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 ket ones norcamphor, camphor and fenchone were measured in the gas phase i n the energy range 3.5-8.5 eV (350-145 nm). The experimental results a re 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 s inglet states involved could be made. In particular, an intense band f ound in the CD spectra at 7.0-7.5 eV is attributed to contributions fr om sigma --> pi valence excited states which is a special feature of c yclic ketones. Relative energy shifts, the sign of the CD bands and si gn inversions as a function of the methyl group substitution pattern i n the three molecules are discussed. A simple model which partitions t he bicyclic structures into an inherently chiral cyclopentanone moiety and perturbing substituents is useful in this respect. (C) 1997 Elsev ier Science B.V.