Methyl phenyl radical pair

Figure 7. Photolytic and thermal decomposition pathways in crystalline ABP. Initial photolysis at 300-400 nm gives methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemi-cally to give methyl-phenyl radical pairs.

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Although we have now measured D and g tensors for four different radical pairs containing the benzoyloxy radical, we ceUinot,yet be certain of the radical s electronic state. If we consider only the g tensors, we would feel confident that the radical in the M-B and P-B pairs is in a Z state with its spin on the oxygen which was originally peroxidic. However, the D tensors would then indicate that the other radical, methyl or phenyl, must undergo surprisingly large and similar) motion after the first decarboxylation but return to its initial location after the second. 

For the 2-methyl-3-phenyl-3-(diphenylphosphatoxy)-2-propyl radical rate constants were obtained for the complete set of processes, including fragmentation to the contact ion pair, collapse of the contact ion pair to the rearranged... 

Very recently Bartlett has found that irradiation of azo-bis-3-methyl-2-phenyl-2-butane, 45, in frozen benzene at 77°K produces a triplet species with an amazingly long lifetime, on the order of weeks.464 The ESR spectrum of the triplet species indicates a very weakly interacting pair of electrons about 7 A apart. If the solution is warmed, the triplet ESR spectrum is replaced by that of a monoradical. The triplet species most likely is the pair of 3-methyl-2-phenyl-2-butyl radicals held apart by a molecule of nitrogen but held rigid by the frozen medium. 

---