Inelastic and deep inelastic neutron spectroscopy of water molecules under ultra-confinement

Water confined within sub-nanometer channels of silicate minerals presents an extreme case of confinement, where the restricted molecules are situated in channels whose diameter is not much larger than the water molecule itself. Recently, we discovered a new quantum tunneling state of the water molecule confined in 5 Å channels in the mineral beryl, characterized by extended proton and electron delocalization. Several peaks were observed in the inelastic neutron scattering (INS) spectra which were uniquely assigned to water quantum tunnelling. In addition, the water proton momentum distribution measured with deep inelastic neutron scattering (DINS) at 4.3 K directly showed coherent delocalization of the water protons in the ground state. The obtained average kinetic energy (E_K) of the water protons was found to be ~30% less than it is in bulk liquid water and ice phases. In the current work we present INS and DINS study of water in single crystal beryl in wider temperature range, T=5–260 K, where we observed significant increase of E_K of the confined water protons with temperature increase. The obtained INS data also indicate that with increasing temperature water molecules are progressively involved in hydrogen bonding (HB) with the beryl cage, while HB is almost absent at low temperatures.