Chromophore Systeme, 1. Konformation und Lichtabsorption von Hexa‐1,5‐dien‐3,4‐dion (Divinylglyoxal)

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 α‐diketones. According to the experimental (UV, IR, 13 C‐NMR, PE spectroscopy and dipole moment) and theoretical (ab initio and molecular mechanics) results, 1 exists predominantly as a C 2h conformer (Θ co/co =180°, Θ Vinyl/co =0°). The dependence of transition energies on CO/CO dihedral angles is similar in shape to that for saturated α‐diketones, but is significantly shifted to lower transition energies. 13 C chemical shifts respond sensitively to CO/CO dihedral angles, and the ab initio calculated curve is supported by a few experimental results.