2023MNRAS.522.4215F

Nearly two decades since the detection of cyclopropenone (c-C₃H₂O) in the interstellar medium (ISM), the understanding of how this molecule comes to be remains incomplete. Many hypotheses place the ubiquitous hydrocarbon c-C₃H₂ at the centre of such discussions. However, insights into c-C₃H₂ chemistry are further complicated by the recent detection of ethynyl cyclopropenylidene (c-C₃HC₂H) and the observation of a radio line possibly belonging to methylenecyclopropene (c-C₃H₂CH₂). In a necessary reconciliation of past and current work on the chemical capabilities of c-C₃H₂ in interstellar environments, the formation pathways of several functionalized cyclopropenes from c-C₃H₂ and a hydrogenated radical are explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations are used to evaluate the energies of reaction and generate structures along the reaction pathway for formation products deemed chemically plausible. Potential energy scans are used to include or rule out certain paths to product formation based on conformation to the necessary requirements of cold interstellar chemistry. Four functionalized cyclopropenes in addition to c-C₃H₂O have net exothermic reactions when forming from c-C₃H₂ (c-C₃H₂CC, c-C₃H₂S, c-C₃H₂NH, and c-C₃H₂CH₂). The former three are found to have reaction profiles favourable for formation in the cold ISM, while c-C₃H₂CH₂ can only form by passage through an association barrier that must be mitigated by an energy source of some kind. c-C₃H₂S and c-C₃H₂NH are the best candidates for new spectroscopic searches. A complete detection of c-C₃H₂CH₂ is necessary to fully understand cyclopropenylidene chemistry in the ISM.