ELECTRONIC ABSORPTION-SPECTRA OF SOME ALKOXYL RADICALS - AN EXPERIMENTAL AND THEORETICAL-STUDY

The visible and UV absorptions of a variety of alkoxyl radicals have been examined by experiment and theory. In most solvents, the tert-butoxyl radical shows only a weak ''tail-end'' absorption in the UV region of the spectrum and no absorption in the visible region. However, a visible absorption band has been observed (lambda(max) = 440 nm) in the presence of 1,3,5-trimethoxybenzene. This absorption band provides the first unequivocal evidence for the existence of alkoxyl radical/electron-rich aromatic pi-complexes. By way of contrast, a solvent-independent absorption in the visible is observed for (arylcarbinyl)oxyl radicals, e.g., benzyloxyl (lambda(max) = 460 nm, epsilon = 1300 M(-1) cm(-1)) and cumyloxyl (lambda(max) = 485 nm, epsilon = 1300 M(-1) cm(-1)). Electron-donating substituents on the aromatic ring shift the visible absorption to the red by 25-30 nm for a 4-methyl group and by 100-105 nm for a 4-methoxy group. However, alpha-substitution has only a minor effect on lambda(max), e.g., 530 and 535 nm for diphenylmethoxyl and 1,1-diphenylethoxyl, respectively. The visible absorption appears to be virtually uninfluenced by the (calculated) magnitude of the dihedral angle, theta, between the C-O-. bond and the plane of the aromatic ring, e.g., indan-1-oxyl, theta - 51 degrees, lambda(max) = 530 nm, and Tetralin-1-oxyl, theta similar to 33 degrees (major conformer), lambda(max) = 520 nm, vs benzyloxyl and cumyloxyl, both of which have theta similar to 0 degrees. All of these experimental observations are well-matched by theory, which reveals that the visible absorptions of (arylcarbinyl)oxyl radicals are due to D-0(A '')-->D-2(A '') (pi-->pi*) transitions which increase the negative charge on the oxygen atom and have respectable oscillator strengths. The unpaired electron is largely localized in the oxygen 2p orbital which is perpendicular to the plane of the aromatic ring.