Gas-Phase Acidities of Nine Sulfur Oxoacids of Composition [H2,S,On] (n = 1–4)

The gas-phase acidities of the most stable conformations of nine sulfur oxo acids were determined by ab initio MO calculations at the CBS-Q, G2(MP2), and G2 levels of theory. The most accurate G2 results are as follows (ΔG°298 of the deprotonation reaction in kJ mol–1): H–S–O–H 1468, H–S–O–H 1449, (HO)2S 1426, H–S(=O)OH 1406, H2S/O 1394, H–S(=O)OH 1361, (HO)2S/O 1324, H2S(=O)2 1321, H2O→S 1294, H–S(=O)2OH 1287, H–S(=O)2OH 1279, (HO)2S(=O)2 1268. These values are in excellent agreement with the few experimental data that can be considered reliable. For molecules of analogous structure, the acidity increases with increasing oxidation number of the sulfur atom. While sulfenic acid, HSOH, is as weak an acid as HOCl, sulfurous acid, (HO)2SO, is as strong a proton donor as nitric acid, HNO3, and sulfonic acid, HS(O)2OH, is even stronger than hydrogen iodide, HI, surpassed only by sulfuric acid, H2SO4. These results are of relevance to phenomena such as acid rain and aerosol formation in the earth′s atmosphere.