A new atmospheric pathway to hydrogen cyanide (HCN) formation is proposed via CH₂NOX intermediates, generated from CH₂NO ^• and reactive radicals. Quantum chemical and kinetic analyses show that among water (H₂O), ammonia (NH₃), hydrochloric acid (HCl), and sulfuric acid (H₂SO₄) as catalysts, H₂SO₄ significantly enhances CH₂NOCH₃ decomposition. The study highlights CH₂NO ^• as a key intermediate in atmospheric nitrogen chemistry.

Hydrogen cyanide (HCN) is a chemically and prebiotically important molecule found in the Earth's atmosphere and other planetary environments. Previous photochemical studies have proposed that HCN could originate from reactions between methane photolysis products, such as methyl radical (CH₃ ^• ) and triplet methylene (³CH₂₎, and reactive nitrogen species like atomic nitrogen (N) and nitric oxide (NO). In this study, we introduce a new atmospheric route to HCN formation involving the decomposition of CH₂NOX intermediates, which are formed via the recombination of CH₂NO ^• with other atmospheric reactive species (X) such as NO, OH ^• , and CH₃ ^• . Using high-level quantum chemical calculations [CCSD(T)//M06-2X/6-311++G(3df,3pd)], we investigate the mechanism of CH₂NOX decomposition towards HCN formation via uncatalyzed and catalyst-assisted (H₂O, NH₃, HCl and H₂SO₄) pathways. Kinetic analysis based on transition state theory (TST) reveals that, while CH₂NONO and CH₂NOOH exhibit significant kinetic barriers under ambient conditions, CH₂NOCH₃ undergoes rapid decomposition, particularly when catalyzed by H₂SO₄. Among all species examined, the H₂SO₄-assisted decomposition of CH₂NOCH₃ shows the highest rate enhancement relative to its uncatalyzed counterpart. This work not only introduces CH₂NO ^• as a novel intermediate in atmospheric nitrogen chemistry but also highlights the key role of CH₃ ^• (a methane photolysis product) and H₂SO₄ in enabling efficient HCN production in both early and modern Earth atmospheres.