Benchmark ab Initio Energy Profiles for the Gas-Phase S{sub N}2 Reactions Y- + CH{sub 3}X {yields} CH{sub 3}Y + X{sup -} (X,Y = F,Cl,Br). Validation of Hybrid DFT Methods

The energetics of the gas-phase S{sub N}2 reactions Y{sup -} + CH{sub 3}X{sup -} {yields} CH{sub 3}Y + X{sup -} (where X,Y = F,Cl,Br), were studied using (variants on) the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. Our computational predictions are in excellent agreement with experimental data where these have small error bars; in a number of other instances reexamination of the experimental data may be in order. Our computed benchmark data (including cases for which experimental data are unavailable altogether) are used to assess the quality of a number of compound thermochemistry schemes such as G2 theory, G3 theory, and CBS-QB3, as well as a variety of density functional theory methods.