Effect of ion hydration in the gas-phase proton-transfer reaction of OH- with HBr

We have examined the role of solvation on intrinsic ion reactivity by measuring rate constants for the reactions of OH-(H2O)(n), 0 less than or equal to n less than or equal to 11, with HBr at 100 K. The reactions all proceed by proton transfer with accompanying solvent transfer. For 0 less than or equal to n less than or equal to 7, the reactions proceed at the collisional rate, while rate constants for the larger cluster reactions decrease slowly with cluster size. The reaction for n = 11 is only one-third efficient. Product ion distributions were measured for the clusters 0 less than or equal to n less than or equal to 7. For clusters containing two or more solvent molecules, the reactions with HBr all yield three product ions of the form Br-(H2O)(m), which differ only in the degree of solvation of the core ion. The average number of solvent molecules lost during the reaction, represented by [n - m], increases with increasing cluster size; the value of [n - m] for n = 7 is 3.4. The significant decrease in reaction efficiency at larger cluster sizes is discussed in terms of reaction energetics and possible structural changes within the primary cluster ion series.