Conformation of 4-ethoxy-4′-biphenylyl cyanide in the isotropic and anisotropic liquid-crystalline state — a rotational isomeric state simulation of 2H NMR spectra

Abstract

The conformation of the ethoxy tail flanking the cyanobiphenyl core has been studied in the liquid-crystalline phase as well as in the isotropic solution. In solution, the vicinal coupling constant 3JCH for the moiety CphOCH (Cph: phenyl carbon) provided the information regarding the rotation around the OC bond. Phenetole was mostly used as a model compound. The conventional rotational isomeric state (RIS) analysis yielded the energy difference between the gauche and trans state to be of the order of 1,6 to 1,4 kcal/mol in CDCl3, C6D6 and dimethyl-d6 sulfoxide. In this treatment, the rotational state around the neighboring bond (CphO) was taken to occur at 0 or π. The deuterated compound, 4-ethoxy-4′-biphenylyl-d7 cyanide, was prepared and studied in the liquid-crystalline state by 2H NMR. The orientational order parameters of the mesogenic core were determined by the analysis of dipolar and quadrupolar splitting data due to aromatic deuterons. The quadrupolar splitting data were also obtained for the deuterated ethoxy tail. The conformation of the tail was elucidated according to the RIS simulation scheme previously proposed. The fraction of the trans conformation ft was estimated to be 0,79 around the nematic-isotropic transition temperature (TNI = 91,9°C). It was suggested that the conformation of the ethoxy tail remains nearly unaffected by the phase transition in the immediate vicinity of TNI. The fraction ft increases moderately toward an asymptotic value as temperature decreases.