Orientational disorder in 1,2,3-trichloro-4,5,6-trimethylbenzene. A single crystal deuterium NMR study of the site populations and dynamics

Abstract

Deuterium NMR measurements on powder and single crystal samples of 1,2,3-trichloro-4,5,6-trimethylbenzene (TCTMB) specifically deuterated at the central methyl group (TCTMB-d₃) are reported. The compound exhibits three solid phases: (I) A high temperature phase, just below the melting point (400 to 499 K), of unknown structure. (II) An intermediate monoclinic phase (268 to 400 K). (III) A low temperature triclinic phase (<268 K). Earlier X-ray investigations of the latter two phases indicate that they are, respectively, well ordered (Phase III at 173 K), and orientationally disordered, with the TCTMB molecules statistically aligned along the six possible orientations of the benzene core (Phase II at 298 K). In contrast to these observations the deuterium NMR results show a continuous growth, with temperature, of orientational disorder already in Phase III. The transition to Phase II involves a discontinuous increase in the disorder, but a considerable degree of order is retained in this phase even at high temperatures. This ordering reflects the non-equal population distribution of the molecular orientation in the crystal lattice sites. A quantitative analysis of the deuterium NMR spectra provided detailed information on this distribution and its temperature dependence in the solid phases of TCTMB. The deuterium NMR spectra also exhibit characteristic dynamic effects. As a function of increasing temperature, four dynamic regions can be distinguished: (i) Below 15 K, the spectrum exhibits features typical of coherent quantum-mechanical tunneling of the CD₃ groups. (ii) Above 15 K, up to about 170 K, there is fast classical (incoherent) reorientation of the methyl groups, but otherwise a rigid molecular lattice. (iii) Between 180 and 250 K, the spectra exhibit dynamic lineshape changes reflecting the onset of six-fold jumps of the TCTMB molecules. (iv) Above 250 K the spectra correspond to a dynamically averaged, but temperature dependent, quadrupolar splitting, reflecting the changes in the orientational distribution of the TCTMB molecules. This region includes the phase transitions, III to II and II to I. T₁ relaxation measurements over the entire temperature range yielded kinetic parameters for the molecular six-fold jumps (k(260 K) = 7 × 10⁶ s⁻¹, ΔE = 38.5 kJ mol⁻¹), as well as for the incoherent methyl group reorientation (k(25 K) = 1 × 10⁹ s⁻¹, ΔE = 2.8 kJ mol⁻¹).