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.

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The time-resolved resonance Raman spectrum of the p-benzosemiquinone radical (H0C6H40•), excited at 415 nm and observed on the microsecond time scale in the pulse radiolysis of moderately acidic solutions of hydroquinone (pH 2-4), is reported. The y7a CO stretching mode appears prominently in the spectrum at 1511 cm"1, showing that the CO bond is very similar to that in phenoxyl where this vibration is observed at 1505 cm"1. Strongly resonance enhanced bands are also observed at 1613 and 1162 cm"1 and are assigned respectively to the v8a ring stretching and 1% CH bending modes. The strong enhancement of these latter vibrations, which are very much weaker in the corresponding spectrum of the phenoxyl radical, and the similarity of the ring stretching frequency (p8a) to that in p-benzosemiquinone radical anion (1620 cm"1) indicate that there is a strong interaction of the OH p (it) electron pair with the phenoxyl radical's ir-electron system. The coefficient for the acid-catalyzed dehydroxylation of the OH adduct of hydroquinone is estimated from the pH and time dependences of the Raman signals of the semiquinone to be 5.4 X 10s M"1 s"1. In strongly acidic solutions (e.g., ~2.3 M H2S04) a second transient species, identified as the hydroquinone radical cation, is observed. The vibrational features of this radical are qualitatively similar to those for p-benzosemiquinone radical anion with the v8a ring stretching mode very strongly resonance enhanced and observed at 1644 cm"1. The v7a CO stretching mode is observed at 1426 cm*1 but is very much weaker than in the corresponding spectrum of the radical anion. The similarity in the vibrational frequencies of these two radicals shows that protonation of the oxygen atoms has relatively little effect on the electronic structure of he semiquinone system.

The resonance Raman spectra of para-substituted phenoxyl radicals (XC6H40•; X = CH3, F, Cl, Br, OCH3, OH), observed by time-resolved techniques in aqueous medium, exhibit a wide variation in spectral features intermediate between phenoxyl and p-benzosemiquinone anion radicals. The v7a (CO stretch) vibration, which is strongly enhanced on Raman excitation in resonance with the electronic transition in the ~400-nm region, appears in a narrow frequency range 1511-1518 cm"1, indicating that the CO bond in the ground electronic state of these radicals is very similar to that of phenoxyl (v7a at 1505 cm"1). The relative intensities of the t>8a bands (CC stretch), observed in the 1552-1613-cm"1 region, change dramatically with the electronic properties of the substituent group. This vibration, which is not apparent in the Raman spectrum of phenoxyl excited at 400 nm, is observed with an intensity comparable to that of the c7a vibration in the p-bromo, p-methoxy, and p-hydroxy derivatives. The Raman intensities show that the electronic structures in the excited states of the p-methyl-, -fluoro-, and p-chloro-substituted radicals are essentially phenoxyl like, while the structures in p-methoxy and p-hydroxy derivatives approach that of p-benzosemiquinone anion radical as a result of strong interaction of the substituent's pir electrons with the phenoxyl it system. The excited state of p-bromophenoxyl radical represents an important intermediate case. The resonance enhancement of the y9a CH bending vibration, observed at ~1160 cm"1, parallels that of the v8a phenyl mode and provides an important diagnostic for assignment of the latter vibration. 1398 vw 1407 vw 1412 w 1396 w 1396 w 1406 w 1432 w 13 CX stretch j 1224 w 1255 w 670 w 613 w j j 9a CH bend 1157 vw 1161 w 1140 w 1163 w-m 1166 m 1163 m-s 1162 m 1/12 ring breath/ CCC bend 1050 vw 1052 vw 1053 vw 1083 vw 1055 m-w j 1039 vw 18a CH bend 990 m 980 w-m 977 w-m 978 w-m 980 m, br 970 w-m 973 w 12/1 CCC bend/ ring breath 840 vw 817 vw