π-SCF studies of the circular dichroism and electronic spectra of alkaloids containing the aniline chromophore. The stereochemical configuration of calycanthine and caracurine-II

Abstract

The absorption and c.d. spectra of calycanthine and caracurine-II have been extended to 53 kK, and the π→π* transition energies and dipole and rotational strengths of the two alkaloids have been calculated in the π-SCF approximation using the dipole-velocity procedure. Theoretical absorption and c.d. spectra have been obtained for the alkaloids from the calculated quantities with a curve-plotter using a Gaussian band-shape function. The spectra computed for the previously assigned configurations of the two alkaloids reproduce the qualitative form of the experimental curves with magnitudes of the correct order. The π-SCF calculations are found to support the general features of the point–dipole exciton treatment of the electronic states and stereochemical configuration of chiral dimeric π-systems in cases where the latter method is applicable, e.g. calycanthine, but the former method is of wider scope and covers cases, e.g. caracurine-II where the latter fails.