N-14 NUCLEAR-QUADRUPOLE DOUBLE-RESONANCE AT ZERO MAGNETIC-FIELD IN THE SMECTIC-C AND NEMATIC MESOPHASES OF HPAB

Pure nuclear-quadrupole-resonance spectra have been measured in both n ematic and smectic C liquid crystalline mesophases of 4-4'-bis-heptylo xy-azoxy-benzene (HpAB). The experiment involves fast-field-cycling NM R of protons and irradiation of the quadrupole N-14 nuclei by means of a second radio-frequency pulse during the zero-magnetic-field period. The obtained spectra show the presence of three inequivalent nitrogen nuclei in tile smectic mesophase and four resolved chemically inequiv alent nitrogens in the nematic. A local microstructure composed of bim olecular units explains the results.