Cooperative Contributions of the Intermolecular Charge Fluxes and Intramolecular Polarizations in the Far-Infrared Spectral Intensities of Liquid Water

The main factors that determine the infrared and farinfrared (terahertz) spectral features of liquid water in the frequency region below 1000 cm(-1) are examined theoretically. By analyzing the modulations of electron populations induced by molecular translations in water clusters, it is shown that there is an approximate linear relation between the derivative of intermolecular electron population transfer (delta rho <- A/delta rO...H) and the hydrogen-bond length (r(O...H)). This relation is used to incorporate the effects of hydrogen-bond fluctuations and defects into the intensity generation mechanisms in the spectral simulations of liquid water based on classical molecular dynamics, showing that intermolecular charge fluxes and intramolecular polarizations contribute cooperatively to the intensity of the band at,similar to 200 cm(-1) (6 THz). It is also shown that the intensity of the band at similar to 700 cm(-1) is reduced by the libration-polarization cross term, in accord with the reduced magnitude of dynamical dipole. The dependence of the calculated spectral profiles on the temperature and potential model is also discussed.