Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most efficient mechanisms in interstellar environments for the formation of H₂CO and CH₃OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores.
Aims: We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters.
Methods: The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules.
Results: Our main result is that the interaction of CO and H₂CO with the water cluster surfaces through H-bonds with the O atoms increases the C-O polarization, thus weakening the C-O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H₂CO and CH₃OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.