Temperature Dependence of the Rate Constants and Branching Ratios for the Reactions of Cl⁻(D₂O)₁₋₃ with CH₃Br and Thermal Dissociation Rates for Cl⁻(CH₃Br)

The rate constants and products for the reactions of Cl - (D 2 O) n + CH 3 Br ( n = 1−3) have been measured. The n = 1 reaction was studied from 238 to 478 K. The rate constant is well described by k = (6.0 × 10 - 10 ) exp(−1270/ T ) cm 3 s - 1 . We determined the reaction mechanism to be ligand switching to produce Cl - (CH 3 Br) followed by thermal decomposition of the complex. Cl - (CH 3 Br) decomposition produces greater than 90% Br - + CH 3 Cl with the remainder being Cl - + CH 3 Br. The Cl - (CH 3 Br) + He rate constant is well described by k = (4.5 × 10 - 10 ) exp(−2260/ T ) cm 3 s - 1 . RRKM theory was used to model the decomposition of Cl - (CH 3 Br). The results are consistent with our experimental results if a central barrier height of 22.5 kJ mol - 1 is used. The n = 2 and 3 reactions also proceed by ligand switching followed by thermal decomposition. The n = 2 reaction was studied from 203 to 298 K. The rate constant is well-described by (4.4 × 10 - 9 ) exp(−1329/ T ) cm 3 s - 1 . The main product observed was Cl - (D 2 O) with a smaller amount of Cl - (D 2 O)(CH 3 Br) also detected. The n = 3 reaction was studied from 188 to 203 K. The reaction was about a factor of 1.8 faster than the n = 2 reaction. The main product was Cl - (D 2 O) 2 with smaller amounts of Cl - (D 2 O) 2 (CH 3 Br) and Cl - (D 2 O)(CH 3 Br) also observed.