Diphenylketyl radical

Photoionization is also an important (although not the major) decay pathway for the excited diphenylketyl radical (161) in polar solvents such as acetonitrile [137] (Scheme 22). This was confirmed with the observation of benzophenone radical anion in laser flash experiments following 515-nm photolysis of ground-state 161. The anion radical is apparently produced as a result of electron ejection followed by trapping of the electron by ground-state benzophenone. 

Table 14 Rate Constants for the Reactions of Excited Diphenylketyl and Substituted Diphenylketyl Radicals with Added Quenchers in Solution at Room...

In the first step the spectroscopically detectable ketyl radical 36 is formed, which then recombines to form benzopinacol (37) (Weiner, 1971). The same pinacol is obtained by reacting benzophenone with 2-propanol, since the dimethylketyl radical (38) produced in the hydrogen abstraction step is a strong reductant and transfers a hydrogen atom to the excess benzophenone to form another molecule of the diphenylketyl radical (36). 

System 26 uses an extrinsic base to tune the reducing power of a-hydroxy radicals [181]. The photochemically generated diphenylketyl radical serves as De/Dp-H, Ae is 1,2,4,5-tetracyanobenzene (TCB), and Ap is a substituted pyridine (py). In the absence of the pyridine to facilitate PT, the ET reaction from the ketyl radical to TCB is endothermic by 0.4 eV. Consequently, the diphenylketyl radical decays slowly to reconstitute the starting reactants with no net reaction taking place. When coupled to PT by pre-associating the... 

The PCET reaction rate for 26 has been slowed by using a less basic proton acceptor, thus reducing the driving force for the concerted PCET reaction. When lutidine is replaced with 2-Cl-py, the driving force diminishes from 0.4 eV to <0.1 eV, and accordingly kp g-i- decreases by a factor of four. A KIE of 3.2 was measured when the deuterated diphenylketyl radical was used and Ap was 2-Cl-py. This magnitude certainly supports the mechanism of concerted PCET. 

A remarkably detailed study in which four pathways for excited radical decay are identified involves diphenylketyl radicals in acetonitrileS5 Electron ejection and dissociative OH cleavage reactions are amongst the alternative primary processes involved. 

In a recent paper some assignments made by Bobrowski and Grodkowski were questioned by Bhasikuttan et al [50]. These workers studied the reduction of a series of CV- and MG-type dyes by diphenylketyl radical and hydrated electrons in pulse radiolysis experiments. They showed that both MG and MGH have two absorption bands at 340 and 400 nm. Also, it was observed that the dye radicals exhibit second-order decay without bleach recovery. This suggested that the radicals probably dimerize rather than disproportionate. 

Critical examination of the coupling constants for the triptycene derivatives, LXIII and LXIV, suggests not only that the direct interaction is important but also that the sign of the coupling constant in this case is negative (55b). In other work, Kulkarni and Trapp investigated the esr spectra of fluorine atom derivatives of the triphenylmethyl and diphenylketyl radicals (131,132). Their observations are presented in Table 22. 

The 02 constants are unusually small compared to the of constants. The spin densities calculated by approximate molecular orbital theory for the 2 and 4 carbon atoms of the trityl and diphenylketyl radicals are, however, essentially the same. Kulkarni and Trapp, accordingly, conclude that a direct transfer of spin density occurs from the radical center to the fluorine atoms in the ortho positions. The interpretation implies that the direct interaction yields negative spin density in the fluorine 2s orbital. 

A lifetime distributions analysis provided important information and revealed an important quenching effect in the case of adsorption from water, comparable to the case of adsorption onto microcrystalline cellulose [7], due to the more efficient hydrogen abstraction reaction from the glycoside rings of cellulose when compared with hydrogen abstraction from the alkyl chains of the modified celluloses. Diffuse reflectance transient absorption spectra revealed the presence of the triplet state of BZP in all supports under study, and also of the diphenylketyl radical and BZP OH radicals. 

A considerable amount of data exists on the luminescence properties of excited benzyl [6,83-90,94-97], arylmethyl [91-93,100-108], diphenylketyl [7,109-116], and heteroradicals [117-123] in low-temperature matrices and in room temperature solution. Tables 6 through 11 tabulate both spectral and lifetime information. Where data is available, these tables also include information on the absorption spectra of the excited radicals. Due to the fact that numerous radicals listed in the tables are partially or totally deuterated, structures indicate the number of hydrogen atoms and/or deuterium atoms present. 

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