Combined crossed-beams and theoretical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments

Acrylonitrile (CH2CHCN) is ubiquitous in space (molecular clouds, solar-type protostars, circumstellar envelopes) and is also abundant in the upper atmosphere of Titan and in comets. The reaction O(3P) + CH2CHCN can be of relevance in the chemistry of extraterrestrial environments because of the abundance of atomic oxygen. The oxidation of acrylonitrile is also relevant in combustion, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including acrylonitrile. Despite its relevance, limited information exists on this reaction. We report on a combined experimental and theoretical investigation of the reactions of acrylonitrile with both ground 3P and excited 1D atomic oxygen. From product angular and time-of-flight distributions in crossed molecular beam experiments with mass spectrometric detection at the collision energy, Ec, of 31.4 kJ/mol, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist interpretation of the experimental results and elucidate the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have been carried out. Combining the experimental and theoretical results, we conclude that the O(3P) reaction leads to two main product channels: (i) CH2CNH (ketenimine) + CO (dominant, BF = 0.87±0.05), formed via efficient intersystem crossing from the entrance triplet PES to the underlying singlet PES, (ii) HCOCHCN + H (minor, BF = 0.13±0.05), occurring adiabatically on the triplet PES. Our study suggests to include this reaction both as a possible destruction pathway of CH2CHCN and a possible formation route of CH2CNH in the interstellar medium. The O(1D) + CH2CHCN reaction mainly leads to formation of CH2CNH + CO adiabatically on the singlet PES. This result can improve models related to the chemistry of interstellar ice and cometary comas, where O(1D) reactions are believed to play a role. Overall, our results are expected to be useful to improve models of combustion and extraterrestrial environments.