Collisional Deactivation of O₂(¹Δ_g) by Solvent Molecules. Comparative Experiments with ¹⁶O₂ and ¹⁸O₂

Abstract

Abstract Phosphorescence spectra of natural singlet oxygen 16 O 2 ( 1 Δ g ) and its heavier isotope 18 O 2 ( 1 Δ g ) have been recorded in several O 2 ( 1 Δ 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 ≤ 16 k XY / 18 k XY ≤ 1.9. The unexpectedly low isotope effect points to the important role of overtone excitations of X—Y during O 2 ( 1 Δ 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 O 2 isotope effect and the very weak temperature effect on k XY .