Comparative H-2 NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

Bent-core liquid crystals showing a nematic phase stable at low temperatures are very attractive for applicative purposes in view of the inherent biaxial nature of the nematic phase. In this work, a typical five-ring bent-core mesogen was investigated by means of H-2 NMR spectroscopy and X-ray diffraction (XRD) methods. These techniques provide complementary information on the structural properties of the nematic phase and the average mesogen conformation: small-angle XRD reveals the presence of short-range positional order in the form of skewed cybotaxis, while a comparison of the orientational order parameters measured by wide-angle XRD and NMR provides an estimate of the molecule bend angle. In addition, H-2 NMR puts in evidence the occurrence of an unexpected transition to a low-temperature tilted phase, having a crystalline or smectic-like character. The results were compared with those of previous C-13 NMR investigations.