Triplets biradicals

A study of the lifetimes of the triplet biradicals D, E, and F, which were generated from the corresponding azo compounds, found that lifetime decreased in the order... 

A few studies have been carried out on the parent four- and five-membered cyclic sulfones—for thietane 1,1-dioxide (30) by Scala and Colon65 and for thiolane 1,1-dioxide (sulfolane) (31) by Honda and coworkers66 and, later, by Schuchmann and von Sonntag67. In the former compound, the major photochemical process, in the vacuum UV range, is the initial production of a trimethylene (C3H6) biradical and S02 (equation 9). In both the solid- (77 K) and gas-phase photolyses, formation of a triplet biradical appears to be favored. As well as the expected cyclopropane and propylene, ethylene is also obtained during these photolyses, presumably by a cycloreversion process (equation 10). 

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

B3LYP/6-31G(d) calculations indicate that the corresponding triplet-biradical 18 is 16.1 kcal/mol higher in energy than zwitterion 17. 

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

Carbenes as divalent carbon species and their analogue carbenoids can form two bonds on a single carbon atom center introducing two different groups at one time or consecutively.1 This is due to the ambiphilic character of singlet carbenes or triplet biradical character, which can be controlled by changing the... 

The methylene triplet adds to ethylene symmetrically through the methylene group (formed from ethylene) is bent downwards by this process (Fig. 10). At this stage rotation or direct ring closure can occur, with loss of stereochemistry following bond formation to yield cyclopropane. The cyclo-addition of the triplet requires only a small activation energy of about 5 kcal/mole 52). 

Ultraviolet irradiation of oxadiazoline (38d) at 333.6 nm (or irradiation using benzophenone as a triplet sensitiser) gave 2-diazopropane and methyl acetate. A triplet biradical intermediate formed by cleavage of the C(OMe)—N bond was postulated <90TL863>. Oxadiazolinone (42) underwent nucleophilic attack at the carbonyl group by methyllithium to give acetate (41) after treatment of the product with acetyl chloride <89CJC1753>. 

Two isomeric ring homologues of naphthoquinomethane also were synthesized during that phase, one (17) a triplet biradical in its ground state, and the other (18), a ground-state quintet tetraradical. ... 

The relative rates of reaction of the singlet TMM derivative 14b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels-Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 14b, on the other hand, follow the reactivity order of their addition reactions with monoradicals. 

In solution, the triplet biradical 14b dimerizes, and the dimeric products are formed with strong chemically induced nuclear polarization. The absolute rate of the dimerization at 146 K, as monitored in viscous solution by ESR spectroscopy, is just about that predicted by the spin-corrected encounter frequency under those conditions. The cycloaddition of the triplet with a typical alkene, acrylonitrile, also can be followed in this way. 

Kinetic observations suggest that of the four species, the biradicals 14b and 14b, and the bicyclic hydrocarbons 28 and 29, the triplet biradical 14b is the global energy minimum. Thus, the two related full-valency molecules 28 and 29 both have a negative bond dissociation energy (BDE). 

In some cases, the structures of oxygenation products have been crucial for assigning the structures of unusual radical cations. Eor example, the endo-peroxides (83 and 85) support the structures assigned to radical cations (24 and 84 ) derived from l,l-diaryl-2-methylenecyclopropane (23) and 2,5-diaryl-l,5-hexadiene, respectively.Time-resolved spectroscopic data suggest that 83 is generated by coupling of triplet biradical (24 ) with (triplet) molecular oxygen. 

Interest in the aza-Bergman reaction and the influence of protonation on 2,5-pyridyne and its derivatives was (once more) directly stimulated by the search for less toxic antimmor drugs. One approach to increase the selectivity is to decrease the reactivity of the biradical intermediate in hydrogen-abstraction reactions.In this regard, Chen has established a simple model that correlates the reactivity of singlet biradicals with their singlet-triplet gap This model is based on the assumption that the reactivity of the triplet biradical is... 

The photochemical decomposition of bicyclic azo compound 43 gave the results shown in Table VII.199 200 The dependence of product composition on the stereochemistry of the starting azo compound, which was observed in direct photolysis, is removed by benzophenone or triphenylene sensitization since the intermediate triplet biradical undergoes randomization of stereo-... 

Alkyl aryl ketones such as alkanophenones, have been studied much more than alkanones and most of our quantitative knowledge of the dynamics of BR formation and decay come from such systems. In spite of the simplifications introduced by the mechanistic importance of only one alkanophenone excited state, the triplet, and presumably the formation of only triplet biradicals from it, a rigorous solution to Eq. 5 in virtually all real systems remains not possible. 

There is a striking similarity in the behavior attributed to the triplet biradicals believed to be formed in the reactions of both triplet methylene and triplet oxygen with the 2-butenes. It should also be pointed out that in the triplet addition reactions of methylene, cis and trans olefins do not give the same cis and trans product ratios, indicating that the rate of spin inversion necessary for closure in the biradical intermediate occurs at a rate comparable to the rate of rotation about C—C bonds. 

The stereospecificity with which dicyanocarbene adds to the 2-butenes in solution decreases as the olefins are diluted with cyclohexane, again suggesting that the first formed reactant species is converted by collisional deactivation into a triplet biradical which adds non-stereospecifically.59... 

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