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A ball rolls up a hill, but it doesn't have enough energy, so it rolls back down. That's classical mechanics for you. But in the quantum world, a tiny particle can sometimes pass right through the hill, as if the barrier weren't there. This trick, tunneling, is business as usual for a light atom like hydrogen.

In cold interstellar clouds, temperatures drop to about ten degrees above absolute zero. There's barely enough energy for chemical collisions, yet this is exactly where complex molecules are born — including aromatic ones, similar to benzene rings. The authors of this study checked how often a hydrogen atom pulls off its quantum stunt to speed up reactions with molecular hydrogen, H₂. They sifted through the large KIDA database and found 64 reactions where tunneling could play a role.

They studied the four most interesting reactions using new calculations of potential energy surfaces and the RRKM method. For the radicals C₂H, OH, and CN, the rates came out to 1.66×10⁻¹⁵, 8.17×10⁻¹⁶, and 3.15×10⁻¹⁶ cm³/s — vanishingly small numbers. But because H₂ is so abundant, these reactions still manage to process a significant fraction of the radicals and compete with other processes. For NH₂, however, tunneling didn't help: the rate is too slow even for cosmic scales.

The researchers then modeled how these accelerated reactions affect the abundance of aromatic molecules. They paid special attention to the cyclopropenylidene ion c-C₆H₅⁺—a key player in forming benzene rings. By varying the rates of reactions involving it, they obtained upper and lower bounds for aromatic concentrations in a typical molecular cloud. Hydrogen tunneling can noticeably shift these estimates, so our models of interstellar chemistry need updating to understand where complex organic molecules come from in space.