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

TL;DR

This work addresses the need for exhaustive, automated exploration of reactive PESs in astrochemistry by applying a fully automated NT2-based workflow (Chemoton) to the CH + oxirane reaction, a highly entangled test case with a large network of possible products. High-level energy refinements ($ ext{DLPNO-CCSD(T)-F12/cc-pVDZ-F12}$) and phase-space-ergodic kinetics (MESS with Ar as collider and PST treatment of entrance channels) yield temperature- and pressure-dependent rate constants for bimolecular channels across ISM-like conditions. The resulting network comprises 212 compounds, 818 reactions, and over 9300 elementary steps, with HCO + C2H4 as the dominant product (~85%), and minor channels such as trans-propenal (~8–9%) and 2H-oxetene (~4–5%); methyl ketene is a minor channel (~0.5%). These results illustrate the power of automated PES exploration to reveal extensive, high-connectivity reaction networks and provide actionable kinetic data to improve astrochemical models, while also offering insights into ISM abundance patterns and the feasibility of ring-opening vs ring-expansion pathways.

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.

Figures (14)

• Figure 1: Degree distribution, i.e., number of incoming + outgoing reactions for each compound, of the reaction network. The molecules corresponding to the highest degrees, namely 42, 46, 47, and 53, are depicted. The degree of 75 corresponds to the hydrogen atom. The second molecule with degree 42 is H2.
• Figure 2: Panel (a) reports the congested network of the reaction between oxirane and the CH radical considering at the center of the figure the most connected species, the H atom. Green points indicate minima (exit products or intermediates), while blue points are weakly bound complexes. The orange arrows indicate the IRC connecting the different intermediates. Panel (b) shows the barrierless approaches between the two reactants and the intermediates/complexes formed.
• Figure 3: Panel (a) shows the interconnection among the four entrance intermediates (INT1a/INT1b, INT2, INT3) and the products directly accessible from them. The entrance channels (already shown in Fig. \ref{['fig:global_entrance']}) are in black, while the new channels are in blue. The dashed lines indicate a barrierless dissociation from vdW wells to products. For simplicity, the species are indicated with the label introduced in Figs. \ref{['figA1']}, \ref{['figA2']}, and \ref{['figA3']} of the Appendix. Panel (b) reports the three main products of the reaction according to the present kinetic simulation.
• Figure 4: Shortest reaction path according to Pathfinder for the formation of (a) s-trans-propenal (P18), (b) methyl ketene (P55), and (c) 2H-oxetene (P17).
• Figure A1: Bimolecular products having the H atom as co-fragment incorporated in the microkinetic modeling performed with MESS. Their ZPE-corrected relative energy (RE) with respect to the oxirane + ^.CH pair is reported in kJ/mol. ZPE correction is at the PBE-D3/def2-SVP level, electronic energy at the DLPNO-CCSD(T)-F12/cc-pVDZ-F12 level. Hydrogen atoms are shown in white, carbon atoms in grey, and oxygen atoms in red. Bonds that cannot be freely rotated, e.g., double or triple bonds, are colored.