Formation of the glycine isomer glycolamide (NH₂C(O)CH₂OH) on the surfaces of interstellar ice grains: Insights from atomistic simulations

¹ Department de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193 Catalonia, Spain
² Università degli Studi di Perugia, Dipartimento di Chimica, Biologia e Biotecnologie, Via dell’Elce di Sotto 8, Perugia 06123, Italy
³ Università degli Studi di Torino, Dipartimento di Chimica, Via Pietro Giuria 7, Torino 10125, Italy
⁴ Department of Chemistry “Giacomo Ciamician”, University of Bologna, Via F. Selmi 2, Bologna 40126, Italy

^★ Corresponding authors: jessica.perrero@unito.it; silvia.alessandrini7@unibo.it

Received: 28 February 2025
Accepted: 7 April 2025

Abstract

Context. Syn-glycolamide, a glycine isomer, has recently been detected in the G+0.693-0.027 molecular cloud. Investigations into its formation in the interstellar medium could offer insights into synthetic routes leading to glycine in prebiotic environments.

Aims. Quantum chemical simulations on glycolamide (NH₂C(O)CH₂OH) formation on interstellar ice mantles, mimicked by a water ice cluster model, are presented.

Methods. In this paper, we modeled glycolamide synthesis considering a stepwise process: the coupling between formaldehyde (H₂CO) and the radical of formamide (NH₂CO^•) occurs first, forming the glycolamide precursor NH₂C(=O)CH₂O^•, which is then hydrogenated to give anti-glycolamide. We hypothesize that anti-to-syn interconversion occurs in conjunction with glycolamide desorption from the ice surface.

Results. The reaction barrier for NH₂C(O)CH₂O^• formation varies from 9 to 26 kJ mol⁻¹, depending on surface binding sites. Kinetic studies indicate that this reaction step is feasible in environments with a T > 35 K, until desorption of the reactants. The hydrogenation step leading to anti-glycolamide presents almost no energy barrier due to the easy H atom diffusion toward the NH₂C(O)CH₂O^• intermediate. However, it competes with the extraction of an H atom from the formyl group of NH₂C(O)CH₂O^•, which leads to formyl formamide, NH₂C(O)CHO, and H₂. Nonetheless, according to our results, anti-glycolamide formation is predicted to be the most favored reactive channel.