Conformation studies by nuclear magnetic resonance spectroscopy in nematic phases: 3-phenyl-1,2,5-oxa-, -thia-, and -selena-diazole

The 100 MHz n.m.r. spectra of 3-phenyl-1,2,5-oxa-(1), -thia-(2), and -selena-diazole (3) partially oriented in the nematic phase have been obtained and analysed. The spectral analysis affords examples of conformational situation in molecules containing two aromatic rings of six and five members respectively. A preferred twisted conformation around the bond between the two rings has been found, the twisting angle θ ranging from 18 to 21°. The conformation is little influenced by the heteroatom in position 2 and by the solvent. The results are discussed in connection with theoretical EHT calculations.