DYNAMICS OF THE HYDROGEN-BONDING ARRANGEMENT IN SOLID TRIPHENYLMETHANOL - AN INVESTIGATION BY SOLID-STATE H-2 NMR-SPECTROSCOPY

Dynamic properties of the hydrogen-bonding arrangement in a selectivel y deuterated sample of solid triphenylmethanol (Ph3COD) have been stud ied by wide-line H-2 NMR spectroscopy. In the crystal structure of Ph3 COD, the molecules form hydrogen-bonded tetramers, with the oxygen ato ms positioned approximately at the corners of a tetrahedron. The tetra mer has point symmetry C-3 (rather than T-d); three of the Ph3COD mole cules (denoted as ''basal'') are related to each other by a 3-fold rot ation axis, and the fourth molecule (denoted as ''apical'') lies on th is axis. Thus, the oxygen atoms from the four molecules in the tetrame r form a pyramidal arrangement with an equilateral triangular base, an d the O ... O distances are consistent with the tetramer being held to gether 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 d ynamic 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 tempera ture 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 deut erons of the basal molecules undergo a two-site 120 degrees jump motio n, by rotation about their C-O bonds. In addition, each deuteron under goes rapid libration (reorientation about the relevant C-O bond) with the libration amplitude increasing as a function of temperature. The b ehavior of the basal molecules is interpreted in terms of the existenc e of two possible hydrogen-bonding arrangements-described as ''clockwi se'' and ''anticlockwise''-on the basal plane of the pyramid. The two- site 120 degrees jump motion for the basal molecules ''switches'' betw een these two hydrogen-bonding arrangements and clearly requires corre lated jumps of the hydroxyl groups of all three basal molecules. On th e assumption of Arrhenius behavior for the temperature dependence of t he jump frequencies, the activation energies for the jump motions of t he 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 ec ho delay and (ii) consideration of H-2 NMR spin-lattice relaxation tim e (T-1) data. A full physical interpretation and justification of this dynamic model is presented.