Low-Frequency Anharmonic Couplings in Crystalline Bromoform: Theory

Theoretical calculations of the low-frequency anharmonic couplings of the beta-phase of crystalline bromoform are presented, based on DFT quantum chemistry calculations. Electrical and mechanical anharmonicities between intra- and intermolecular modes are calculated, revealing that electrical anharmonicity dominates the cross peaks intensities in the 2D Raman-THz response and crystalline, as well as liquid, bromoform. Furthermore, the experimentally observed difference in relative cross peak intensities between the two intramolecular modes of bromoform and intermolecular modes can be explained by orientational averaging, taking into account the double-degeneracy of the lower-frequency intramolecular mode. The good agreement with experimental results provides further evidence for our interpretation that the 2D Raman-THz response of bromoform is indeed related to the anharmonic coupling between intra- and intermolecular modes.