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

Deuterium NMR measurements on powder and single crystal samples of 1,2,3-tr ichloro-4,5,6-trimethylbenzene (TCTMB) specifically deuterated at the centr al methyl group (TCTMB-d(3)) are reported. The compound exhibits three soli d phases: (I) A high temperature phase, just below the melting point (400 t o 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, respective ly, well ordered (Phase III at 173 K), and orientationally disordered, with the TCTMB molecules statistically aligned along the six possible orientati ons of the benzene core (Phase II at 298 K). In contrast to these observati ons the deuterium NMR results show a continuous growth, with temperature, o f orientational disorder already in Phase III. The transition to Phase II i nvolves a discontinuous increase in the disorder, but a considerable degree of order is retained in this phase even at high temperatures. This orderin g reflects the non-equal population distribution of the molecular orientati on in the crystal lattice sites. A quantitative analysis of the deuterium N MR spectra provided detailed information on this distribution and its tempe rature dependence in the solid phases of TCTMB. The deuterium NMR spectra a lso exhibit characteristic dynamic effects. As a function of increasing tem perature, four dynamic regions can be distinguished: (i) Below 15 K, the sp ectrum exhibits features typical of coherent quantum-mechanical tunneling o f the CD3 groups. (ii) Above 15 K, up to about 170 K, there is fast classic al (incoherent) reorientation of the methyl groups, but otherwise a rigid m olecular lattice. (iii) Between 180 and 250 K, the spectra exhibit dynamic lineshape changes reflecting the onset of six-fold jumps of the TCTMB molec ules. (iv) Above 250 K the spectra correspond to a dynamically averaged, bu t temperature dependent, quadrupolar splitting, reflecting the changes in t he orientational distribution of the TCTMB molecules. This region includes the phase transitions, III to II and II to I. T-1 relaxation measurements o ver the entire temperature range yielded kinetic parameters for the molecul ar six-fold jumps (k(260 K)=7x10(6) s(-1), DeltaE=38.5 kJ mol(-1)), as well as for the incoherent methyl group reorientation (k(25 K)=1x10(9) s(-1), D eltaE=2.8 kJ mol(-1)).