Radicals and Biradicals

Closs G L, Miller R J and Redwine O D 1985 Time-resolved CIDNP applications to radical and biradical chemistry Acc. Chem. Res. 18 196-202... 

An anchor, as defined above, contains stable molecules, conformers, all pairs of radicals and biradicals formed by a simple bond fission in which no spin re-pairing took place, ionic species, and so on. Figure 1 shows some examples of species belonging to the same anchor. Thus, an anchor is a more general and convenient temi used in the discussion of spin re-pairing. 

The extended Hiickel method has been used in a discussion of properties and reactivity of radicals and biradicals (75). We have found it possible to correlate the basicity constants, pKbh. of radical anions with extended Hiickel data (76). 

Give examples of simple radical and biradical reactions - combination, disproportionation, hydrogen abstraction and fragmentation. 

The chemical details of the reactions of representative alkyl radicals, alkoxy radicals, and biradicals with oxygen should be established. Both the rate constants and the immediate products are needed to construct realistic mechanisms for the model. 

Scheme 3 Chemical stmctures of neutral tc-radical and biradical molecules of organic conductors...

If a biradical intermediate is involved in the reaction path, it must be the one shown above and clearly is not the most stable possibility. However, the initial addition to the isolated double bond does proceed to form a five-membered ring as would appear to be common with radical and biradical species. 

In compiling the information in this chapter, I have relied heavily on several very comprehensive reviews that have appeared over the past few years [1-7]. In particular, the 1978 review by T irro et al. [1] is extremely thorough in describing the intra- and intermolecular photophysics and chemistry of upper singlet and triplet states. In fact, rather than reproduce the same details here, I direct the reader to this review for a summary of upper state behavior reported prior to 1978. (A description of azulene and thione anomalous fluorescence is included since these systems are the best-known systems that display upper state behavior.) I also direct readers to the reviews by Johnston and Scaiano [2] and Wilson and Schnapp [3] which focus on the chemistry of both upper triplet states and excited reaction intermediates as studied by laser flash photolysis (one- and two-color methods) and laser jet techniques. Also, Johnston s thorough treatment of excited radicals and biradicals [4] and the review of thioketone photophysics and chemistry by Maciejewski and Steer [5] are excellent sources of detailed information. 

One may safely conclude that both the electronic and steric structures of the radical and biradical play a predominant role in the spin inversion and reaction rates involving the radical and biradicals. The electronic structure of the photoexcited triplet molecules obviously influences greatly their reactivity, such as the difference between n, it and it, it triplets. However, the effect of substitutions is also a major influence on reactivity. A recent example is provided by Levy and Cohen (93) in the study of photoreduction of various substituted nitroben-zenes in isopropanol. Photo-CIDNP was observed if the substituent is electron-withdrawing but not when it is electron-releasing. The result was interpreted by the authors in terms of more efficient ISC and the higher reactivity of the triplet as the electron-withdrawing power of the substituents increases. 

As shown in this figure, the quartet and doublet states consisting of the cation radical and the anion biradical ( A "-Ri ) are generated after the electron transfer. From both of the quartet and doublet states, the radical and the biradical escape from solvent cages, forming the escape radical and biradical. On the other hand, the cage recombination only occurs from the doublet state, forming D and A-Ri . 

A very useful review of time resolved CIDNP and its application to radical and biradical chemistry has been prepared by Closs et al... 

The study of the 1-naphthylcarbene system is an example of the use of probe kinetics, first employed in great detail by Scaiano and coworkers in the study of radicals and biradicals. [23]... 

The probe kinetic methodology developed by Scaiano [23] to study radicals and biradicals by laser flash photolysis (LFP) methodology is readily extended to carbenes. LFP (308, 337, 351, or 355 nm) of diazirine or diazo compounds in the presence of pyridine produces carbenes, which generally react rapidly lO M sec" ) to form ylides. [28,29] Ylides are much easier to monitor than the carbenes because they have intense UV-Vis absorption and microsecond lifetimes. Pyridine ylide methodology has enabled LFP studies of alkyl, dialkyl, alkylhalo, dihalo and carbonylcarbenes. It is now a standard tool and will be used as long as LFP studies of simple carbenes are performed. 

Spectroscopic Evidence for Bisected Trimethylenemethane Cation Radical and Biradical Intermediates... 

The most characteristic feature of the photoinduced electron transfer degenerate methylenecyclopropane rearrangement is die intervention of both cation radical and biradical intermediates. In this case, highly endothermic recyclization of d2-S2a tod -TIa and d -TIeT allows the operation of the important back electron transfer. Nevertheless, such a back electron transfer process may operate generally if a highly stabilized cation radical intermediate is formed in a highly exothermic process. In fact, the photoinduced electron transfer degen-... 

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