Diffusion And Percolation Of Radical Pairs In Zeolite Media - A Product Analysis Study

The photochemistry of dibenzyl-d5 ketone (DBK-d5) adsorbed in the zeolite NaX was investigated as a function of substrate loading. The cage effect and the relative yields of 1,2-diphenylethane (DPE), o-methyl-beta-phenylacetophenone (o-MAP), and p-methyl-beta-phenylacetophenone (p-MAP) were found to depend dramatically on the loading of the starting material present. These results and the variations observed in the percent cage effect are described in terms of local and global effects that determine the influence of the zeolite media. Changes in reactivity as a function of reactant loading are explained in terms of percolation theory by using the model "ants in a labyrinth" proposed by de Gennes. The diffusing radicals play the role of the ants and the disposition of the reactant in the regular zeolite topology determines the nature of the labyrinth. This model implies that the diffusion coefficient of the radicals is larger than the diffusion coefficient of the starting ketone. The model is supported by trapping experiments with an oxygen scavenger and by experiments carried out at -20-degrees-C where the diffusion of the radicals is largely diminished.