Volume 120. number 6 CHLMICAL PHYSICS LE-I-KRS 25 October 1985 AN UNUSUAL PHASE OF SOME THERMOTROPIC LIQUID CRYSTALS A CARBON-13 NMR STUDY M ahinda GANGODA and B M. FUNG 1 Introduction Llquld crystals have local orientational order. Ne- matlc liquid crystals can orient mdcroscoplcally m a magnetic field and pve moderately sharp carbon-13 NMR peaks [l-S] _ Therr chenucal shifts are quite dlf- ferent from those in the rsotroplc phase because of large chemical shift anisotropy. Some hquld crystals have a cubic phase, in which the molecules have rapid isotropic translational dlffuslon and short range hquld- like disorder but long range order [6] _ The cubic phase shows sharp, isotropic or nearly rsotroplc NMR signals [6] Most cubic phases are formed either between two mesophases [6,7] or between a cholesteric phase and an rsotropic hquid (blue phase) [S] _ WC do not know of nematrc hquld crystals forming a cubic phase before or during crystallization Molecules in many plastic sohds undergo isotropic rotatrons and also show sharp, isotroprc NMR peaks without magc angle spinning [9] _ Recently two different groups have shown that some thermotropic liqurd crystals can form a new phase which shows isotropic or nearly Isotropic carbon- 13 NMR specta [lO,ll]. It was suggested that the new phase IS a plastic phase [lo,1 I], but the results can also be explamed by the formatlon of a cubic phase. Since then, we have found that three other liquid crys- tals which have structures sunilar to one of the com- pounds previously studred [IO] also behave sirmlarly The results are reported here 2. Experunental All hqurd crystals were obtamed from EM Chenu- cals, Hawthorne, NY The 13C NMR spectra were measured wrth a Vanan XL-300 NM R spectrometer at 75.43 MHz. A phase-alternating broadband decouplmg sequence, ALPHA-3 [12,13], was used for the hquid crystalline and solid phases with a proton decoupling power of 13 kHz. The samples were spun along an axis aparallel to the superconducting field at ~15 Hz for the liquid and liqurd crystalline phases The solid samples were not spun. 3 Results and discussion The compounds studied were 4-alkoxyphcnyl-trans- 4-alkylcyclohexylcarbonates havmg the structure S1496 R, = C,H,. R2 = C2H,, nematlc range 48-78”C, S1563- R, = C,Hg, R2 = C,H, ,nematlc range 37-75°C; S1495. R, = C,H,, , R2 = CH3;nematlc range 40-71°C The 13C spectra of S1496 are shown in fig 1 The peaks are sharp m the hquld phase (fig la)_ The tern- perature of the sample was lowered m steps of l°C 0 009-2614/85/S 03.30 0 Elsevier Science Pubhshers B V. (North-HoIland Physics Publishing Division) 527