Browsing by Author "Land, E. J."

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    Pulse Radiolysis of 9,10-Anthraquinones: Part 1.—Radicals
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (10), 1972) Hulme, B. E.; Land, E. J.; Phillips, G. O.
    Radicals derived from 1- and 2-substituted 9,10-anthraquinones have been studied using pulse radiolysis. In aqueous solution, both sodium anthraquinone-1- and -2-sulphonatcs react rapidly with hydrated electrons, and more slowly with formate radicals, to form anthrasemiquinonc radical ions (A*~). The absorption spectra and extinction coefficients of both A*- radical ions were very similar; the corresponding protonated semiquinones (AH-) had similar spectral properties also. No significant difference was found in the reactivities of both A*- radicals towards oxygen, but the pK values for the equilibria A-_ + H+^AH- differed significantly. OH radicals react rapidly with the 1- and 2-sulphonate to form transient products which have similar spectral properties. Kinetic salt effect studies indicate that such transient products are OH adducts rather than semi-oxidized anthraquinones. The rates of reduction of 1- and 2-piperidinoanthraquinones in alcoholic solution by solvated electrons, or by CH2O~ radicals, to form the corresponding semiquinones, arc similar. Reduction by CH2OH in methanol acidified with 10~2 M H2SO4 is much slower for the 1-piperidino derivative than for the 2- derivative, due to differences in the states of protonation of the two quinones. The absorption spectra were found for both unprotonated and protonated forms of these scmiquinoncs, and the corresponding species derived from 1- and 2-aminoanthraquinone.
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    Pulse Radiolysis of 9,10-Anthraquinones: Part 2.—Triplet Excited States
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (10), 1972) Hulme, B. E.; Land, E. J.; Phillips, G. O.
    The triplet excited states of anthraquinone, 1- and 2-piperidinoanthraquinone and 1- and 2- aminoanthraquinonc have been studied in benzene using pulse radiolysis. The triplet-triplet absorption spectra and extinction coefficients were characterized via energy-transfer techniques for all but the 1-piperidino derivative. The lowest triplet energy levels of the piperidino- and aminoanthraquinones were similarly located. The lifetime of anthraquinone triplet in benzene is short (r = 180 ns), and it reacts with oxygen and propan-2-ol with rate constants of 1.5 x 109 and 2.1 x 107 M-1 s-1, respectively. No reaction of the triplet with its own ground state could be detected. The triplet states of 2-piperidino-, 1-amino- and 2-amino-anthraquinone are much longer-lived in benzene than anthraquinone. 2-Piperidinoanthraquinone triplet reacts with oxygen with a rate constant of 1.9 x 109 M_1 s-1. No reaction of 2-piperidinoanthraquinone triplet with its own ground state or with propan-2-ol could be detected. The different properties of the piperidino- and amino-anthraquinone triplets, in comparison with anthraquinone triplet itself, are considered to arise because their lowest triplet levels are charge-transfer in character, rather than /nr*.