Temperature Dependences of the Rate Constants and Branching Ratios for the Reactions of OH-(H2O)0-4 + CH3Br

Various effects of solvation, isotopic substitution, and temperature on the reactions of OH-(H2O)n=0-4 with CH3Br are reported. Increasing solvation is found to decrease reactivity dramatically as has been observed previously. For n = 0 the rate constant is large, and the temperature dependence is small, T-0.58; Br- is the only product. For n = 1 the rate constant is about a factor of 1.5 smaller than that for the n = 0 reaction, and the temperature dependence is T-1.6. This reaction produces about 90% Br- and 10% Br-(H2O), with this ratio roughly independent of temperature. The rate constant for the n = 2 reaction is almost 2 orders of magnitude lower than that for n = 1. The n = 2 reaction is complicated in that the low-temperature behavior depends on the identity of the buffer gas. In a He buffer at all temperatures, only Br- and Br-(H2O) products are observed, while in a H2 buffer another product, OH(H2O)2CH3Br-, is observed at low temperatures in addition to the above products. The temperature depend...