Abstract
The energetics of the stationary points of the gas‐phase reactions CH~3~X+F^−^→CH~3~F+X^−^ (X=F, Cl, CN, OH, SH, NH~2~ and PH~2~) 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$\rm{_{F,F}^{,b}}$=−0.8, E$\rm{_{F,Cl}^{,b}}$=−12.2, E$\rm{_{F,CN}^{,b}}$=+13.6, E$\rm{_{F,OH}^{,b}}$=+16.1, E$\rm{_{F,SH}^{,b}}$=+2.8, E$\rm{_{F,NH_{2}}^{,b}}$=+32.8, and E$\rm{_{F,PH_{2}}^{,b}}$=+19.7 kcal mol^−1^. For the reverse reactions E$\rm{_{F,F}^{,b}}$=−0.8, E$\rm{_{Cl,F}^{,b}}$=+18.3, E$\rm{_{CN,F}^{,b}}$=+12.2, E$\rm{_{OH,F}^{,b}}$=−1.8, E$\rm{_{SH,F}^{,b}}$=+13.2, E$\rm{_{NH_{2},F}^{,b}}$=−1.5, and E$\rm{_{PH_{2},F}^{,b}}$=+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 (FCH~3~⋅F^−^ and ClCH~3~⋅F^−^), and those with substantial covalent character (NCCH~3~⋅F^−^, CH~3~OH⋅F^−^, CH~3~SH⋅F^−^, CH~3~NH~2~⋅F^−^ and CH~3~PH~2~⋅F^−^). All of the product complexes are of the form FCH~3~⋅X^−^ and are primarily electrostatic.