The effect of pressure on the near-neighbor distance in water

When liquid water is compressed, the O--O hydrogen bonds tend to contract because of the direct effect of the pressure, and the O--O--O angles tend to deviate more from the tetrahedral. The coupling between the O--O stretching and O--O--O angle bending is such that the change of the angles tends to increase the bond lengths. The net effect of pressure is the resultant of the direct contraction and the indirect expansion. It is suggested that the actual contraction can be measured by the effect of pressure on the O--H or O--D stretching frequencies of HDO in dilute solution in D2O or H2O. The application of this suggestion to literature data shows that the bond contracts only about 0.3 of the rate that the hydrogen bond in ice contracts. No doubt the contraction due to the direct effect of pressure is largely compensated by the expansion due to the distortion of the O--O--O angles