We report measurements of the Raman spectrum of supercooled water in the hindered translational region (20–400 cm−1) down to a temperature of −20 °C. The spectra are analyzed after correcting for the effects of Boltzmann factor and harmonic oscillator coupling, i.e., in the reduced R(ν̄) representation of Shuker and Gammon. Spectral deconvolution shows that in addition to the previously observed 0-0-0 bending mode (≂60 cm−1) and the 0-0 stretching mode (≂190 cm−1), there is a weak feature at 260 cm−1 whose intensity increases by almost an order of magnitude as temperature decreases from 40 to −20 °C. A plausible interpretation of the 260 cm−1 band is that it is analogous to the 310 cm−1 band seen in ice I and probably arises because of differing electrostatic interactions in different configurations of coupled H bonds of neighboring H2O molecules. The 0-0 stretching band at 190 cm−1 changes in many respects as temperature decreases from 40 to −20 °C: (i) Its peak intensity increases almost four times; (ii) integrated intensity increases three times; (iii) bandwidth decreases about 30%; and (iv) peak maximum increases linearly from 176 cm−1 at 40 °C to 202 cm−1 at −20 °C. In contrast, the 0-0-0 bending at 60 cm−1 is quite insensitive to changes in temperature. The increase in the intensity of the 190 and 260 cm−1 bands is consistent with the idea that four-coordinated H2O molecules contribute directly to these spectral features and the fraction of such molecules increases with decreasing temperature. This effect on intensity is further enhanced by the strong coupling of the motion of a few four-coordinated water molecules, as seen in the small-angle x-ray scattering data of Bosio et al. We also observe a limiting value to the width of the 190 cm−1 band at temperatures below the melting point, suggesting that the local structure of supercooled water is approaching some limiting structure.

Intermolecular coupling of OD oscillators from HOD in H2O has been investigated by Raman spectroscopy. The data indicate that at concentrations of HOD greater than 10 mol % the effects of intermolecular coupling of OD ⋅⋅⋅ OD pairs become noticeable. Difference spectra show a characteristic derivativelike feature with an increase of intensity around 2400 cm−1 as intermolecular coupling increases. The peak frequency of the OD stretching vibration in HOD decreases from 2525 cm−1 at infinite dilution to 2500 cm−1 in 50 mol % HOD, while the width of the OD stretching band increases from 150 cm−1 (infinite dilution) to 178 cm−1 (50 mol % HOD). Depolarization measurement indicates that the band at ∼2500 cm−1 is polarized. By comparing these difference spectra with the spectrum of OD oscillators from D2O we suggest that the major features of the D2O spectrum in the liquid state can be obtained by considering intermolecular coupling of OD oscillators.

We report Raman spectroscopic measurements on supercooled water down to a temperature of −20 °C. The spectral region investigated is the uncoupled OD stretching vibration (2000–3000 cm−1) from a 5% solution of D2O in H2O. The results are consistent with an interpretation in terms of two categories of OD bonds, those that are hydrogen bonded and those that are essentially free. The fraction of H-bonded OD bonds varies from 0.9 to 0.95 as the temperature decreases from 0 to −20 °C. We observe that the frequency of the bonded OD component decreases with temperature at a slightly slower rate in the supercooled region than in the normal liquid region. With decreasing temperature, a decrease in width of this component band is observed, and this decrease appears to have a much larger slope in the supercooled region as compared to that observed previously in the normal temperature region.

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