Vibrational line shapes in liquids. The role of resonant intermolecular coupling

The role of intermolecular coupling on vibrational line shapes is studied in terms of a mapping procedure relating the full spectrum with a spectrum corresponding to vanishing coupling. This spectral transform is based on a single site approximation. It exhibits an asymmetric line, exchange narrowing, and a shift of the line center depending on the average intermolecular coupling. Fluctuations of the total amount of intermolecular coupling at a reference molecule lead to line broadening. Fluctuations of the intermolecular coupling leaving the total amount of coupling at a reference molecule constant, invoke motional narrowing. The consequences of the model are compared with available data on isotopic dilution experiments.