Photochemical behavior

Reaction (7) was believed to be the result of an initial bimolecular displacement step, (7a) NF + N2p4- FN NF2-NF2 N2F3+NF2, AH -5 kcal/mol, followed by step (6). Earlier reaction schemes for NF2 decomposition at 253.7 nm irradiation included steps (1), (3), (9) F + (see pp. 351/2), and (10) 2NF N2p2 [3] or (1), (3), (6), (10), and (7a) [4], In these schemes, emphasis was laid on the formation of N2F2 which was an abundant product in the photolysis of pure N2F4/NF2 or in photochemical reactions thereof. Step (2) and (3) played an important role in HF lasing reactions where N2F4 had been used as the fluorine donor, see for example, [5] and Fluorine Suppl. Vol. 3, pp. 128/50. 

Illumination of NF2 in a 4 K Ar or Kr matrix by a mercury lamp leads to NF and NF3 as was observed by the disappearance of NF2 IR bands and concomittant growth of product bands, cis- and trans-N2F2 formed when the photolyzed matrix was warmed to 20 K [10]. 

The irradiation of solid sulfur allotropes or of solid solutions of sulfur rings produces not only free radicals but also new absorbing and unstable species the identity of which is however not always certain (for a discussion, see [61]). The properties of the radicals as well as their mode of generation are summarized in Table 23 [172, 173]. In addition to the formation of radicals and colored species the irradiation of sulfur induces also luminescence and enhanced electrical conductivity [174]. 

The three types of radicals observed in irradiated sulfur samples have been interpreted as follows Radical /I is a sulfur chain-end with the terminal 

Sample Radical generation Sample temperature g tensor values Radical type Reference 

Bombardment of thin sulfur films, cooled to low temperatures, by He+ ions of MeV energy (sputtering) results in erosion and loss of material by sublimation [175]. These results are of relevance to the surface properties of Jupiter s moon lo large parts of which are covered by elemental sulfur [176] which is constantly bombarded by fast particles from the magnetosphere of nearby Jupiter. 

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Little is known about the photochemical behavior of 2-imino-4-thiazolines the only data concerns the photolysis of 3,4-diphenyl-2-nitrosoiniino-2,3-dihydrothia2ole (403) (Scheme 231) (722). 

Photochemical Reactions. Increased knowledge of the centraUty of quinone chemistry in photosynthesis has stimulated renewed interest in their photochemical behavior. Synthetically interesting work has centered on the 1,4-quinones and the two reaction types most frequentiy observed, ie [2 A 2] cycloaddition and hydrogen abstraction. Excellent reviews of these reactions, along with mechanistic discussion, are available (34,35). 

Specialized training is an absolute requirement for technical service personnel. A typical example is a person involved in supporting a polymer for which the use is the manufacture of rotationaHy molded consumer products. The technical service person is expected to be reasonably familiar with topics such as polymer rheology evaluations, gel-permeation chromatography, rotational mol ding, color science, regulatory requirements for use, mechanical and photochemical behavior of the pigmented polymer, optics, and so forth. Expertise of this variety caimot be expected to be obtained without careful... 

The photochemical behavior of the isomeric 3-methyl-2-phenyl-2-allyl-l-azirine (66) system was also studied. Irradiation of (66) in cyclohexane gave a quantitative yield of azabicyclohexenes (67) and (68). Control experiments showed that (65) and (66) were not interconverted by a Cope reaction under the photolytic conditions. Photocycloaddition of (66) with an added dipolarophile afforded a different 1,3-dipolar cycloadduct from that obtained from (65). The thermodynamically less favored endo isomer (68b) was also formed as the exclusive product from the irradiation of azirine (66b). 

In contrast to the well-defined photochemical behavior of 1-azirines the thermal reactions of these compounds have been studied less thoroughly (68TL3499). The products formed on photolysis of azirines can best be rationalized in terms of an equilibration of the heterocyclic ring with a transient vinylnitrene. Thus, products formed from the thermolysis of azirines are generally consistent with C—N cleavage. For example, the vinylnitrene generated from the thermolysis of azirine (149) can be trapped with phosphines (72CCS6S). 

Evidence for the formation of oxabicyclobutane (6) in the irradiation (S) was negative (72JA1193). The striking difference in photochemical behavior of enones such as (5) and the analogous conjugated dienes, which readily form bicyclobutanes, is attributed to the substantially greater endothermicity of the isomerization in the oxa series. 

PHOTOCHEMICAL BEHAVIOR OF THE INSECTICIDE METHOMYL IN ORGANIC SOLVENTS... 

Diphenylcyclohexadiene shows divergent photochemical behavior, depending on whether the reaction is induced by direct irradiation or by photosensitization. On direct irradiation, the electrocyclic conversion to 1,1-diphenylhexatriene is dominant, whereas a... 

The azo compounds A and B were prepared and the thermal and photochemical behavior of these materials was investigated. The results are summarized in the equations below. Discuss how these results m relate to the photochemical di-rc-methane rearrangement. (See Section 12.1.4 for some indications of the reactivity of... 

The photochemical behavior of butadienes has been closely studied. When these compounds are exposed to light, they move from the ground state to an excited state. This excited state eventually returns to one of the ground state conformations via a process that includes a radiationless decay (i.e., without emitting a photon) from the excited state potential energy surface back to the ground state potential energy surface. 

The irradiation of imidazole derivatives such as 81 gave isomeric compounds 82 (Scheme 30) (67TL5315 69T3287). Dewar isomers are invoked to justify the observed photochemical behavior. 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

The irradiation of isothiazole with a low-pressure mercury arc leads to the formation of thiazole [69JCA(CC)1018], The photochemical behavior of alkyliso-thiazoles has been studied. 3-Methylisothiazole gave 2-methylthiazole in a low yield. 4-Methylisothiazole was converted into 4-methylthiazole, and 5-methyliso-thiazole gave a mixture of 5-methylthiazole (55%) and 4-methylisothiazole (Scheme 38) (72T3141 93JOC3407). Either a ZI (72T3141) or an ICI mechanism was invoked to justify these reactions (93JOC3407). 

Recently, the photochemical behavior of 4-phenylisothiazole has been reanalyzed (98JOC5592). This compound, on irradiation in ether, gave ring-opening products and 4-phenylthiazole (3% yield) (Scheme 40). The irradiation in methanol... 

In an attempt to explain the peculiar photochemical behavior of 2,2,4,6-tetra-phenyldihydro-l,3,5-triazine, Maeda and coworkers carried out extensive studies on the annular tautomerism of this compound and its variously substituted derivatives both in solution and in the solid state. Thus, spectroscopic studies ( H NMR, IR, and UV) of 2,2,4,6-tetraphenyldihydro-l,3,5-triazine 105 (R = Ph) and some... 

The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxides has been examined by Still and coworkers192-194. These authors also report that rearrangement to cyclic sulfenates, with subsequent reaction by homolysis of the S—O bond, appears to be a particularly favorable process. For example, ultraviolet irradiation of a solution of 8-methylthiochroman-4-one 1-oxide (133) in benzene for 24h afforded a single crystalline product which was assigned the disulfide structure 134 (equation 54). More recently, Kobayashi and Mutai195 have also suggested a sulfoxide-sulfenate rearrangement for the photochemical conversion of 2,5-diphenyl-l,4-dithiin 1-oxide (135) to the 1,3-dithiole derivatives 136 and 137 (equation 55). 

The photochemistry of sulfoxides and sulfones, which was first comprehensively reviewed in 19691, continues to be an area of active research interest. In this early review some 30 to 40 primary publications on the photochemistry of sulfoxides and sulfones were described. Since that date, interest in this field has continued at a steady, rather than accelerated, pace but further reviews of the general area of photochemistry of organic sulfur compounds have appeared2,3. The present review will focus on the main areas of interest for both sulfoxides and sulfones which, in spite of their apparent similarity, exhibit quite different photochemical behavior. 

In an interesting contrast to the photochemical behavior of the simpler sulfine system, the photolysis of thioketene S-oxides such as 25 in carbon tetrachloride leads to the corresponding thioketenes in excellent yield36. This photochemical deoxygenation is... 

Co2(CO)8 has also been studied in low-temperature matrices (19,20), the photochemical behavior of which led to the identification of three isomeric forms of the dimer complex (19). Two of these are the accepted forms, 1 and 2, whereas the third has no bridging, CO ligands. The structure most... 

The biological activity of several halogenated herbicides in water is destroyed by ultraviolet irradiation (18). Irradiation seems to be a promising method for decontaminating small quantities of pesticides. The chemical similarity between the chlorinated dioxins and other chlo-rinted aromatic compounds suggested that if there were parallels in their photochemical behavior, sunlight might destroy dioxins in the environment. 

While the examples in Scheme 7.16 hinted at the practicality of the solid state photodecarbonylation of ketones, the factors controlling this reaction remained unknown until very recently. As a starting point to understand and predict the photochemical behavior of ketones in terms of their molecular structures, we recall that most of the thermal (kinetic) energy of crystals is in the form of lattice vibrations. 

Photochemical Behavior of Organosilicon Polymers Bearing Phenyldisilanyl Units... 

The TT-electron system-substituted organodisilanes such as aryl-, alkenyl-, and alkynyldisilanes are photoactive under ultraviolet irradiation, and their photochemical behavior has been extensively studied (1). However, much less interest has been shown in the photochemistry of polymers bearing TT-electron substituted disilanyl units (2-4). In this paper, we report the synthesis and photochemical behavior of polysiloxanes involving phenyl(trimethylsilyl)-siloxy units and silicon polymers in which the alternate arrangement of a disilanylene unit and a phenylene group is found regularly in the polymer backbone. We also describe lithographic applications of a double-layer system of the latter polymers. 

In order to learn more about the photocrosslinking process, we synthesized 1,1-bis(trimethylsiloxy)-1-phenyl(trimethyl)disilane (5) as a model compound and examined its photochemical behavior in solutions. Compound 5 could readily be prepared by cohydrolysis of 1,1-dichloro-1-phenyl(trimethyl)disilane with a large excess of chloro-trimethylsilane in high yield. 

We have investigated the photochemical behavior of polymer 1 under various conditions in air and found that UV irradiation of the thin liquid films with a thickness of less than 10 pm, indeed produced transparent solid films. However, when the films with a thickness of 100 pm were irradiated with a mercury lamp, cross-linking leading to the solid films occurred only on the surface of the films, but inside remained as liquid after prolonged irradiation. In thses cases, tha surface of the films was found to be slightly opaque. Therefore, most of the light would not be transmitted to the inside of the films. 

Since crosslinking of the polymer occurs as the result of the initial formation of silyl radicals, the siloxane polymer containing both phenyldisilanyl units and functional groups which undergo radical polymerization should produce solid material whatever the thickness of the films. To ascertain this, we have examined the photochemical behavior of the polymers 2-4. 

The interaction of UV radiation with nucleic acids is of great importance since it can lead to UV-induced damage in DNA with profound consequences, including photocarcinogenesis [1,2]. The nucleobases are the primary chromophores in DNA and RNA, and consequently, the photophysical and photochemical behavior of the nucleobases has been the focus of extensive theoretical and experimental work over the years [4, 6, 81, 82],... 

In fact, one does not need an anthracene-like nucleus to observe similar photochemical behavior 42-45 ... 

That the high degree of torsional and other types of strain inherent in the triplet states or trans conformers of cyclohexene and cycloheptene may be responsible for their photochemical behavior is suggested by the reactions of compound (50), a moderately twisted olefin according to molecular models. Compound (50) quantitatively yields bicyclo[3.3.1]non-l-yl acetate (51) within 15 sec after being dissolved in glacial acetic acid(83> ... 

In this chapter we will endeavor to answer the following questions What does the photochemist do in preparing to investigate the photochemical behavior of a molecule What equipment does he use to carry out his experiments Once he has determined the results of the reaction, how can he develop a mechanism to account for these results In answering these questions we will be concerned mainly with the photochemistry of anthracene and related compounds. 

The photochemistry of the polynucleotides has been elucidated primarily by studies of the photochemical behavior of the individual pyrimidine and purine bases (the ribose and phosphate groups would not be expected to undergo photochemical reactions in this wavelength range). These studies have shown the pyrimidines (cytosine and thymine) to be roughly ten times more sensitive to UV than the purines (adenine and guanine.) Thus we would expect most of the photochemistry of the nucleic acids to result from the action of light on the pyrimidines. 

These findings clearly illustrate how the results of so-called "accelerated-aging" tests can be affected by the high temperatures in ordinary xenon- and carbon-arc equipment and can, therefore, lead to erroneous conclusions regarding the photochemical behavior of materials at near-normal temperatures (23). 

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