Hydrogen bond dynamics in solid triphenylsilanol

Dynamic properties of the hydroxyl groups in a selectively deuterated polycrystalline sample of triphenylsilanol (Ph3SiOD) have been studied using variable-temperature solid-state H-2 NMR spectroscopy. The crystal structure of triphenylsilanol contains eight crystallographically independent molecules, which are arranged in two tetrameric building units. Within each of these tetrameric units, the four silicon atoms are arranged in the form of a slightly distorted square, with the O atoms of the four hydroxyl groups involved in O-H...O hydrogen bonding. The temperature dependence of the quadrupole echo 2 H NMR line shape in the temperature range 213-358 K and H-2 NMR spin-lattice relaxation time measurements at 368 K demonstrate that the hydrogen-bonding arrangement is dynamic. From the H-2 NMR line-shape analysis, the dynamic process is interpreted as interconversion between "clockwise" and "anticlockwise" hydrogen-bonding arrangements within each tetrameric unit, via a two-site jump motion of each hydroxyl deuteron about the Si-OD bond. It is assumed that the motions of the hydroxyl groups,in each hydrogen-bonded tetramer are highly correlated. The temperature dependence of the jump frequency, exhibits Arrhenius behavior, with the activation energy for the dynamic process estimated to be 35 +/- 2 kJ mol(-1). High-resolution solid-state Si-29 NMR spectroscopy has also been used to identify crystallographically inequivalent silicon sites.