20% of the toluene reacted on a molar basis),16 in agreement with previous studies,16"18 while in irradiations of simulated polluted atmospheres with the xylene isomers added one at a time, -dicarbonyls were observed only for o-xylene addition.43 This latter observation is probably due to a combination of the expected rapid reaction of the -dicarbonyls glyoxal and methylglyoxal, which are presumably formed from m-and p-xylene, with OH radicals [analogous to the aldehydes31,44] together with analytical difficulties for the gas chromatographic detection of these two a-dicarbonyls.
References and Notes(1) A.
dominates at early time, and an activation energy for diffusion, Ediff, which dominates at late times. Application of the model to the data yields Ecomb = 9.6 and Ebbi = 19.3 kcal/mol. The activation energies correspond well to those reported by other workers from NMR measurements of rotational and diffusional motion in PAA. The model also allows estimation of the diffusion coefficient provided r0 in the kinetic equation can be approximated. Taking r0 as the largest lattice dimension (20.5 Á) one obtains a value of D « 1 X 10-16 cm2/s at 0.96 of the phase transition temperature. Such considerations lead to the assignment of Ediff to an extrinsic, defect controlled diffusion process and Ecomb to a rotation of the radical about the long molecular axis necessary to reach a favorable orientation for combination with another radical. The general role of radical shape and ease of rotation in influencing the decay kinetics is discussed.
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,pazoxyanisole (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 previously 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 °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.
ESR studies indicate that about 70% of the radicals formed by X-radiolysis of poly crystalline samples of n-alkoxyazoxybenzenes at 77 K are produced as closely spaced pairs. Upon warming, the radicals in pairs combine leaving behind a smaller population of chemically different independent radicals. Good correlation between the yield of N2 gas (G(N2) = 0.021) and the yield of radical pairs (G(pairs) ~0.02) in p-azoxyanisole (PAA), as well as spectral differences in deuterated samples and other considerations, indicate the pairs are probably formed by an intramolecular process involving destruction of the azoxy linkage to leave two substituted phenyl radicals separated by a light gas molecule. Overall radical pair decay is "stepwise". Plots of fractional decay vs. time for different doses indicate that pair decay is a correlated process; the method of initial rates gives a reaction order of 0.9 and activation energy 2.7 kcal/mol for recombination in p-azoxyphenetole (PAP); and the pair separation distance is constant during decay, all indicating that the decay occurs by a single jump recombination process involving geminate partners.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.