Beiträge zur Chemie der Pyrrolpigmente, 8. Mitt.: ZumBrunings—Corwin-Effekt bei Pyrromethenderivaten

TheBrunings—Corwin-effect of pyrromethene derivatives is discussed on the basis of thePPP-SCF-LCAO-MO-CI method. To this end the electron absorption spectra of pyrromethenes and their corresponding positively charged ions were calculated for several torsional angles of the methine bridge. In the case of the free base the calculation predicts a weak hypsochromic effect and decreasing intensity of the first transition with increasing torsion. The cations are predicted to exhibit a strong bathochromic shift and decreasing intensity. The latter result is in excellent qualitative and quantitative agreement with the experimental results, assuming a torsional angle of approximately 30° for both methine bridge bonds.