SOLVENT ISOTOPE EFFECT ON THE RATE CONSTANTS OF SINGLET-OXYGEN QUENCHING BY EDTA AND ITS METAL-COMPLEXES

The reactivity of singlet oxygen (O-2((1) Delta(g),)) with edta and it s metal complexes with Al3+, CU2+, Fe3+, and Mn2+ was investigated. Th e emission of singlet oxygen at 1270 nm in D2O was measured in order t o determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constants (k(r) k(q)) of the chemical reaction between singlet oxygen and the accepto r (k(r)) and of the physical quenching of singlet oxygen by the accept or (k(q)) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H2O were used to determine quantum yields of th e sensitized photooxidation, and the combined results of these experim ents allowed the determination of k(r) and k(q) separately. A strong i sotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical p rocedure used. The isotope effect, as well as the reactivity of edta a nd its metal complexes, depend markedly on the complexed metal ion.