Phenylcyclohexadienyl radicals

Photoconductivity induced by single-photon excitation of aromatic molecules in liquid hydrocarbons is thought to involve Rydberg states.A number of photophysical studies on glasses or crystals have appeared these include 2,4,5-trimethylbenzaldehyde in durene single crystals, identification of phenylcyclohexadienyl radicals in benzene crystals, hydronaphthyl radicals in dehydronaphthalene crystals,1-hydronaphthyl radicals in naphthalene crystals, phenylcyclohexadienyl radicals in biphenyl crystals, ... 

H-abstraction by C02 from the aromatic ring could also lead to the formation of benzoic acid, if the intermediate phenyl radical were subsequently to react with C02. However, phenyl radicals are highly reactive and may alternatively react with benzene (k = 4.5 x 10 ImoP s ) to yield a phenylcyclohexadienyl radical (Ph-CHD), which upon disproportionation would lead to biphenyl (observed as a reaction product, see above) and phenylcyclohexadiene. The latter product not being detected in our experiments, we conclude that biphenyl is formed by an addition mechanism rather than by an H-abstraction. 

The formation of C02 -CHD has been postulated in order to account for the generation of benzoic acid as a reaction product (see above). We expect that the lifetime of this radical is longer than 50 ps in order to be able to undergo disproportionation to benzoic acid. The observed absorption at A >340 nm may in fact be attributed to the C02 -CHD radical, because of the agreement of its absorption spectrum with that of the structurally related H adduct of benzoic acid showing, both, absorption maxima at around 350 nm. Despite the fact that Ph-CHD phenylcyclohexadienyl radicals also show an absorption with a maximum between 330 and 340 nm, its contribution to the observed spectrum is neglected, because the recombination product, phenylcyclohexadiene, was not found (see above). Consequently, the time-resolved experiments with benzene support both the addition and electron transfer pathways of reaction. 

Benzene as solvent also competes for phenyl radicals at low concentrations of PBN to produce non-detected phenylcyclohexadienyl radicals ... 

In the nitrosoacetanilide system, Binsch and Riichardt recognized that a relatively high concentration of a persistent radical forms, which efficiently oxidizes the phenylcyclohexadienyl intermediate to biphenyl (equation 39) and produces a second species, from which the radical is regenerated in a catalytic cycle. Two possibilities, 63 and 64, exist for the identity of the pesistent radical, and both of these can reach ESR-detectable... 

The decomposition of 31 in benzene, like that of nitrosoacetanilide, does not give phenylcyclohexadienyl dimers, since the outcome is again governed by the presence of a persistent radical, in this case triphenylmethyl. A limited... 

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