Breakup of the Hydrogen Bond Network in Water: The Momentum Distribution of the Protons

Neutron Compton Scattering measurements presented here of the momentum distribution of hydrogen in water at temperatures slightly below freezing to the supercritical phase show a dramatic change in the distribution as the hydrogen bond network becomes more disordered. Within a single particle interpretation, the proton moves from an essentially harmonic well in ice to a slightly anharmonic well in room temperature water, to a deeply anharmonic potential in the supercritical phase that is best described by a double well potential with a separation of the wells along the bond axis of about .3 Angstroms. Confining the supercritical water in the interstices of a C60 powder enhances this anharmonicity. The changes in the distribution are consistent with gas phase formation at the hydrophobic boundaries.