The ν(AH) band formation in H-bonded complexes RAH⋯B in the gas and liquid phases. Theory and experiment

The mechanisms involved in the ν(AH) band formation in the H-bonded complexes RAH⋯B and the approximations used to reconstruct the experimental band shapes are discussed. It is emphasized that the high-precision studies of H-bonded systems in the gas phase are important in determining the geometrical and electro-optical parameters of the complexes necessary, in particular, for calculating their properties in condensed phases. The gas phase studies of ν(AH) band profiles at room temperature allow one to derive the characteristics of low frequency modes. Analysis of the shape of strong ν(AH) stretching bands is promising in this respect. Consideration of the H-bonded systems whose spectra are structured, e.g. complexes of HF with HCN and H2O, can provide a larger amount of information. It is shown that for the H2O⋯HF complex not only the electro-optical parameters, but also the assignment of spectral features can be reliably obtained only from nonempirical calculations, which are quite feasible at present. The studies of H-bonded systems in mixtures with rare gases performed at densities varying from a dilute gas to liquid are especially promising for understanding the mechanisms of band shape formation in different states of aggregation. It is shown that the theory of spectral exchange between the rovibrational bands describes essential features of the evolution of the ν(AH) bands upon increasing the density.