A comparative X-ray diffraction study of two liquid crystalline 2,5-disubstituted 1,3,4-oxadiazole and 1,3,4-thiadiazole derivatives in their smecticphases

X-ray diffraction study of two liquid crystalline 2,5-disubstituted 1,3,4-o xadiazole and 1,3,4-thiadiazole derivatives reveals the existence of a part ially bilayered smectic Ad phase in the terminally bromo substituted deriva tives, with the lower level of molecular overlapping in the formation of th e dimer and more pronounced smectic fluctuations on the dimeric density wav e observed for the former compound.