Photochemistry free radical production

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

In the following, an overview of the experimental approaches is presented, including the production and detection methods of free radicals and the techniques for studying free radical photodissociation in the molecular beam. The photochemistry of the free radical systems investigated recently will then be discussed in detail. 

It is clear that one of the major challenges in the experimental studies of free radicals is the preparation of radicals. The experimental designs (production of radicals and detection of radicals and photoproducts) are largely dependent on the particular radicals of interest. Nevertheless, many approaches have been taken, as seen in this review, to study the free radical photodissociation, and a great number of systems have been examined during the last couple of years. The sophistication in the experimental studies of free radical photochemistry has reached the level that has been available for the stable molecules. State-to-state photodissociation dynamics of free radicals have been demonstrated for a few small systems. Many more advances in the field of photodissociation dynamics of radicals are expected, and it is hoped that a more systematic and sophisticated understanding of free radical photochemistry can be developed. 

The photochemical reaction of 2,3,4,6-tetra-0-acetyl-/3-D-gluco-pyranosyl sulfone (57) in benzene under nitrogen has been carefully studied, and a number of products identified116 (see Scheme 22). A mechanism that involves a photochemically initiated series of free-radical processes has been proposed that is consistent not only with product formation but also with the extent of incorporation of deuterium found in the various products following irradiation of 57 in benzene-d6. The mechanism shown in Scheme 22 is compatible with proposals offered to explain sulfone photochemistry in noncarbohy-... 

In addition to the limitations enumerated above, which are inherent in the photochemistry, there are other side reactions of a free-radical nature which may compete seriously with the desired reaction. It is a simple matter to determine which of the products are derived from these reactions as they can be formed in the dark, using free-radical chain initiators. For example, chain reactions where the propagating steps are of the following type are fairly general. 

Absorption of sunlight induces photochemistry and generates a variety of free radicals that drive the chemistry of the troposphere as well as the stratosphere. This chapter focuses on the absorption spectra and photochemistry of important atmospheric species. These data can be used in conjunction with the actinic fluxes described in the preceding chapter to estimate rates of photolysis of various molecules as well as the rate of generation of photolysis products, including free radicals, from these photochemical processes. 

The most important aspect of 03 photochemistry for the troposphere is the yield and wavelength dependence of ( D) production in reaction (5) since it is a source of hydroxyl free radicals via its reaction with water vapor ... 

The restricted motion of molecules and of fragments such as free radicals formed by photodissociation results in interesting differences in the photochemistry of some molecules in solution or as guests in inclusion compounds. To take one example, the aliphatic ketone 5-nonanone can yield fragmentation or cyclization products via the biradical formed through intramolecular hydrogen atom abstraction (Figure 8.18). In the photolysis of the inclusion compound the cyclization is the preferred reaction, and there is a marked selectivity in favour of the ay-isomer of the cyclobutanol. 

Experimental evidence of the part played by free radicals in a chemical reaction was soon forthcoming. In 1934 Frey24 found that butane decomposed very slowly at 525° but that if one per cent of dimethyl mercury was introduced the decomposition proceeded rapidly. In the same year Sickman and Allen25 found that acetaldehyde was stable at 300° but that it was decomposed completely when a few per cent of azomethane was added. The introductions of dimethyl mercury or azomethane at these temperatures apparently liberated free radicals which initiated chains. Moreover when mixed gases decomposed simultaneously they did not do so independently. The products contained groups from each in a way that could be easily explained on the assumption of the liberation and recombination of free radicals. Again the appearance of butane from the decomposition of propane is difficult to explain on any hypothesis except on the assumption that some free radicals of CH3 are split out and that they become attached to propane molecules. More direct examples will be given later in the discussion of photochemistry. 

Studies were made both by 7-radiation and by UV photolysis (Table III) [4]. There were substantial differences in the ratio of G values for these two products. In UV photochemistry, excitation is to lower vibrationally excited states and the free radical (Type I) yields are really quite low. Most of the reaction can be attributed to the type II rearrangement rather than the radical process. On the other hand, the G value for type I products in y-radiolysis is very much higher. It was suggested that in radiolysis, higher excited states were involved vdiich eventually provided greater translational velocity to separate the free radical pairs, thus giving rise to higher yields of chemical products. 

UV-induced skin carcinogenesis by azthioprine (216), photochemical interaction between triamterene and hydrochlorthiazide (217) photochemistry of diclofenac (218), kinetic treatment of photochemical reactions (219) and a direct electron paramagnetic resonance (EPR) and spin-trapping study of light-induced free radicals from 6-mercaptopurine and its oxidation products (220). 

Benzil has frequently been used as a means of generating free radicals in polymerization systems subjected to ultra-violet irradiation 11, 16, 56—58). In studies of the benzil-photoinitiated polymerizations of methyl methacrylate, and vinyl acetate, Melville (16) assumed that initiation was brought about by fragmentation of photoexcited benzil into two benzoyl radicals. However a survey of the photochemistry of benzil 34) indicates that such a cleavage does not in fact take place in solution studies of the products formed on irradiation of benzil in cyclohexane (59), cumene and isopropanol (60) can be rationalised on the basis of initial hydrogen abstraction from solvent by photoexcit i benzil, e.g. 

Interest in the photochemistry of the phthalimide systems has continued. The phthalimide derivatives (316) are phot ochemically reactive and on irradiation in acetone yields the cyclized products (317). The reaction involves hydrogen abstraction to yield the biradical (318) which subsequently bonds to afford the observed products. A recent study has examined the behaviour of the anion (319) in an attempt to reduce electron transfer processes. In t-butanol irradiation affords the solvent addition product (320) as the principal product presumably by a free radical path. Minor products (321) and (322) are also formed but are probably artefacts of the work-up procedure. Irradiation of (319) in methanol with added cyclohexene follows a different reaction path. In this system the reaction with methanol is minor while the dominant reaction is addition of the alkene to afford the adduct (323) in 20 % yield. The Dewar benzene derivative (324) is photocheraically unstable and irradiation affords t etramet hyl cyclobutadiene. ... 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

FIGURE 4-36 Simplified photochemistry of propene. In this diagram, a few of the pathways by which propene can be oxidized are shown. Numerous reactive free-radical intermediates are produced these free radicals can react with molecular oxygen and also can oxidize NO to N02, thus enhancing ozone production. 

---