Quantum Chemical Computations of Gas-phase Glycolaldehyde Deuteration and Constraints on Its Formation Route

Abstract

Despite the detection of numerous interstellar complex organic molecules (iCOMs) for decades, it is still a matter of debate whether they are synthesized in the gas phase or on the icy surface of interstellar grains. In the past, molecular deuteration has been used to constrain the formation paths of small and abundant hydrogenated interstellar species. More recently, the deuteration degree of formamide, one of the most interesting iCOMs, has also been explained with the hypothesis that it is formed by the gas-phase reaction NH2 + H2CO. In this paper, we aim at using molecular deuteration to constrain the formation of another iCOM, glycolaldehyde, which is an important prebiotic species. More specifically, we have performed dedicated electronic structure and kinetic calculations to establish the glycolaldehyde deuteration degree in relation to that of ethanol, which is its possible parent species according to the suggestion of Skouteris et al. We found that the abundance ratio of the species containing one D atom over the all-protium counterpart depends on the produced D isotopomer and varies from 0.9 to 0.5. These theoretical predictions compare extremely well with the monodeuterated isotopomers of glycolaldehyde and that of ethanol measured toward the solar-like protostar IRAS 16293–2422, supporting the hypothesis that glycolaldehyde could be produced in the gas phase for this source. In addition, the present work confirms that the deuterium fractionation of iCOMs cannot be simply anticipated based on the deuterium fractionation of the parent species but necessitates a specific study, as already shown for the case of formamide.

JWST Observations of Young protoStars (JOYS+): Detecting icy complex organic molecules and ions

“…Boudin et al (1998) also estimated upper limits for molecules such as ethane (C 2 H 6 ), acetylene (C 2 H 2 ), hydrazine (N 2 H 4 ), hydrogen peroxide (H 2 O 2 ) and the hydrozonium ion (N 2 H + 5 ). In parallel, the gas phase formation of COMs through ionmolecule reactions has been proposed as well (e.g., Balucani et al 2015;Skouteris et al 2018;Vazart et al 2022). The recent detection in a protoplanetary disk of gas phase CH + 3 (Berné et al 2023), an important intermediate in such reactions, supports the idea that COM formation is not exclusively realized on icy grains.…”

Spiers Memorial Lecture: Astrochemistry at high resolution

“…In addition, the measured deuteration of glycolaldehyde and acetaldehyde, and their isomers, are also in good agreement with those predicted by QM computations. 158,190 Unfortunately, no predictions of the deuteration for these two molecules exist in the literature. Therefore, while the gas-phase origin hypothesis has passed the possible tests so far available, the grain-surface one can not be excluded for lack of similar precise predictions from QM computations.…”

Tracing Prebiotic Molecules: Rotational Spectroscopy of Deuterated Glycolaldehyde and (Z)-1,2-Ethenediol

“…19 These observations prompted new studies on the deuterium fractionation of glycolaldehyde with the aim of understanding whether the formation route of this COM involves a gas-phase path or occurs on dust grains. 20 These two hypotheses are still highly debated, and several chemical models have been proposed and compared with the observed abundances in various regions of the ISM. 21−23 Very recently, the less stable enol form of glycolaldehyde, namely, (Z)-1,2-ethenediol (CHOH�CHOH), has also been detected in the ISM toward the G+0.693−0.027 molecular cloud located in the galactic center.…”

“…In 2016, all possible forms of singly deuterated glycolaldehyde (CH 2 OD–CHO, CH 2 OH–CDO, and CHDOH–CHO) were detected as well in the ISM, more precisely in the Class 0 protostellar binary IRAS 16293-2422B . These observations prompted new studies on the deuterium fractionation of glycolaldehyde with the aim of understanding whether the formation route of this COM involves a gas-phase path or occurs on dust grains . These two hypotheses are still highly debated, and several chemical models have been proposed and compared with the observed abundances in various regions of the ISM. − …”