Formation of acetonitrile (CH3CN) under cold interstellar, tropospheric and combustion mediums

The knowledge of the formation of gas-phase carcinogenic acetonitrile (CH₃CN) is limited in interstellar, troposphere, and combustion mediums; thus, its formation and fate are of great importance for all these gas-phase environments when accessing its toxicity and its wide range of applications. In this work, we propose a mechanism for the formation of CH₃CN from the reaction of OH/O₂ on ethanimine (CH₃CH=NH) using ab initio/Density Functional Theory (DFT) potential energy surface in combination with microcanonical variational transition state theory (µVTST) and Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation to predict the rate constants and branching fraction in the temperature range of 100 K to 1000 K and pressure range of 0.0001 bar to 100 bar. The reaction starts with cis (Z) and trans (E) CH₃CH=NH isomer with OH radical followed by spontaneous formation via pre-reactive complex, forming the carbon and nitrogen-centered radicals. The O₂ radical then attacks the carbon and nitrogen-centered radicals to form acetonitrile (CH₃CN) and HO₂ radicals. The results show that N-H and C-H dominate the H-atoms abstraction by OH radicals is similar to its isoelectronic analogous reaction system, i.e., CH₃CHO + OH/O₂ and CH₃CHCH₂ + OH/O₂ and similar to methanimine (CH₂NH) systems. The calculated rate constants for OH-initiated oxidation of CH₃CH=NH are in the range of ~ 10^-11 cm³ molecule^-1 s^-1 (at 300 K) and are in very good agreement with previous experimental values of its isoelectronic reaction system. The atmospheric lifetime due to the loss of CH₃CHNH by OH radical (10 to 11 h) is in very good agreement with the similar pollutants in the troposphere temperature range between 200 and 320 K. The results indicate that its contribution to global warming is negligible. However, the formation of products such as CH₃CN may interact with other atmospheric species, which could lead to the production of potentially hazardous compounds such as cyanogen (N₂C₂) and hydrogen cyanide (HCN).