A Mechanistical Study on the Formation of Dimethyl Ether (CH₃OCH₃) and Ethanol (CH₃CH₂OH) in Methanol-containing Ices and Implications for the Chemistry of Star-forming Regions

Abstract

Abstract The underlying formation mechanisms of complex organic molecules (COMs)—in particular, structural isomers—in the interstellar medium (ISM) are largely elusive. Here, we report new experimental findings on the role of methanol (CH 3 OH) and methane (CH 4 ) ices in the synthesis of two C 2 H 6 O isomers upon interaction with ionizing radiation: ethanol (CH 3 CH 2 OH) and dimethyl ether (CH 3 OCH 3 ). The present study reproduces the interstellar abundance ratios of both species with ethanol to dimethyl ether branching ratios of (2.33 ± 0.14):1 suggesting that methanol and methane represents the key precursor to both isomers within interstellar ices. Exploiting isotopic labeling combined with reflectron time-of-flight mass spectrometry (Re-TOF-MS) after isomer selective vacuum ultra-violet (VUV) photoionization of the neutral molecules, we also determine the formation mechanisms of both isomers via radical–radical recombination versus carbene (CH 2 ) insertion with the former pathway being predominant. Formation routes to higher molecular weight reaction products such as ethylene glycol (HOCH 2 CH 2 OH), dimethyl peroxide (CH 3 OOCH 3 ), and methoxymethanol (CH 3 OCH 2 OH) are discussed briefly as well.