## Abstract

We have assigned the rotational spectrum of H$_2$O $\cdots$ HF in vibrationally excited states associated with the hydrogen bond out-of-plane and in-plane bending modes $\nu_{\beta(o)}$ and $\nu_{\beta(i)}$ respectively. This has allowed us to decide whether the equilibrium configuration at the oxygen atom is planar (C$_{2\nu}$ 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 the electric dipole moment on the quantum numbers $v_{\beta(o)}$ and $v_{\beta(i)}$ (from the Stark effect). The $v_{\beta(o)} = 1 \leftarrow 0$ separation in D$_2$ O $\cdots$ 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 $\nu_{\beta(o)}$ is governed by a one-dimensional potential function, a quantitative analysis of all available data leads to the expression $V(\phi)/\mathrm{cm}^{-1} = 328\phi^4 - 406\phi^2,$ where $\phi$ is the angle between the O $\cdots$ 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 $\phi$ is now available for the series H$_2$O$\cdots$ HF ($\phi$ = 46$^\circ$), (CH$_2$)$_3$O$\cdots$ HF ($\phi$ = 54$^\circ$) and (CH$_2$)$_2$O$\cdots$ HF ($\phi$ = 72$^\circ$). The variation of $\phi$ 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.