The saddle point of the nucleophilic substitution reaction Cl-+CH3Cl:: results of large-scale coupled cluster calculations

On the basis of large-scale coupled cluster calculations including connectedz triple substitutions in a perturbative way, the geometrical parameters of the D-3h Saddle point of the Walden inversion reaction Cl- + CH3Cl --> ClCH3 + Cl'(-) are predicted to be R-s (C-Cl) = 2.301 Angstrom and r(s) (C-H) = 1.069 Angstrom. The barrier height with respect to the reactants is recommended to be 11.5 +/- 1.0 kJ mol(-1) Connected triple substitutions lower the barrier height by almost a factor of 2, but have very little influence on the geometric structure of the saddle point.