- Laser Flash Photolysis of tert-Butyl Aroylperbenzoates: Kinetics of the Singlet and Triplet States and the Aroylphenyl Radicals
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tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm, pulse width ~350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (λmax ~ 625 nm; τ ~ 20 and ~7.9 ps, respectively). The triplet state of each (λmax ~ 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 ~0.74 ns). The ~550 nm absorption obtained from the 355 nm LFP (pulse width ~7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4′-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 × 107 to 1.68 × 109 M-1 s-1 in CCl4 at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.
- Shah, Bipin K.,Neckers, Douglas C.
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- Application of a new kinetic method in the investigation of cleavage reactions of haloaromatic radical anions
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A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constants of radical anions over the range of 107 - 5 × 109 s-1 in aprotic solvents. The standard potential for the formation of the radical anions may be extracted from the kinetic analysis as well. The method employs electrochemical steady-state or optical detection techniques and is an extension of the redox catalysis approach described previously in the literature. The applicability of the method is illustrated through a systematic study of the cleavage reactions for a number of short-lived haloaromatic radical anions. Interestingly, the radical anion of iodobenzene is found to be an intermediate in the homogeneous reduction of iodobenzene, even though recent investigations have shown that the corresponding heterogeneous reduction at an electrode surface proceeds by a concerted electron transfer-bond cleavage process. The nature of the cleavage reactions is discussed in terms of the activation driving force plot of the cleavage rate constants versus the relevant Gibbs energies. While the exergonic cleavage reactions follow a simple decay mechanism taking place at the halogen site, the endergonic processes are best described as intra-molecular electron transfers from the substituent to the carbon-halogen bond. Nevertheless, the overall intrinsic barrier is found to be relatively small (27-39 kJ mol-1) and it is suggested that the endergonic reactions may proceed by a stepwise mechanism, in which a σ* radical anion is formed as an intermediate prior to the formation of the dissociated products, the aryl radical and the halide. The above conclusions were supported by semi-empirical PM3 calculations of structures and charge distributions in the radical anions.
- Enemaerke, Rasmus J.,Christensen, Torben B.,Jensen, Henrik,Daasbjerg, Kim
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p. 1620 - 1630
(2007/10/03)
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- Rapid Cleavage Reactions of Haloaromatic Radical Anions Measured with Fast-Scan Cyclic Voltammetry
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Cyclic voltammetry at scan rates from 10 mV/s to 1E6 V/s has been used to characterize the reduction of aryl and benzyl halides in acetonitrile solutions containing 0.6 M tetraethylammonium perchlorate.The use of very rapid scan rates is made possible with electrodes of micrometer dimensions.The kinetics and mechanism of the reduction process have been evaluated by digital simulation of the recorded voltammograms.The radical anion generated at the electrode surface is found to have a half-life ranging from less than 100 ns in the case of 4'-bromoacetophenone to 70 ms for the case of m-nitrobenzyl chloride.The reduction mechanism for the aryl halides is consistent with the ECE-DISP1 mechanism.Thus following initial one-electron reduction, halogen bond cleavage occurs resulting in a radical that is subsequently reduced.For the compounds with relatively long half-lives (>1ms) the results in thus work are in good agreement with prior investigations with cyclic voltammetry.Good agreement is also obtained with compounds that have a short half-life ( ca. 1 μs) which have been characterized by the technique of redox catalysis.However, the measured rate constants differ with previously reported values which were estimated or measured at the extreme time limits of classical electrochemical techniques.
- Wipf, David O.,Wightman, R. Mark
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p. 4286 - 4291
(2007/10/02)
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- Intramolecular Electron Transfer and Dehalogenation of Anion Radicals. 4. Haloacetophenones and Related Compounds
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Halogen-substituted acetophenones and several other carbonyls and carboxyl derivatives were reduced with eaq- in irradiated aqueous solutions to produce the anion radicals.In certain cases, reduction by (CH3)2C.O- was also used.The anion radicals undergo itramolecular electron transfer and dehalogenation to yield inorganic halide ions.The rates of these reactions varied over a wide range (1E1-1E7 s-1) and were strongly dependent on the nature of the halogen and the other substituent and their relative positions in the molecule.The results obtained here are discussed along with previous measurements on cyano and nitro derivatives.The rate of dehalogenation is found to be dependent on the electron affinity of the other substituent on the ring.Hammett's substituent constants do not correlate with the observed rates.However, spin density distribution in the anion radicals, as derived from ESR parameters, yields a good qualitative correlation.Increased spin density on the ring carbon bearing the halogen is accompanied by an increase in the rate of C-X bond scission.The spin density represents in this case the negative charge density, since protonation of the anion radicals results in minor changes in spin densities but causes a dramatic decrease of the rate of dehalogenation.
- Behar, D.,Neta, P.
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p. 2280 - 2283
(2007/10/02)
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