Dynamics of the Hydrogen-Bonding Arrangement in Solid Triphenylmethanol:  An Investigation by Solid-State ²H NMR Spectroscopy

Abstract

Dynamic properties of the hydrogen-bonding arrangement in a selectively deuterated sample of solid triphenylmethanol (Ph3COD) have been studied by wide-line 2H 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 2H 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 2H NMR spectra across the full temperature range studied. In this model, the deuteron of the apical molecule undergoes a three-site 120° 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° 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° 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 2H NMR line shape on the echo delay and (ii) consideration of 2H NMR spin−lattice relaxation time (T 1) data. A full physical interpretation and justification of this dynamic model is presented.

Solid triphenylmethanol: A molecular material that undergoes multiple internal reorientational processes on different timescales

“…Thus, the timescales for the hydrogen bond dynamics and the phenyl ring dynamics differ by at least five orders of magnitude, and it is clear that the dynamics of the hydrogen bonding arrangement and the dynamics of the phenyl rings are not correlated to one another. We note that the 1801-jump motion of each phenyl ring interconverts two orientations that are indistinguishable by diffraction techniques, and thus the structures determined from single-crystal X-ray diffraction [1] and single-crystal neutron diffraction [2] provide no hint for disorder with respect to the orientations of the phenyl rings (unlike the situation for the disorder in the hydrogen bonding arrangement, which is suggested from the single-crystal X-ray diffraction data and proven by the single-crystal neutron diffraction data). Finally, we note that the 2 H NMR technique probes the dynamic properties of individual deuterons, and does not provide any insights on whether the 1801 jumps of the three phenyl rings in a given molecule occur in a concerted fashion or whether they are independent of one another.…”

“…2.9 Å ) within the tetramer are consistent with the tetramer being held together by O-H?O hydrogen bonds. Although the hydrogen atoms of the hydroxyl groups were not located in the structure determination from single-crystal X-ray diffraction data (suggestive of possible disorder of the hydrogen bonding arrangement), subsequent single-crystal neutron diffraction studies [2] confirmed that the hydrogen bonding arrangement in this material is indeed disordered (each hydroxyl hydrogen atom is disordered between different sites with fractional occupancies, each corresponding to the formation of an O-H?O hydrogen bond along a different O?O edge of the O 4 pyramid).…”

“…In the crystal structure of triphenylmethanol (Ph 3 COH; denoted TPM) [1,2], the molecules form hydrogen-bonded tetramers ( Fig. 1) with the oxygen atoms positioned approximately at the corners of a tetrahedron.…”

Explorations in the Dynamics of Crystalline Solids and the Evolution of Crystal Formation Processes

“…As there are six O⋅⋅⋅O edges of the tetramer but only four O−H⋅⋅⋅O hydrogen bonds, there are several different permutations for arranging the O−H⋅⋅⋅O hydrogen bonds, and disorder of the hydrogen‐bonding arrangement may be anticipated, consistent with the fact that hydrogen‐atom positions were not reported for the hydroxyl groups in the crystal structure determined from single‐crystal X‐ray diffraction data at ambient temperature. To explore whether the disorder of the hydrogen‐bonding arrangement is dynamic, solid‐state 2 H NMR studies were carried out on the selectively deuterated material Ph 3 COD.…”

Site-resolved 2H relaxation experiments in solid materials by global line-shape analysis of MAS NMR spectra

“…A large number of studies have been performed using 2 H as the probe of molecular properties; examples are dynamics of silanol groups in silica gels and nanoparticles [3,4], methylene group dynamics [5], conformations of membrane lipids [6], dynamics of different segments of polymer chains [7], distinguishing among and characterizing polymorphs [8] and hydrogen-bond arrangements in various materials [9,10]. Typically, such studies concerned 2 H populations at respective single sites in which case the whole spectrum reported on the same phenomenon.…”