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

Abstract

In a recent article [Pietropaolo et al., Phys. Rev. Lett.100, 127802 (2008)] we discussed the results of a deep inelastic neutron scattering experiment aimed at the measure of mean kinetic energy, <E(k)>, and momentum distribution of protons in supercooled water. The observed excess of proton mean kinetic energy, with respect to theoretical predictions and measurements in water stable liquid and solid phases, suggested a possible link between the anomalous temperature dependence of water density and the temperature dependence of <E(k)>. Nevertheless, the limited number of data did not allow a more quantitative description. Here we report on new measurements, above and below the temperature of maximum density and in the supercooled phase. These show two <E(k)> maxima: The first one, in the supercooled phase in the range of 269-272 K, and a second one at 277 K, the temperature of the maximum density of water.

Quantum effects and the local environment of water hydrogen: Deep inelastic neutron scattering study

“…shows the proton and deuteron mean kinetic energy as a function of temperature, combining results of the present study with earlier ones 4,9,10. DINS experiments on bulk H 2 O have indeed shown an intriguing temperature dependence 10 around the TMD (T = 277.15 K), with a clear maximum in energy of about 164 meV, well above the proton zero-point energy 22 (E zp = 144 meV, black dashed line in…”

“…5,6 Quantum effects on these two quantities could be large and a number of recent DINS experiments has evidenced these effects in supercooled bulk water, in nanoconfined water, and in a protein or DNA hydration shell, mostly visible as a proton delocalization along the H bond and as a temperature-dependent excess of kinetic energy compared to its classical value. [7][8][9][10][11][12] It is important to ask whether or not these experimental results can be adequately described by state-of-the-art computer simulations. As things stand, it is fair to state that none of the existing electrostatic models, which treat the intermolecular potential as unchanged by the hydrogen bonding, are accurate enough to fully reproduce the experimentally determined momentum distribution and kinetic energy, 13,14 despite impressive progress that has been achieved with path-integral Car-Parrinello molecular dynamics simulations.…”

Kinetic energy of protons in ice Ih and water: A path integral study

“…This excess was associated to a coherent delocalization of the proton between neighbor oxygen atoms. 8,9 However, this tentative explanation has been criticized by noting that measured proton momentum distribution implies a contracted proton wave function which is inconsistent with the purported increased proton delocalization. 7 Moreover, coherent delocalization (i.e., tunneling) of a proton is associated to a resonance of the particle between two localized states.…”

“…7 Very recently a new set of DINS experiments have been performed for water at temperatures in the range 272-285 K, and seemingly what appears as a unique set of measurements have been published by two different groups. 8,9 From an analysis of their experimental data these authors found a maximum in K H at about 277 K that was considered as an evidence of a new water structural anomaly related to the well-known existence of a maximum in the density of water at the same temperature. 8 DINS derived values of K H have been also reported at 5 K and at 269 K for the stable phase of ice at atmospheric pressure (ice Ih).…”

Water Proton Environment: A New Water Anomaly at Atomic Scale?

“…A full description of the original experiments, along with a comment and a reply, can be found in Refs [17][18][19][20][21][22]. Figure 1 shows the temperature dependence of light water proton's E k measured in both stable and metastable (supercooled) phases of bulk water in the temperature range from 269 to 300K (see top axis).…”