Single radical activated chemistry on cold surfaces: an experimental and theoretical study of glycolaldehyde formation

Abstract In this work, we investigate the possibility of forming interstellar complex organic molecules (iCOMs) through reactions involving a single radical and a closed-shell molecule on interstellar ices, using both experimental and theoretical approaches. Specifically, we studied the case of the reaction between formyl radical (HCO^. ) and formaldehyde (H2CO). By means of Fourier-transform infrared spectroscopy, quadrupole mass spectrometry, and matrix-isolation techniques, we demonstrate that HCO^. reacts with H2CO in the solid state to form glycolaldehyde (HOCH2CHO) at temperatures between 35 and 40 K, during the evaporation of the Ar matrix. Quantum chemical calculations simulating the experimental conditions were carried out to identify possible reaction pathways. As previous theoretical studies, we found that the reaction HCO^. + H2CO possesses a relatively large activation barrier (∼15 kJ/mol), which is likely overcome in the experiment because of the energy provided by the sublimation of the matrix. We discuss how the results of this study can be transposed to the interstellar conditions.