Optical activity of oriented molecules. α,β-unsaturated steroid ketones and their sector rules

Abstract

Polarized spectroscopy is applied to the nπ* transition of α,β-unsaturated ketosteroids. The anisotropy of the CD of the oriented molecules (ACD) is shown to be strong for all compounds analyzed. Two different vibrational band progressions, namely an “allowed” and a “forbidden” one, determine most of the absorption of this transition. The CD of the “allowed” progression is positive whereas that of the “forbidden” one is negative. In general, for these compounds the CD of the isotropic solution Δϵ is determined by the “forbidden” progression. The variation of the intensity of the “allowed” and “forbidden” CD contributions by substitution in the vicinity of the enone chromophore may provide an explanation of a failure of the helicity rule for the enone system. As an example for such a failure the behaviour of some substituted α,β-unsaturated ketosteroids is discussed where the CD of the “allowed” progression determines the sign of Δϵ. Herewith, it is suggested - which is also plausible from the theoretical point of view - that helicity or sector rules can be applied only in such cases where the CD of the compound to be analyzed belongs to the same transition | Nn〉 → | Kk〉, e.g. the same vibronic transitions n, k in N and K as the CD of the compounds from which the rule is derived, a fact which is not always taken into account in literature.