Effects of Intermolecular Charge Transfer in Liquid Water on Raman Spectra

The
low-frequency vibrational spectrum of liquid water is composed
of contributions from the intermolecular librational and translation
modes. The existence of these two modes introduces difficulty into
the simulation of experimentally obtained Raman spectra. We constructed
a polarizability function for a water model that includes intramolecular
charge flow (CF) effects, intermolecular charge transfer (CT) effects,
and intermolecular dipole-induced-dipole (DID) effects. We computed
the one-dimensional (1D) Raman and terahertz (THz) spectra with all
of these effects included (CFCT-DID) and compared with experimental
spectra. We find that the CFCT-DID function provides a better description
of the experimental results, because the CT effects reduce the polarizability
only for translational motion to which parallelly polarized (VV) and
perpendicularly polarized (VH) Raman spectra are sensitive. In our
calculations of two-dimensional (2D) Raman and THz-Raman spectra,
we observe the enhancement of echo signals in both cases. The details
of the CFCT-DID function, along with its source code, are provided
in the Supporting Information.