Microscopic study of proton kinetic energy anomaly for confined water*

Several anomalies, related to structural and dynamical transition, have been reported for water at different thermodynamic conditions and environments. Of particular interest, the reported anomalies of the proton mean kinetic energy, Ke(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), are a longstanding problem related to proton dynamics in hydrogen-bonded systems. We used classical MD method to deduce Ke(H) by calculating the proton vibrational density of states, H-VDOS, for the case of water inside single wall carbon nanotubes (SWCNT) of varying diameters. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, D_CNT. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke(H), by treating each phonon state as a quantum harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke(H) at 5K for D_CNT < ~12Å, while absent for larger diameters. The results also reveal a very significant blue and red shifts of the stretching and bending modes respectively compared to those in bulk ice, in agreement with experiment.