DIRECT AND INDIRECT SOLVENT COUPLING VIBRATIONAL DEPHASING MECHANISMS IN HYDROGEN-BONDED MOLECULES

Infrared absorption spectral bandshapes are examined theoretically for the OH stretching vibration in hydrogen-bonded complexes in solution. Two distinct dephasing mechanisms are considered: the indirect mechanism in which the OH vibration is coupled to one or more internal vibrations in the complex, which are in turn coupled to the solvent, and the direct mechanism in which the OH vibration is directly coupled to the solvent. Attention is focused on intramolecularly H-bonded complexes for which extensive spectral data are available for a range of solvent polarity. It is concluded for the complexes considered that the direct mechanism is dominant. In particular, good fits to the experimental data are obtained via this mechanism for spectral width features in a given solvent and as a function of solvent polarity. Some distinctions between intra- and intermolecularly H-bonded complexes are given, and some suggestions for further work are made.