Die optische Aktivität von chiralen Dienen, Enonen und α‐Diketonen

Abstract

Abstract The ground state and excited states of butadiene, acrolein and glyoxal are calculated by the SCF‐CNDO‐CI method. The gauche conformers of these molecules, obtained by a twist about the formal CC single bond, have served as models for the study of the optical activity of dienes, α,β‐unsaturated ketones and α‐ and β‐diketones. An analysis of the molecular orbitals of the ground state and of excited states leads to the conclusion that the sign of a Cotton effect is determined primarly by the nodal properties and the helicity of the ground and excited state orbitals [20]. In cyclopentenones the longest wavelength Cotton effect does not obey the rule deduced from the twisted acrolein model. This apparent anomaly is explained on the basis of the altered nodal properties of cyclopentenones as compared to cyclohexenones. Concerning diones, the comparison of experimental CD. and UV. spectra with the calculated data allows the assignment of the five lowest Cotton effects of camphor quinone. It is found that α‐diketones have only two singletsinglet transitions in the region from 500 nm to 250 nm. In contrast, the next strong UV. absorption between 220 nm and 190 nm consists of three distinct electronic transitions, the nature of which is discussed. These findings are correlated with the previously calculated [8] symmetry properties and ordering of the energy levels of the ground state and excited states of planar trans and cis glyoxal. From these results the most likely conformations of a number of α‐diketons are derived.