Beiträge zur Chemie der Pyrrolpigmente, 61. Mitt.

The parameters of aPPP-SCF-LCAO-MO-CI treatment adjusted earlier for studies in the bilatriene-abc series are checked by means of a 3,4-dihydropyrromethenone of known conformation. This method is then applied to the calculation of -electron densities, bond orders, reactivities and absorption spectra of the various tautomeric forms, diastereomers and conformations of the 2,3-dihydrobilatrieneabc chromophor, which is of interest as the prosthetic group of the plant photoreceptor phytochrome and of antenna pigments of algae. The results are compared to¹³C-,¹⁵N-NMR and X-ray data as well as absorption spectra of available tautomeric model compounds and diastereomers with conformations known from experiment. The monomeric nature of the absorbing species is checked in some cases by vapour pressure osmometry. In case of the 2,3-dihydrobilatrienes-abc in their N₂₃-H tautomeric form (which was consolidated experimentally earlier) isomerization of the (Z,Z,Z) configured parent compound doesnot yield significant changes of the type of absorption spectra. The stretched chromophore criterion deduced byMoscowitz et al. for bilatrienes-abc (i.e., the intensity of the long wavelength band exceeds that of the short wavelength band) is refined for the 2,3-dihydro-derivatives, as only conformational changes fromsyn toanti at the single bonds 10–11 and 14–15 yield this effect. This change at 10–11 may be discriminated from the one at 14–15 by a strong secondary band in the long wavelength region.