The structure and orientational ordering of 4-methoxy-4′-cyanobiphenyl in the nematic mesophase

Abstract

The proton N.M.R. spectra of two partially deuteriated samples of the mesogen 4-methoxy-4′-cyanobiphenyl (1-OCB) have been obtained. The spectra were simplified by decoupling the interactions between protons and deuterons and analysed to yield sets of dipolar couplings, [Dtilde]_ij . The [Dtilde]_ij are used to investigate the angle φ between the two ring normals and the conformation of the methoxy group relative to the attached phenyl ring. If the potential governing rotation, V(φ), is assumed to have a deep minimum such that only structures with the minimum value of φ are appreciably populated, then φ_min is determined to be 30·4° ± 0·4°. However, adopting a continuous form for V(φ) changes φ_min to 36°. For the methoxy group the data are consistent with the potential governing rotation about the ring-oxygen bond being zero when the carbon atom is in the ring plane and infinite otherwise; in addition, we have investigated the consequences for the structure and ordering of adopting more realistic forms for this potential. The 24 structures of equal, minimum energy, which are generated by rotation about the inter ring bond, the phenyl-O bond and the O—CH3 bond, all have the same shape, and hence each of these conformations has the same ordering matrix, with principal elements S_zz and S_xx — S_yy . The magnitudes of S_xx and S_xx — S_yy have been obtained, together with the orientation of x, y and z with respect to a frame fixed in the methoxylated ring, with the assumption that only these minimum energy structures are populated. The magnitudes of these principal order parameters are used to test the form of a potential of mean torque.