The reaction mechanism of sulfur vapor (S) with nitrite ion (NO2 −) has been investigated theoretically on the triplet and singlet potential energy surfaces (PESs). All stationary points for the title reaction have been optimized at the B3LYP/6-311+G(3df) level. The energetic data have been obtained at the CCSD(T)//B3LYP level employing the 6-311+G(3df) basis set. Five stable collision complexes, 3IN1 (S–ONO−), 3IN2 (cyclic SONO−), 1IN1 (cis S–ONO−), 1IN2 (S–NO2 −), and 1IN3 (trans S–ONO−), have been considered on the triplet and singlet PESs through barrier-less and exothermic processes. By starting from these complexes, a simple mechanism has been obtained on the triplet PES while a complex mechanism has been considered on the singlet PES. The calculated results show that there are no favorable paths for the reaction of S with NO2 − on the singlet PES. Therefore, the S + NO2 − reaction proceeds only on the triplet PES to produce 3SO + 3NO− as main products. The results from the comparative study of S + NO2 − reaction mechanism with S + O3 (as isoelectronic and isostructure reactions) on the singlet PES show similarities in the overall trend of reaction mechanism and atom connectivity and differences in the stability of intermediates and the energy barriers of transition states.
