Depletion Kinetics of Chromium Atoms by Sulfur Dioxide

Abstract

The gas-phase depletion kinetics of Cr(a7S3, a5S2, a5DJ) in the presence of SO2 are reported. Chromium atoms were produced by the 248 nm photodissociation of chromium carbonyl and were detected by laser-induced fluorescence. The ground state of Cr, a7S3, was found to react rapidly via a termolecular mechanism with SO2. At 297 K, the limiting low-pressure third-order rate constant is ko = (2.68 ± 0.41) × 10-28 cm6 molecule-2 s-1, and the limiting high-pressure second-order rate constant is k∞ = (2.73 ± 0.38) × 10-10 cm3 molecule-1 s-1; the uncertainties represent ±1σ in precision. ko was found to decrease with increasing temperature. The binding energy of CrSO2 is estimated at 53 kcal mol-1 from combining the kinetic results with unimolecular rate theory and density functional theory. Cr(a5S2) depleted with a rate constant of k = 6.32 × 10-10 cm3 molecule-1 s-1, and the a5DJ spin/orbit states depleted at the collision rate of 2.97 × 10-10 cm3 molecule-1 s-1; overall uncertainties are estimated at ≤35%. Both chemical and physical quenching are likely for the excited states. The present work completes a study on the depletion kinetics of group 6 transition metal atoms by SO2. Results are interpreted in terms of long-range attractive forces between Cr and SO2 and the orbital occupancies of the Cr atomic states.