influence of the level of electron correlation and different basis sets on the structures and energies was studied by calculations at various levels of theory up to QCISD(T). The stabilities of the halosulfite ions were found to be -46.9 kcal/mol for the fluorosulfite ion (1) and -19.8 kcal/mol for the chlorosulfite ion (2). Higher complexes were optimized at the HF/6-31+G* and MP2/6-31+G* levels, and two different complexation patterns were obtained: complexation at the halogen or at an oxygen of the halosulfite ion. Almost independently of the type of complexation, each SO2 ligand afforded a stabilization energy of 10-18 kcal/mol, thus proving that the high solvation energy is responsible for the ionogenic character of this solvent. The bonding situation in the complexes was studied by NBO analyses, and it was found that the stabilization is mainly due to electrostatic interactions, while charge transfer, covalent bonding, and orbital interactions make only minor contributions.