Carbon-13 NMR studies of liquid crystals
High resolution, proton decoupled /sup 13/C nmr are observed for a series of neat nematic liquid crystals, the p-alkoxyazoxybenzenes, and a smectic-A liquid crystal, diethylazoxydibenzoate in a magnetic field of 23 kG. The (uniaxial) order parameters S = less than P/sub 2/(costheta) greater than are found to be about 0.4 and 0.9 for the nematic and smectic-A phase respectively at the clearing points. The order parameter increases with decreasing temperature in the nematic phase but is constant, or nearly so, with temperature in the smectic-A phase. In the nematic series studied, the ordering exhibits an even-odd alternation along the series and qualitative agreement with a recent theory due to Marcelja is found. In both phases, the spectra show that the molecule rotates rapidly about its long axis. Tentative conclusions about molecular conformational motion and /sup 14/N spin relaxation are presented for both nematic and smectic-A phases. In the smectic-A phase, the sample is rotated about an axis perpendicular to H/sub 0/ and the resulting spectra are dicusssed. The theory of observed chemical shifts in liquid crystals is discussed and equations are derived which relate the nmr spectra of liquid-crystals to the order parameters. A model for the smectic-C phase due to Luz and Meiboom and Doane is described and lineshapes are determined on the basis of this model for special cases. The dependence of the order parameters on the molecular potential which give rise to the various degrees of order in the different liquid crystalline phases is examined. To a good approximation the functional dependence of the order parameters on the molecular potential is shown to be a simple one in the limit of small tilt angle in the smectic-C phase.
- Research Organization:
- California Univ., Berkeley (USA). Lawrence Berkeley Lab.
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 7324880
- Report Number(s):
- LBL-4506
- Country of Publication:
- United States
- Language:
- English
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360602* -- Other Materials-- Structure & Phase Studies
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201 -- Chemical & Physicochemical Properties
ALKOXY RADICALS
ALKYL RADICALS
AROMATICS
BARYONS
BENZENE
BENZOIC ACID
CARBON 13
CARBON ISOTOPES
CARBOXYLIC ACIDS
CHEMICAL SHIFT
CRYSTALS
DECOUPLING
ELEMENTARY PARTICLES
ETHYL RADICALS
EVEN-ODD NUCLEI
FERMIONS
FLUIDS
HADRONS
HYDROCARBONS
ISOTOPES
LIGHT NUCLEI
LIQUID CRYSTALS
LIQUIDS
MAGNETIC FIELDS
MAGNETIC RESONANCE
MATHEMATICAL MODELS
MOLECULAR MODELS
MONOCARBOXYLIC ACIDS
NITROGEN 14
NITROGEN ISOTOPES
NUCLEAR MAGNETIC RESONANCE
NUCLEI
NUCLEONS
ODD-ODD NUCLEI
ORDER PARAMETERS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORIENTATION
PROTONS
RADICALS
RELAXATION
RESONANCE
SPIN-LATTICE RELAXATION
STABLE ISOTOPES
TEMPERATURE DEPENDENCE