Properties of clusters in the gas phase: V. Complexes of neutral molecules onto negative ions

Ion–molecules association reactions of the form A−(B)n−1+B=A−(B)n where studied over a range of temperatures in the gas phase using high pressure mass spectrometry. Enthalpy and entropy changes were determined for the stepwise clustering reactions of (1) sulfur dioxide onto Cl−, I−, and NO2− with n ranging from one to three or four, and onto SO2− and SO3− with n equal to one; and (2) carbon dioxide onto Cl−, I−, NO2−, CO3−, and SO3− with n equal to one. From these data and earlier hydration results, the order of the magnitude of the enthalpy changes on the association of the first neutral for a series of negative ions was found to parallel the gas-phase basicity of those anions. For any given ion, the relative order of the addition enthalpies among the neutrals was found to be dependent on the polarizabilities of the neutrals and on the covalency in the ion-neutral bond. Dispersion of charge via covalent bonding was found to significantly affect the succeeding clustering steps.