, 2374 (1985). It is shown that there will be strong orienting effects in the collision of two radicals which will invalidate attempts to calculate disproportionation-recombination ratios by simple geometric arguments.L. K. COHEN et H. 0 . PRITCHARD. Can. J. Chem. 63, 2374 (1985). On dCmontre qu'il existe des effets directifs importants dans la collision de deux radicaux et que ces effets rendent invalides tous les essais de calculer les rapports de disproportionation-recombinaison par des arguments gtornCtriques simples.[Traduit par le journal]It is possible to take two extreme views of the disproportionation process which occurs between a pair of alkyl radicals, e.g.At one extreme, one can consider that the methyl radical abstracts a hydrogen atom from the difluoromethyl radical (1); at the other extreme, that the products are the "dissociated" species in a chemical activation process, with the recombined product (C2H4F2 in this case) being the "stabilised" fraction of the initial flux (2). It is the purpose of this note to throw some light on the mechanisms of these processes by performing trajectory calculations for the approach of two bodies under the influence of a strongly directional attractive potential, such as is characteristic of a p-orbital in an aliphatic free radical. We find that in such collisions, provided that the radicals are not rotating too strongly, there is a propensity for them to orient themselves so that they are "facing" each other. Thus, rather like a pair of dancers, they remain facing each other during the whole of the intimate encounter, regaining independent rotational motion only after they have receded beyond a critical value of the interparticle separation; however (to continue with the dancing analogy), collisions in which one radical "turns its back" on the other are not entirely excluded. These observations complicate attempts, such as those of Benson recently (3), to calculate disproportionation-recombination ratios by simple geometric arguments based on the spatial characteristics of the radicals.We are faced with a multitude of possible disproportionation reactions (4) and so, for our first examination of this problem, we restrict ourselves to a carefully chosen model of the collision between two methyl radicals which retains, we believe, sufficient physical reality to enable the salient physical effects to be discerned; extension to real reactive systems is then only a matter of detail and of computational complexity.
The model and the potentialThe dominant interactions are the bonding attraction between the two radical centres, and the non-bonded repulsions between the valence-saturated atoms in the system. For the attractive potential we chose, simulating ethane, a Morse curve In order to test our conjecture that most, if not all, of the initial rotational angular momentum is converted into overall angular momentum during closest approach, it is not necessary to perform a full three-dimensional calculation with two radicals: the physical picture remains the same if on...