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

Abstract This work investigates the synthesis of complex organic molecules with special focus on acetic acid (CH 3 COOH) via experiments involving the processing of astrophysical model ices of carbon dioxide–methane (CO 2 –CH 4 ) by low doses of ionizing radiation, exposing the initial bond-breaking processes and successive reactions initiated by energetic electrons generated in the track of galactic cosmic-ray particles penetrating ice-coated interstellar grains, deep inside molecular clouds in their early stages of evolution. The key results were obtained through single photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) and exploiting isotopically labeled samples (C 18 O 2 –CH 4 ; CO 2 –CD 4 ). Not only acetic acid (CH 3 COOH), along with fragments of acetic acid dimers (CH 3 COOH) 2 , but also the hitherto elusive interstellar methyl hydroperoxide (CH 3 OOH) and the hydrocarbons ethane (C 2 H 6 ) and butane (C 4 H 10 ), along with species belonging to C 2 H 4 O, C 2 H 6 O, and C 3 H 6 O 2 isomers, are swiftly formed via suprathermal reactions at doses of only 0.88 ± 0.12 eV per molecule of carbon dioxide and 0.32 ± 0.04 eV per molecule of methane, which is equivalent to doses deposited in just (2.0 ± 0.5) × 10 6 yr in a typical molecular cloud. The results suggest further that the search for acetic acid dimers (CH 3 COOH) 2 toward star-forming regions has a significant potential to be successful. Finally, methyl hydroperoxide (CH 3 OOH) and dimethyl peroxide (CH 3 OOCH 3 ), as identified previously in our laboratory, are predicted to be present in the interstellar medium, thus providing a homologous series of peroxides—HOOH, CH 3 OOH, and CH 3 OOCH 3 —to shed light on the interstellar oxygen chemistry.