Theoretical mechanistic study on the ion-molecule reactions of CCN⁺/CNC⁺ with H₂S

A detailed [C₂H₂NS⁺] potential energy surface in singlet, including 45 minimum isomers and 57 transition states, is built up at the B3LYP/6-311G(d,p) and CCSD(T)/6-311G(2df,p) (single-point) levels in order to explore the mechanisms of the important ion-molecule reactions between CCN⁺/CNC⁺ and H₂S. For the reactions of both CCN⁺ and CNC⁺ toward H₂S, product HCS⁺ + HNC may be the most abundant followed by the much less HCNH⁺ + CS and then HCS⁺ + HCN. Significant discrepancies on the product distributions are found between our calculated results and previous experimental finding. On the other hand, the reaction of HCNH⁺ + CS to form HCS⁺ + HCN/HNC is also considered by the [C₂H₂NS⁺] PES. For such reaction, the barrierless association may lead to the adduct HNC(H)CS⁺, while the proton-transfer may barrierlessly lead to product HCS⁺ + HCN/HNC via the hydrogen-bound complexes SCH⋯NCH⁺/SCH⋯CNH⁺. The computations reported in this paper may represent the first theoretical study on the chemical reactivity of the C₂N⁺ ion and may thus provide a useful guide for understanding the mechanisms of the other analogous reactions such as those of C₂N⁺ with H₂O, CH₄, NH₃, CH₃OH, etc. The present calculations may also provide useful information for future laboratory investigations on the HCNH⁺ + CS reactions that have not been previously studied. Interstellar implications of the title reactions are discussed.