Calculation of the Absorption Spectrum of Wurster's Salts

The so-called Wurster's salts, radicals which occur in the oxidation of the paraphenylene diamines, show a strong absorption in the visible. The frequency and the intensity were calculated by the method of completely antisymmetrized products of molecular orbitals, with the introduction of but one empirical constant, namely, the difference between the ionization potential of the non-bonding nitrogen electron in ammonia, and the ionization potential of benzene. The results are in good agreement with the observed absorption band. The calculated intensity corresponds to an electron number of about 0.3, the observed one to about 0.1. The shift of the band to longer wave-length, and the intensification upon substitution of C${\mathrm{H}}_3$ groups on the nitrogen is explained qualitatively.