The resonance Raman spectrum of phenoxyl radical

Abstract

The Raman spectra of phenoxyl, phenoxyl-d5, and phenoxyl-2,4,6-d3 radicals produced pulse radiolytically in aqueous solutions and observed by time resolved resonance Raman methods are reported. Excitation was mainly in the narrow and moderately intense (0,0) phenoxyl absorption band at 400 nm. These Raman spectra are superimposed on a broad fluorescence in the region of 410–440 nm which is also ascribed to the phenoxyl radical. A very intense Raman band, which is assigned to a mode principally involving the CO stretch (Wilson 7a), is observed at 1505 cm−1. In the fully and partially deuterated radicals this band is at 1489 and 1487 cm−1, respectively, indicating that in the latter instance the vibration is shifted toward a lower frequency, probably by Fermi resonance with an underlying weak vibration. In the protonated radical moderately intense bands are also observed at 990 and 528 cm−1. The higher of these, by virtue of its considerably lower frequency in the deuterated radicals, is assigned to a CH bending mode (Wilson 18a). The other is affected very little by the substitution and is assigned to a CCC bending mode (Wilson 6a). Weak bands at 1157, 1056, and 865 cm−1 in phenoxyl, phenoxyl-2,4,6-d3, and phenoxyl-d5 radicals are assigned to a second CH bending mode (Wilson 9a). Two weak bands are also reported at 1398 and 1331 cm−1 and are attributed to the nontotally symmetric 19b and 14 modes. No Raman band is, however, observed in the 1550–1650 cm−1 region where the Wilson 8a ring stretching motion prominent in semiquinone radicals is expected. The excitation profile of the 1505 cm−1 emission largely follows the narrow absorption spectrum of the radical in the region of 400 nm. Below 390 nm, where the electronic excitation in resonance is to an upper vibrational level, resonance enhancement of the Raman signals is reduced by more than an order of magnitude. The decay of the radical at 10−4 M, as monitored by its Raman signal, corresponds to its loss mainly in second order processes having a rate constant of (2.6±0.3)×109 M−1 s−1, in agreement with measurements by absorption methods at a 50-fold lower concentration.