Definitive ab initio studies of model SN2 reactions CH3X+F- (X = F, Cl, CN, OH, SH, NH2, PH2)

The energetics of the stationary points of the gas-phase reactions CH3X + F- --> CH3F + X- (X = F, Cl, CN, OH, SH, NH2 and PH2) have been definitively computed using focal point analyses. These analyses entailed extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher-order electron correlation [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and addition of auxiliary terms for core correlation and scalar relativistic effects. The final net activation barriers for the forward reactions are: E-F,F(b)=-0.8, E-F,Cl(b) = -12.2, E-F,CN(b) = +13.6, E-F,OH(b) = +16.1, E-F,SH(b) = + 2.8, E-F,NH2(b) = +32.8, and E-F,PH2(b) = +19.7 kcal mol(-1). For the reverse reactions E-F,F(b) = -0.8, E-ClF = + 18.3, E-CN,F(b) = +12.2, E-OH,F(b) = -1.8, E-SH,F(b) = +13.2, E-NH2,F(b) = -1.5 and SH F NH2,F E-PH2,F(b) = +9.6 kcal mol(-1). The change in energetics between the CCSD(T)/aug-cc-pVTZ reference prediction and the final extrapolated focal point value is generally 0.5-1.0 kcal mol(-1). The inclusion of a tight d function in the basis sets for second-row atoms, that is, utilizing the aug-cc-pV(X+d)Z series, appears to change the relative energies by only 0.2 kcal mol(-1). Additionally, several decomposition schemes have been utilized to partition the ion-molecule complexation energies, namely the Morokuma-Kitaura (MK), reduced variational space (RVS), and symmetry adapted perturbation theory (SAPT) techniques. The reactant complexes fall into two groups, mostly electrostatic complexes (FCH3.F- and ClCH3.F-), and those with substantial covalent character (NCCH3.F-, CH3OH.F-, CH3SH.F-, CH3NH2.F- and CH3PH2.F-). All of the product complexes are of the form FCH3.X- and are primarily electrostatic.