Formation of complex molecules in translucent clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via “nonenergetic” processing of C₂H₂ice

Abstract

Context. Complex organic molecules (COMs) have been identified toward high-and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C 2 H n O, such as acetaldehyde (CH 3 CHO), vinyl alcohol (CH 2 CHOH), ketene (CH 2 CO), and ethanol (CH 3 CH 2 OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts. Aims. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H 2 O ice under translucent cloud conditions (1≤A V ≤5 and n H ∼10 3 cm −3 ). We focus on the interactions of C 2 H 2 with H-atoms and OH-radicals, which are produced along the H 2 O formation sequence on grain surfaces at 10 K. Methods. Ultra-high vacuum (UHV) experiments were performed to study the surface chemistry observed during C 2 H 2 + O 2 + H codeposition, where O 2 was used for the in-situ generation of OH-radicals. These C 2 H 2 experiments were extended by a set of similar experiments involving acetaldehyde (CH 3 CHO) -an abundant product of C 2 H 2 + O 2 + H codeposition. Reflection absorption infrared spectroscopy (RAIRS) was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a Quadrupole mass spectrometer (TPD-QMS) was used as a complementary analytical tool. The IR and QMS spectral assignments were further confirmed in isotope labeling experiments using 18 O 2 . Results. The investigated 10 K surface chemistry of C 2 H 2 with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C 2 H 4 ) and ethane (C 2 H 6 ), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C 2 H 2 , acting as a molecular backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides a so far experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed.

Formation of complex organic molecules in molecular clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via the “energetic” processing of C2H2 ice

“…deformation) (Shimanouchi 1972;Bennett et al 2006;Kaiser et al 2014). This finding is fully in line with the solid-state hydrogenation scheme of C 2 H 2 → C 2 H 4 → C 2 H 6 , proposed in the literature (Tielens 1992;Hiraoka et al 1999Hiraoka et al , 2000Kobayashi et al 2017;Abplanalp & Kaiser 2020;Chuang et al 2020). The absence of C 2 H 3 and C 2 H 5 implies a low concentration of intermediate radicals, which have been suggested to associate with H-atoms following H 2 O dissociation.…”

“…The IR features around 1140 cm −1 correspond to the CO stretching mode of vinyl alcohol and CC stretching mode of acetaldehyde as has been confirmed in a previous isotope-labeled study using C 2 H 2 + (18) OH + H (see Fig. 2 in Chuang et al 2020). In the present work, therefore, a spectral deconvolution is applied using Gaussian fitting.…”

“…Unlike the case of molecules of the form C 2 H n O 2 , the formation chemistry of species of the form C 2 H n O, is still under debate (Enrique-Romero et al 2016. Several solid-state mechanisms, such as radical associations, O-atom additions, and hydroxylation, or gas-phase routes have been proposed (Charnley 2004;Balucani et al 2015;Taquet et al 2016; A85, page 1 of 13 A&A 650, A85 (2021) Chuang et al 2020). For example, Bennett et al (2005a) reported acetaldehyde formation through radical association reactions between CH 3 and HCO, which immediately form upon the electron bombardment of a CO:CH 4 ice mixture.…”

“…Therefore, the direct OH-radical attachment to the available C 2 H 2 ice offers an alternative reaction route forming COMs without the need for suprathermal O-atoms originating from energetic processing (i.e., dissociation reactions). Very recently, the surface chemistry of C 2 H 2 with "non-energetic" OH-radicals and H-atoms has been experimentally investigated under translucent cloud conditions (Chuang et al 2020). Here, the term "non-energetic" refers to "thermalized" species reacting with interstellar ice and has been conventionally used to contrast with energetic chemical triggers, such as photons and fast ions as well as electrons (Watanabe & Kouchi 2008;Linnartz et al 2015;Arumainayagam et al 2019).…”

O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: Detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3

“…(Abplanalp et al 2016;Bergantini et al 2017), while C 2 H 3 CHO is produced after the electron irradiation of CO/C 2 H 4 ices (Abplanalp et al 2015). Non-energetic processing of C 2 H 2 ices, in which reactions with H atoms and OH radicals occur on the surface, also produces C 2 H 3 OH (Chuang et al 2020). It remains uncertain, however, whether these experimental setups (e.g., in terms of irradiation fluxes and ice composition) resemble those of cold dark clouds.…”