The proton momentum distribution in water and ice

Abstract

Deep Inelastic Neutron Scattering (Neutron Compton Scattering), is used to measure the momentum distribution of the protons in water from temperatures slightly below freezing to the supercritical phase. The momentum distribution is determined almost entirely by quantum localization effects, and hence is a sensitive probe of the local environment of the proton. The distribution shows dramatic changes 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 0.3 Angstrom. Confining the supercritical water in the interstices of a C 60 powder enhances this anharmonicity and enhances the localization of the protons. The changes in the distribution are consistent with gas phase formation at the hydrophobic boundaries and inconsistent with the formation of ice there.

Temperature dependence of the zero point kinetic energy in ice and water above room temperature

“…4) also the data for the liquid from previous measurements from room temperature to the supercritical phase [4,51], where we recall that the liquid at room temperature has been measured also in Refs. [52,34,38], with consistent results. These start from an initial value lower than ice at T = 271 K and then show an increase with a stronger T-dependence (approximately 0.1 meV/K) with respect to the solid.…”

“…Moreh and Nemirovski [24] calculated hE K ðTÞi of the proton in ice Ih between 5 K and 269 K, using optical vibration frequencies from the literature, assuming the harmonic approximation and decoupling between the degrees of freedom of translation, rotation-libration, and internal vibrations. They obtained good agreement with DINS data at 5 K [37] but not at 269 K [38], assuming free rotation of the entire molecule. More recently, Finkelstein and Moreh [39] reported new calculations on ice at ambient (and high) pressure, which account for a revised evaluation of the librational component from the literature, obtaining a $ 10 meV increase in the calculated kinetic energy.…”

Quantum effects in water: proton kinetic energy maxima in stable and supercooled liquid

“…For example, differences in n(p) between liquid water and ice re ect the breaking and distortion of hydrogen bonds that occurs upon melting [6,9].…”

“…The Figure reports <E K > values in the solid [6], in the metastable phase -supercooled phase from T = 269.00 K [15] to T = 272.95 K (present data) -in the region of the density maximum from present study -at T = 274.15 K, T = 277.15 K, T = 281.15 K, T = 285.15 K, at room temperature [6,7,11] up the supercritical point [5,7]. The <E K > exhibits two local maxima, one in the stable liquid phase at …”

“…Indeed present data clearly indicates that linear behavior of <E K > versus T is lost in both the stable phase, around the density maximum, and in the metastable state (see Figure 1). [19,6] (full circle) at T = 293.00 K, T = 296.15 K and from ref. [7] (triangle) at T = 300.00 K. Insert shows proton <E K > for stable water at T = 423.00 K, T = 523.00 K, T = 573.00 K, T = 673.00 K from ref.…”