Sulfur Dioxide and Water: Structures and Energies of the Hydrated Species SO2•nH2O, [HSO3]-•nH2O, [SO3H]-•nH2O, and H2SO3•nH2O (n=0-8)

The structures of a large number of hydrates of sulfur dioxide (SO2 center dot nH(2)O), of the sulfonate ion ([HSO3](-)center dot nH(2)O), of the tautomeric hydrogensulfite anion [SO3H](-) **), and of sulfurous acid (H2SO3 center dot nH(2)O) with up to eight water molecules attached to these species have been optimized at the B3LYP/6-31G(2df,p) level of theory (DFT). The calculated vibrational frequencies allow the definite assignment of certain characteristic modes, and in this way a convincing interpretation of published spectra of aqueous SO2 as well as Of SO2 adsorbed on very cold ice crystals has been achieved for the first time. Single-point calculations at the G3X(MP2) level of theory were used to calculate the binding energies of the water molecules in SO2 center dot nH(2)O as well as the relative stabilities of the isomeric anionic species ([HSO3](-)center dot nH(2)O) and (SO3H](-)center dot nH(2)O). Generally, the water molecules tend to stick together forming clusters, whereas the particular sulfur-containing molecule remains at the surface of the water cluster, but it is always strongly hydrogen-bonded. Only when there are more than six water molecules are the anions more or less completely surrounded by water molecules. DFT calculations erroneously predict that the gaseous hydrated sulfonate ions are more stable than the isomeric hydrogensulfite ions, even when hydrated with six water molecules. However, if these hydrated species are calculated as being embedded in a polar continuum simulating the aqueous phase, the hydrogensulfite ions are more stable than the sulfonate ions, in agreement with various spectroscopic observations on aqueous sulfite solutions. On the other hand, at the higher G3X(MP2) level, the gaseous hydrated-hydrogensulfite anions are more stable than the corresponding sulfonate ions only if the number of water molecules is larger than four, whereas for the weakly hydrated anions the order of relative energies is reversed. The possible implications of these results for the enzymatic oxidation of "sulfite ions" ([HSO3](-) and [SO3H](-)) by sulfite oxidase are discussed. The conversion of SO2 center dot 6H(2)O into its isomer ([HSO3](-)center dot nH(2)O) is predicted to be exothermic (Delta H-298 degrees = -56.1 kJ mol(-1)) and exergonic (Delta G(298)degrees = -22.5 kJ mol(-1)). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)