Raman frequency and intensity studies of liquid H2O, H218O and D2O

Raman frequency and intensity measurements have been performed on liquid water as H2O, D2O and H218O to study the effects of isotope substitution. Intensity data were collected digitally, normalized to account for the temperature and frequency factors and presented in isotropic and anisotropic forms. The frequency and intensity changes are consistent with the predictions of simple reduced mass calculations. In particular, there was no evidence to support the reported breakdown of the Born—Oppenheimer approximation as has been reported for H218O. The isotope invariant sum rule was checked for H216O, H218O and D2O by relative intensity studies for the OH stretching region against an internal sulfate reference peak. Identical values for the isotope invariant sum were obtained for H216O and H218O but the value for D2O was about 30% larger. The difference appears to have its origin in the more highly structured nature of D2O due to smaller anharmonic effects. Accurate frequency shifts among H216O, H218O and D2O are also presented. Surprisingly, the greatest frequency shift which accompanied 18O substitution was in the low‐frequency hydrogen‐bonded region where the band at 192 cm−1 for H2O shifted by 15 cm−1 to 177 cm−1 for H218O. This result confirms previous observations and establishes the origin of this band as a hydrogen‐bonded symmetric stretching mode which involves primarily oxygen displacement. Further support for this assignment comes from the observation that the band at 192 cm−1 is slightly polarized. The effects of intermolecular coupling contribute to the band structure of the internal modes. Frequency differences in the OH stretching region of H2O and H218O suggest that only about 50% of the anisotropic intensity is due to the Raman activity of the ν3 antisymmetric stretching mode while the remainder is due to the symmetric stretching modes of intermolecularly coupled water molecules. A point‐by‐point comparison of the OH stretching region for the isotropic scattered intensity of H2O and H218O revealed that the complete region from 2800 to 3800 cm−1 was shifted equally by 7 cm−1, a fact that suggests that the peak maximum at about 3250 cm−1 is just part of the ν1 symmetric stretching mode and is not primarily due to 2ν2. The effect of intermolecular coupling in the ν2 region of liquid water was confirmed by the difference in the frequency for the isotropic and anisotropic components for each of the isotopic forms of water. For H218O the peak maxim