NMR AND LOW-FREQUENCY RAMAN STUDIES OF THE LIQUID AND SOLID-PHASES OF BICYCLOHEXYL

The H-1 and C-13 NMR spectra and spin-lattice relaxation times (T-1) of bicyclohexyl (BCH) were measured in phase I (277-274 K) and in the high-temperature region of phase II (274-256.4 K), A small fraction of the BCH molecules has exhibited a new phase which shows sharp NMR peaks superimposed on the broad signals of phase I. This new phase is most probably a disordered (plastic) solid phase, The relatively broad NMR signals of phase I of BCH, obtained without rapid sample spinning: indicate that this is a liquid-crystalline phase rather than a plastic phase, The C-13 spectral shape of phase I resembles, but is considerably wider than, the aliphatic region of neat trans,trans-4'-propylbicyclohexyl-4-carbonitrile in the nematic phase. This suggests that phase I of BCH is a smectic rather than a nematic phase, The magic angle spinning technique is used to obtain high-resolution C-13 NMR spectra of phases I and II of BCH. Raman spectra of the liquid and various solid phases of BCH have been recorded below 300 cm(-1), revealing phase I to be highly disordered; the Rayleigh wing is quite similar to that of the liquid. In phase II, a single broad Raman band at 42 cm(-1) is observed indicating that all the degrees of freedom are not frozen although the phase contains molecules only in the ee anti conformer, In phase III, a series of sharp peaks are observed, revealing an ordered crystalline solid with molecules in the ee gauche conformer, The C-13 spin-lattice relaxation times (T-1) have been measured for the liquid and narrow-line component of neat BCH. The observation that T-1 is continuous across the melting point is again indicative of a disordered phase, The activation energies and correlation times for the composite motions of the different C-H vectors are within 19-22 kJ mol(-1) and 23-29 ps (at 277 K), respectively, The large values of these parameters suggest that the overall tumbling motion in the liquid and disordered phases is considerably hindered.