Contribution to the study of the structure of a complex non rigid molecule in the liquid phase : Case of nematic para-azoxyanisole

Abstract

The whole set of presently available proton and deuterium NMR data concerning the nematic phase of para-azoxyanisole (a molecule with five rigid moieties connected by single covalent bonds) is used to test a model in which the molecule is assumed to exist essentially in one single conformation, this conformation being as close as possible to that existing in the solid phase. It is found that such model can describe satisfactorily the data provided large amplitude internal motions that leave the geometrical shape unchanged are introduced : π flips of the phenyl rings, methyl group rotations and (probably) exchange with the mirror image conformation. Further, it must be assumed that some dihedral angles between rigid moieties change and that other do not. Comparison with theoretical calculations concerning the isolated molecule reveals that these changes occur for those angles whose values in the solid phase do not correspond to a minimum of the intramolecular potential. Small amplitude librations of the rigid moieties about the most probable positions also occur. Concerning the molecular orientational order, it is found that the main principal axis of the ordering matrix is very close to that of the direction of maximum elongation of the molecule. The results are compared with those obtained with models in which several different conformations are allowed to exist with comparable probabilities. We give reasons why the single conformation model should be preferred.