Photochemical behavior fragmentation

The photochemical behavior of cyclobutanone (IS) contrasts sharply with that of other ketones. Cyclobutanone undergoes a cleavage also from the (n, r ) state, with subsequent fragmentation to ketene and olefin, decarbonylation to cyclopropane or cyclization to oxacarbene (16), whose concerted formation has also been proposed on the basis of stereochemical observations (Stohrer et al., 1974). In contrast, cyclohexanone cleaves exclusively from the triplet state and undergoes disproportionation reactions. The photochemical activity of cyclobutanone persists even at low temperatures (77 K) where cyclohexanone is photostable. 

Fig. 12. Schematic representation of the photochemical behavior of the Cr(CO)5 fragment in a mixed Ne/Xe matrix at low temperatures. represents the Clu fragment in the excited lE state [reproduced with permission from (60), p. 136],...

A tentative correlation between the mass spectral fragmentation of 3,5-diphenylisoxazole, 2-phenyl-3-benzoyl-l-azirine, and 2,5-diphenyloxazole, and their photochemical behavior has been suggested by Japanese workers.287... 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

Only excimer emissions were observed during laser pulse, a broad plasma-like emission was detected later, and fragmented radicals became distinct. Ablation behavior can be interpreted in terms of photophysical and photochemical processes, including Si - Si annihilation. 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

Successive introduction of nitrogen atoms into benzene causes a gradual reduction in aromatic stabilization. The diazines still show typical aromatic behavior in that in most of their reactions they revert to type. However, with the triazines and tetrazines, decreasing aromaticity increases the ease of both thermal and photochemical fragmentations and rearrangements, and of cyclic transition state reactions with other reagents. 

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