Abstract
Laser Raman spectra of H2O and D2O ice clathrate of type II structure containing tetrahydrofuran molecules as guests have been measured in the frequency ranges 100–350, 2800–3500 and 2200–2800 cm−1, temperature range 250–275 K and pressure range 1–1500 bar. The O─H stretching frequency decreases with increasing pressure and decreasing temperature. The frequency of the main peak in the translational vibration region increases with increasing pressure and decreasing temperature. The Grüneisen constant, -(δ In v/δ ln V) T , is 0·41 for O─H and −07·39 for O─D stretching, and 2·40 for translational vibrations, in both H2O and D2O clathrate.
The increase in the O─H stretching frequency of the clathrate over hexagonal ice is about one-eighth, or less, of that expected on the basis of the increased average distance between the nearest-neighbour water molecules in it. This is attributed partly to the anharmonicity associated with the correlated motions of O─H stretching and O─H─O bending, and it is suggested that the anharmonic shift in the clathrate is much less than in ice. Nearly 35% of the increase in the O─H stretching frequency with temperature is attributable to an accompanying increase in volume, and the rest of the increase to an increase in temperature alone. This indicates a thermally induced bending of the O─H─O linkages. A comparison with the corresponding values for hexagonal ice indicates that ∼20% of the increase in the frequency with temperature at a constant volume is caused by a decrease in the intermolecular coupling of O─H oscillators, and ∼ 80% by an increase in the length of the hydrogen bond on bending of the O─H─O linkage. Nearly 40% of the decrease in the frequency of hindered translational vibrations with increasing temperature is caused by an accompanying increase in volume and 60% by an increase in temperature alone. It is pointed out that no simple relationship between the O─H stretching frequencies and near-neighbour distances can be obtained. Therefore, the scaling of the frequency with distance, and the reverse, which are generally used in modelling the structure of liquid and amorphous solid water, are seen to be serious oversimplifications.
