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).