Molecular theory of low-frequency Raman spectrum of water in the translational-band region

The depolarized low-frequency Raman spectrum (RS) arising in water in the translational-band (T-band) region, viz. near the frequency ν about 180 cm− 1, is calculated using an analytical representation of the dipolar autocorrelation function. The proposed model comprises the dimer of H2O or D2O molecules, connected by the hydrogen bond (HB) and suffering elastic vibrations along the HB direction. The HB polarizability is assumed to be proportional to the square of the elastic dipole moment and varies in time due to HB longitudinal vibration. The correct frequency of the RS-peak intensity is obtained due to parameterization of the HB force constant k. A contribution to RS due to elastic reorientation of the H-bonded polar molecule is also accounted for. An interplay of these two molecular mechanisms generates, in agreement with the experimental data, two overlapping bands located in the 180 cm− 1 region. The presented calculation also explains the effect of temperature on the Raman spectra in the T-band. The frequency dependence of the RS intensity correlates with the absorption νε″ (ν) and loss ε″ (ν) spectra calculated in the T-band region for the same model parameters.