Resonance Raman spectra of p-benzosemiquinone radical and hydroquinone radical cation

Abstract

The time-resolved resonance Raman spectrum of the p-benzosemiquinone radical (HOC/sub 6/H/sub 4/O), 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 nu/sub 7a/ CO stretching mode appears prominently in the spectrum at 1511 cm/sup -1/, showing that the CO bond is very similar to that in phenoxyl where this vibration is observed at 1505 cm/sup -1/. Strongly resonance enhanced bands are also observed at 1613 and 1162 cm/sup -1/ and are assigned respectively to the nu/sub 8a/ ring stretching and nu/sub 9a/ 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 (nu/sub 8a/) to that in p-benzosemiquinone radical anion (1620 cm/sup -1/) indicate that there is a strong interaction of the OH p(..pi..) electron pair with the phenoxyl radical's ..pi..-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 10/sup 8/ M/sup -1/ s/sup -1/. In strongly acidicmore » solutions (e.g., approx. 2.3 M H/sub 2/SO/sub 4/) 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 nu/sub 8a/ ring stretching mode very strongly resonance enhanced and observed at 1644 cm/sup -1/. The nu/sub 7a/ CO stretching mode is observed at 1426 cm/sup -1/ but is very much weaker than in the corresponding spectrum of the radical anion.« less