CHROMOPHORIC SYSTEMS .1. CONFORMATION AND ABSORPTION OF LIGHT IN HEXA-1,5-DIENE-3,4-DIONE (DIVINYLGLYOXAL)

Divinylglyoxal (1) is investigated by several spectroscopic and theoretical methods in order to set up a correlation between the CO/CO dihedral angle and the transition energies in unsaturated alpha-diketones. According to the experimental (UV, IR, C-13-NMR, PE spectroscopy and dipole moment) and theoretical (ab initio and molecular mechanics) results, 1 exists predominantly as a C2h conformer (THETA-CO/CO = 180-degrees, THETA-Vinyl/CO = 0-degrees). The dependence of transition energies on CO/CO dihedral angles is similar in shape to that for saturated alpha-diketones, but is significantly shifted to lower transition energies. C-13 chemical shifts respond sensitively to CO/CO dihedral angles, and the ab initio calculated curve is supported by a few experimental results.