LINEAR AND CIRCULAR-DICHROISM STUDIES OF PI -] PI STAR TRANSITIONS IN STEROIDAL DIENES AND ALPHA,BETA-UNSATURATED KETONES

The diene and α,β-unsaturated keto chromophore in different conformations and orientations in a number of steroids were studied by circular dichroism (CD) in solution and linear dichroism (LD) in a stretched polyethylene matrix. The geometries of the chromophores were obtained from force-field calculations, and the energies, transition moments, and oscillator strengths for the ύ Ⱄ ύ transitions were calculated by the VESCF-Cl method including all singly and doubly excited configurations. CD, supported by LD, confirms the presence of an additional transition around 200 nm (6 eV), not observed in the U V spectra of dienes and α,β-unsaturated ketones. An analysis of the spectra shows, in agreement with the calculations, that the first two ύ Ⱄ ύ transitions occur close to each other in the region 200-260 nm. The importance of taking doubly excited configurations into account is demonstrated by the fact that the second transition is otherwise obtained at very high energy. The experimental and calculated transition moments were in reasonable agreement, and a rule for the transition moment direction of the first absorption band was provided by the line between the end atoms of the diene or enone moieties, even for twisted chromophores. The circular dichroism is discussed with respect to chromophore dihedral angle. Unexpectedly large rotational strengths shown by the planar conformations of the chromophore indicate a very strong influence from substituents. © 1979, American Chemical Society. All rights reserved.