The uncoupled O–H stretch in ice VII. The infrared frequency and integrated intensity up to 189 kbar

The integrated absorptivity of the O–H stretching vibration of a dilute solution of HDO in D2O ice VII and the peak frequencies of the O–H stretching, bending, and rotational vibrations of HDO have been measured in diamond anvils in the range 23.6–189 kbar. The integrated absorptivity of the O–H stretching vibration increases from 55 to 153 cm μmol−1 in this range, i.e., by a factor of 2.78, and the effective charge increases from 0.61 to 0.94 electronic charges, i.e., by the factor 1.54. The effective charge of the O–H stretch as a function of pressure can be correlated within the experimental precision by a simple model in which the dipole moment of an ice crystal changes sinusoidally as the protons are moved in phase along the O––O lines. It predicts that the change of dipole moment with oxygen–proton distance, when the oxygen–proton distance is small, is 1.1 D Å−1. The theory could be tested by calculating the dipole moment of a neon atom caused by adding a small dipole moment to the nucleus. The uncoupled O–H stretching frequency changes from 3402 to 3059 cm−1 in the range 23.6–189 kbar. This change is interpreted as an largely as indirect effect of the hydrogen bonding, which appears to increase the O–H distance by 0.183 Å Mbar−1 and so reduces the effective force constant because of the anharmonicity. By combining the frequency of the uncoupled O–H stretch with the frequency of the strongest Raman band reported by others, the sum k12+4k13 of the intramolecular and first-neighbor intermolecular O–H coupling constants was obtained over a range of O––O distances.