NCCN and NCCCCN Formation in Titan's Atmosphere:  2. HNC as a Viable Precursor

Quantum chemical calculations, at the CCSD(T)/aug-cc-pVXZ//B3-LYP/6-311G** (X = D, T) levels of theory, have been used to characterize stationary points on the C2HN2 and C4HN2 potential energy surfaces. The calculations permit evaluation of the reactions of CN (X 2Σ+) and C3N (X 2Σ+) with the isomers HNC and HCN, which have been proposed as possible sources of the dicyanopolyynes NCCN and NCCCCN within Titan's upper atmosphere. In keeping with previous studies, we find that the reaction of CN (X 2Σ+) with HCN is inhibited by a significant activation energy barrier for all feasible product channels, while CN (X 2Σ+) + HNC lacks an overall barrier to formation of NCCN + H (2S) and to HCN + CN (X 2Σ+) with the NCCN product channel likely dominant. The C4HN2 surface, studied here for the first time, does not possess overall barriers for the processes C3N (X 2Σ+) + HNC → NCCCCN + H (2S), C3N (X 2Σ+) + HNC → C3N (X 2Σ+) + HCN, C3N (X 2Σ+) + HCN → NCCCCN + H (2S), and HC3N + CN (X 2Σ+) → NCCCCN + H (2S). We discuss the implications of these results, and the implied high efficiency of dicyanopolyyne formation in the reactions found to lack overall barriers, for the upper atmospheric chemistry of Titan.