A molecular orbital and ESR study of nitro-substituted anion [pi]-radicals

Abstract

The work in this dissertation was undertaken to study intramolecular and intermolecular rate processes in nitro substituted pi-anion radicals, and to improve and test a semiempirical quantum mechanical theory. Spin densities and ESR hyperfine coupling constants of six nitro-substituted radicals were calculated using Huckel molecular orbital theory. This simple theory yielded a good correlation between the calculated and experimental values. However, it was found that the correlation was poorest for meta-substituted radicals and radicals with strong electron-releasing para substituents. A modified form of the w'-technique has been developed that is convergent for all radicals tested. The self-consistent spin densities are in very good agreement with the experimental values. Visible transition energies calculated from the self-consistent eigenvalues are found to be in good agreement with visible spectra of the radicals. The ESR spectra of the 2-trifluoromethylnitrobenzene and 3-trifluoromethyl-4-nitrophenol anion radicals have been found to exhibit extreme linewidth alternation. This result has been successfully explained in terms of the Fraenkel theory of linewidths. Solvent and temperature variations have been found to have a significant effect on the spectral linewidths. A specific model is proposed to explain these observations. Spin exchange has been studied in nitro-substituted [2.2] paracyclophane and biphenyl anion radicals. The ESR spectra of the anion radical of 4,16-dinitro[2.2]paracyclophane indicate that spin exchange is more rapid than the hyperfine intervals. In the 4-nitro 2.2 paracyclophane anion. radical spin exchange is slow (~10^6 cps). Electrolysis of 2-trifluoromethylnitrobenzene and 2,2' -dinitrobiphenyl at potentials greater than their second d.c. polarographic E 1/2 produces paramagnetic species yielding ESR spectra not consistent with the structure of the original molecule. Certain aspects of the reduction process have been discussed and general mechanisms have been postulated.