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 dependence of this reaction is flat in a He buffer over the entire experimental temperature range and in a H2 buffer in the range 300 K and above. The temperature dependence in a H2 buffer is about T -4 for temperatures below 300 K. The n = 3 reaction is about a factor of 10 slower than the n = 2 reaction at 163 K and proceeds entirely by association. For n = 4 an upper limit for the rate constant of k ≤ 2 × 10-13 cm3 s-1 is found at 163 K. For the reaction of OH-(H2O) with CH3Br eight isotopic variants were studied: OH-(H2O) with 0−3 deuterium atoms and both CH3Br and CD3Br. No isotopic effect was found between CD3Br and CH3Br. For OH-(H2O) isotopic substitution, the reactivity order was H2DO2 - ≥ HD2O2 - > H3O2 - ≥ D3O2 -. The ions group into two categories: those incorporating both hydrogen and deuterium and those with only hydrogen or deuterium. The former group was found to react about 10% more rapidly than the latter group.