Photolysis, radical formation

The reaction of the peroxyl radical with the sulfonyl radical was studied by pulse photolysis technique [38]. Both radicals were generated photochemically by a light pulse (A = 270 -380 nm) in the system DBP-cyc/o-CgHnSCkCI ryc/o-CgH lRH) air (T 293 K). The reactions of free radical formation were the following ... 

It should be noted that at the excitation wavelength employed, the absorbance of MDI-PUE is 1.1 while the absorbance of the solvent THF is 0.3. This is a condition dictated by polymer solubility considerations and choice of excitation wavelength. We are confident that the spectral results for the photolysis at 248 nm are derived from radicals generated by direct excitation, as opposed to radical abstraction by solvent radicals, since the kinetic curves indicate no delay in radical formation of the transients. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

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,... 

The degradation can be photochemically induced (a) homolytic or (b) heterolytic cleavage at the weaker bonds. The photolysis of the type (a) may lead to elimination reactions and the type (b) may lead to free radical formation. The point of bond cleavage may not be the seat for light absorption. The energy can migrate from unit to unit until it finds itself at the seat of reaction. 

From pulse radiolysis lifetimes of phenol radical cations between 300 and 500 ns are known [4, 9]. Laser photolysis (3 ns, 266nm up to 15 mJ) ofN2-purged solutions of up to 10 3 mol dm 3 phenols yields phenoxyl radicals as dominating products (Figure 2). In the spectrum only a little hint for the phenol radical cations exists. The inset shows that the phenoxyl radical formation does not depend linearly from the energy but appears by biphotonic absorption contradictory to the fs-experiments described above. 

Using the polymerized viologen bilayers, the viologen units were removed from the outer surface to which they were attached by ester linkages by reaction with imino ethanol. Photolysis with [Ru(bipy)3]2+ in the bulk phase then led to viologen cation radical formation on the inner surface. Presumably the iminoethanol groups acted as electron donors.342... 

Glaze et al. (1995) proposed a kinetic model for the UV/H202 oxidation using radical and the direct photolysis of the organic compounds. The model utilized the literature values of the rate constants for radical formation and substrate oxidation. It was applied to interpret the data from the oxidation of l,2-dibromo-3-chloropropane (1,2-DBCP) at low levels (less than 500 pg/ L) in simulated and actual groundwater. 

Mark G, Schuchmann MN, Schuchmann H-P, von Sonntag C (1990) The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol a simple actinometer for 254 nm radiation. J Photochem Photobiol A Chem 55 157-168 Mark G, Korth H-G, Schuchmann H-P, von Sonntag C (1996) The photochemistry of aqueous nitrate revisited. J Photochem Photobiol A Chem 101 89-103 Mark G, Tauber A, Laupert R, Schuchmann H-P, Schulz D, Mues A, von Sonntag C (1998) OH-radical formation by ultrasound in aqueous solution, part II. Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield. Ultrason Sonochem 5 41-52 MarkG, Schuchmann H-P, von Sonntag C (2000) Formation of peroxynitrite by sonication of aerated water. J Am Chem Soc 122 3781-3782... 

Indirect Photolysis in Aqueous Solution Involving Hydroxyl Radical Formation... 

The cyclization of 2,4,6-tri-/-butylthiobenzaldehyde to the isothiochroman 493 has been accomplished both thermally <1996BCJ719> and by photolysis (Scheme 181) <1991J(P2)1045> it appears that the reaction involves radical formation. 

This was an attempt at a crossover experiment, possibly relevant to the mechanism of photolysis, Eqs. (8)—(11) a trace of Sn(NR )3 was also present, but not detected with 1 1 SnR2 Sn(NR2)2, nor any mixed alkyl-amido-radical (142). Irradiation of Sn(NRj)2 with Sn(C5H5-i))2 yielded Sn(NR2)(C5H5-i7), but no paramagnetic species. Radical formation was also not detected by UV irradiation of Sn(C5H6-7))j, Snl2, SnCI(NR ), or Zn(NR )2. 

These results reveal clearly that the polymerization is extremely affected by the dielectric constant of the solvent, and also the affinity with water and the mechanisms of primary radical formation by photolysis. 

The reaction of ozone and hydrogen peroxide in its ionic form and photolysis of both oxidants constitute the initiation reactions leading to a mechanism of hydroxyl radical formation in water. This mechanism is basically the same for all these advanced oxidation systems, whereas the main differences lie in the initiating steps. These oxidation technologies have been applied for the treatment of pollutants in water for more than two decades. 

As previously discussed, the concentration of Fe2+ is an important factor in the rate of hydroxyl radical formation from hydrogen peroxide. Consequently, any process that can speed the reduction of Fe3+ to Fe2+ will increase the formation rate of hydroxyl radical. UV or visible radiation can play this role by photoreducing iron. However, the photo-Fenton process involves three additional mechanisms that can contribute to pollutant degradation (a) direct photolysis of H202 to yield two hydroxyl radicals (Eq. (22)) (b) photolysis of Fe(OH)2+ to form hydroxyl radical (Eq. (23)) and (c) degradation of pollutants by direct photolysis (i.e., absorption of a photon by the pollutant molecule followed by decomposition of the photoexcited pollutant molecule). 

Fischer and Warneck [175] investigated the photolysis of nitrite over the wavelength range of 280 and 390 nm and determined that the quantum yield for hydroxyl radical formation 0oh was decreased with increasing wavelength from 0.069 at 280 nm to 0.022 at 390 nm, in agreement with previous... 

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