Carbon-13 NMR of nematic and smectic liquid crystals with magic-angle spinning

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Abstract
Carbon-13 NMR of liquid crystals with magic-angle spinning (MAS) has been studied. Nematic liquid crystals with Δχ>0 orient parallel to the field at low spinning rates, and orient along the spinning axis at high spinning rates (ν). At a field of 7.05 T, the C-13 peaks of EBBA are very sharp and there are no spinning sidebands at ν≳1000 Hz. Spin lattice relaxation time (T1) and nuclear Overhauser effect (NOE) of individual carbon atoms of EBBA have been measured and the results give valuable information on the motional behavior of EBBA in its nematic phase. Nematic liquid crystals with Δχ<0 show rather broad peaks even at high ν. Smectic liquid crystals do not orient along the spinning axis, but can be made to do so when they are cooled through the nematic phase while the sample is being spun. With MAS at high ν, cross polarization is ineffective for nematic liquid crystals, and is not very effective for smectic liquid crystals.