Stepwise Hydration and Multibody Deprotonation with Steep Negative Temperature Dependence in the Benzene•+−Water System

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Abstract
We studied the stepwise hydration and solvent-mediated deprotonation of the benzene•+ cation (Bz•+) and found several unusual features. The solvent binding energies ΔH on-1,n for the reactions Bz•+(H2O)n-1 + H2O → Bz•+(H2O)n are nearly constant at 9 ± 1 kcal mol-1 for n = 1 to 8. We observed a remarkable sudden decrease in the entropy of association accompanying the formation of Bz•+(H2O)7 and Bz•+(H2O)8, indicating strong orientational restraint in the hydration shells of these clusters consistent with the formation of cagelike structures. We observed the size-dependent deprotonation of Bz•+ in a cooperative multibody process, where n H2O molecules (n ≥ 4) can remove a proton from Bz•+ to form protonated water clusters. We measured, for the first time, the temperature dependence of such a process and found a negative temperature coefficient of a magnitude unprecedented in any chemical reaction, of the form k = AT-67± 4, or in an Arrhenius form having an activation energy of −34 ± 1 kcal mol-1. The temperature effect may be explained by Bz•+ and four H2O molecules needing to be assembled from gas-phase components to form the reactive species. Such large temperature effects may be therefore general in solvent cluster-mediated reactions.

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