Subsequent\nReaction of CH<sub>2</sub>(<sup>1</sup>A) with N<sub>2</sub> Molecule\nas a Potentially Important Source\nof HCN in the Atmosphere of Titan: Studies by Quantum-Chemical and\nStatistical Rate Theories

In this theoretical research, the possibility of the\nformation\nof the significant prebiotic hydrogen cyanide molecule and other important\nspecies in Titan’s atmosphere through the subsequent reactions\nof singlet methylene species, <sup>1</sup>CH<sub>2</sub>, with the\nN<sub>2</sub> molecule is investigated. The stationary points geometries\nand energies of species involved in the studied reaction are calculated\nby high-level quantum-chemical methods such as W1RO and CCSDT(Q) methods.\nNext, the rate coefficients for the formation of products are computed\nby sophisticated statistical rate theories including RRKM and VRC-TST.\nIt is inferred from a previous theoretical study that CH<sub>2</sub>NN is produced predominantly from the reaction of <sup>1</sup>CH<sub>2</sub> with N<sub>2</sub> in the atmosphere of Titan [Xu, S.; Lin, M.\nC.J. Phys. Chem. A 2010, 114, 5195–5204]. The reactive CH<sub>2</sub>NN molecules\nreact with other atmospheric species like <sup>1</sup>CH<sub>2</sub> to produce new species. According to the present study, <sup>1</sup>CH<sub>2</sub> species add to CH<sub>2</sub>NN molecules through\nrelatively fast barrierless processes to produce some chemically activated\nintermediates. These intermediates rapidly decompose to yield 2 NCH<sub>2</sub>, HCN + CH<sub>2</sub>NH, and C<sub>2</sub>H<sub>4</sub> +\nN<sub>2</sub> products. The calculated data reveal that HCN and C<sub>2</sub>H<sub>4</sub> are efficiently produced from the subsequent\nreaction of <sup>1</sup>CH<sub>2</sub> with N<sub>2</sub> molecules\nin the atmosphere of Titan. The following rate constant expressions\nare suggested for the computed rate coefficients for the production\nof 2 NCH<sub>2</sub> (<i>k</i><sub>1</sub>), HCN + CH<sub>2</sub>NH (<i>k</i><sub>2</sub>), and C<sub>2</sub>H<sub>4</sub> + N<sub>2</sub> (<i>k</i><sub>3</sub>) from <sup>1</sup>CH<sub>2</sub> + CH<sub>2</sub>NN reaction over the temperature\nrange 200–700 K: <i>k</i><sub>1</sub> = 5.48 ×\n10<sup>–10</sup> (T/300)<sup>0.258</sup> exp (255/T) <i>k</i><sub>2</sub> = 1.15 × 10<sup>–14</sup> (T/300)<sup>0.901</sup> exp (355/T) <i>k</i><sub>3</sub> = 3.10 ×\n10<sup>–10</sup> (T/300)<sup>−0.428</sup> exp (130/T).