n →π*‐Übergänge in Dicarbonylverbindungen Der Einfluss der n , n ‐ und π*, π*‐Wechselwirkung

By CNDO‐CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO‐models. The dominant features of the long‐wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases: Large splitting (about 2 eV) both between the n and the π* orbitals. The longest‐wavelength n → π* transition is shifted to the red. The higher transition remains roughly where it would appear in the monocarbonyl compound. No or very small splitting between either the two n , or the two π* orbitals, or both. Both n → π* transitions merge, roughly, in the wavelength region of the corresponding monocarbonyl compound. Intermediate (0.5–1.0 eV) splitting between the two n orbitals on one hand, the two π* orbitals on the other. Spectral properties between A. and B.The splitting between the two n orbitals and the two π* orbitals depends not only on the distance between the two carbonyl groups, but also very strongly on their mutual position and on the nature of the molecular fragment connecting them. As one would expect, in α‐dicarbonyl compounds the splitting between the n orbitals is large, namely 1.8 eV. In some γ‐diketones such as P ‐quinone, one still finds, however, a value of 0.7 eV. The splitting between the n orbitals gets significantly lowered if the two carbonyl groups are coaxial, but their 2 p π nodal planes are perpendicular to each other. In planar β‐diketones the splitting between the π* orbitals is small, but may increase to an effective value of 0.4 eV when they are skewed. Our calculations agree well with measured electronic spectra and CD. data. An electrochromism experiment on camphorquinone convincingly supports our conclusions on α‐dicarbonyl compounds.