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

Periodic intensity variations in the measured Compton profile anisotropies of ordinary ice $\mathrm{I}h$ correspond to distances of 1.72 and 2.85 Å, which are close to the hydrogen bond length and the nearest-neighbor O-O distance, respectively. We interpret this result as direct evidence for the substantial covalent nature of the hydrogen bond. Very good quantitative agreement between the data and a fully quantum mechanical bonding model for ice $\mathrm{I}h$ and the disagreement with a purely electrostatic (classical) bonding model support this interpretation and demonstrate how exquisitely sensitive Compton scattering is to the phase of the electronic wave function.