Hydroxyl radical halide ions

The Matrix TiOa photocatalytic treatment system is a technology that destroys dissolved organic contaminants in water in a continuous-flow process at ambient temperature. The technology uses ultraviolet (UV) light and a titanium dioxide (TiOa) semiconductor catalyst to break hydroxide ions (OH ) and water (H2O) into hydroxyl radicals (OH ). The radicals oxidize the organic contaminants to form carbon dioxide, water, and halide ions (if the contaminant was halogenated). 

If the assumption of this reaction sequence is correct, the photolysis of tetraphenylphosphonium chloride must then only lead to biphenyl, diphenylphosphine, ethyl diphenyl-phosphinate and triphenylphosphine and its oxidation products. After 2 h of irradiation, biphenyl, diphenylphosphine and its oxidation products, triphenylphosphine and triphenylphosphine oxide, in a ratio of 3 1 5, along with raw material, are obtained. Ethyl diphenylphosphinate was detected in trace amounts7. These results support the postulate of the reversibility of phosphoranyl radical formation in such systems and indicate one-electron transfer processes15 in the formation and decomposition of the tetraarylphosphonium cation. This reaction is comparable to the observation of an electron transfer from halide ions to hydroxyl radicals or hydrogen atoms in aqueous solutions16,... 

Hydroxyl radicals react with many halide (pseudohalide) ions at close to diffusion-controlled rates thereby forming a three-electron-bonded adduct radical [e.g., reaction (1) k = 1.1 x 1010 dm3 mol-1 s 1 Zehavi and Rabani 1972], These adducts may decompose into OH" and the halide (pseudohalide) radical which then complexes with another halide (pseudohalide) ion yielding the dihalogen radical anion [reactions (2) and (3) k2 = 4.2 x 106 s"1 k3 1010 dm3 mol"1 s"1 for resonance Raman spectra of such intermediates, see Tripathi et al. 1985]. 

Bansal KM, Patterson LK, Schuler RH (1972) Production of halide ion in the radiolysis of aqueous solutions of the 5-halouracils. J Phys Chem 76 2386-2392 Barnes JP, Bernhard WA (1994) One-electron-reduced cytosine in acidic glasses conformational states before and after proton transfer. J Phys Chem 98 887-893 Barvian MR, Greenberg MM (1992) Independent generation of the major adduct of hydroxyl radical and thymidine. Examination of intramolecular hydrogen atom transfer in competition with thiol trapping.Tetrahedron Lett 33 6057-6060... 

Hybrid solvation Implicit solvation plus Explicit solvation microsolvation subjected to the continuum method. Here the solute molecule is associated with explicit solvent molecules, usually no more than a few and sometimes as few as one, and with its bound (usually hydrogen-bonded) solvent molecule(s) is subjected to a continuum calculation. Such hybrid calculations have been used in attempts to improve values of solvation free energies in connection with pKp. [42], and also [45] and references therein. Other examples of the use of hybrid solvation are the hydration of the environmentally important hydroxyl radical [52] and of the ubiquitous alkali metal and halide ions [53]. Hybrid solvation has been surveyed in a review oriented toward biomolecular applications [54]. 

In addition, the hydrated electron acts as a nucleophile, especially with organic molecules that contain halogen atoms (Eq. 6-16). This reaction results in rapid elimination of a halide ion from the initially formed negatively charged organic species. The reaction of Eq. 6-16 is of special interest for the degradation of per-halogenated saturated hydrocarbons that are usually not affected by hydroxyl radicals (Sun et al, 2000). 

Throughout the literature, many authors argue for the high reduction potential of hydroxyl radicals being responsible for the oxidative reactions observed in AOPs. However, simple electron transfer reactions such as those of Eq. 6-21 seem to be unlikely because of the large solvent reorganization energy involved in the formation of the hydrated hydroxide ion (Buxton et al., 1988). Instead, in the case of halide ions X or pseudo-halide ions, the formation of intermediate adducts with hydroxyl radicals is observed (Eq. 6-24). 

Srivastava and Ghosh report that the kinetics are first-order with respect to peroxodisulphate and zero-order with respect to formic acid, but Kappana reports first-order kinetics with respect to each reactant. The effect of trace amounts of metal ions and of oxygen on the rate is uncertain, and discussion of the mechanism is of doubtful significance at present. However, the reported observations definitely indicate a chain mechanism. Thus Srivastava and Ghosh found an induction period in the oxidation, and report that halide ions inhibit the reaction (inhibition by halide ions is a feature of reactions involving hydroxyl radicals). In a study of the silver ion-catalysed oxidation, Gupta and Nigam found that the reaction is approximately first-order with respect to both peroxodisulphate and the catalyst, and zero-order with respect to the substrate. 

In some cases when oxidising conditions are required, milder oxidants may be needed, because the hydroxyl radical can react with the solute forming adducts as well as via electron transfer. Hydroxyl radicals can be converted into milder (one-electron) oxidants by the addition of halides, thiocyanate or azide ions (reactions 8.15-8.17). In fact, halide radical reactions occur in atmospheric chemistry, particularly in urban cloud droplets, as well as in marine water radical reactions [29]. 

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