Combined crossed-beams and theoretical investigation of the O( 3 P) + acrylonitrile reaction: Dominant formation of ketenimine (CH 2 CNH) via intersystem crossing Giacomo Pannacci 1 , Luca Mancini 1 , Pengxiao Liang 1 , Gianmarco Vanuzzo 1 , Demian Marchione 1 , Pedro Recio 1 , Marzio Rosi 2 , Dimitrios Skouteris 3 , Piergiorgio Casavecchia 1 and Nadia Balucani 1 1 Dept. of Chemistry, Biology and Biotechnology, Italy, 2 Dept. of Civil and Environmental Engineering, University of Perugia, Italy, 3 Master-Tec Srl, Italy The relevance of the oxidation of acrylonitrile, CH 2 CHCN (also termed cyanoethene or vinylcyanide or 2-propenenitrile), a widespread nitrile compound, covers areas ranging from combustion to astrochemistry. As a matter of fact, thermal combustion and selective catalytic combustion are the main treatments to reduce acrylonitrile emissions into the atmosphere following its industrial uses, being CH 2 CHCNa volatile compound with toxic/carcinogenic properties. Most notably, CH 2 CHCN was the first molecule with a C=C double bond to be detected in the Interstellar Medium (ISM) and the inclusion in models of its oxidation (O is the third most abundant element in the ISM) could open new insights on the formation/destruction processes of biologically relevant species. In this context, we have investigated the O( 3 P)+CH 2 CHCN reaction by combining synergistically crossed molecular beam (CMB) experiments and theoretical calculations. This combined effort is needed because the reactions of O( 3 P) with unsaturated hydrocarbons are multichannel reactions where, after the initial attack of the O-atom to the electron rich unsaturated bond(s), the triplet diradical intermediate can undergo a unimolecular decomposition adiabatically on the triplet Potential Energy Surface (PES), or nonadiabatically on the singlet PES via intersystem crossing (ISC), and many different products can be formed. The CMB technique with mass spectrometric detection and time-of-flight analysis, coupled to theoretical calculations of the relevant triplet/singlet PESs and statistical (RRKM/Master Equation) estimates of product branching fractions (BFs) with inclusion of ISC, is the best resource to unveil the micro-mechanism of O( 3 P) reactions and to identify the primary products and their relative yields (BFs), as reported in numerous publications concerning the reactions between O( 3 P) and unsaturated 1,2 as well as aromatic hydrocarbons. 3 By applying a similar combined CMB/theoretical approach for the study of the O( 3 P)+acrylonitrile reaction, we have found that the reactive flux is dominated by ISC from the triplet to the singlet PES following a barrierless addition of O( 3 P) to the methylenic carbon of acrylonitrile, with CO+CH 2 CNH (ketenimine) being the most favored product channel (BF ∼ 0.9). The reaction leads also, to a minor extent, to H+HCOCHCN adiabatically on the triplet PES (BF ∼ 0.1). 4 Because the title reaction represents a possible destruction pathway of CH 2 CHCN and a possible formation route of ketenimine, a key intermediate towards the formation of biologically relevant molecules, also at the typical temperature of SgrB2(N) hot cores, our results are expected to contribute to the improvement of not only current combustion models, but also current astrochemical models. Acknowledgments : This work was supported by MUR and University of Perugia (Department of Excellence- 2018-2022-Project AMIS) and MUR (PRIN 2017, MAGIC DUST, Prot. 2017PJ5XXX). P.L. acknowledges support from the Marie Sklodowska-Curie project "Astro-Chemical Origins" (Grant No. 811312). D.M. thanks ASI (DC- VUM-2017-034, Grant No. 2019-3 U.O Life in Space). References 1. C. Cavallotti, et al., Faraday Discuss."Unimolecular Reactions" (2022), in press (DOI: 10.1039/D2FD00037G).A. Caracciolo, et al., J. Phys. Chem. A 123 , 9934 (2019). 2. G. Vanuzzo, et al., J. Phys. Chem. A 125 , 8434 (2021).G. Pannacci, et al., Phys. Chem. Chem. Phys. , in preparation.
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