Hydrogen Bonding Effects on Vibrational Dynamics and Photochemistry in Selected Binary Molecular Complexes

This brief review presents an overview and analysis of the experimental studies performed recently in the laboratory of the authors demonstrating the role of hydrogen bonding as a promotional factor for intermolecular vibrational energy relaxation, and as a driving force for the occurrences of specific reaction channels in binary molecular complexes. Both vibrational and electronic spectroscopic methods have been used, and measurements have been performed in the supersonic jet expansion, in cold inert gas matrixes and also in suitable liquids at room temperature. Pure and mixed dimers of two types of aromatic chromophores, 7-azaindole and phenol, have been used as the model molecular systems, which involve O–H···O, O–H···N and N–H···N types of hydrogen bonds. The promotional effect of vibrational relaxation has been demonstrated showing large broadening of either of the infrared X–H stretching fundamentals of the donor groups in the hydrogen-bonded X–H···Y network of the binary complexes or broadening of the fluorescence spectra upon excitations to single vibronic levels of the isolated complexes in the gas phase and making comparisons with the corresponding spectra of the monomers. A similar approach has been adopted to demonstrate the promotion of photodissociation and charge transfer reactions in binary complexes by hydrogen bonding.