The fluctuation hypothesis of hydrogen bonding

Abstract

The paper is devoted to calculations of the contour shapes in the infra-red and Raman spectra of the 1 : 1 and 1 : 2 type complexes of ordinary and heavy water with two organic bases, dioxane and dimethyl sulphoxide, by means of a computer. The calculations have been made using the mathematical formalism developed in the previous papers of this series which takes into account the coupling between two stretching vibrations of the water molecule and the overtone of its bending vibration for a set of the liquid molecules variously perturbed by non-equal H-bonds. The basic parameters of the model are obtained from the spectra of HOD molecules involved in the same complex as the one considered, and from empirical correlations discussed in the previous papers. Starting from a set of parameters determined in this manner we calculate three spectra simultaneously for each of the above-mentioned complexes, namely the infra-red spectrum and the isotropic and anisotropic components of the Raman spectrum. Every spectrum is composed of three contours belonging to the overtone of the bending vibrations, and also to the in-phase and out-of-phase stretching vibrations respectively. These contours are asymmetrical and have different half-widths, maximum frequencies and intensities in all three types of spectra. The model demonstrates the inadequacy of the widespread interpretation of such spectra in terms of the symmetric and antisymmetric vibrations of undistorted water molecules characterized by the symmetry C _2v . Good agreement between calculated and experimental spectra is considered to provide strong evidence of the fluctuation model.