The fluctuation hypothesis of hydrogen bonding

Abstract

On the basis of a fluctuation model of hydrogen bonding which takes into account the non-equivalence of H-bonds due to fluctuations of local environments in liquids, a mathematical formalism is developed to calculate the fluctuation spectral contours for the case of coupling of two stretching vibrations and the overtone of the bending mode (Fermi resonance). Computer calculations of the contour shape are performed for the infra-red and Raman spectra of the 1 : 2 complexes formed by an XH₂ molecule with two proton-acceptor molecules. The fluctuation model is shown to explain in a quantitative manner some important empirical regularities established for the infra-red spectra of various complexes of water and primary amines with organic bases, namely the convergence of the frequencies of two observed maxima and the equalization of their intensities, both of which accompany the strengthening of H-bonds. The calculations provide the logical connection between numerous empirical correlations inherent in essentially diverse systems : crystalline hydrates and complexes of ordinary and semi-deuterated water. The important role of Fermi resonance in the formation of the spectral contour is ascertained even in the cases when the bending overtone does not in practice appear in the spectrum.