Chain ordering in cyanobiphenyls and cyanophenylcyclohexanes

Abstract

Two-dimensional carbon-13 n.m.r. has been used to study the chain ordering of two cyanobiphenyls (6CB and 7CB) as a function of temperature. The C—H bond order parameters of 6CB and 7CB were calculated with the method of the potential of mean torque, using parameters obtained from 5CB and 8CB. The results agree very well with the experimental data. The orientational ordering of three 4′-cyanophenylcyclohexanes, two of which contain alkenyl side chains, has also been investigated. Trends of calculated C—H order parameters agree with those of experimental values, but the numerical agreements are less satisfactory than those for the cyanobiphenyls. The discrepancies are attributed to simplifications in the model. The two major contributing factors are the assumption of a cylindrical core and a simplified potential form for the rotation about the C—C bond joining the cyclohexane ring and the aliphatic chain. Calculated C—C bond order parameters of the three cyanophenylcyclohexanes and four cyanobipnenyls are also presented, and their trends are discussed.