COLLISIONAL DEACTIVATION OF O-2(1-DELTA-G) BY SOLVENT MOLECULES - COMPARATIVE EXPERIMENTS WITH O-16(2) AND O-18(2)

Phosphorescence spectra of natural singlet oxygen O-16(2)(1-DELTA(g)) and its heavier isotope O-18(2)(1-DELTA(g)) have been recorded in several O2(1-DELTA(g)) weakly deactivating solvents. Lifetimes of singlet oxygen were determined for both isotopes in 21 solvents, From these data rate constants k(XY) of deactivation of singlet oxygen by terminal oscillators X-Y of the solvent molecules are calculated revealing a graduated isotope effect of 1.0 less-than-or-equal-to 16k(XY)/18k(XY) less-than-or-equal-to 1.9. The unexpectedly low isotope effect points to the important role of overtone excitations of X-Y during O2(1-DELTA(g)) deactivation. A modified collisional deactivation model including overtone excitation of the energy accepting oscillator reproduces the experimental findings: the tremendeous graduation of k(XY) of almost 5 orders of magnitude, the heavy atom effect, the O2 isotope effect and the very weak temperature effect on k(XY).