About the Origin of the Chiroptical Properties of the Planar Diene Chromophore in Cyclohexylidenepropene Derivatives

The circular dichroism of the lowest energy π-π* transition in cyclohexylidenepropenes (a class of s-trans dienes) has been investigated theoretically. Two calculation methods, viz. the De Voe coupled oscillators theory and a semiempirical MO-SCF method (CNDO/S), have been employed. CD signs opposite to those experimentally found by Walborsky and co-workers have been obtained for every molecule studied. A possible origin of this disagreement cannot be found in a twist of the diene chromophore on the basis of the theoretical conformational analysis (MMP2 and ab initio SCF calculations give planar diene structure); the origin of the optical activity of these compounds seems then to be an open question.