Universiteit Leiden

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Towards molecular complexity in birth places of stars: Formaldehyde formation from carbon atoms reacting with water ice

Scientists at Leiden University (Netherlands), Stuttgart University (Germany) and Ural Federal University (Russia) have successfully put forward a novel, computed, reaction mechanism that was experimentally tested and show that formaldehyde is formed at much earlier stages in the birthplaces of stars than was previously believed.

Origin of formaldehyde in interstellar ices

The surface formation of formaldehyde (H2CO) has – up-to-now – been thought to occur in the later stages of the life of molecular clouds in which stars are born. This is a direct consequence of the hydrogenation of carbon monoxide on top of cosmic dust grains, which is only abundantly present towards the end of the cloud lifetime. Formaldehyde is not only a crucial molecule in this hydrogenation sequence, but being the simplest carbonyl compound, it plays a central role in the formation of complex organic compounds and is one of the sources for the formation of pre-biotic molecules in space.

Water-catalyzed reaction on ices

The reaction between a water molecule and a carbon atom is a potential other route leading to H2CO being formed, but this reaction is known to require a large amount of energy, which in cosmic nebulae at a temperature of -263oC, is not available. Here, a novel and alternative mechanism is proposed allowing this reaction step to take place, circumventing the highly energetic route, by making use of water molecules in the vicinity of the carbon atom to act as a catalyst. This way formaldehyde can indeed be formed by the interaction of atomic carbon with amorphous solid water ice.

Reaction mechanism explained

Molecules inside the water ice that covers submicrometer-sized cosmic dust particles can react with a carbon atom that accretes from the gas phase on the surface. This way, a complex between a carbon atom and a water molecule is formed. Water molecules, however, are bound to each by hydrogen bonds that act simultaneously as donors and acceptors of protons (positively charged hydrogen) and as such they carry out a pseudo-transfer of protons from the oxygen atom of the formed complex to its carbon atom. This coordinated transfer of protons along the chain of hydrogen bonds between water molecules
catalyzes the reaction, accelerating the formation of formaldehyde. 

The closest analogy of this mechanism from our everyday life is the translation of switches on railway tracks. The rails remain in place, but the logistics of the connections are completely changed. Such transformations are easy and do not require a lot of energy, which explains why formaldehyde is expected to be formed, despite the difficulty of the reaction taking place in absence of water.

Linking to laboratory and observational work

Experiments at the Laboratory for astrophysics at the Leiden Observatory confirm the ease with which H2CO is formed when a H2O ice is ‘irradiated’ with C-atoms.

In the near future, the presence of formaldehyde in water-rich ices will hopefully be probed by the James Webb Space Telescope, which was recently launched into orbit, and is set out, amongst others, to observe ices in space. 

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