Dynamics of the hydrogen-bonding arrangement in solid triphenylmethanol:: An investigation by solid-state 2H NMR spectroscopy

Dynamic properties of the hydrogen-bonding arrangement in a selectively deuterated sample of solid triphenylmethanol (Ph3COD) have been studied by wide-line H-2 NMR spectroscopy. In the crystal structure of Ph3COD, the molecules form hydrogen-bonded tetramers, with the oxygen atoms positioned approximately at the corners of a tetrahedron. The tetramer has point symmetry C-3 (rather than T-d); three of the Ph3COD molecules (denoted as "basal") are related to each other by a 3-fold rotation axis, and the fourth molecule (denoted as "apical") lies on this axis. Thus, the oxygen atoms from the four molecules in the tetramer form a pyramidal arrangement with an equilateral triangular base, and the O ... O distances are consistent with the tetramer being held together by O-H ... O hydrogen bonds. The H-2 NMR line shape for Ph3COD varies with temperature (in the range 97-373 K), demonstrating clearly that the hydrogen-bonding arrangement is dynamic. Several plausible dynamic models are proposed, and it is found that only one model gives a good fit to the experimental H-2 NMR spectra across the full temperature range studied. In this model, the deuteron of the apical molecule undergoes a three-site 120 degrees jump motion by rotation about the C-O bond (with equal populations of the three sites), whereas the deuterons of the basal molecules undergo a two-site 120 degrees jump motion, by rotation about their C-O bonds. In addition, each deuteron undergoes rapid libration (reorientation about the relevant C-O bond) with the libration amplitude increasing as a function of temperature. The behavior of the basal molecules is interpreted in terms of the existence of two possible hydrogen-bonding arrangements-described as "clockwise" and "anticlockwise"-on the basal plane of the pyramid. The two-site 120 degrees jump motion for the basal molecules "switches" between these two hydrogen-bonding arrangements and clearly requires correlated jumps of the hydroxyl groups of all three basal molecules. On the assumption of Arrhenius behavior for the temperature dependence of the jump frequencies, the activation energies for the jump motions of the apical and basal deuterons are estimated to be 10 and 21 kJ mol(-1) respectively. This dynamic model is further supported by (i) analysis of the dependence of the quadrupole echo H-2 NMR line shape on the echo delay and (ii) consideration of H-2 NMR spin-lattice relaxation time (T-1) data. A full physical interpretation and justification of this dynamic model is presented.