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Abstract:
The title compound, 1,2-dichloro-3,4,5,6-tetramethylbenzene (DCTMB) exhibits three solid phases, III, II, and I, of which two, Phase III (<170 K) and Phase II (170–381 K), have been investigated in the present study by means of x-ray diffraction and deuterium NMR. The latter measurements were performed on powder and single crystal samples of perdeuterated DCTMB (DCTMB-d12). Phase III is “right–left” disordered, with the molecular para axes (the axes having two methyls in para positions) well ordered in the crystal. The right–left disorder is manifested by the fact that the chlorine and ortho methyls have each an occupancy number of 12, thus ensuring an average inversion symmetry at the molecular site. The NMR results also indicate that at least one of the ortho methyls, and probably the second one too, are slightly distorted, randomly up and down, relative to the benzene plane. Except for fast methyl group rotation the molecules in this phase are static on the NMR time scale. Phase II is much more mobile and disordered, with the molecular para axes distributed over all six local crystallographic orientations. A detailed analysis of the single crystalNMR results shows that the fractional populations in the various orientations are biased with some orientations more populated than others (while keeping average inversion symmetry). It is shown that this bias is due to differences in both the enthalpy and entropy associated with the various orientations. NMR line shape and T1 relaxation data in Phase II (and I) indicate rapid jumps between the different orientations in the high temperature range of the measurements. The estimated activation parameters for the reorientational jumps are Ea=33 kJ/mol and k(260 K)=4.5×107 s−1.
