Quantum Effects in Water: Proton Kinetic Energy Maxima in Stable and Supercooled Liquid

A strong temperature dependence of proton mean kinetic energy was observed for liquid water around the density maximum and for moderately supercooled water. Line shape analysis of proton momentum distribution, determined from deep inelastic neutron scattering measurements, shows that there are two proton kinetic energy maxima, one at the same temperature of the macroscopic density maximum at 277 K, and another one in the supercooled phase located around 270 K. The maximum at 277 K is a microscopic quantum counterpart of the macroscopic density maximum, where energetic balance giving rise to the local water structure is manifest in the temperature dependence of kinetic energy. The maximum in the supercooled phase, with higher kinetic energy with respect to stable phases, is associated to changes in the proton potential as the structure evolves with a large number of H-bond units providing both stronger effective proton localization, as well as proton quantum delocalization.