Molecular modelling of chiral nematics

The surface model that was originally developed for evaluating the twisting power of chiral solutes is generalised to the ease of pure chiral nematics to rationalise the cholesteric pitch on the basis of the molecular structure. Its application to cyanobiphenyl nematogens having one chiral centre in the flexible chain, demonstrates that an important role is exerted by the conformational distribution, since the chirality contributions of conformers of a given compound differ both in magnitude and in handedness. By using reasonable estimates of the conformer populations and structure, we obtain predictions of cholesteric pitch in fair agreement with the experimental values. On the other hand, given the large number of conformers to be analysed and because of compensation effects between contributions with opposite handedness, the relation between the macroscopic chirality and the molecular structure becomes less evident than in the case of rigid solutes. Therefore we have developed a procedure which allows the identification of the contribution of specific molecular groups to the macroscopic chirality. Such a procedure, when applied to the examined cyanobiphenyl nematogens, clearly shows that the cholesteric pitch is controlled by the size of the substituents at the chiral centre.