Oxygen isotope fractionation in H2O and the structure of liquid water

The difference between the atomic force constant for oxygen in ice I and liquid water, and that in the vapor phase is determined from an analysis of H2 16O/H2 18O isotope fractionation factors. It is shown that the force constants in liquid water depend explicitly upon the hydrogen-bond distribution in the liquid phase, and upon fusion the average number of hydrogen bonds per H2O decreases from 4 to 3. This result and the number of hydrogen bonds in water above the ice point are compared with the numbers obtained from molecular dynamics calculations and the cluster and lattice–gas theories of liquid water.