Coupled cluster calculations for the S(N)2 reaction Cl-+CH3Br -> ClCH3+Br-

Coupled cluster calculations including single and double excitation operato rs plus a perturbative treatment of connected triples have been carried out for five stationary points of the potential energy surface of the S(N)2 re action Cl- + CH3Br --> ClCH3 + Br-. The ion-dipole complexes have well dept hs of 11.2 kcal mol(-1) in the reactant and 18.2 kcal mol(-1) in the produc t channel, respectively, and are separated by a barrier which is lower than the asymptotic reactant energy by 2.2 kcal mol(-1). The enthalpy of reacti on at 298 K is calculated to be -8.15 kcal mol(-1). (C) 2000 Elsevier Scien ce B.V.