Low-frequency anharmonic couplings in crystalline bromoform: Theory

Abstract

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.

Molecular Dynamics of Artificially Pair-Decoupled Systems: An Accurate Tool for Investigating the Importance of Intramolecular Couplings

“…The correlated/coupled motion of the nuclei in a molecular system is an ultrafast phenomenon, and, as such, it must be studied using either experimental ultrafast techniques, which include pulse probe methods, or computer simulation methods aimed at the interpretation and simulation of two-dimensional spectra. ,− In theoretical chemistry, the identification of the uncoupled degrees of freedom is useful for computational methodologies that calculate the vibrational spectrum in reduced dimensionality, such as, for instance, semiclassical approaches, − QM/MM calculations, tensor-trains and sum of products of basis functions methods − and also the Multi-Configuration Time-Dependent Hartree method (MCTDH) − and methods based on MCTDH-like ansatz . Applications of all the aforementioned methods imply either that part of a system is partially independent of another or that the two parts have an artificial interaction.…”