Definitive Ab Initio Studies of Model S_N2 Reactions CH_3X + F~- (X = F, Cl, CN, OH, SH, NH_2, PH_2)

摘要

The energetics of the stationary points of the gas-phase reactions CH_3X + F~- -> CH_3F + 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 high-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, NH_2)~b = + 32.8, and E_(F, PH_2)~b = + 19.7 kcal mol~(-1). For the reverse reactions E_(F,F)~b = 0.8, E_(Cl, F)~b = + 18.3, E_(CN, F)~b = + 12.2, E_(OH, F)~b = - 1.8, E_(SH, H_)~b = +13.2, E_(NH_2, F)~b = - 1.5, and E_(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~- and CH_3PH_2·F~-). All of the product complexes are of the form FCH_3·X~(-1) and are primarily electrostatic.