Simple cage process in liquid phase radical reactions

Abstract

The cage combination of CF3• and CH 3• radicals formed by the photolysis of hexafluoroazomethane and azomethane, respectively, has been investigated in various nonpolar solvents in the temperature range of 0-120°C [K. Chakravorthy, J. M. Pearson, and M. Szwarc, J. Am. Chem. Soc. 90, 283 (1968); 0. Dobis, J. M. Pearson, and M. Szwarc, J. Am. Chem. Soc. 90, 278 (1968)]. In this work, the experimental results are interpreted by using the self-diffusion constants and self-diffusion energies of the solvents as parameters for the radical cage processes. It is presumed that the ratio of the two competitive elementary cage processes-the combination of the geminate radical pair and the escape of radicals by diffusion-is a function of the solvent microstructure. The first approach to separate the two coexistent cage processes is finding a correlation between radical escape probability and the self-diffusion parameters of solvents calculated on the basis of the body-centered geometrical model of liquid microstructure. The coarse graining description of the liquid allows dividing the random motion of the radical pair in the cage into four states of different energy levels. In the steady state limit, this four-state random motion of radicals can be treated according to Markov's kinetics. The temperature dependence of the radical transition probabilities between the consecutive states points to energetic interaction within the radical pair and a similar interaction between radicals and solvent molecules. The prediffusion and diffusion states of radicals give a good correlation with the quasicrystalline structure of solvents, but the precombination state and the state of the radical pair combination are better correlated with the continuous model of solvent structure. Copyright © 1976 American Institute of Physics.