Merged and separate band profiles arising from resonantly coupled vibrational modes of liquid mixtures: theoretical study

Abstract

AbstractVibrational band profiles of equimolar liquid mixtures in the presence of intermolecular resonant vibrational coupling according to the transition dipole coupling (TDC) mechanism are studied by performing calculations on a model liquid system. It is shown that the bands of the two species in the mixture merge into a single band as the extent of resonance (closeness of the wavenumber positions as compared with the magnitude of the coupling) between the vibrations of the two species becomes greater and the bandwidth becomes larger. In the separate‐band case, intensity redistribution is seen in the isotropic Raman spectra because of the intermolecular vibrational coupling. The merged/separate behavior of the band profiles observed for isotopic mixtures of liquid formamide is reasonably reproduced by the calculations on the model liquid with the values of parameters in realistic ranges. Ab initio molecular orbital (MO) calculations on a linearly hydrogen‐bonded cluster of formamide are also carried out to support the discussion. Copyright © 2008 John Wiley & Sons, Ltd.