Rotational correlation motion in liquid and adsorbed water

Abstract

The rotational correlation functions of liquid water, of water adsorbed on a silica surface, and of water contained in a zeolite cavity have been determined from the infrared spectra of these systems. In all cases except in critical water and zeolite water, the rotational motion of water molecules is strongly vibrationally perturbed, which results in a periodic acceleration and slowing down (modulation) of the rotary motion. First order perturbation theory shows that the frequency of this modulation is determined by the difference between the average perturbation of the excited and ground vibrational levels. The average modulation frequencies are compared with those of the intermolecular bands of water and of the librations of ice. It is shown that tailing of vibration-rotation bands to low frequencies, observed in infrared and Raman bands of liquid water and to a lesser degree, also in adsorbed water, can be attributed to an attractive perturbation of the rotary motion, which is thought to be a precursor to an oriented hydrogen bond.