OBSERVATION AND MODELING OF THE RECOMBINATION KINETICS OF DIPHENYLMETHYL RADICALS IN THE CAVITIES OF NA-X ZEOLITE

An experimental and theoretical study is reported of the recombination of diphenylmethyl radicals generated from precursors included in Na-X zeolite. These precursors, 1,1,3,3-tetraphenylacetone and 1,1-diphenylacetone, are decomposed by flash photolysis, yielding a radical pair in a triplet configuration. The diphenylmethyl radical concentration, monitored by time-resolved diffuse reflectance spectroscopy, decreases roughly linearly with logarithmic time, indicative of a reaction rate that decreases gradually. The overall decay pattern depends only weakly on temperature, precursor, or laser dose. Product analysis indicates that geminate recombination, which requires a radical pair in a singlet configuration, is the dominant decay mechanism, consistent with the known mobility of diphenylmethyl radicals in the Na-X lattice. This face-centered lattice consists of large cavities each connected by channels to four other cavities situated at the corners of a tetrahedron. To describe the diffusion of the radicals in this lattice, a random walk model is adopted and solved by computer simulation. For short times (200 ns-10-mu-s) and low precursor concentrations, the rate of geminate recombination generated by the model yields a satisfactory reproduction of the observed time dependence, but for long times the fraction of radicals surviving geminate recombination falls well below the theoretical limit of 51%. There is evidence that quenching processes other than nongeminate recombination are responsible for this behavior. It is found that the spin flip required to allow recombination is fast on the time scale of radical hopping and that the hopping rate of the order of 10(6)-10(7) s-1 at room temperature is thermally activated with a substantial activation energy (approximately 2 kcal/mol in the range 244-327 K).