Formaldehyde chemistry in cometary ices: the case of HOCH2OH formation

Abstract

Laboratory experiments devoted to simulate the chemistry occurring in interstellar and cometary ice analogues are of paramount importance to understand the formation of complex organic molecules that are detected throughout the universe. These laboratory simulations provide relevant hints on the fundamental physical and chemical steps associated with the increase of the molecular complexity in space and, moreover, give benchmark results for dedicated space missions. In the present work, we study the thermally promoted reactivity of H₂O-dominated and D₂O-dominated cometary ice analogues that contain various amounts of H₂CO and NH₃ by means of Fourier-transform infrared spectroscopy (FTIR), mass spectrometry and DFT calculations. Experimental measurements show that methyleneglycol (HOCH₂OH) and D₂-methyleneglycol (DOCH₂OD, the corresponding isotopologue) are formed from the H₂O- and D₂O-dominated ices, respectively, only if ammonia is present. We also reported for the first time the mass spectrum of methyleneglycol and D₂-methyleneglycol. B3LYP calculations have also been used to characterize the potential energy surface of the mechanistic steps associated with the formation of HOCH₂OH as well as to simulate the IR spectrum of this compound. The fruitful interplay between theory and experiment has allowed us to elucidate the exact role of ammonia during the warming, which essentially stands for the formation and stabilization of the NH₄⁺/OH⁻ ion pair, thus enabling the OH⁻ species to react with formaldehyde. The present results reproduce the heating of circumstellar ices in star formation regions and can be applied to the late thermal evolution of comets. In addition, the mass spectrum of methyleneglycol represents a benchmark for the analysis of the data coming from the ROSINA on-board instrument of the Rosetta mission.