Time-resolved study of acetyl radical in zeolite NaY by step-scan FT-IR spectroscopy

Step-scan FT-infrared spectroscopy of l-naphthyl acetate and pinacolone pho todissociation in zeolite NaY revealed a transient at 2125 cm(-1) assigned to acetyl radical. Comparison of the intensities of transient and final pro duct absorptions, 2-acetyl-1-naphthol in the case of naphthyl acetate and a cetaldehyde for pinacolone, indicates that the acetyl radical represents th e main, and probably only, reaction channel. The decay of the radical at ro om temperature is best described by a single-exponential law with a lifetim e of 71 +/- 15 mu s (1-napthyl acetate) and 315 +/- 30 mu s (pinacolone). T his constitutes the first detection and kinetic study of a small transient radical in a zeolite. The kinetic result is interpreted in terms of complet e separation of the photogenerated pairs from the parent supercage, followe d by random walks in subspaces of the zeolite lattice imposed by the much l ess mobile precursor molecules. These force the geminate radicals to react and thereby contribute to the high selectivity of these photoreactions.