Charge Transfer Interaction and Complex Formation of Free Radicals

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In this project, the complex formation of the benzophenone ketyl radical with amines and the decay rates of radical complexes have been studied with the laser flash photolysis technique. The benzophenone ketyl radicals were formed by the hydrogen atom abstraction reaction of triplet benzophenone from solvent molecules, cyclohexane, or amines. The excitation laser used was a 308 nm XeCl excimer laser. When aliphatic amines were added to the cyclohexane solutions of benzophenone, the peak position of transient absorption shifted to red by 5-10 nm and the spectral shapes became broad compared with that of free benzophenone ketyl radical. These facts indicate that the ketyl radical makes complexes with aliphatic amines. On the other hand, one of the aromatic amines, N,N-dimethylaniline, did not show the transient absorption of the ketyl radial, which suggested that the ketyl radical would react with N,N-dimethylaniline.The decays of complexes of the ketyl radical were measured by the trans … More ient absorption at the peak position. The decay rate became slow by the complex formation ; under our experimental conditions, the free ketyl radical almost completely disappeared within about 30μs, but the complexes of amine-ketyl radical survived after 40μs. The kinetic analyses were carried out on triethylamine and tri-n-propylamine complexes. The decays of both radical complexes followed the second order decay law, which indicates that the radical complexes disappeared through the recombination reaction forming benzpinacole like the free ketyl radicals do. The rate constants obtained are 0.5 and 0.45 times smaller than that of the free ketyl radical, respectively. It is revealed that the rate constant decreases about half of the free ketyl radical by the complex formation and that of the tri-n-propylamine complex is smaller than that of triethylamine complex, indicating more bulky amine makes the recombination rate slower. It is concluded that the amines work to hinder the reaction center and supress the recombination reaction. Less