Modeling gas-grain chemistry with quantum chemical cluster calculations. I. Heterogeneous hydrogenation of CO and H₂CO on icy grain mantles

Formaldehyde (H₂CO) and methanol (CH₃OH) are thought to be produced in the interstellar medium by the successive hydrogenation of carbon monoxide (CO) on grain surfaces. In the gas phase, the steps in which H adds to CO and H₂CO possess modest barriers and are too inefficient to account for the observed abundances. Recent laboratory work has confirmed that formaldehyde and methanol are formed when H atoms are deposited on CO ice at 12 K. The present study employs ab initio quantum chemical calculations to investigate the impact of water ice on the sequential hydrogenation of CO. The most favorable pathway is CO → HCO (formyl radical) → H₂CO → CH₃O (methoxy radical) → CH₃OH. There is sufficient reaction energy in the final step to fragment CH₃OH into methyl and hydroxyl radicals, which can be hydrogenated to yield methane and water, as observed in the experimental work. The emphasis here was on the two steps with barriers, H + CO and H + H₂CO, with both addition and abstraction considered for the latter. Calculations with up to four explicit water molecules were performed, as well as further modeling to incorporate bulk effects. While ice was found to have a nearly negligible impact on H + CO → HCO, it modestly enhances the addition reaction H + H₂CO → CH₃O and hinders the abstraction reaction H + H₂CO → H₂ + HCO. The deuterium-substituted reactions D + CO → DCO and D + H₂CO → CDH₂O were found to be slightly favored over the corresponding H reactions, particularly in the latter case. Overall, the energetics are not favorable: water ice is evidently not a good catalytic substrate for H + CO or H + H₂CO addition reactions at very cold temperatures.