CH₃NO as a potential intermediate for early atmospheric HCN: a quantum chemical insight

Hydrogen cyanide (HCN) has played a central role in the production of several biological molecules under prebiotic conditions on primitive Earth. Previously, K. J. Zahnle (J. Geophys. Res.: Atmos., 1986, 91, 2819) and Tian et al. (Earth Planet. Sci. Lett., 2011, 308, 417) emphasized that HCN production in the early Earth's CH₄-rich atmosphere could have been possible through the reaction between active nitrogen atoms (N) and methane photolysis products. Here, we have proposed alternative pathways for the formation of early atmospheric HCN via the decomposition of CH₃NO as an intermediate. In the early Earth's O₂-free atmosphere, CH₃˙ could preferentially attach to NO, which was generated via early atmospheric volcanism or lightning and photochemical processes. We have quantum chemically explored both unimolecular and bimolecular decomposition pathways of CH₃NO via the assistance of another CH₃NO molecule and via H₂O, NH₃, HCl, HCOOH, HNO₃ and H₂SO₄ catalysis. Both energetic and kinetic analyses reveal that H₂SO₄ is more efficient in this regard than other atmospheric species. Overall, it has been suggested that the proposed bimolecular decomposition pathways might have been alternative pathways for the formation of HCN under certain conditions on prebiotic Earth, while the unimolecular decomposition of CH₃NO could lead to the formation of HCN in the high temperature volcanic environment on early Earth.