The effect of large amplitude motions on the vibrational intensities in hydrogen bonded complexes

We have developed a model to calculate accurately the intensity of the hydrogen bonded XH-stretching vibrational transition in hydrogen bonded complexes. In the Local Mode Perturbation Theory (LMPT) model, the unperturbed system is described by a local mode (LM) model, which is perturbed by the intermolecular modes of the hydrogen bonded system that couple with the intramolecular vibrations of the donor unit through the potential energy surface. We have applied the model to three complexes containing water as the donor unit and different acceptor units, providing a series of increasing complex binding energy: H2O⋯N2, H2O⋯H2O, and H2O⋯NH3. Results obtained by the LMPT model are presented and compared with calculated results obtained by other vibrational models and with previous results from gas-phase and helium-droplet experiments. We find that the LMPT model reduces the oscillator strengths of the fundamental hydrogen bonded OH-stretching transition relative to the simpler LM model.