A combined crossed molecular beam and theorerical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments

Abstract

Acrylonitrile (CH₂CHCN) is ubiquitous in space (molecular clouds, solar-type star forming regions, and circumstellar envelopes) and is also abundant in the upper atmosphere of Titan. The reaction O(³P) + CH₂CHCN can be of relevance in the chemistry of the interstellar medium because of the abundance of atomic oxygen. The oxidation of acrylonitrile is also important in combustion as 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 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 a collision energy, E_c, of 31.4 kJ mol^-1, 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 interpret 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 with a BF of 0.87 ± 0.05), formed via efficient intersystem crossing from the entrance triplet PES to the underlying singlet PES, and (ii) HCOCHCN + H (minor, with a BF of 0.13 ± 0.05), occurring adiabatically on the triplet PES. Our study suggests the inclusion of this reaction 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 the 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 can play a role. Overall, our results are expected to be useful for improving the models of combustion and extraterrestrial environments.