Sequential hydration energies of the sulfate ion, from determinations of the equilibrium constants for the gas-phase reactions:: SO4(H2O)n2- = SO4(H2O)n-12-+H2O

Sequential hydration energies of SO4(H2O)(n)(2-) were obtained from determinations of the equilibrium constants of the following reactions: SO4(H2O)(n)(2-) = SO4(H2O)(n-1)(2-) + H2O. The SO42- ions were produced by electrospray and the equilibrium constants K-n,K-n-1 were determined with a reaction chamber attached to a mass spectrometer. Determinations of K-n,K-n-1 at different temperatures were used to obtain Delta G(n,n-1)(0), Delta H-n,n-1(0) and Delta S-n,n-1(0) for n = 7 to 19. Interference of the charge separation reaction SO4(H2O)(n)(2-) = HSO4(H2O)(n-k)(-) + OH(H2O)(k-1)(-) at higher temperatures prevented determinations for n < 7. The Delta S-n,n-1(0) values obtained are unusually low and this indicates very loose, disordered structures for the n >= 7 hydrates. The Delta H-n,n-1(0) values are compared with theoretical values Delta E-n,E-n-1, obtained by Wang, Nicholas, and Wang. Rate constant determinations of the dissociation reactions n,n - 1, obtained with the BIRD method by Wong and Williams, showed relatively lower rates for n = 6 and 12, which indicate that these hydrates are more stable. No discontinuities of the Delta G(n,n-1)(0) values indicating an unusually stable n = 12 hydrate were observed in the present work. Rate constants evaluated from the Delta G(n,n-1)(0) results also fail to indicate a lower rate for n = 12. An analysis of the conditions used in the two types of experiments indicates that the different results reflect the different energy distributions expected at the dissociation threshold. Higher internal energies prevail in the equilibrium measurements and allow the participation of more disordered transition states in the reaction.