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 S8. 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 + S2 + 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-1/298.15)]. The second-order rate constant of S + S2 → S3 is 6.47 × 10-14 cm3 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-1/298.15)] 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. [ABSTRACT FROM AUTHOR]