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