Automated Exploration of Radical-Molecule Chemistry: The Case of Oxirane + CH in the ISM

TL;DR

This study employs an automated quantum chemistry workflow to comprehensively explore the reaction pathways of oxirane and methylidyne in the interstellar medium, revealing key products and kinetic behaviors relevant to astrochemistry.

Contribution

First application of automated reactive potential energy surface exploration to a complex astrochemical reaction network involving oxirane and CH.

Findings

Main product is HCO + ethene.

Formation of acrolein and 2H-oxene is also possible.

s-trans-propenal is more accessible than methyl ketene in the ISM.

Abstract

Quantum chemistry provides accurate and reliable methods to investigate reaction pathways of reactive molecular systems relevant to the interstellar medium. However, the exhaustive exploration of a reactive network is often a daunting task, resulting in unexplored reactive channels that affect kinetic outcomes and branching ratios. Here, an automated workflow for exploring reactive potential energy surfaces (PESs) is employed for the first time to study the oxirane (C$_2$H$_4$O) plus methylidyne ($^.$CH) reaction. The ultimate goal is to comprehensively map its PES and, subsequently, derive rate constants for the most important reaction channels. In addition to its astrochemical relevance, this reaction has been considered because it is a challenging test case, its network being very extended, with 60 exothermic bimolecular products lying below the reactant's energy. Kinetic simulations indicate that the main product of the reaction is the HCO radical plus ethene (C$_2$H$_4$), while formation of s-trans-propenal (acrolein) and 2H-oxene is also possible, but to a lesser extent. Based on the present study and other references in the literature, we suggest that the slightly higher relative abundance of s-trans-propenal compared to methyl ketene in the interstellar medium is a gas-phase kinetic effect, s-trans-propenal being a more easily accessible product on the C$_3$H$_5$O$^.$ PES.