Study of the isotropic-nematic-solid transitions in a liquid crystal by carbon-13—proton double resonance

Carbon-13 NMR spectra have been obtained for p-methoxy benzylidene p^'-n-butylaniline (MBBA) in the isotropic, nematic, and solid phases. Sharp, resolved spectra were obtained in the nematic phase with high-power proton-spin decoupling. The isotropic-nematic transition is accompanied by a discontinuous change in ¹³C absorption frequencies due to the molecular ordering and anisotropic chemical shielding. It is concluded that the benzene rings reorient rapidly about the para axes. Employing average values from studies of ¹³C chemical shielding tensors in model aromatic solids, absolute values of the order parameter S are estimated and normalized values are compared with previous results. The nematic-solid transition is accompanied by sudden and severe line broadening due to anisotropic chemical shielding.