Spectroscopic investigations of hydrogen bonding interactions in the gas phase. Vll. The equilibrium conformation and out-of-plane bending potential energy function of the hydrogen-bonded heterodimer H ₂ O • • • HF determined from its microwave rotational spectrum

Abstract

We have assigned the rotational spectrum of H 2 O • • • HF in vibrationally excited states associated with the hydrogen bond out-of-plane and in-plane bending modes v β(0) and v β(i) respectively. This has allowed us to decide whether the equilibrium configuration at the oxygen atom is planar (C 2v symmetry) or pyramidal (C s symmetry) by combining three types of information: vibrational spacing (from relative intensity measurements of vibrational satellites), the dependence of rotational constants on vibrational excitation (from frequency measurements), and the dependence of rotational constants on vibrational excitation (from frequency measurements), and the dependence of the electric dipole moment on the quantum numbers v β(0) and v β(1) (from the Stark effect). The v β(0) = 1 ← 0 separation in D 2 O • • • DF has also been obtained and used in reaching the conclusion that the equilibrium configuration at oxygen is pyramidal. Under the assumption that the mode v β(0) is governed by a one-dimensional potential function, a quantitative analysis of all available data leads to the expression V ( ϕ ) / c m − 1 = 328 ϕ 4 − 406 ϕ 2 , where (ϕ) is the angle between the O • • • HF line and the bisector of the angle HOH. This function implies a barrier height of 126 cm -1 or 1.5 kJ mol -1 to inversion of the configuration at oxygen. The equilibrium angle (ϕ) is now available for the series H 2 O • • • HF ((ϕ) = 46°), (CH 2 ) 3 O • • • HF ((ϕ) = 54°) and (CH 2 ) 2 O • • • HF (ϕ = 72°). The variation of (ϕ) along the series is interpreted in terms of the angle between the conventionally viewed non-bonding pairs on the oxygen atom in relation to the angle between the chemical bonds at that atom.

General features of geometries and potential constants of hydrogen bonded dimers

“…However, while the equilibrium geometry is pyramidal at the oxygen atom, this geometry is only 1.5 kJ mol-l more stable than the planar configuration [28]. It has been suggested that it is understandable on the basis of the potential functions that, in the solid state, statistical analyses of 0 * * H -0 hydrogen bonds show a preference for hydrogen bonding within the plane of the lone-pair directions, but there is no observed preference for the lone-pair directions within this plane [27].…”

Semiclassical model calculations of weak, strong, and short H-bonds

“…9). We had found (6) a similar superharmonic case with the sum of intermolecular Lennard-Jones potentials of nearest neighbours in condensed phases, and Millen and co-workers were able to establish this effect experimentally with H-bond vibrations (57). The first, meanwhile more or less forgotten, theoretical "superharmonic" example was given by Poschel and Teller (58) in 1933.…”

Electrostatic Potential and a Simple Extended Electric Dipole Model of Hydrogen Fluoride as Probes of Non-Bonding Electron Pairs in the Cyclic Ethers 2,5-Dihydrofuran, Oxetane and Oxirane

“…Similar behaviour was reported in Ref. [7] and here (Figure 1) for the variation of V(φ) with r for the H2O molecule, that is, when r was set to the experimental hydrogen bond length r(O⋯H) = 1.74 Å of H2O⋯HF, both the height of the potential barrier at φ = 0°(0.8 kJ mol −1 ) and φmin = ~±30° were underestimates when compared with the experimental potential energy function governing inversion of the configuration at O in H2O⋯HF [9], which has a potential energy barrier of 1.5 kJ mol −1 to the planar molecule and ϕmin = ±46°. In a recent detailed study, Alonso and co-workers [17] reinvestigated the rotational spectrum of 2,5-dihydrofuran⋯HF at very high resolution and discovered a small doubling in Figure 6.…”

“…Note that now φe = ±55°. A detailed analysis of the rotational spectrum of H 2 O· · · HF [9] in the ground state and in excited states associated with the out-of-plane, low frequency bending mode of the molecule led to a similar experimental potential energy function V(φ) versus φ, with φ e = ±46 (8) • . An even better match with the experimentally determined function V HF (φ) is obtained if an extended electric dipole model of HF is used.…”

“…The geometries of simple hydrogen-bonded complexes B· · · HX (where X is a halogen atom, a CN group or C≡CH) isolated in the gas phase at very low pressure have been extensively investigated by microwave and infrared spectroscopy over many years. The systematic variation of B led to a set of A detailed analysis of the rotational spectrum of H2O⋯HF [9] in the ground state and in excited states associated with the out-of-plane, low frequency bending mode of the molecule led to a similar experimental potential energy function V(φ) versus φ, with φe = ±46 (8)°. An even better match with the experimentally determined function VHF(φ) is obtained if an extended electric dipole model of HF is used.…”