Carbon Atom Reactivity with Amorphous Solid Water: H₂O-Catalyzed Formation of H₂CO

We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H₂CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H₂CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions ³C + H₂O → ³HCOH and ¹HCOH → ¹H₂CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers (or the absence thereof) explain the very small kinetic isotope effect in our experiments when comparing the formation of H₂CO to D₂CO. Our results reconcile the disagreement found in the literature on the reaction route C + H₂O → H₂CO.