Kinetics of radical decay. 5. Single crystal n-alkoxyazoxybenzenes

Abstract

Decay kinetics of free radicals produced by x-radiolysis of single crystal samples of three members of the homologous series of n-dialkoxyazoxybenzenes has been studied in the range between room temperature and the solid-to-liquid crystal phase transition temperature. Radical decay was analyzed using a model due to Waite which assumes a random initial spatial distribution of radicals and separate activation energies for radical diffusion and recombination. In single crystal samples the activation energies (in kcal/mol) were 23 and 34 for p, p'-azoxyanisole (PAA), 17 and 40 in p,p'-azoxyphenetole (PAP), and 19 and 31 in heptyloxyazoxybenzene (HAB) for combination and diffusion, respectively. The activation energy for combination is attributed to a rotational reorientation of the radicals. The activation energy for diffusion is attributed to an intrinsic bulk self-diffusion process, in contrast to the grain boundary diffusion process previusly reported for radical decay in polycrystalline PAA. The effect of changing length of the alkoxy group on the activation energies is discussed. A solid-solid phase transition was discovered at 91/sup 0/C in PAP. Radicals in solvent-grown crystals decayed rapidly at the transition temperature, while radicals in samples which had been annealed above the transition temperature before irradiation showed normal decay behavior.