Benchmark ab Initio Energy Profiles for the Gas-Phase SN2 Reactions Y- + CH3X → CH3Y + X- (X,Y = F,Cl,Br). Validation of Hybrid DFT Methods

The energetics of the gas-phase SN2 reactions Y^- + CH₃X \ CH₃Y + X^- (where X, Y = F, Cl, Br), were studied using (variants on) the recent W1 and W2 \ ab initio computational thermochemistry methods. Our computed benchmark data (including cases for which experimental data are unavailable altogether) are used to assess the quality of a number of semiempirical compound thermochemistry schemes such as G2 theory, G3 theory, and CBS-QB3, as well as a variety of density functional theory methods. Upon applying some modifications to the level of theory used for the reference geometry (adding diffuse functions, replacing B3LYP by the very recently proposed mPW1K functional), the compound methods appear to perform well. Only the 'half-and-half' functionals BH the other functionals fail to find a transition state in the F/Br case. BH this problem is resolved in HCTH-120. mPW1K appears to exhibit the best performance of the functionals considered, although its energetics are still inferior to the compound thermochemistry methods.