A NOVEL MECHANISM FOR REACTIONS OF THIIRANE WITH THE THIIRANE RADICAL-CATION - AN EXPERIMENTAL AND AB-INITIO STUDY

An experimental and computational study of the ion-molecule reactions initiated by the reaction of SC2H4.+ with SC2H4 has been carried out. The dimer ion is observed in a hybrid drift tube/ion source under chemical ionization conditions at low temperatures and high pressures. However, the ion intensity is much lower than expected for a product containing a stable two-center three-electron (2c-3e) S=S bond. Above room temperature, ion intensities for m/z 92 S2C2H4.+, m/z 124 S3C2H4.+, m/z 156 S4C2H4.+, and m/z 184 S4C4Hg.+ dominate. The first three ions result from sequential association of SC2H4.+ with SC2H4 followed by elimination of ethylene, while the latter ion (m/z 184) is due to the association of S3C2H4.+ with SC2H4 Without loss of ethylene. As the temperature is lowered, clustering is promoted relative to reaction channels involving loss of ethylene. The experimental results are not in full agreement with a published ab initio study which found that the product channel was 33-kJ/mol endothermic with respect to reactants. The present ab initio calculations indicate that another reaction channel is possible, resulting in a four-membered-ring C-S2C2H4.+, which is exothermic by 43.0 kJ/mol. The potential energy surface for this channel, including transition states, was computationally studied in detail. An interesting aspect of this mechanism is the stepwise addition of sulfur atoms forming successively larger rings which continues until the six-membered-ring S4C2H4.+ is formed.