NMR and low-frequency Raman studies of the liquid and solid phases of bicyclohexyl

Abstract

Abstract The 1 H and 13 C 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 13 C 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 highresolution 13 C 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 13 C 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.