Effect of pressure and temperature on the O?H and O?D stretching, and translational vibrations in the Raman spectrum of ice clathrate

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Abstract
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.