Orientation and dynamics of water molecules in beryl

Behavior of individual molecules of normal and heavy water in beryl single crystals was studied by ¹H and ²H nuclear magnetic resonance spectroscopy. From temperature dependences of the spectra, we deduce that type-I water molecules embedded in the beryl voids are oriented quite differently from the view established in the literature: Different from the earlier assumptions, their H-H lines deviate by about 18° from the hexagonal axis. We suggest that this is due to the molecules attaching to the oxygen atoms forming the beryl structural voids by a hydrogen bond. Our analysis shows that the molecules perform two types of movement: (i) rapid librations around the axis of the hydrogen bond and (ii) less frequent orientational jumps among the 12 possible binding sites in the beryl voids. The frequencies of the librational motions are evaluated from a simple thermodynamic model, providing good quantitative agreement with the frequencies of librations from optical experiments reported earlier.