High-temperature reactions of O + COS and S + SO₂. Abstraction versus substitution channels

The main concern of this study is to investigate the reaction mechanism of O + COS → products (1). Experiments are conducted by use of an excimer laser photolysis shock tube technique, where mixtures of COS and SO₂ diluted in Ar are photolyzed behind reflected shock waves. Time-resolved measurements of O and S atoms are conducted by use of atomic resonance absorption spectroscopy, and the overall rate constant of (1) is determined by the O atom decay at temperatures 1250-1600 K, i.e., k₁ = 10^-10.18±0.26 exp[-(22.5 ± 7.1) kJ·mol^-1/RT] cm³ molecule^-1 s^-1, which is in good agreement with the former recommendation. It is confirmed in this experiment that the S atom is a direct product of reaction 1. By analysis of time profiles of the S atom, the branching fraction of the S production channel O + COS → S + CO₂ (1b), α, is determined against the main channel O + COS → CO + SO (1a), where the S atom consumption reactions, S + SO₂ → 2SO (2) and S + COS → S₂ + CO (3), are inevitably taken into account. The present experimental result is expressed as α = (0.40 ± 0.10) - (202 ± 137)/T(T= 1120-1540 K). Also, in this kinetic analysis, the rate constant of (2) is simultaneously determined to be k₂ = l0^-11.01±0.33 exp[-(37.8 ± 8.2) kJ·mol^-1/RT]. The reaction mechanism of (1) is examined by comparing the experimental results with those of ab initio potential energy surface/transition state theory calculations.