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Abstract:
We report a 2H nuclear magnetic resonance (NMR) investigation of 2,3,7,8,12,13-hexakis(octadecanoyloxy)-
truxene (TxHA17) specifically labeled in the α-position of the alkyl chain. Results from these measurements,
together with polarizing microscopy, show that the substance is in a crystalline state up to 90 °C. At this
temperature the compound melts and a uniaxial discotic mesophase appears. The liquid crystalline phase
persists until the clearing point at 191 "C. The differential scanning calorimetry thermograms confirmed
these transitions but exhibited additional peaks below 90 °C. The phase behavior at low temperatures (<90 °C) differs significantly from previous reports by Warmerdam et al. (Liq. Cryst. 1988, 3, 149 and 1087). Solid-state deuterium line shape simulations were used to examine the motions of the labeled methylene site. A Gaussian libration model gave excellent agreement between experimental and calculated 2H NMR spectra. A pronounced spatial inhomogeneity of the motion was found in the high-temperature region of the solid
phases. By combining line shape simulations with nuclear spin relaxation measurements, mean correlation
times for the α-site could be estimated. It was shown that there exist, even at -100 °C, fast motions of the α-deuterons with rates relatively close to the Larmor frequency. Deuterium NMR spectra of benzene-d6 dissolved in protonated TxHA17 are also reported. These spectra were interpreted using a model in which the benzene molecules occupy two solvation sites: (i) intercalated within the columns of TxHA17 and (ii) dissolved in the aliphatic chains. The exchange of C6D6 between the two sites is slow on the NMR time scale in the crystalline phase whereas it is fast in the mesophase.
