Determination of absolute configuration of helicenes and related biaryls from calculation of helical twisting powers by the surface chirality model

The sign and the magnitude of the helical twisting power beta for a series of helicenes have been calculated by the Surface Chirality model. The principal contribution to beta derives from the helicity in the direction perpendicular to what can be defined as the main molecular plane; the cholesteric axis is also predicted to be along this direction, in agreement with a view of the cholesteric induction by helical molecules based on empirical observations. In the case of non-rigid biphenanthryl derivatives, the value of beta is predicted to vary with the dihedral angle between the phenanthryl moieties, changing sign at about 90 degrees where the conformation passes from s-cis to s-trans, i.e. when the stereochemical descriptor of the biaryl moiety exchanges the M and P indices. Comparison between experimental and calculated values indicates an s-trans conformation for the flexible dopants, in agreement with a previous conclusion drawn from empirical correlations. The Surface Chirality model appears to be a promising technique to assess the absolute configuration of rigid molecules by the comparison of experimental and calculated beta values. For flexible molecules, the quality of the information depends critically on the degree of knowledge of their conformational freedom.