Evidence for Covalency of the Hydrogen Bond in Ice: A Direct X-Ray Measurement

Hydrogen bonds play a crucial role in determining many of the distinctive properties of water and biological complexes. In particular, in ice, hydrogen forms two distinct types of bonds with neighboring oxygen. The shorter (1.00 angstroms) covalent bond is a typical molecular sigma-bond between the oxygen and hydrogen. It has been appreciated since Pauling that the longer (1.75 angstroms) bond is probably partly covalent. Even so, a microscopic quantitative understanding of the hydrogen bonds covalent character remains experimentally untested and controversial. Here, we report on high momentum transfer (Compton) inelastic x-ray scattering measurements which are exceptionally sensitive to the hydrogen bonds in ice I sub h. The predominant feature of the spectra, periodic intensity variations, corresponds to a distance of 2.68 angstroms, very close to the O-O distance (2.75 angstroms). Striking quantitative agreement between the data and a density functional theory for ice I sub h, and the remarkable disagreement with a purely electrostatic bonding model are interpreted as direct evidence for the substantial covalent character of the hydrogen bond.