Terahertz Spectrum of Water at Varying Temperatures from 260 to 340 K: Contributions from Permanent and Induced Dipole Correlations at Different Length Scales.

Abstract

The terahertz (THz) absorption spectrum of water is calculated at varying temperatures from 260 to 340 K from molecular dynamics simulations using a polarizable potential model of water. The current calculations produce the known experimental features of the THz spectrum of water such as the hydrogen-bond stretch mode at ∼200 cm-1 and librational mode at ∼600 cm-1. The peak positions generally show a red shift with an increase of temperature due to the weakening of the hydrogen bonds at higher temperatures. Overall, the changes of the spectrum with temperature are found to be in good agreement with experimental results. The total THz spectrum at a given temperature is dissected into self- and cross-correlation contributions and also into contributions from permanent dipoles, induced dipoles, and permanent-induced dipole correlations. It is shown that while the peak at ∼200 cm-1 due to hydrogen-bond stretching primarily comes from fluctuations of induced dipoles, the librational peak at around 600 cm-1 originates mainly from fluctuations of the permanent dipoles. Also, through calculations of self- and cross-correlations, it is shown that the broad librational peak arises from the superposition of several components like the antisymmetric libration, symmetric libration, and also self-dipole correlations. The length-scale-resolved calculations of cross-correlations reveal the contributions from different solvation shells to the total cross-component of the THz spectrum and how such length-scale-resolved components change with temperature. Results are also presented for the dielectric relaxation of water over different length scales and temperatures.