| Abstract
| - Sequential hydration energies of SO4(H2O)n2- were obtained from determinations of the equilibrium constantsof the following reactions: SO4(H2O)n2- = SO4(H2O)n-12- + H2O. The SO42- ions were produced byelectrospray and the equilibrium constants Kn,n-1 were determined with a reaction chamber attached to amass spectrometer. Determinations of Kn,n-1 at different temperatures were used to obtain ΔG0n,n-1,ΔH0n,n-1, and ΔS0n,n-1 for n = 7 to 19. Interference of the charge separation reaction SO4(H2O)n2- =HSO4(H2O)n-k- + OH(H2O)k-1- at higher temperatures prevented determinations for n< 7. The ΔS0n,n-1values obtained are unusually low and this indicates very loose, disordered structures for the n ≥ 7 hydrates.The ΔH0n,n-1 values are compared with theoretical values ΔEn,n-1, obtained by Wang, Nicholas, and Wang.Rate constant determinations of the dissociation reactions n,n − 1, obtained with the BIRD method by Wongand Williams, showed relatively lower rates for n = 6 and 12, which indicate that these hydrates are morestable. No discontinuities of the ΔG0n,n-1 values indicating an unusually stable n = 12 hydrate were observedin the present work. Rate constants evaluated from the ΔG0n,n-1 results also fail to indicate a lower rate forn = 12. An analysis of the conditions used in the two types of experiments indicates that the different resultsreflect the different energy distributions expected at the dissociation threshold. Higher internal energies prevailin the equilibrium measurements and allow the participation of more disordered transition states in the reaction.
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