Interstellar Enolization‐Acetaldehyde (CH₃CHO) and Vinyl Alcohol (H₂CCH(OH)) as a Case Study

Abstract

Owing to the unique conditions in cold molecular clouds, enols—the thermodynamically less stable tautomers of aldehydes and ketones—do not undergo tautomerization to their more stable tautomers in the gas phase because they cannot overcome tautomerization barriers at the low temperatures. Laboratory studies of interstellar analog ices have demonstrated the formation of several keto–enol tautomer pairs in astrochemically relevant ice mixtures over the last years. However, so far only one of them, acetaldehyde−vinyl alcohol, has been detected in deep space. Due to their reactivity with electrophiles, enols can play a crucial role in our understanding of the molecular complexity in the interstellar medium and in comets and meteorites. To study the enolization of aldehydes in interstellar ices by interaction with galactic cosmic rays (GCRs), we irradiated acetaldehyde ices with energetic electrons as proxies of secondary electrons generated in the track of GCRs while penetrating interstellar ices. The results indicate that GCRs can induce enolization of acetaldehyde and that intra‐ as well as intermolecular processes are relevant. Therefore, enols should be ubiquitous in the interstellar medium and could be searched for using radio telescopes such as ALMA. Once enols are detected and abundances are established, they can serve as tracers for the non‐equilibrium chemistry in interstellar ices thus eventually constraining fundamental reaction mechanisms deep inside interstellar ices.

Identification of Glycolaldehyde Enol (HOHC═CHOH) in Interstellar Analogue Ices

“…63 Recently, Kleimeier and Kaiser have demonstrated that enolization of acetaldehyde can be induced by 5 keV electron irradiation as proxy for galactic cosmic rays by inter-and intramolecular hydrogen shifts, indicating that this pathway is feasible for the enolization of aldehydes. 64 Based on the infrared spectroscopic detection of the formyl radical, the most plausible reaction leading to 1,2-ethenediol in reactions involving two 13 C 18 O molecules (m/z = 66) is the dimerization of formyl to glyoxal (HCOCHO, 10) followed by reduction through addition of two (suprathermal) hydrogen atoms (pathway 3 in Figure S7; Scheme 1). An experimental study by Chuang et al 65 as well as surface carbon monoxide hydrogenation studies by Fedoseev et al 66 indicate that glyoxal formation from formyl and subsequent hydrogenation are the dominant contributors to glycolaldehyde as well as ethylene glycol formation.…”

Bottom-Up Synthesis of 1,1-Ethenediol (H2CC(OH)2)─The Simplest Unsaturated Geminal Diol─In Interstellar Analogue Ices

“…Thermodynamically unfavorable molecules such as enols can form at abundances orders of magnitude exceeding those expected due to the nonequilibrium nature of these reactions with excess external energy . Furthermore, GCRs can induce enolization by both inter- and intramolecular hydrogen shifts . In the absence of external energy sources, the low temperatures do not allow the enols to overcome the tautomerization barrier, keeping these reactive species available for (nearly) barrierless reactions to form more complex molecules .…”

“…13 Furthermore, GCRs can induce enolization by both inter-and intramolecular hydrogen shifts. 14 In the absence of external energy sources, the low temperatures do not allow the enols to overcome the tautomerization barrier, keeping these reactive species available for (nearly) barrierless reactions to form more complex molecules. 15 Thus, far, the only detected tautomer pair in space is acetaldehyde (CH 3 CHO)−vinyl alcohol (H 2 CCHOH).…”

Characterization of the Coriolis Coupled Far-Infrared Bands of syn-Vinyl Alcohol

“…Ground vibrational state anti- and syn- VA were first observed in the ISM by Turner and Apponini, following which potential formation mechanisms were investigated, both in astrophysical ices − and in the gas phase. , In this section, we speculate on both the possibility for detecting rotational emission from vibrationally excited syn -VA in the ISM, and on the possibility of observing it in Titans atmosphere.…”