Homolysis of water

Unlike the H2O2-UV process the VUV oxidation of organic substrates in water (H2O-VUV) is initiated by the photochemical homolysis of water producing a series of reactive species (cf Fig.s 5-9 and 7-3, and Equations 5-18 to 5-20). In many cases water itself is the prominent absorber of VUV radiation at 172 nm (cf Ex-... 

Knowing this, the results of Hterature studies are explicable which found only very hmited elimination of MTBE from any water type by pure UV irradiation [28,33,67,70,76,89-93]. Mostly, the sHght reduction in concentration observed (between 5 and 10%) coiild be attributed to the formation of radicals either by the homolysis of water or from other water ingredients. Mofidi et al. [67] exphcitly stated that UV irradiation alone is not effective in removing MTBE. 

Pliotolytic. Bussacchini et al. (1985) studied the photolysis (7, = 254 nm) of phenmedipham in ethanol, ethanol/water, and hexane as solvents. In their proposed free radical mechanism, homolysis of the carbon-oxygen bond of the carbamate linkage gave the following photoproducts 3-(hydroxylphenyl)carbamic acid methyl ester, m-toluidine, 2-hydroxy-4-aminomethyl benzoate, 3-hydroxy-5-aminomethyl benzoate, 2-amino-4-hydroxymethyl benzoate, and 2-amino-6-hydroxymethyl benzoate. 

Merenyi G, Lind J, Goldstein S, Czapski G (1999) Mechanism and thermochemistry of peroxynitrite decomposition in water. J Phys Chem A 103 5685-5691 Merenyi G, Lind J, Goldstein S (2002a) Thermochemical properties of a-hydroxy-alkoxyl radicals in aqueous solution. J Phys Chem A 106 11127-11129 Merenyi G, Lind J, Goldstein S (2002b) The rate of homolysis of adducts of peroxynitrite to the C=0 double bond. J Am Chem Soc 124 40-48... 

Photolysis of water leads to the splitting of water into uncharged parts with unpaired electrons (formation of radicals through homolytic splitting (homolysis) see also dissociation, note 339) ... 

The photolysis of Co (III) alkyls has attracted considerable attention (59), because homolysis of such a bond may well be implicated in enzymatic isomerization reactions catalyzed by vitamin B12 coenzyme (see Refs. 27, 98,115a, 193). Early evidence was based on characterization by UV or ESR of the d1 Co(II) corrinoid. However, recently the organic fragment has been spin-trapped and identified by ESR for the case of (i) the coenzyme (5 -deoxyadenosylcobalamin) (10-3 M) with Bu NO (10-2 M) and photolysis in water at 50° to yield Ar(5 -deoxyadenosyl)XO (XXI)... 

Direct photolysis of 2-halopyridines in methanol, ethanol or acetonitrile (with 1% water) affords pyridine, together with alkoxy-, or hydroxypyridines376. The formation of the substitution products will be discussed in Section IV.C. The reductive dehalogenation is considered to proceed via homolysis of the carbon-halogen bond. The efficiency of pyridine formation decreases in the order Br > Cl > I. The 3- and 4-halopyridines produce pyridine exclusively. 

An investigation of the spectral and kinetic characteristics of radical cations of para-halogeno-A,A-dimethylanilines and / ara-halogenodiphenylamines in water/terr-butyl alcohol has been performed in view of the fact that such species are probable intermediates in photonucleophilic substitution of halogen638. Bathochromic shifts in the absorption maxima of the radical cations were observed in the order H=F < Cl < Br. The disappearance of the radical cations obeys second-order kinetics and the rate constants are close to the diffusion-controlled values. The radical cations arise from the singlet excited states of the halogenoarylamines and homolysis of the carbon-halogen bond competes with their formation. 

It is possible to use a direct photolytic process for the treatment of waters and effluents, without the addition of chemical reagents. It is worthwhile to bear in mind that, for example, a 254-nm photon is equivalent to 4.89 eV, enough energy to produce homolytic or heterolytic breakages in the molecules. Direct irradiation leads to the promotion of a molecule from the fundamental state to an excited singlet state, which may then intersystem cross to produce triplets. Such excited states can undergo homolysis, heterolysis or photoionization, among other processes. In most cases, homolytic rupture produces radicals ... 

The bromide ion apparently has trapped the excimer. One might consider homolysis of the C-Cl bond in the triplet, followed by electron transfer from the aryl radical to chlorine to generate aryl cation which then reacts with bromide ion, using as a model the mechanism put forward to explain the phenol products from the photolysis of 4-chlorobiphenyl in water [25]. However, this mechanism seems unlikely, since the excited chloroarene is in the micellar interior and such a mechanism has not been observed in hydrocarbon solvents. In addition, this... 

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