Theoretical study on the ion–molecule reaction of HCN+ with NH3

A detailed theoretical study is carried out at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels as an attempt to investigate the mechanism of the little understand ion–molecule reaction between HCN+ and NH3. Various possible reaction pathways are considered. It is shown that six dissociation products P1(NH3+ + HCN), P2(NH4+ + CN), P3(NH3+ + HNC), P9(HCNH+ + NH2) P10(NCNH3+ + H), and P12(HNCNH2+ + H) are both thermodynamically and kinetically feasible. Among these products, P1 is the most competitive product with predominant abundance. P3 and P9 may be the second feasible products with comparable yields. P12 may be the least possible product followed by the almost negligible P2 and P10. Because the isomers and transition states involved in the HCN+ + NH3 reaction all lie below the reactant, the title reaction is expected to be rapid, which is consistent with the measured large rate constant in experiment. The title reaction may have a potential relevance in Titan’s atmosphere, where the temperature is very low. Furthermore, our calculated results are compared with the previous experimental findings.