The reaction of sulfur atoms with carbon disulfide: Potential energy surface features

The lowest singlet and triplet potential energy surfaces of the reaction, S+CS₂→CS₃→CS+S₂, were investigated by the 6-31G* ab initio self-consistent-field (SCF) method with the inclusion of electron correlation by Møller-Plesset perturbation theory. The triplet reactants and products [S(³P)+CS₂(¹Σ⁺_g),S₂(³Σ^-_g)+CS(¹Σ⁺)] are predicted to be more stable than their singlet counterparts [S(¹D)+CS₂(¹Σ⁺_g),S₂(¹Δ_g)+CS(¹Σ⁺)] in agreement with experiment. However, the CS₃ complex is more stable in its singlet as opposed to triplet state, leading to interesting surface crossings in the intermediate regions of the surfaces. The triplet surface contains shallow wells corresponding to cis- and trans-CS₃ chain isomers. A low-lying singlet C_2v ring structure, carbon trisulfide, was connected to a chain molecule, carbon disulfide S-sulfide, by a relatively low-lying transition state. Another transition state was found with a modest barrier, which joined the C_2v ring structure to another relatively low-lying minimum, a symmetric D_3h structure. Relative to the singlet ring compound, the singlet chain isomer and the D_3h structure are 14.3 and 4.1 kcal/mol higher in energy, and the triplet cis and trans minima 31.2 and 34.1 kcal/mol higher in energy. At the unrestricted Hartree-Fock level (UHF or UMP2), transition states join the chain structure to both reactants and products on the singlet surface. From the experimental evidence and the predictions of this work, an intersystem crossing from the triplet potential energy surface to the singlet one is expected in the region of those transition states leading to the products, CS+S₂.