Geometry, electronic structure and optical spectrum of azocompounds

1 GEOMETRY, ELECTRONIC STRUCTURE AND OPTICAL SPECTRUM OF AZOCOMPOUNDS 

Structural parameters for some azocompounds reported in the literature from electron or X-ray diffraction studies are collected in Table 1. Trans- and cis-NjFj have planar structures and the carbon-nitrogen skeleton of azomethane is also planar. In N2F2 the cis isomer is 3 kcal.mole more stable than the trans and in N2H2 the cis isomer is probably also the more stable . In alkyl or aryl substituted derivatives, however, for steric reasons the trans form lies below the cis in energy. The molecular structure of Irons-azobenzene is known to be planar in the solid state while c/j-azobenzene deviates from planarity and a propeller shaped conformation has been proposed, wherein the phenyl rings are rotated approximately 30° out of plane . 

MO calculations could only be carried out on the diimide molecule and substituent effects which become evident in the higher energy transitions of the spectrum cannot be accounted for. The lowest energy transitions where most of the photochemical studies of azoalkanes have been carried out, are of the ( +, n ) type. 

---