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 - (H 2 O) n =0 - 4 with CH 3 Br 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 - (H 2 O), 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 - (H 2 O) products are observed, while in a H 2 buffer another product, OH(H 2 O) 2 CH 3 Br -, 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 H 2 buffer in the range 300 K and above. The temperature dependence in a H 2 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 cm 3 s -1 is found at 163 K. For the reaction of OH - (H 2 O) with CH 3 Br eight isotopic variants were studied: OH - (H 2 O) with 0−3 deuterium atoms and both CH 3 Br and CD 3 Br. No isotopic effect was found between CD 3 Br and CH 3 Br. For OH - (H 2 O) isotopic substitution, the reactivity order was H 2 DO 2 - ≥ HD 2 O 2 - > H 3 O 2 - ≥ D 3 O 2 - . 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.