Radicals photochemical generation

When these polymers are subjected to light of A = 365 nm in bulk vinyl monomer, (MMA or styrene) grafted or extensive crosslinking polymers were produced. The photografting or photocrosslinking occurs through the macro-radicals photochemically generated on the backbone of the polymer ... 

Its chemical composition, for example, is still uncertain. The powerful H-atom abstracting ability of MP" was overlooked in the otherwise careful early work and has yet to be controlled convincingly. The so-called second phase of MP-TCNQ turned out to be HMP-TCNQ, which as summarized in Section 4.1 has typical mixed stacks of D and A ion radicals. Photochemical generation of HMP and the concomitant degradation of MP in solution are apparently general phenomena 119,120) gio crystallization to enhance crystal perfection then becomes counterproductive. Sandman has addressed such questions about the kinetic control... 

Suginome, H., Reaction and Synthetic Application of Oxygen centered Radicals Photochemically Generated from Alkyl Hypohalites. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, FL, 2004, Chapter 109, pp. 1 16. 

Dolinova, J., Ruzicka, R., Kurkova, R., Klanova, J., Klan, R, Oxidation of Aromatic and Aliphatic Hydrocarbons by OH Radicals Photochemically Generated from H202 in Ice, Environ. Sci. Technol. 2006, 40, 7668 7674. 

The NOs- radicals can also be generated by flash photolysis of CAN in acetonitrile (Del Giacco et al., 1993). These photochemically produced nitrate free radicals have been used to study the one-electron oxidation of methylbenzenes. Depending on the oxidation potential of the substrate, radical cations or benzyl-type neutral radicals were formed. Formation of radical cations was observed for most of the methylbenzene derivatives that were studied, whereas formation of the benzyl radical was observed with toluene and with ortho- and meta-xylene. This reaction is not useful for synthetic applications. The reactivity of propellane C-C bonds towards free nitrate radicals photochemically generated from ammonium hexanitrato-eerate(IV) in acetonitrile was studied from both theoretical and experimental points of view (Fokin et al., 2000). Baciocchi and coworkers reported that the photochemical reaction of CAN in acetonitrile with cyclohexene, 1-octene and styrene derivatives leads to the formation of 1,2-dinifrate adducts in high yields (scheme 65) (Baciocchi et al., 1988b). 

Reaction and Synthetic Application of Oxygen-Centered Radicals Photochemically Generated from Alkyl Hypohahtes... 

Forbes M D E, Schulz G R and Avdievich N I 1996 Unusual dynamics of micellized radical pairs generated from photochemically active amphiphiles J. Am. Chem. Soc. 118 10 652-3... 

Electrochemically generated trifluoromethyl radicals add to 1-hexyne to give a 1 4 mixture of ( )- and (Z)-l,l,l-trifluoro-2-heptene [22] Kinetic data on the addition of photochemically generated trifluoromethyl radicals to acetylene and substituted acetylenes were reported [2J]. Alcohols and aldehydes add to hexa-fluoro-2-butyne in the presence of peroxide and y-ray initiation [24] (equations 16 and 17). 

The reactions of /-butoxy radicals are amongst the most studied of all radical processes. These radicals are generated by thermal or photochemical decomposition of peroxides or hyponitrites (Scheme 3.75). 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

In cases where hydrogen atom transfer gives primarily reduced products, Bu3Sn-SnBu3 under photochemical generates the radical that can cyclize, but a halogen atom transfer agent such as iodoethane is usually present (see 15-44). 

Phosphonate ester (10) is made from chloride (12) available by direct chlorination with photochemical generation of radicals. 

These are complexes formed between tungstates and molybdates, and silicate or phosphate, and have been used to generate hydroxyl radicals photochemically. The tungstates PWj204q and SiWj204o have been used most frequently. 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Wu, Y., L. Piekara-Sady et al. (1991). Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem. Phys. Lett. 180 573-577. 

The Period 1930-60. Following the work of Paneth (J ), there was a long period of active work on free radicals, mostly of importance in organic chemistry but with some relevance to organometallic chemistry. The radicals were generated by thermal, photochemical or discharge... 

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

The main feature of the PCL measuring method is combination of the simple and reliable photochemical generation of free radicals with their very sensitive... 

Initiation by light accelerates oxidation due to the photochemical generation of free radicals, which was noticed by Backstrom [16] and repeated by many others [9,11 — 13], The quantum yield (photooxidation products is sufficiently higher than unity. Here are several examples [12]. 

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 ... 

Endrin ketone may react with photochemically generated hydroxyl radicals in the atmosphere, with an estimated half-life of 1.5 days (SRC 1995a). Available estimated physical/chemical properties of endrin ketone indicate that this compound will not volatilize from water however, significant bioconcentration in aquatic organisms may occur. In soils and sediments, endrin ketone is predicted to be virtually immobile however, detection of endrin ketone in groundwater and leachate samples at some hazardous waste sites suggests limited mobility of endrin ketone in certain soils (HazDat 1996). No other information could be found in the available literature on the environmental fate of endrin ketone in water, sediment, or soil. 

The photochemical reduction of Barton ester 40 is depicted in Scheme 12. A series of hydrogen atom donors were screened. A stoichiometric amount of benzenethiol at - 78 °C provided the product in 86% ee (entry 3). This implies that, in the presence of an efficient hydrogen atom donor, radical trapping is competitive with the ring/radical inversion, generating an enantiomeri-... 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

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