A simulation study on Raman cross sections of OH and OD stretches in isotopically pure and diluted liquid water

The Raman scattering activity spectra of isotopically pure and diluted liquid water are theoretically calculated to examine the mole-fraction dependence of the OH- and OD-stretch cross sections. This mole-fraction dependence was once claimed to be anomalous and attributed to nuclear quantum correlations of protons and deuterons when it was discovered experimentally. The present study demonstrates that the experimental data of the mole-fraction dependence of the cross sections, as well as the Raman scattering activity spectra in the stretch region, are well reproduced by the theoretical calculations according to the quantum/classical mixed approach in which the nuclear quantum correlations are not taken into account. This suggests that the anomalous dependence does not serve as evidence of the nuclear quantum correlations. The present study provides a more plausible interpretation that the OH- and OD-stretch cross sections depend on the mole fraction because of the inter- and intra-molecular vibrational couplings.