Quantum behaviour of water molecule in gemstone: terahertz fingerprints

We have shown that a weak interaction of a lone H₂O molecule with the ''walls'' of nano-sized crystalline cage of gemstone (beryl) results in emergence of a rich set of molecular vibrational states. By analogy with translational and librational bands in liquid water and ice corresponding absorption bands are explained as due to translational (T) and librational (L) movements of the H₂O molecule which is hydrogen bonded to the cage walls. In beryl crystal lattice, however, the six-fold symmetry of the cage brings about additional effect of splitting of the T and L bands into fine structure due to tunnelling within the six-well potential relief. The presented results will be of use for analysis of more complicated systems with confined water molecules like H₂O chains in carbon nano-tubes, molecular clusters in e.g. zeolites, clays, silica gels and other natural or synthetic frameworks, as well as for interfacial water in biological systems.