Pressure and temperature dependence of the O—H and O—D stretching vibrations in the Raman spectrum of ice

Laser-excited Raman spectra of H2O, D2O and 2% isotopically dilute HDO ice have been measured in the frequency ranges 2100–2600 cm−1 and 2800–3600 cm−1, temperature range 250–272 K and pressure range 1–1·6 kbar. The frequency of the main peak attributable to O—H and O—D stretching vibrations in H2O and D2O ice, and in 2% H2O in D2O and 2% D2O in H2O ice, all decrease linearly with decreasing temperature and with increasing pressure. The Grüneisen constant, — (∂ln v/∂ ln V), for O—H stretching vibrations is -0·36 for H2O and -0·20 for HDO ice. Values of the temperature coefficient of the frequency at a constant pressure and at a constant volume, and of the pressure coefficient at a constant temperature, have been determined, as have the changes in frequency with respect to intermolecular distance at constant temperature and constant pressure. These show that at 255 K (1) the various coefficients have approximately the expected isotopic effect in that the ratio of their values in H2O to those in D2O is ∼ 1·4, (2) all coefficients decrease on isotopic decoupling of the O—H and O—D stretching modes, (3) nearly 28% of the increase in v 1 and v OH with temperature arises from the change in the volume of ice, and nearly 70% from the change in temperature alone, and (4) the Grüneisen constant is relatively unaffected by isotopic substitution. These results are interpreted as suggesting a thermally induced bending of the O—H—O linkage in ice. It is also shown that any relation between the O—H stretching frequency and the distance between nearest oxygen atoms, r OO, must take into consideration changes with r OO of the intermolecular coupling and of the anharmonic effects. The 30% decrease in the Grüneisen constant on isotopic decoupling of the O—H oscillator suggests a substantial change in the intermolecular coupling with volume, which also needs to be considered in a theory of the spectrum of ice. The magnitude of the widths of the spectrum indicates that the disorderinduced variation of r OO in ice is decreased by ∼ 30% on deuteration.