BENZENE PHOTOSENSITIZATION AND DIRECT PHOTOLYSIS OF CYCLOBUTANONE AND CYCLOBUTANONE-2-T IN GAS PHASE

The fluorescence of the 1B2u-state benzene is quenched by cyclobutanone with a quenching cross section (πσ2) of 4 Å2, and this singlet energy transfer produces an excited singlet cyclobutanone which subsequently decomposes to yield primarily ethylene (plus ketene). On the other hand, the 3B1u-state benzene transfers its electronic energy to cyclobutanone with a quenching cross section (πσ2) of 1 Å2 to produce an intermediate which subsequently decomposes to yield a vibrationally excited cyclopropane (plus CO). The yield of cyclopropane is lowered by the addition of cw-2-butene which is a quencher of the triplet benzene, and therefore it is proposed that cyclopropane is a characteristic decomposition product of a triplet cyclobutanone. Sensitization quantum yields have been determined, and they are near unity. The comparison of the decomposition product distribution in the benzene-photosensitized samples with that observed at varying excitation energies (wavelength) in the direct photolysis of cyclobutanone indicates that, on the average, about 96 kcal/mole of excitation energy is delivered to cyclobutanone in the singlet energy transfer while about 80-85 kcal/mole of excitation energy is delivered to cyclobutanone in the triplet energy transfer. © 1968, American Chemical Society. All rights reserved.