In recent years, astronomers have detected some simple organic chemicals in the disks of material surrounding some stars. In our own Solar System, these seem to have undergone reactions that converted them into more complex molecules—some of them crucial for life—that have been found on meteorites. So, understanding the reactions that can take place in space can help provide an indication of the sorts of chemistry available to start life both here and around other stars.

Based on a publication in Nature Chemistry, it seems that the chemistry that can take place in the cold clouds of gas of space is much more complex than we had predicted. Reactions that would be impossible under normal circumstances—simply because there's not enough energy to push them forward—can take place in cold gasses due to quantum mechanical effects. That's because one of the reactants (a hydrogen nucleus) can undergo quantum tunneling between two reactants.

The key to understanding the work is the idea of activation energy. Many reactions that are energetically favorable (think burning wood) simply don't happen spontaneously. That's because the intermediate steps of the reaction are higher energy states. You need some additional energy (like a lit match) to push things over the activation energy barrier and get things to run downhill to the product state.

This, as you might imagine, is a problem in a cold gas cloud. With very little energy around, there's nothing available to hop a reaction over an activation energy barrier. On energetic considerations alone, there are some reactions that are simply impossible in that environment. And yet the authors of the new paper actually found that the reaction rate went up as the temperature went down.

The reaction the authors were looking at involved methanol, which has been found in gas clouds, and a hydroxyl radical. The latter is a water molecule with one of the hydrogens stripped away, leaving an unpaired electron. When these two molecules react, the favored outcome is to strip a hydrogen off the methanol, forming water and leaving a methoxy radical behind. Both hydroxyl and methoxy radicals have been detected in space.

Under normal circumstances, the intermediates of the reaction are energetic molecules with two oxygens bound to methanol's lone carbon. They require a fair bit of energy to create, which means there's a large activation energy to the reaction.

Once the temperature drops sufficiently, however, things start to change. At temperatures below 70K, rather than forming a covalently bonded intermediate, the two molecules can form a hydrogen bond. And at these temperatures, that bond will be relatively stable, keeping the two molecules in close proximity for extended periods of time. The proximity allows for quantum tunneling, in which small objects pass through a large energy barrier without occupying the intermediate, high energy states. In this case, one of the protons from the methanol simply tunnels over to the hydroxyl radical to form water, leaving a methoxy radical behind.

Methanol has four hydrogens, but the regular chemical reaction favors the transfer of specific ones when forming the water molecule. The authors found that the preference went away at low temperatures, confirming that something other than standard chemistry was going on here.

The fact that quantum tunneling allows reactions that would never take place in their own right is pretty impressive. But the results are also important because they give us a clearer picture of what's likely to be going on in the neighborhood of distant stars. Because of their distance, it's hard to detect anything other than raw materials around them. To infer the actual chemistry of the gas clouds, we have to look at the raw materials and the conditions, then figure out what reactions are likely to take place. By confirming that otherwise-impossible reactions can take place in these gas clouds, the authors have greatly expanded the range of chemistry we can expect to be taking place. And that can tell us something about the chemicals that are likely to be present in any planets formed under similar conditions.

Nature Chemistry, 2013. DOI: 10.1038/NCHEM.1692 (About DOIs).