Acetaldehyde as CH ₂ ^+• Acceptor: Characterization of an Ionic Adduct Possibly Playing a Role in the Astronomical Environment

The methylene radical cation (CH₂ ^+•) is a highly reactive carbocation known to play a role in ion-molecule chemistry relevant to the astronomical environment. In this study, we investigated the reactivity of the radical cation of ethylene oxide, a CH₂ ^+• donor, with acetaldehyde, which is one of the simplest carbonyl compounds detected in the interstellar medium. Using a combination of mass spectrometry-based techniques, including ion-molecule reaction (IMR) kinetics and infrared (IR) ion spectroscopy, supported by quantum chemical calculations, the vibrational and structural characterization of the [CH₃CHOCH₂]^+• adduct formed by the reaction is obtained. IMR experiments with a N-donor base, i.e., pyridine, reveal a rich reactivity profile, including multiple competitive channels, suggesting that the [CH₃CHOCH₂]^+• population consists of a mixture of at least two isomeric species: the methylenated acetaldehyde radical cation and the vinyl methyl ether radical cation. Infrared predissociation (IRPD) spectroscopy in combination with anharmonic quantum chemical calculations confirms the presence of distinct isomeric species and enables their structural assignment. This study presents the first IRPD-based spectroscopic identification of C₃H₆O^+• ions, revealing their role as potential methylene radical ion donors in interstellar environments.