Raman frequency and intensity studies of liquid H 2 O, H 2 18 O and D 2 O

Abstract Raman frequency and intensity measurements have been performed on liquid water as H 2 O, D 2 O and H 2 18 O 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 H 2 18 O. The isotope invariant sum rule was checked for H 2 16 O, H 2 18 O and D 2 O 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 H 2 16 O and H 2 18 O but the value for D 2 O was about 30% larger. The difference appears to have its origin in the more highly structured nature of D 2 O due to smaller anharmonic effects. Accurate frequency shifts among H 2 16 O, H 2 18 O and D 2 O are also presented. Surprisingly, the greatest frequency shift which accompanied 18 O substitution was in the low‐frequency hydrogen‐bonded region where the band at 192 cm −1 for H 2 O shifted by 15 cm −1 to 177 cm −1 for H 2 18 O. 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 H 2 O and H 2 18 O suggest that only about 50% of the anisotropic intensity is due to the Raman activity of the ν 3 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 H 2 O and H 2 18 O 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 ν 1 symmetric stretching mode and is not primarily due to 2ν 2 . The effect of intermolecular coupling in the ν 2 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 H 2 18 O the peak maximum in the isotropic spectrum was at 1619 cm −1 whereas the peak maximum in the anisotropic spectrum was at 1637 cm −1 with the result that in the measured I ‖ spectrum the peak maximum was at 1629 cm −1 .