Tunneling systems in molecular crystals: studies by optical spectroscopy and neutron scattering

Two-level tunneling systems have been identified in a number of pure and doped molecular crystals. The tunneling dynamics of translations of protons along symmetric hydrogen bonds and of hindered rotations of methyl groups have been characterized by a variety of experimental techniques: optical as well as vibrational spectroscopy, neutron scattering, and proton NMR T1 relaxation. Molecular mechanics/dynamics calculations have been made to quantify the potential energy surface of two-level-systems and to identify substitutional sites of guest molecules in host crystals and to evaluate the structural and energetic disorder of these sites.