Temperature Dependences of the Rate Constants and Branching Ratios for the Reactions of F⁻(H₂O)₀₋₅ with CH₃Br

The effects of solvation, isotopic substitution, and temperature on the reactions of F - (H 2 O) n =0 - 5 with CH 3 Br have been studied. The reaction of n = 0 produces Br - as the exclusive ionic product and has a rate which is fast, approaching the collision rate, with a slight negative temperature dependence of T -0.9±0.1 . The reaction of n = 1 is a factor of 3 slower with a slightly larger negative temperature dependence, T -1.1±0.05 . The main ionic product is Br -, with smaller amounts of Br - (H 2 O) also formed. Substituting D 2 O for H 2 O causes the n = 1 rate constant to increase slightly. The n = 2 reaction has a much slower rate and produces three ionic products: Br -, Br - (H 2 O), and F - (H 2 O) 2 (CH 3 Br). The n = 3 reaction is immeasurably slow. The rate constants for the n = 4 and 5 reactions are greater than those for the n = 3 reaction. The ionic products of these reactions are F - (H 2 O) 3 and F - (H 2 O) 4, respectively.