from abstract ideas.,.fundamental knowledge at Esso Research
Existence of the "cage" effect, an im portant concept in solution kinetics, has been confirmed directly by two Esso Research chemists, Dr. Richard K. Lyon and Mr. Donald H. Levy. T h e Franck-Rabinowitch cage effect is the reverse of the diffusion controlled recombination reaction. T w o highly reactive species, A and B, may react with each other so readily that the rate of reaction of an initially separate pair A and Β is controlled by their diffusion towards each other. Conversely, if A and Β are produced very close together (for example by the reaction AN2B—» N 2 + A + B), they may react with each other before they can diffuse apart. Figuratively, as depicted in the illustration, the solvent molecules form
a cage holding the reactants together. Numerous observations have been ex plained by invoking this " c a g e " effect with varying degrees of certainty. In the experiments at Esso Research, mixtures of azomethane and ds-azomethane were photolyzed both in gas phase and in isooctane solution. Primary steps formed C H 3 and C D 3 radicals, followed by hydrogen abstraction to methane or recombination to ethane. Gas phase recombination occurs ran domly among methyl radicals forming ethane in proportions such that ( C H 3 CD 3 ) 2 / ( C 2 H 6 ) (C 2 D 6 ) = 4. In solu tion, however, no detectable CH3CD3 forms, but appreciable C 2 H 6 and C 2 D 6 do form, experimentally proving the " c a g e " effect
Separate experiments have shown " c a g e " effects occurring in high pres sure gas phase, the diffusion coefficient being somewhat larger than in solution. These hitherto unknown " c a g e " effects may have a marked influence on high pressure gas phase reaction kinetics. Consideration of this effect in both high pressure gas phase and solution reactions may have great significance in determining the optimum conditions for new or existing chemical processes.
... adapted from the scientist's notes Esso Research and Engineering Company (P.O. Box 45B, Linden, New Jersey)
