Structure of the p-aminophenoxyl radical

Abstract

Experimental results cited here clearly indicate that the electronic and vibrational structures of p-aminophenoxyl radical are close to those of the isoelectronic p-benzosemiquinone radical anion and p-phenylenediamine radical cation. Recently published ab initio calculations by Raymond and Wheeler (J. Chem. Soc., Faraday Trans., 1993, 89, 665), which predict a benzene-like structure for the p-aminophenoxyl radical, grossly underestimate the Raman frequencies of the Wilson 8a (ring stretch) and 7a (CO/CN stretch) modes. In particular, the latter, which is observed at 1434 cm^–1, demonstrates that the p-aminophenoxyl radical has a semiquinone-like structure. The referenced calculation also places the unpaired spin population almost entirely on the oxygen atom. This conclusion is inconsistent with the spin distribution exhibited in the observed EPR hyperfine constants which demonstrate quite conclusively that there is a significant fraction of the unpaired spin on the ring and also on the amino nitrogen atom. The delocalization of the unpaired spin manifest in both the Raman and EPR spectra is also reflected in the reduction potential of the radical (0.21 V vs. NHE) which is lower than that of bona fide phenoxyl radicals by ca. 0.6 V. It is clear that ab initio calculations at the level used by Raymond and Wheeler do not give a realistic picture of the electronic structure of p-aminophenoxyl-like radicals.