A new water anomaly: the temperature dependence of the proton mean kinetic energy

TL;DR

This study reveals an anomalous temperature dependence of proton kinetic energy in water, with maxima near the maximum density temperature and evidence of proton delocalization, challenging classical models of water's microscopic behavior.

Contribution

It provides new experimental insights into proton dynamics in water across different phases, highlighting anomalies near the density maximum and supercooled states.

Findings

Proton kinetic energy peaks at 277 K, coinciding with water's maximum density.

Anomalous increases in proton energy occur in supercooled and above 273 K.

Evidence of proton delocalization between hydrogen-bonded oxygens in supercooled water.

Abstract

The mean kinetic energy of protons in water is determined by Deep Inelastic Neutron Scattering experiments, performed above and below the temperature of maximum density and in the supercooled phase. The temperature dependence of this energy shows an anomalous behavior, as it occurs for many water properties. In particular two regions of maximum kinetic energy are identified: the first one, in the supercooled phase in the range 269 K - 272 K, and a second one above 273 K. In both these regions the measured proton kinetic energy exceedes the theoretical prediction based on a semi-classical model. Noteworthy, the proton mean kinetic energy has a maximum at 277 K, the temperature of the maximum density of water. In the supercooled metastable phase the measured mean kinetic energy and the proton momentum distribution clearly indicate proton delocalization between two H-bonded oxygens.