The rate constants for oxygen quenching of aromatic hydrocarbon triplets have been measured by the laser flash photolysis technique. The quenching of high triplet energy compounds is characterised by rates which are inversely proportional to triplet energy. The reaction probabilities, which are as small as in hexane, have been found to increase in polar or viscous solvents. Molecules with low triplet energies are quenched at one-ninth the measured diffusion controlled encounter rate.These data are analysed in terms of the non-radiative transitions of a collision complex of the aromatic triplet and ground state oxygen. The importance of restrictive Franck-Condon factors which are determined by the hydrocarbon is discussed. The data are shown to be consistent with electronic matrix elements for the energy transfer quenching processes that are dependent on orbital symmetry matching and charge transfer interactions.
Raman spectroscopy experiments found the V@O stretching frequency for the supported VO 4 species to decrease with increasing catalyst temperature. Calculations on the vibrational frequencies of several models using density functional theory show that a consistent description of the experimental data can be obtained if we assume that the VO 4 species are anchored to the oxidic surface by one V-O bond only, in contrast to the traditional pyramidal model, which assumes three V-O support bonds and one V@O. The proposed VO 3 structure points away from the surface and consists of one V@O unit and an active oxygen ÔmoleculeÕ loosely bound to the vanadium atom, a peroxide species.
Recent measurements of the quenching of aromatic triplet states by oxygen are discussed. The dependence of the quenching rate constant on triplet energy is analysed in terms of a nonradiative transition within the collision complex between a ground state oxygen and a triplet state aromatic molecule. The Franck-Condon factor and electronic matrix element for this transition are considered. It is shown that the data are consistent with symmetry restrictions in the formation of 'Zi oxygen. Quantum yields of lZ; formation are calculated from the observed quenching rate constants and compared with experiment.
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