STABILITIES OF ANIONIC MIXED CLUSTERS OF SULFUR-DIOXIDE AND WATER

The thermochemistry of the mixed anionic clusters SO2-. (SO2)n.(H2O)m for clusters up to rank r = n + m = 6 was determined from equilibrium measurements. The gradual substitution of the SO2 molecule by the H2O molecule in a cluster of a given rank r is always stabilizing. The step-wise build-up of SO2-.(SO2)n.(H2O) clusters shows a slight shell effect after n = 3, consistent with the filling of a first solvent shell about the dimer anion SO2-.(SO2). Electrostatic calculations and symmetry considerations seem to support this shell effect. The enthalpy changes for the second and the third hydration steps for SO2-.(SO2)n.(H2O)m decrease smoothly over the range m = 1-3, suggesting a similar process for the H2O attachment and thus the favored topology places H2O molecules near the anion dimer.