In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN3 as competitors to be 1.2 × 1010 dm3 mol-1 s-1 (8.0 × 109 dm3 mol-1 s-1). A transient appears after a short induction period and shows a well-defined absorption spectrum with λmax = 400 nm (ε = 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has λmax = 450 nm (ε = 6560 dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with λmax = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 × 105 dm3 mol-1 for thiourea (7.6 × 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 × 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 × 109 dm3 mol-1 s-1), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 × 107 dm3 mol-1 s-1) to a new intermediate with a broad maximum at 510 nm (ε = 750 dm3 mol-1 cm-1). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 × 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH- with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 × 109 dm3 mol-1 s-1 for thiourea and 3.8 × 109 dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 × 108 dm3 mol-1 s-1 for tetramethylthiourea), and even azide ion (4 × 106 dm3 mol-1 s-1 for thiourea and ∼106 dm3 mol-1 s-1 for tetramethylthiourea). With O2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 × 107 dm3 mol-1 s-1) and reversibly (2 × 103 s-1).

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The peroxone process is one of the AOPs that lead to (•)OH. Hitherto, it has been generally assumed that the (•)OH yield is unity with respect to O3 consumption. Here, experimental data are presented that suggest that it must be near 0.5. The first evidence is derived from competition experiments. The consumption of 4-chlorobenzoic acid (4-CBA), 4-nitrobenzoic acid (4-NBA) and atrazine present in trace amounts (1 μM) has been followed as a function of the O3 concentration in a solution containing H2O2 (1 mM) and tertiary butanol (tBuOH, 0.5 mM) in excess over the trace compounds. With authentic (•)OH generated by γ-radiolysis such a competition can be adequately fitted by known (•)OH rate constants. Fitting the peroxone data, however, the consumption of the trace indicators can only be rationalized if the (•)OH yield is near 0.5 (4-CBA: 0.56, 4-NBA: 0.49, atrazine: 0.6). Additional information for an (•)OH yield much below unity has been obtained by a product analysis of the reactions of tBuOH with (•)OH and dimethyl sulfoxide with (•)OH. The mechanistic interpretation for the low (•)OH yield is as follows (Merényi et al. Environ. Sci. Technol. 2010, 44, 3505-3507). In the reaction of O3 with HO2(-) an adduct (HO5(-)) is formed that decomposes into O3(•-) and HO2(•) in competition with 2 O2 + OH(-). The latter process reduces the free-radical yield.