A Time-Resolved EPR Study of the Electron-Spin-Polarization Pathways of p-Benzosemiquinone

Jäger, Martin; Norris, James R.

doi:10.1006/jmre.2001.2315

Cited by 14 publications

(9 citation statements)

References 24 publications

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“…This oxidation−reduction system is applied to a quinhydrone electrode to measure proton concentration in solutions. A reaction intermediate between hydroquinone and benzoquinone was identified as p -benzosemiquinone radical, as shown in the Scheme . − For example, Trinpathi and Schuler observed a time-resolved resonance Raman spectrum of transient species in the pulse radiolysis of hydroquinone solution and identified the observed spectrum of p -benzosemiquinone radical by a vibrational analysis . On the other hand, Jäger and Norris investigated laser flash photolysis of hydroquinone and benzoquinone by time-resolved electron spin resonance spectroscopy and detected the signals of the semiquinone radical …”

Section: Introductionmentioning

confidence: 99%

UV Photolysis of 1,4-Diaminobenzene in a Low-Temperature Argon Matrix to 2,5-Cyclohexadiene-1,4-diimine via 4-Aminoanilino Radical

2003

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UV photolysis of 1,4-diaminobenzene isolated in a low-temperature argon matrix has been investigated by Fourier transform infrared spectroscopy with the aid of the density-functional-theory calculation. Infrared bands of an intermediate produced from 1,4-diaminobenzene upon UV irradiation (λ < 350 nm) are assigned to a semiquinone-type radical, 4-aminoanilino radical. A final product produced from the 4-aminoanilino radical upon shorter-wavelength irradiation (λ < 310 nm) is assigned as 2,5-cyclohexadiene-1,4-diimine. Optimized structures of 1,4-diaminobenzene, 4-aminoanilino radical, and 2,5-cyclohexadiene-1,4-diimine are compared with one another, resulting in changes of the π-conjugated system similar to those of the hydroquinone and 1,4-benzoquinone system. In addition, trans − cis isomerization of 2,5-cyclohexadiene-1,4-diimine upon UV irradiation is observed in photoequilibrium, where the cis/trans population ratio depends on the irradiation wavelength.

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Section: Introductionmentioning

confidence: 99%

UV Photolysis of 1,4-Diaminobenzene in a Low-Temperature Argon Matrix to 2,5-Cyclohexadiene-1,4-diimine via 4-Aminoanilino Radical

2003

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“…These signals were attributed to two radicals, quoted as s1 (marked by open squares in Figures −3) and s2 (marked by open triangles), which are derived from ethylene glycol. The spectrum of s1 , composed of 12 resolved lines, identifies s1 as being HO−CH 2 −C•H−OH upon comparison of the observed hcc's and the characteristic time decay τ value (see Table ) with those reported in the literature. − The spectrum of the radical s2 is a triplet of doublets. Its hcc's (see Table ) allow us to identify s2 as being H 2 C•OH by comparison with published data. , …”

Section: Resultsmentioning

confidence: 91%

Study of PhotoinducedN-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

et al. 2004

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The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).

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“…While in the low pH region chemical exchange broadened hf lines are obvious, a superposition of the neutral and the anionic semiquinone is detected. A spectrum taken in ethylene glycol exhibits the neutral radical with traces of the anion. ,, On monitoring the time domain at various magnetic fields that correspond to the arrows in the frequency representation, cf. Figure c−e, the transients are found to reflect the effects of different types of magnetization transfer.…”

Section: Resultsmentioning

confidence: 99%

Computation of Time-Resolved EPR Spectra of Systems Exhibiting Electron Spin Polarization Complicated by Magnetization Transfer

Jäger

Norris

2002

J. Phys. Chem. A

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In order that chemically induced dynamic electron polarization (CIDEP) can provide a means to explore dynamics in arbitrary systems such as inclusion compounds or sequential chemical reactions, the Bloch equations are extended for use with a system exhibiting electron spin polarization (ESP). Since CIDEP depends on time and external magnetic field and in this paper is observed by time-resolved electron paramagnetic resonance spectroscopy (TREPR), an algorithm for the general calculation of EPR signals that display ESP is presented. In particular, processes inducing magnetization transfer between hyperfine (hf) states are described by means of a kinetic matrix that is incorporated into the Bloch relaxation matrix. The utilized approach takes into account the complete set of hyperfine states of all involved species and can be applied to such different phenomena as chemical exchange, electron transfer, and secondary radical generation that affect the time dependence of the observed signal. Solutions to the linear differential equations are found numerically. Hence, other than the normal restrictions on the Bloch equations, no additional approximations are assumed, rendering the algorithm more generally applicable than previous analytical treatments. Only chemical diffusion is not included. Specific examples are discussed. The TREPR spectra of p-benzoquinone are analyzed in terms of magnetization transfer. Excellent agreement between the experimental data and the computations using the numerical method is achieved. With rate constants depending on pH and solvent, chemical exchange between the neutral and/or anionic semiquinones is found present in the signals of the radicals generated by laser flash photolysis.

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A Time-Resolved EPR Study of the Electron-Spin-Polarization Pathways of p-Benzosemiquinone

Cited by 14 publications

References 24 publications

UV Photolysis of 1,4-Diaminobenzene in a Low-Temperature Argon Matrix to 2,5-Cyclohexadiene-1,4-diimine via 4-Aminoanilino Radical

UV Photolysis of 1,4-Diaminobenzene in a Low-Temperature Argon Matrix to 2,5-Cyclohexadiene-1,4-diimine via 4-Aminoanilino Radical

Study of PhotoinducedN-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

Computation of Time-Resolved EPR Spectra of Systems Exhibiting Electron Spin Polarization Complicated by Magnetization Transfer

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