Photoelectron spectra, electronic structure and long-range electronic interaction in some steroids

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Abstract
Alicyclic molecular frameworks are useful for the study of electronic through-bond (TB) and through space (TS) interactions of functional groups over fixed distances and/or a specified number of C-C bonds. Steroids, being readily available, stable and stereochemically well-defined, are especially suited for such investigations. Indeed, their use in the study of the stereochemical aspects and the long-range effects of groups situated at specified distances from chemically reactive centers has been well documented over the last 20 years. Since steroids are compounds of considerable biological importance, their activity being dramatically dependent on construction, conformation and electronic structure, it is surprising that our knowledge of their electronic structure, whether by quantum chemistry or photoelectron (PE) spectroscopy, is so sparse. Having recorded the first PE spectra of steroids (refs. 1,2) we believe that such studies may resolve many problems concerning long-range intramolecular interaction since (i) PE spectroscopy is a gas phase method and its results pertain to the free molecule and (ii) steroids by virtue of their fixed geometry are unable to form intramolecular (head-to-tail) adducts in which both TB and TS effects might separately propagate. Indeed, one should expect the presence of TB interactions only. We report the PE spectra of several androstane derivatives and derive their electronic structure using empirical arguments and of quantum chemical, MNDO calculations. Emphasis is placed on the long range effects of a carbonyl group located at the biologically important 3-, 11- and 17- positions and the enhancement of these effects by interpolated, localized double bonds. The position and fine structure of the lowest energy PE bands and the shift of the .sigma.-onsets are the gauges used to estimate these effects. These long range effects seem to exert considerable influence on confirmation, activity and, particularly, on fast intramolecular electron transfer (ET) such as has been observed recently in steroid solutions.