Isotopic exchange in gamma-irradiated mixtures of C24H5 and C24D5: Evidence of free radical migration in the solid state

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Abstract
Irradiation of a 1:1 co‐melted mixture of tetracosane‐h50 and tetracosane‐d50 in the solid state resulted in isotopic exchange in very high yield [G(H–D)=25]. Exchange was not observed for a similar isotopic mixture irradiated in the liquid phase. A mathematical model, which operates by iterative computer calculations, has been constructed to simulate the complex changes in isotopic peak patterns which arise in the irradiated solid mixture. The simulations show that the observed isotopic exchange is not random, but rather that an individual molecule which has once undergone an isotopic exchange has an enhanced probability of undergoing additional exchanges. The results are very consistent with the occurrence of a localized chain‐reaction process specific to the solid phase, and they provide evidence supporting the hypothesis that free radical transport in solid organics occurs extensively by a repetitive ‘‘hydrogen‐hopping’’ mechanism involving successive hydrogen abstractions. It can be estimated that the average number of H–D exchanges occurring per radical pair, from formation to termination, is 14.3.