Diverse aspects of the potential surface for the proton-transfer reaction CH3OH+F−→CH3O−+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Mo/ller–Plesset perturbation theory from second through 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 excitations [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2p1d;2s1p] to [14s9p6d4f;9s6p4d], including as many as 482 atomic orbitals for the CH3OHF− system. The ion–molecule complex on the proton-transfer surface is a tight, hydrogen-bonded structure of CH3OH⋅F− character, exhibiting a nearly linear -OHF− framework, an elongated O–H distance of 1.07(1) Å, and a small interfragment separation, r(H–F)=1.32(1) Å. Improved structural data for F−⋅H2O are obtained for calibration purposes. A large fluoride affinity is found for the CH3OHF− adduct, D0=30.4±1 kcal mol−1, and a bonding analysis via the Morokuma decomposition scheme reveals considerable covalent character. The harmonic stretching frequencies within the -OHF− moiety are predicted to be 421 and 2006 cm−1, the latter protonic vibration being downshifted 1857 cm−1 relative to ω1(O–H) of free methanol. A systematic thermochemical analysis of the reactants and products on the CH3OHF− surface yields a proton-transfer energy of 10.6 kcal mol−1, a gas-phase acidity for methanol of 381.7±1 kcal mol−1, and D0(CH3O–H)=104.1±1 kcal mol−1, facilitating the resolution of previous inconsistencies in associated thermochemical cycles. A minimum-energy path in geometric configuration space is mapped out and parametrized on the basis of constrained structural optimizations for fixed values of an aptly chosen reaction variable. The evaluation of numerous energy points along this path establishes the nonexistence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O−⋅HF type. The mathematical considerations for a classical multipole analysis of reaction path asymptotes are outlined for ion–dipole systems and applied to the CH3OHF− surface with due 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 suitable dynamical model is tested which involves an effective, triatomic hydrogen pseudobihalide anion, [-OHF]−. Converged variational eigenstates of [-OHF]− to one-half its dissociation limit are determined using vibrational configuration interaction expansions in terms of self-consistent-field modals. The fundamental stretching frequencies of the CH3OHF− complex predicted by the [-OHF]− model are 504 (+84) and 1456 (−549) cm−1, the corresponding anharmonicities appearing in parentheses.

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Electron photodetachment spectra of three alkoxide ions, 2,2-dimethyl-3-pentoxide, 3,3-dimethyl-2-butoxide, trifluoroethoxide, and two complex ions, 3,3-dimethyl-2-butanol/fluoride, 2,2-dimethyl-3-pentanol/fluoride, have been measured using an ion cyclotron spectrometer to generate, trap, and detect the ions. The electron affinities of trifluoroethoxide, 2,2-dimethyl-3-pentoxide, and 3,3-dimethyl-2-butoxide were measured as 58.9 ( 0.1, 47.8 ( 0.3, and 45.7 ( 0.1 kcal mol -1 , respectively. Using known proton affinities of the three alcohols allows us to determine the O-H bond dissociation energies. The threshold detachment energies for 2,2-dimethyl-3-pentanol/fluoride and 3,3-dimethyl-2-butanol/fluoride complex ions are 70.3 ( 0.5, and 70.1 ( 0.5 kcal mol -1 , respectively. The fluoride binding energies are thus on the order of 30 kcal mol -1 . We show that the observation or non-observation of photodetachment for alcohol/fluoride ions depends only on the alcohol acidity. In complex ions of the form ROHF -, if ROH is more acidic than HF the structure is RO -‚HF, and if ROH is less acidic than HF the structure is ROH‚F -. A flat potential energy surface model is consistent with the experimental results. The experimental results show no evidence for unusual structural change or stabilization when a hydrogen bond forms between two acids with similar acidities.

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