NCCN and NCCCCN formation in Titan's atmosphere:: 1.: Competing reactions of precursor HCCN (3A) with H (2S) and CH3 (2A′)

Quantum chemical calculations, at the CCSD(T)/aug-ce-PVTZ//B3-LYP/6-311G** level of theory, have been used to characterize stationary points on the C2H2N and C3H4N potential energy surfaces. Reactions which occur on these surfaces include the processes H (S-2) + HCCN ((3)A) and CH3 (2A') + HCCN ((3)A), which are previously unconsidered but very important potential loss processes for the proposed key intermediate, HCCN ((3)A), implicated in the formation of NCCN and NCCCCN within the upper atmosphere of the Saturnian satellite Titan. We find that both H (S-2) + HCCN ((3)A) and CH3 (2A') + HCCN ((3)A) have exothermic product channels lacking positive activation energy barriers, and thus should occur with high efficiency at the low temperatures (similar to200 K) characteristic of Titan's atmosphere. We suggest that the occurrence of these competing reactions poses severe problems for the viability of existing mechanisms of Titanian NCCN and NCCCCN formation.