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

Syn-glycolamide, a glycine isomer, has recently been detected in the G+0.693-0.027 molecular cloud. Investigations on its formation in the interstellar medium could offer insights into synthetic routes leading to glycine in prebiotic environments. Quantum chemical simulations on glycolamide (NH₂C(O)CH₂OH) formation on interstellar ice mantles, mimicked by a water ice cluster model, are presented. Glycolamide synthesis has been here modeled 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 will occur in conjunction with glycolamide desorption from the ice surface. 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 towards 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.