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

Abstract

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.

A difference Fourier-transform infrared study of two redox-active tyrosine residues in photosystem II.

“…Upon perdeuteration, the C-O stretching mode of the phenoxyl radical in vitro downshifted 16 cm-' (29). There has been no comparable study of v8a for the phenoxyl radical, but the v8a mode of the p-benzosemiquinone radical shifted 25 cm-' upon perdeuteration (32). There is little existing information about the effects of 13C labeling of phenoxyl radicals in vitro.…”

Radiation Chemistry in Chemically Amplified Resists

“…159,160) Radical cations of phenol derivatives, which are precursors of phenoxy radicals in many cases, had been more elusive than phenoxy radicals. 158,[161][162][163][164][165][166][167][168] The absorption maxima of radical cations of phenol and p-cresol are at 440 and 430 nm, respectively. 169) The radiation chemistry of the model compounds for protected PHS such as methoxybenzene (anisole), methoxytoluene, and dimethoxybenzene has also been intensively investigated using pulse radiolysis, [170][171][172][173] photolysis, 174,175) chemical oxidation, [176][177][178] and electrochemical methods.…”

The free radical site in pea seedling copper amine oxidase probed by resonance Raman spectroscopy and generated by photolysis of caged substrate

“…The resonance Raman spectrum of the radical in PSAO is strikingly similar to those of semiquinone radicals (reviewed by Tripathi [21]). It may be interpreted by analogy with resonance Raman spectra ofpara-substituted phenoxyl [22][23][24][25], including p-aminophenoxyl and benzosemiquinone anion, and metasemiquinone radicals [26] in addition to radicals derived from one-electron oxidation of 1,2,4-benzenetriol [27]. The strong band at 1647 cm -~ is clearly the Wilson Vsa C-C stretch which is strongly enhanced in phenoxyl radicals having para-substituents, such as -OMe, NH 2 and -OH, with p~ electrons interacting with the phenoxyl radical g-electron system.…”

“…The strong band at 1647 cm -~ is clearly the Wilson Vsa C-C stretch which is strongly enhanced in phenoxyl radicals having para-substituents, such as -OMe, NH 2 and -OH, with p~ electrons interacting with the phenoxyl radical g-electron system. In such radicals, the vs, vibration is observed at relatively high frequency, such as 1620 cm -~ in the p-benzosemiquinone anion [25] and 1636 cm -~ in the p-aminophenoxyl radical [23], compared with 1552 cm -~ in the phenoxyl radical [28]. The resonance Raman spectrum of the m-benzosemiquinone radical anion shows a very weak band at 1570 cm ' assigned to vs~ and two equally intense bands at 1093 and 1519 cm-L The latter are both assigned to C O stretching vibrations and indicate the C-O groups are non-equivalent.…”