Conformational and Orientational Characteristics of Chain Molecules Placed in Nematic Fields. Part II

Abstract

Deuterium NMR quadrupolar splittings and 2 H - 2 H dipolar couplings observed from perdeuterated 1,6-dimethoxyhexane (CD 3 O(CD 2 ) 6 OCD 3 ) dissolved in a liquid crystal 4'-methoxybenzylidene-4-η-butylaniline (MBBA) were analyzed using the single-ordering-matrix (SOM) and the Photinos-Samulski-Toriumi (P-S-T) models. Within the framework of the rotational isomeric state approximation, intramolecular interactions up to the third-order (between atoms and groups separated by five bonds) were considered. The geometrical parameters and intramolecular interaction energies determined from ab initio molecular orbital calculations were employed. The two simulation models yielded fair agreement between theory and experiment, but were found to give quite different images of the solute chain: (the SOM model), the solute becomes rigid and extended to conform itself to the nematic field; (the P-S-T model), the solute has almost the same degree of flexibility as in the free state, but populations of the anisotropic conformers are selectively enhanced. To reveal the true conformational characteristics of chain molecules in nematic fields, experimental techniques to enable direct and quantitative measurements of the bond conformations are necessary.