LCAO-MO calculations on verdazyls

Abstract

Simple Hückel MO theory satisfactorily explains several experimental results, amongst them the near equivalence of the four nitrogen atoms, whose hyperfine coupling constants are identical at least within the resolution of the spectra, as determined by the linewidths. The biradicals III and IV and the triradical V have a node in the odd electron orbitals at the aryl system connecting the radical parts and should consequently exhibit little intramolecular spin exchange. The same result can be derived from polarographic experiments and the broadening of lines in the ESR spectra can be attributed mainly to dipol-dipol interaction, not electron exchange. In contrast to the radicals III to V, in which the unpaired electrons occupy energetically degenerate orbitals, the radical electrons in VI occupy closely neighboring levels. The temperature-dependent paramagnetism of this substance results from the temperature-dependent population of these two levels. The calculated bond orders and charge densities show a delocalization of the remaining π-electrons over the whole molecule. Optical spectra cannot be interpreted within the HMO theory.