The electronic structure and optical activity of conjugated dienes: 1,3‐Butadiene and α‐ and β‐phellandrene

The optical activity of conjugated dienes is investigated by means of ab initio SCF–CI calculations. The computed electronic spectrum of trans ‐1,3‐butadiene is shown to be in good agreement with the results of more rigorous calculations of the valence transitions and in satisfactory agreement with experiment. The optical rotatory strengths of the lower electronic transitions of twisted 1,3‐butadiene as a function of dihedral angle are presented and simulated CD spectra are produced. The N → V 1 (π 2 → π 3 *) transition is predicted to have a positive rotational strength for all dihedral angles that correspond to a right‐handed twist of the chromophore, in accord with the empirically deduced “diene rule” although for a twist angle of 60°, the rotatory strength is calculated to be almost zero. The role of the orientation of allylic bonds is investigated in the model system 1‐butene in which the rotational strength of the π → π* transition as a function of rotation about the 2,3 bond is determined. The effect of allylic bond disposition in dienes on the optical activity of the long‐wavelength π 2 → π 3 * transition is simulated by use of the exciton coupling model of Harada and Nakanishi in which two 1‐butene molecules with suitable geometries are coupled via interactions of the electric dipole transition moments of their π → π* transitions. The model systems 1,3‐butadiene and 1‐butene are used to rationalize the apparently anomalous optical activity of (−)‐α‐phellandrene and (−)‐β‐phellandrene, both of which should have a diene chromophore with a right‐handed twist in their most stable conformers and so should be dextrorotatory. The experimental CD spectrum of α‐phellandrene is determined at several temperatures down to −180°C. The observed variation of the apparent rotational strength of the N → V 1 transition is in good agreement with that predicted by use of the exciton coupling model.