Chiroptical Properties of cisoid Enones from Circular Dichroism (CD) and Anisotropic Circular Dichroism (ACD) Spectroscopy
✍ Scribed by Jadwiga Frelek; Wojciech J. Szczepek; Stephan Neubrech; Bernd Schultheis; Joachim Brechtel; Hans-Georg Kuball
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 283 KB
- Volume
- 8
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Substituted cisoid 4-en-6-one steroids with isotropically distributed and partially oriented molecules were analyzed by circular dichroism (CD) and anisotropic circular dichroism (ACD) spectroscopy, respectively. CD and ACD data supplement their respective phenomenological information. For a series of C3-substituted enones 1 to 7, the difference of CD (De) and ACD (De A ) values, that is, De À De A , vary in the n ± p* transition region in the same direction, independently of the nature and position (3a or 3b) of the substituent. For 7a-bromo-substituted enones 5 and 6 the sign of the n ± p* CD band is opposite to that predicted by the enone helicity rule. The ACD data indicate that this behavior is a consequence of the effect of vibronic coupling caused by the 7a-bromo substituent. In contrast to the results obtained for the series of C7unsubstituted compounds 1 to 4, the intensity of the CD bands for 5 and 6 is determined by the vibrational progressions of a different symmetry. Therefore, the helicity rule must fail in both cases because the rule can only be applied to those vibrational transitions for which the rule was developed. The sign of the coordinates De* ii , estimated from the ACD data, yields additional stereochemical information that cannot be obtained from the CD data alone. The CD and ACD spectra in the region of the p ± p* transition vary for enones 1 to 4 in a different fashion and indicate dependence upon spatial orientation (3a or 3b) of substituents. This dependence may lead to the possibility of extracting additional stereochemical information from the ACD spectra. Furthermore, the experimental findings indicate that the second CD band located at about 220 nm belongs to a forbidden transition and not to an allowed p ± p* transition.