Carbonate radical (CO3*-) is a powerful oxidant that is present in sunlit surface waters and in waters treated by advanced oxidation processes. The production of CO3*- in aqueous solution through oxidation of carbonate anion by excited triplet states of aromatic ketones was investigated in this study to provide new methods for the determination of rate constants and to explore a possible photoinduced pathway of CO3*- formation in the aquatic environment. Rate constants for triplet quenching by carbonate anion of up to 3.0 x 10(7) M(-1) s(-1) and CO3*- yields approaching unity, determined using laser flash photolysis, allowed us to conclude that such a formation mechanism might be significant in sulit natural waters. Kinetic methods based on either flash photolysis or steady-state irradiation and on the use of aromatic ketones as photosensitizers gave bimolecular rate constants in the range of 4 x 10(6) to 1 x 10(8) M(-1) s(-1) for the reaction of CO3*- with several s-triazine and phenylurea herbicides. For various anilines and phenoxide anions, rate constants determined by these methods agreed well with published values. Moreover, it could be shown for the first time by a direct method that dissolved natural organic matter (DOM) reduces the lifetime of CO3*- and a second-order rate constant of (280 +/- 90) (mg of C/L)(-1) s(-1) was obtained for Suwannee River fulvic acid.
p-Hydroxyphenacyl is an effective photoremovable protecting group, not least due to the fast release of its substrates, accompanied by a photo-Favorskii rearrangement of compounds 1 to p-hydroxyphenylacetic acid (2) that is transparent down to 300 nm. First used for the release of ATP from 1 (X = ATP) a decade ago, 1 the reaction has been employed in a variety of fields as diverse as neurobiology, 2 enzyme catalysis, 3 and biochemistry. 4 The nature and timing of the bond-making and bond-breaking events have not been fully elucidated despite extensive experimental and theoretical efforts by our group 5 and others. 6,7 We now report observation of the primary photoproduct, the triplet biradical 3 3, and of a new side product, p-hydroxybenzyl alcohol (6), that is formed by decarbonylation of the putative spirodione intermediate 4 at moderate water concentrations (Scheme 1). Solvent kinetic isotope effect (SKIE) studies by nanosecond laser flash photolysis (LFP) provide significant information on the role of water in the photo-Favorskii rearrangement of p-hydroxyphenacyl diethyl phosphate 1a to p-hydroxyphenylacetic acid (2). Anderson and Reese first reported the intriguing photoreaction 1(X = Cl) → 2 + HCl and suggested that the skeletal rearrangement may proceed via a spirodione intermediate 4, 8 which has yet to be detected. Intersystem crossing (ISC) of diethyl phosphate 1a is very fast, k ISC = 4 × 10 11 s −1 , 5 and we have established that the rearrangement proceeds from the triplet state, T 1. 1,4b,5 This was confirmed by Phillips et al. 7a-c Hydroxylic solvents play a major role in the rearrangement. The lifetime of T 1 decreases from several μs in degassed, dry CH 3 CN to about 0.4 ns in aqueous CH 3 CN (50% by vol). 5,7c,d The lifetime of T 1 of 1a is further reduced to 100 ± 10 ps in 87% aqueous CH 3 CN (Figure 1) and to 63 ± 10 ps in wholly aqueous solution. This was the key to revealing that the decay of T 1 left weak absorptions at 445, 420, and 330 nm, which decayed with a somewhat longer lifetime of ca. 0.6 ns. Pump-probe spectra obtained with other derivatives of 1 with good leaving groups (1b,c: X = tosylate, mesylate) also displayed the transient species possessing
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