Untersuchung von einfachen π-Elektronensystemen mit Hilfe von Elektronenspin-Resonanz und Polarographi

Abstract

In order to test the results of the MO-theories of HÜCKEL and MCLACHLAN, spin densities and energies of unpaired electrons of 10 aromatic hydrocarbon radicals have been measured by EPR and polarography. Eight negative ion radicals have been prepared electrolytically in solvents of high dielectric constant. This technique is found to yield “free” solvated radical ions with spin densities and π-orbital energies not noticeably affected by the medium. By eliminating solvent effects HÜCKEL values of the COULOMB and resonance integrals α₀ and β₀ of the free ions could be measured. They turn out to depend strongly on the sign of the excess charge. The measurements of the splitting constants indicate that the COLPA-BOLTON equation possibly overestimates charge effects on the σ-system at the expense of influences from non-neighbouring 2p_z-AO’s. In the case of alternant radical ions the HÜCKEL MO’s are better than those derived by MCLACHLAN’S theory, though qualitatively McLACHLAN’S theory predicts negative spin densities correctly. The g-factor measurements, which are accurate within 2 ppm, confirm the linear relation between Δg and the HÜCKEL coefficient m₀ of the lowest halfoccupied π-orbital recently deduced by STONE. On the other hand neutral odd-AH-radicals have g-factors about 4 · 10^-5 smaller than the theoretical values for m₀=0. A linear dependence has been found between Δg and the half-wave potential U_½ which provides the possibility to measure g-factors with an accuracy of about 10 ppm by measuring U_½-values. This method enables one to measure g-factors even for those AH-radical ions which could not yet be detected by EPR.