DIFFUSE-REFLECTANCE SPECTROSCOPY OF VISIBLE ALKENE-O-2 CHARGE-TRANSFER ABSORPTIONS IN ZEOLITE-Y AND DETERMINATION OF PHOTOOXYGENATION QUANTUM EFFICIENCIES

Very large red shifts of charge-transfer bands of 2,3-dimethyl-2-butene.O-2, 2-methyl-2-butene.O-2, and trans-2-butene.O-2 complexes in zeolite NaY have been observed by diffuse reflectance spectroscopy. Absorption tails are shifted by over 10 000 cm(-1) compared to gas phase, solution, or solid oxygen matrix. In zeolite BaY with its divalent counterions, the 2-butene.O-2 absorption is further red shifted by several thousand wavenumbers. This points to stabilization of the excited charge-transfer states by the very strong electrostatic fields inside the zeolite cages as the origin of the observed red shifts. Reaction quantum efficiencies of previously reported selective photooxygenations of these alkenes by visible light are estimated on the basis of these spectra and found to be in the range 0.1-0.3.