A Mechanistic Study on the Formation of Acetic Acid (CH₃COOH) in Polar Interstellar Analog Ices Exploiting Photoionization Reflectron Time-of-flight Mass Spectrometry

Acetic acid (CH₃COOH) is considered a key molecule in the formation of the simplest amino acid, glycine, and consequently peptides. It is ubiquitous in the interstellar medium and has been detected toward hot cores, in the coma of comets, and on the surface of the comet 67P/Churyumov–Gerasimenko by the Rosetta mission. Here we present the isomer-selective formation of acetic acid in polar ice mixtures of water/acetaldehyde upon exposure to ionizing radiation in the form of energetic electrons as a proxy for secondary electrons generated once Galactic cosmic rays pass through interstellar ices. Acetic acid is formed even at low irradiation doses of only 0.13 eV molecule⁻¹ (deuterium oxide) and 0.29 eV molecule⁻¹ (acetaldehyde), representing molecular cloud lifetimes of 1 × 10⁶ yr. Isotopic substitutions reveal that the dominant formation pathway is the barrierless radical–radical recombination of acetyl (CH₃CO) with hydroxyl-d ₁ radicals (OD), whereas oxygen insertion does not yield any detectable amounts of acetic acid. This hitherto unknown reaction pathway will influence the relative abundances of distinct C₂H₄O₂ isomers in chemical models aiming to constrain the reaction conditions by comparing these abundances. In contrast to its formation in nonpolar model ices, the formation in this polar binary ice is isomer-selective and produces acetic acid only.