The kinetics study of the S + S₂ → S₃ reaction by the chaperone mechanism

The recombination of S atoms has been found to be stepwise from the smallest unit, the elemental S atom, to the most abundant molecule S ₈. The reaction between S S₂ → S₃ has not been reported either experimentally or by theory, but may be a key intermediate step in the formation of sulfur aerosols in low-O₂ atmospheres. In this work, the kinetics of this reaction is reported with Ar gas used as the chaperone molecule in the production of S₃ via two complex intermediates: SAr + S₂ and S₂Ar + S. Quasi-classical and classical trajectory methods are used. The rate constant of the S + S ₂ + Ar → S₃ + Ar reaction is determined to be 2.66 × 10^-33 cm⁶ mol^-1 s^-1 at 298.15 K. The temperature dependence of the reaction is found to be 2.67 × 10^-33 exp[143.56(1/T-1298.15)]. The second-order rate constant of S + S₂ → S₃ is 6.47 × 10^-14 cm ³ molecule^-1 s^-1 at 298.15 K and the Arrhenius-type rate constant is calculated to be 6.25 × 10^-14 exp[450.15(1/T-1298.15)] cm³ molecule^-1 s^-1. This work provides a rate coefficient for a key intermediate species in studies of sulfur formation in the modern Venus atmosphere and the primitive Earth atmosphere, for which assumed model rate coefficients have spanned nearly 4 orders of magnitude. Although a symmetry-induced mass-independent isotope effect is not expected for a chaperone mechanism, the present work is an important step toward evaluating whether mass-independence is expected for thiozone formation as is observed for ozone formation.