- Photoreduction of p-Benzoquinones: Effects of Alcohols and Amines on the Intermediates and Reactivities in Solution
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The photochemistry of 1,4-benzoquinone (BQ) and alkyl-, Cl- and related derivatives, e.g. methyl-, 2,6-dimethyl-, chloro-, 2,5-dichloro-1,4-benzoquinone, duroquinone and chloranil, was studied in nonaqueous solvents by UV-vis spectroscopy using nanosecond laser pulses at 308 nm. The reactivity of the triplet state (3Q*) of the quinones with 2-propanol in the absence of water is largest for BQ and depends mainly on the quinone structure, whereas the rate constant of electron transfer from amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane, is close to the diffusion-controlled limit for BQ and most derivatives. Photoinduced charge separation after electron transfer from amines to 3Q* and the subsequent charge recombination or neutralization are supported by time-resolved conductivity measurements. The half-life of the decay kinetics of the semiquinone radical (.QH/Q.-) depends significantly on the donor and the medium. The photoconversion into the hydroquinones was measured under various conditions, the quantum yield, λirr = 254 nm, increases with increasing 2-propanol and TEA concentrations. The effects of quenching of 3Q*, the .QH/Q.- radicals and the photoconversion are outlined. The mechanisms of photoreduction of quinones in acetonitrile by 2-propanol are compared with those by TEA in benzene and acetonitrile, and the specific properties of substitution are discussed.
- Goerner, Helmut
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p. 440 - 448
(2007/10/03)
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- Triplet Mechanism of CIDNP in the Photolysis of Benzoquinones As Studied by Dynamic Nuclear Polarization
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CIDNP and CIDNP-detected ESR spectra are observed in the photolysis of benzoquinone and its methyl derivatives.In low concentrations of solutions, the sign of the CIDNP of the ring and methyl protons of benzoquinones is in accord with the prediction from
- Meng, Qing-Xiang,Suzuki, Kouei,Maeda, Kiminori,Terazima, Masahide,Azumi, Tohru
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p. 1265 - 1269
(2007/10/02)
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- DNP Spectrum Observed in the Photolysis of Benzoquinone and Some of Its Methyl Derivatives. The Triplet Mechanism in CIDNP
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CIDNP and dynamic nuclear polarization (DNP) are observed in the photolysis of benzoquinone and its methyl derivatives.In low concentrations of solutions, the signs of the CIDNP of the ring proton of benzoquinone and methyl protons of methyl derivatives o
- Meng, Q.,Yamakage, Y.,Maeda, K.,Azumi, T.
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- Photochemical Dediazoniation of Arene Diazonium Salts Catalyzed by Quinones
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The rate constants of the hydrogen abstraction of triplet-excited p-benzoquinone (BQ), duroquinone (DQ), p-chlorophenyl-p-benzoquinone (PQ) and 2,5-di(p-chlorophenyl)-p-benzoquinone (P2Q) from n-heptane and acetonitrile are determined by laser flash photolysis as well as the rate constants for the reactions with oxygen. The semiquinones formed are in equilibrium with their respective semiquinone anions; at pH 3 and pH 7, respectively, it was possible in acetonitrile/water solution to determine separately the rate constants for their disproportionation affording quinones and hydroquinones and the rate constants for electron transfer to p-chlorbenzene diazonium tetrafluoroborate.It comes out that the electron transfer to the diazonium compound only competes favorably with the disproportionation in the case of the semiquinone anions.
- Becker, H. G. O.,Jirkovsky, J.,Fojtik, A.,Kleinschmidt, J.
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p. 505 - 511
(2007/10/02)
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