The formation of reactive singlet oxygen molecule ( 1 O 2 ), the a 1 D g state of excited O 2 , in the photocatalytic TiO 2 aqueous suspension system was shown to be evident by detecting the nearinfrared phosphorescence at 1270 nm by means of a gated photon counting method. The lifetimes of O 2 ( 1 D g ) in three different media (water, ethanol, 1 : 1 water-ethanol mixture) were elucidated and compared with those reported in corresponding homogeneous media. Based on the comparison of the lifetime with those of the other reactive species such as OH radicals and trapped holes, the contribution of 1 O 2 to the actual photocatalytic procedure was discussed.

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Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energy surface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with an exothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxy radical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through the four-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transition states (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in addition to the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggested that the rate constant for the backward reaction (E) of C6H5CH2OO-->C6H5CH2+O2 was several orders of magnitude higher that those for the product dissociation channels (B-D) for temperatures 300-1500 K and pressures 0.1-10 atm; therefore, it was also suggested that the dissociation of benzylperoxy radicals proceeded with the partial equilibrium between the benzyl+O2 and benzylperoxy radicals. The rate constants for product channels B-D were also calculated, and it was found that the rate constant for each dissociation reaction pathway was higher in the order of channel D>channel C>channel B for all temperature and pressure ranges. The rate constants for the reaction of benzyl+O2 were computed from the equilibrium constant and from the predicted rate constant for the backward reaction (E). Finally, the product branching ratios forming CH2O molecules and OH radicals formed by the reaction of benzyl+O2 were also calculated using the stationary state approximation for each reaction intermediate.