| Résumé | The 308-nm laser flash photolysis of dibenzoyl peroxide and some ring-substituted derivatives yields broad, structureless absorptions in the range 500-800 nm. These are assigned to the corresponding aroyloxyl radical, in part by analogy with the previously studied 3 4-methoxybenzoyloxyl radical. Absolute rate constants for reaction of four aroyloxyls with their parent peroxide and with six organic substrates have been measured at ambient temperatures. In general, the rate constants increase along the series 4-CH 3OC 6H 4CO 2• < 4-CH 3C 6H 4CO 2• ≲ C 6H 5CO 2• < 4-ClC 6H 4CO 2• both for hydrogen atom abstractions (e.g., with cyclohexane: 5.3 × 10 5, 2.1 × 10 6, 1.4 × 10 6, and 1.2 × 10 7 M -1 s -1, respectively) and for additions (e.g., with benzene: 2.3 × 10 6, 2.2 × 10 7, 7.8 × 10 7, and 2.2 × 10 8 M -1 s -1, respectively). The rates of decarboxylation of aroyloxyl radicals increase along the series (4-FC 6H 4CO 2• ≤) 4-CH 3OC 6H 4CO 2• < 4-CH 3C 6H 4CO 2• ∼ 4-ClC 6H 4CO 2• < C 6H 5CO 2• < 3-ClC 6H 4CO 2•. Rate constants, k 2, for some of these decarboxylations have been determined over a range of temperatures; e g for C 6H 5CO 2•, log (k 2/s -1) = 12.6 - 8.6/θ, where θ = 2.3RT kcal/mol. The structure of aroyloxyl radicals is considered and it is concluded that the long-wavelength absorption (ε 720nm ∼ 290 M -1 cm -1 for 4-CH 3OC 6H 4CO 2•) 3 is most probably due to a transition from the 2B 2 ground state to the 2A 1 potential energy surface. The effects of ring substituents on intermolecular reactivity and decarboxylation rates are rationalized in terms of an aroyloxyl structure in which the aromatic ring and carboxyl group are probably coplanar or nearly so and of the contributing valence-bond canonical structures. Some spin-trapping experiments using C 6H 6 and C 6F 6 have also been performed. The production of some phenyl radicals in the direct photolysis of dibenzoyl peroxide is indicated by an enhanced yield, relative to the thermal decomposition, of the geminate cage product, phenyl benzoate. However, it is concluded that the yield of phenyl radicals formed in the photolysis is probably considerably less than has been presumed previously. |
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