Subsequent Reaction of CH₂(¹A) with N₂ Molecule as a Potentially Important Source of HCN in the Atmosphere of Titan: Studies by Quantum-Chemical and Statistical Rate Theories

In this theoretical research, the possibility of the formation of the significant prebiotic hydrogen cyanide molecule and other important species in Titan’s atmosphere through the subsequent reactions of singlet methylene species, 1CH2, with the N2 molecule is investigated. The stationary points geometries and energies of species involved in the studied reaction are calculated by high-level quantum-chemical methods such as W1RO and CCSDT(Q) methods. Next, the rate coefficients for the formation of products are computed by sophisticated statistical rate theories including RRKM and VRC-TST. It is inferred from a previous theoretical study that CH2NN is produced predominantly from the reaction of 1CH2 with N2 in the atmosphere of Titan [Xu, S.; Lin, M. C. J. Phys. Chem. A 2010, 114, 5195–5204]. The reactive CH2NN molecules react with other atmospheric species like 1CH2 to produce new species. According to the present study, 1CH2 species add to CH2NN molecules through relatively fast barrierless processes to produce some chemically activated intermediates. These intermediates rapidly decompose to yield 2 NCH2, HCN + CH2NH, and C2H4 + N2 products. The calculated data reveal that HCN and C2H4 are efficiently produced from the subsequent reaction of 1CH2 with N2 molecules in the atmosphere of Titan. The following rate constant expressions are suggested for the computed rate coefficients for the production of 2 NCH2 (k1), HCN + CH2NH (k2), and C2H4 + N2 (k3) from 1CH2 + CH2NN reaction over the temperature range 200–700 K: k1 = 5.48 × 10–10 (T/300)0.258 exp (255/T) k2 = 1.15 × 10–14 (T/300)0.901 exp (355/T) k3 = 3.10 × 10–10 (T/300)−0.428 exp (130/T).