THE PROTON-TRANSFER SURFACE OF CH3OHF-

Diverse aspects of the potential surface for the proton-transfer react ion CH3OH +F--->CH3O-+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Moller-Plesset perturbation theory from second thr ough fourth order (MP2-MP4), the configuration interaction and coupled -cluster singles and doubles methods (CISD and CCSD), and CCSD theory augmented by a perturbative correction for connected triple excitation s [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2pld;2slp] to [14s9p6d4f;9s6p4d], including as man y as 482 atomic orbitals for the CH3OHF- system. The ion-molecule comp lex on the proton-transfer surface is a tight; hydrogen-bonded structu re of CH(3)0H.F- character, exhibiting a nearly linear -OHF- framework , an elongated O-H distance of 1.07(1) Angstrom, and a small interfrag ment separation, r(H-F)=1.32(1) Angstrom. Improved structural data for F- H2O are obtained for calibration purposes. A large fluoride affini ty is found for the CH3OHF- adduct, D-o=30:4+/-1 kcal mol(-1) and a bo nding analysis via the Morokuma decomposition scheme reveals considera ble covalent character. The harmonic stretching frequencies within the -OHF(-)moiety are predicted to be 421 and 2006 cm(-1), the latter pro tonic vibration being downshifted 1857 cm(-1) relative to omega(l)(O-H ) of free methanol. A systematic thermochemical analysis of the reacta nts and products on the CH3OHF- surface yields a proton-transfer energ y of 10.6 kcal mol(-1), a gas-phase acidity for methanol of 381.7+/-1 kcal mol(-1), and Do(CH3O-H)= 104.1+/-1 kcal mol(-1), facilitating the resolution of previous inconsistencies in associated thermochemical c ycles. A minimum-energy path in geometric configuration space is mappe d out and parametrized on the basis of constrained structural optimiza tions for fixed values of an aptly chosen reaction variable. The evalu ation of numerous energy points along this path establishes the nonexi stence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O-.HF type. The mathematical considerations f or a classical multipole analysis of reaction path asymptotes are outl ined for ion-dipole systems and applied to the CH3OHF- surface with du e concern for bifurcations in the exit channel for the proton-transfer process. A global analytic surface for vibrational stretching motion in the -OHF- moiety of the CH3OHF- system is constructed, and a suitab le dynamical model is tested which involves an effective, triatomic hy drogen pseudobihalide anion, [-OHF](-). Converged variational eigensta tes of [-OHF](-) to one-half its dissociation limit are determined usi ng vibrational configuration interaction expansions in terms of self-c onsistent-field medals. The fundamental stretching frequencies of the CH3OHF- complex predicted by the [-OHF](-) model are 504 (+84) and 145 6 (-549) cm-1 the corresponding anharmonicities appearing in parenthes es.