The proton-transfer surface of CH3OHF−

Abstract

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.

A high level ab initio map and direct statistical treatment of the fragmentation of singlet ketene

“…The extent of the decay of the successive terms toward zero is a gauge of the reliability of the final electronic structure predictions. As observed repeatedly, 48,49 the focal-point construction isolates in large part the slow basis-set convergence of differential correlation effects in the second-order increment ␦ ͓MP2͔, exposing the more rapid absolute convergence of analogous higher-order terms and facilitating the dual extrapolation of the final energy change (⌬E e [ f p]) toward the one-and n-particle limits. The changes in the binding energy increments upon the QZ(2d,2p)→QZ(2d1 f ,2p1d) augmentation exemplify this general behavior; 73 specifically, the shifts ⌬͑␦ ͓RHF͔, ␦ ͓MP2͔͒ϭ͑474, 795͒ cm Ϫ1 substantially exceed their counterparts ⌬͑␦ ͓MP3͔, ␦ ͓CCSD͔, ␦ ͓CCSD͑T͔͒͒ϭ͑103, 3, 66͒ cm Ϫ1 .…”

“…The carbon and oxygen 1s core orbitals were excluded from the active space in all correlation treatments together with the highlying, core-localized virtual orbitals appearing above selected energy thresholds. 47 Final energetic predictions for the dissociation curve were derived from focal-point extrapolations 48,49 of the dual one-particle basis and higherorder correlation dependence of the CH 2 •••CO interaction energies. The total energies for the focal point analysis at each level of theory are given in the supplementary material at the end of this article ͑Table XII͒.…”

“…The foundation for estimating these corrections is a focal-point extrapolation 48 of both basis set dependence and higher-order correlation terms augmented by an empirically based bond-additivity correction ͑BAC͒. Several recent thermochemical studies 48,49 exemplify the original technique, which is extended here to directly include the effects of connected quadruple excitations and to account for the decay of the BAC with increasing interfragment separation.…”

“…Indeed, convergence of higher-order increments even to 0.1 kcal mol Ϫ1 for basis sets of this type does not appear uncommon. 48,49 In accord with such observations, the higherorder correlation terms for the [13s8 p6d4 f ,8s6p4d] basis are taken from corresponding QZ(2d1 f ,2p1d) predictions in the f p extrapolation procedure, as signified by the brackets in Table IV. The connected quadruples ͑Q͒ correction for the dissociation energy, whose evaluation is feasible only with the 6-31G* basis, is Ϫ0.31 kcal mol Ϫ1 .…”

Nature of the Bonding in the Bifluoride Anion, FHF–

“…Although the current results are limited to the FHF – anion, it is almost certain that this type of bonding will be found in other bihalide anions, a subject that is currently under investigation. It is also likely that this new bonding motif will also be found in other anions with strong, short “hydrogen bonds. − …”