Theoretical and experimental investigations of the electronic circular dichroism and absorption spectra of bicyclic ketones

The absorption and circular dichroism (CD) spectra of the bicyclic ketones norcamphor, camphor and fenchone were measured in the gas phase in the energy range 3.5-8.5 eV (350-145 nm). The experimental results are compared with theoretical density functional calculations combined with a configuration interaction treatment employing extended AO basis sets. This approach gives a quantitative agreement between theory and experiment for the excitation energies and transition moments of four separated bands. On this basis a complete assignment of the excited singlet states involved could be made. In particular, an intense band found in the CD spectra at 7.0-7.5 eV is attributed to contributions from sigma --> pi valence excited states which is a special feature of cyclic ketones. Relative energy shifts, the sign of the CD bands and sign inversions as a function of the methyl group substitution pattern in the three molecules are discussed. A simple model which partitions the bicyclic structures into an inherently chiral cyclopentanone moiety and perturbing substituents is useful in this respect. (C) 1997 Elsevier Science B.V.