Molecular models for recombination and disproportionation of radicals

Abstract

The modified Gorin model of recombination as a long range interaction of weakly bound radicals, constrained only by steric forces, is rederived to give a collision impact parameter 11% larger than rmax, the separation at the centrifugal maximum. This larger impact parameter resolves some of the earlier difficulties with secondary and tertiary radicals. By imposing a new constraint, that the orientation of the radicals be such as to permit exchange forces to act, a new model is developed in which kr = 1/4 Zr α, where a represents the product of fractions of active surface areas of each radical available for bonding. Excellent agreement is obtained in estimating values of kr.Analysis of disproportionation is made to show that it cannot be described by the tight transition state but must also be governed by similar, long range exchange forces between the bond being attacked and the same "active surface" of the abstracting radical. Using an ad hoc radius for [Formula: see text]equal to 4 Å it is possible to reproduce the data on alkyl radical disproportionation to within ± 10%.The model predicts a small decrease in rate of recombination with increasing temperature. This decrease is more pronounced for bulkier radicals and at higher temperatures (> 900 K).