Photochemical Oxidation of Water by 2-Methyl-1,4-benzoquinone:  Evidence against the Formation of Free Hydroxyl Radical

Abstract

Photolysis of 2-methyl-1,4-benzoquinone (toluquinone) in aqueous solution results in the oxidation of water to create either hydroxyl radical or some species capable of transferring a hydroxyl radical. Trapping of the latter with dimethyl sulfoxide (DMSO) creates a methyl radical which in turn can be trapped by the stable radical 3-amino-2,2,5,5-tetramethyl-1-pyrrolidinyloxy. Competitive trapping studies using DMSO and either nitrite anion or salicylate anion show that the hydroxylating species is much more selective in its reactions than free hydroxyl radical. Laser flash photolysis experiments on toluquinone in aqueous solution show formation of a transient species immediately (<150 ns) following the excitation pulse that had previously been assigned to the excited triplet state of the quinone. This spectrum differs from the authentic triplet state spectrum generated in less reactive organic solvents (carbon tetrachloride and acetonitrile). The same intermediate is shown to react with the hydroxyl radical traps, cupric ions, and benzoate anion to yield the semiquinone radical. On the basis of these experiments it is argued that this transient species is a hydroxylating intermediate, probably best described as a complex between the semiquinone radical and the hydroxyl radical. It is further argued that this species is responsible for the hydroxyl radical trapping reactions.

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

“…[5] In water and in the absence of hydrogen donors, however, Q is efficiently photohydroxylated (quantum yield of photodecomposition F = 0.31, [19] F = 0.5, [37] ), and as resulting products 2-hydroxy-1,4-benzoquinone, HOQ, and hydroquinone, QH 2 , are formed in equal amounts (F = 0.25). [37] This type of reaction is also given by other quinones, for example, anthraquinonesulfonates, [21, 52±59] 2-methyl-1,4-benzoquinone, [60] 2-methoxy-1,4-benzoquinone [47] and 2,6-dimethylbenzoquinone. [61] Photohydroxylation is suppressed by the addition of halide ions.…”

“…[4,54,55] In a recent study, the formation of a free COH has been rejected, and instead a kind of ™krypto COH∫ (COH complexed to CQH) has been favoured. [60] If COH were formed their addition to Q would indeed result in the formation of the observed products, HOQ and QH 2 {reactions (5)± (7) and (3); for details of the complex kinetics see ref. [71]}.…”

“…[32] In this study, [32] where the UV absorption spectrum of the triplet was given in the range between 350 and 500 nm, the transient displayed a maximum at 410 nm, but the much stronger one at 282 nm (cf. Figure 5) had been missed, and also in the more recent study on 2-methyl-1,4-benzoquinone, [60] no attempt has been made to measure the triplet spectrum below 330 nm. The shapes of the short-wavelength absorption band when put on an energy scale shows a good Gaussian distribution, and based on this analysis its maximum is located at 282 nm.…”

“…This is in contrast to a recent study on the photolysis of 2-methyl-1,4-benzoquinone in aqueous DMSO solutions. [60] There, the formation of methyl radicals was concluded based on fluorescent material formed in the presence of a nitroxyl radical trap. [60] Our more stringent data suggest that DMSO acts overwhelmingly as a physical quencher.…”

Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

“…The concentrations of the stock solutions in our experiments were typically 1×10 -3 mol dm -3 . 1,4-Benzoquinones are photosensitive compounds in solution, 7,8,[31][32][33][34][35][36][37] therefore fresh solutions were prepared for all experiments in brown glass volumetric flask.…”