A Photoionization Reflectron Time-of-flight Mass Spectrometric Study on the Formation of Acetic Acid (CH 3 COOH) 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.