33574-05-9Relevant academic research and scientific papers
A Time-Resolved EPR Study on Photodecomposition of Dibenzoyl Peroxides in Carbon Tetrachloride
Yamauchi, Seigo,Hirota, Noboru,Takahara, Shigeru,Misawa, Hiroaki,Sawabe, Ken,et al.
, p. 4402 - 4407 (2007/10/02)
We have investigated the photodecomposition of a series of dibenzoyl peroxides in carbon tetrachloride by means of time-resolved EPR spectroscopy.The EPR spectra of the intermediate benzoyloxyl and trichloromethyl radicals were identified and the reaction mechanisms were clarified.It is shown that the decarboxylation takes place mostly from the intermediate benzoyloxyl radicals whose lifetimes are on the order of 1 μs at room temperature.The rate constants for the decarboxylation determined from the transient EPR signals were in good agreement with those obtainedfrom the transient absorption.On the basis of the observed polarization of the EPR signals, it is concluded that the spin states of the intermediate radicals are in thermal equilibrium within 0.5 μs after the laser excitation.In aerated systems additional radicals with larger g values were detected and tentatively assigned as phenylperoxyl radicals.
Spectroscopic and Kinetic Characteristics of Aroyloxyl Radicals. 2. Benzoyloxyl and Ring-Substituted Aroyloxyl Radicals
Chateauneuf, J.,Lusztyk, J.,Ingold, K. U.
, p. 2886 - 2893 (2007/10/02)
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 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-CH3OC6H4CO2. . . . both for hydrogen atom abstractions (e.g., with cyclohexane: 5.3 x 105, 2.1 x 106, 1.4 x 106, and 1.2 x 107 M-1s-1, respectively) and for additions (e.g., with benzene: 2.3 x 106, 2.2 x 107, 7.8 x 107, and 2.2 x 108 M-1s-1, respectively).The rates of decarboxylation of aroyloxyl radicals increase along the series (4-FC6H4CO2. . . ca. 4-ClC6H4CO2. . ..Rate constants, k2, for some of these decarboxylations have been determined over a range of temperatures; e.g., for C6H5CO2., log(k2/s-1) = 12.6-8.6/τ, where,τ = 2.3RT kcal/mol.The structure of aroyloxyl radicals is considered and it is concluded that the long-wavelength absorption (ε720nm ca. 290 M-1cm-1 for 4-CH3OC6H4CO2.) is most probably due to a transition from the 2B2 ground state to the 2A1 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 C6H6 and C6F6 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.
