Laser flash photolysis oxygen reactions

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Roberts, C. B. Zhang, J. Chateauneuf, J. E. Brennecke, J. F. Laser Flash Photolysis and Integral Equation Theory to Investigate Reactions of Dilute Solutes with Oxygen in Supercritical Fluids. J. Am. Chem. Soc. 1995, 117, 6553-6560. 

Laser flash photolysis has been used to study the primary photochemical reactions involving the excited state of the photosensitizers as well as the photochemically generated intermediate species. A sequence of reactions leading to oxygen photoreduction, with the concomitant formation of hydrogen peroxide, is proposed in every case. 

A profound understanding of the reaction mechanism through which H202 is being formed could obviously enable optimization and lead to a maximal efficiency of such photochemical systems. Laser flash photolysis was thus employed to study the primary and secondary photochemical reactions in which lumiflavin and Ru (Il)-tris (2, 2 -bipyridine) are involved. Fig. 2 shows that photoexcited lumiflavin in its triplet state (picture a) is efficiently quenched by semicarbazide (picture c) this leads to flavosem iquinone formation (picture b). Further addition of molecular oxygen allows oxidation of flavosemiquinone radicals (picture d). On the other hand, Fig. 3 shows that photoexcited Ru (Il)-tris (2, 2 -bipyridine) is efficiently quenched by... 

A recent study by laser flash photolysis showed that a triplet state is not involved in the coloration mechanism of spirooxazines when in solution. A similar conclusion was reported, noting that the photocoloration occurs only in the excited singlet state because of the independence of the reaction to the presence of oxygen.29... 

An earlier experimental study involved the generation of benzylchlorocarbene, phenylchlorocarbene, methoxy-phenylcarbene, dimethylcarbene, cyclobutylidene, adamantylidene, and fluorenylidene by means of laser flash photolysis and their reactions with oxiranes and thiiranes <1998JA8681>. Absolute rate constants ranging from 10 to 10 ° M s at 22 °C were measured in acetonitrile and cyclohexane. Thermally generated dimethoxycarbene was found to be inefficient at abstracting oxygen or sulfur from an oxirane or a thiirane. 

Reactions of OH, 0 and SO " radicals with cresols were studied in detail by pulse radiolysis and laser flash photolysis techniques combined with product analysis.The rate constants of the OH reaction with cresols are very high (fe 1 x 10 ° dm mol s ) whereas 0 was found to be less reactive k 2.4 x 10 dm mol s ). The second-order rate constants of the reaction for SO " reaction with cresols are in the range of (3 to 6) x 10 dm mol s" The transient absorption spectra obtained in the reaction of OH with isomers of cresols have peaks in the region 295-325 nm. Merga et al and Choure et carried out a detailed product analysis by HPLC on radiolysis of chlorotoluenes and cresols. Table 2 lists the products obtained in deoxygenated and oxygenated solutions of these systems. 

Carbon-oxygen bond heterolysis is responsible for the observed photolyses of 9-aryl-9-xanthenols ° and llH-benzo[b]fluoren-11-ol. - - Evidence for the formation of the 9-fluorenol radical cation as well as the 9-fluorenyl cation has been obtained from a laser flash photolysis study of 9-fluorenol. 1, l-Di-2-thienylethanol undergoes light-induced dehydration to give i,i-di-2-thienylethylene.Single electron transfer pathways, however, are implicated in the ring cleavage reactions of a,j8-epoxy ketones in the presence of allyltributyltin - or alkylamines. 

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