Beiträge zur Chemie der Pyrrolpigmente, 24. Mitt. Über die Beziehung zwischen Lichtabsorption und Struktur von Bilatrienen-abc

Abstract

Using the recently established solution structure of a bilatriene-abc derivative the parameter set of thePPP-SCF-LCAO-MO-Cl model was refined. The general trend, that molecular configuration itself does not so much determine the absorption spectra as was deduced by investigation of partial bile pigment structures, was confirmed for the bilatrienes-abc. Instead, the configuration at a particular double bond induces a certain torsional angle at the adjacent single bond which leads to dramatic spectral changes on isomerization of this double bond. Isomerization without this torsional effect brings about different relative intensities of the two main spectral bands. Comparing measured spectra of several bilatrienes-abc and calculated spectra a helicalsyn, syn, syn-conformation with three torsional angles at the methine fragments of appr. 20° was deduced for systems with (Z, Z, Z)-configuration. For a recently prepared (E, Z, Z)-derivative a nearly helicalsyn, syn, syn-conformation with torsional angles of 40°, 20° and 20° at the methine single bonds is the most probable.