Carbon Atom Reactivity with Amorphous Solid Water: H$_2$O Catalyzed
  Formation of H$_2$CO
  We report new computational and experimental evidence of an efficient and
astrochemically relevant formation route to formaldehyde (H$_2$CO). 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 H$_2$CO 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 $^3$C + H$_2$O ->
$^3$HCOH and $^1$HCOH -> $^1$H$_2$CO can take place with low or without
barriers, contrary to the high-barrier traditional internal hydrogen migration.
These low barriers or absence thereof explain the very small kinetic isotope
effect in our experiments when comparing the formation of H$_2$CO to D$_2$CO.
Our results reconcile the disagreement found in the literature on the reaction
route: C + H$_2$O -> H$_2$CO.
