The temperature dependence and the mechanism of the SO₂ (³<i>B</i>₁) quenching reactions

Abstract

Abstract The Arrhenius parameters have been determined for the SO 2 ( 3 B 1 ) quenching reaction (9), SO 2 ( 3 B 1 ) + M → (SO 2  M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO 2 ( 3 B 1 ) molecules was accomplished by two very different methods: (1) a 3829 Å laser pulse generated the triplet directly through absorption within the “forbidden” SO 2 ( 3 B 1 ) → SO 2 ( 1 A 1 ) band; (2) a broadband Xe‐flash system generated SO 2 ( 3 B 1 ) molecules and triplets were formed subsequently by intersystem crossing, SO 2 ( 1 B 1 ) + M → SO 2 ( 3 B 1 ) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N 2 , 2.9 ± 0.4; 0 2 , 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO 2 , 2.8 ± 0.4; CO, 2.7 ± 0.4; CH 4 , 2.5 ± 0.6. This energy corresponds to the first region of the SO 2 ( 3 B 1 ) → SO 2 ( 1 A 1 ) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO 2 ( 3 B 1 ) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n ‐butane, cyclohexane, and isobutane, which parallels closely the decrease in CH bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge‐transfer complex and subsequent chemical interactions between the SO 2 ( 3 B 1 ) molecule and the π‐system of these compounds.