Scientists harness light to generate reactive hydrogen atoms for organic synthesis

Hydrogen is the simplest thing in chemistry—one proton, one electron—and yet it has always been a problem to work with. The element itself is everywhere, but getting it to do what you want in a laboratory is another matter entirely. For more than a century, chemists have struggled with this paradox: the smallest atom is also one of the most unruly.

A team spanning the Max Planck Institute of Colloids and Interfaces, University College London, and Imperial College London has now found a way around the problem. They use light to generate individual hydrogen atoms under conditions mild enough for practical chemistry, and they do it without relying on metals. The work, published in the Journal of the American Chemical Society, represents a shift in how chemists might approach synthesis at the molecular level.

The core difficulty is this: hydrogen exists in nature as H₂, two atoms locked together in a stable bond. A single hydrogen atom, by contrast, is a different creature entirely. "From a chemical point of view, a hydrogen atom is incredibly simple, but it is also extremely reactive," says Nils J. Flodén, the first author of the study. "Therefore, we had to find a way to generate hydrogen atoms under conditions where we can channel their reactivity to do what we want." The challenge is not generating the atoms—it is controlling them once they exist.

The researchers' approach hinges on light. They combine hydrazine with a thiophenol derivative and expose the mixture to light. The photons provide energy that triggers an electron transfer between the two molecules. This transfer creates what chemists call a Rydberg radical, an exotic intermediate that exists for only about 13 picoseconds—13 trillionths of a second. As this Rydberg molecule decays, it releases a hydrogen radical that can then be put to work.

The history of hydrogen atom chemistry stretches back to 1912, when Irving Langmuir, an American chemist, discovered that hydrogen molecules break apart into individual atoms when exposed to a heated tungsten wire inside a light bulb. That process required temperatures exceeding 2,000 Kelvin. Later methods used electrical discharges or mercury UV radiation to generate atomic hydrogen, allowing chemists to study its properties in detail. But these approaches were too extreme for practical organic synthesis. They destroyed other molecules in the reaction mixture and made it impossible to control where the hydrogen atoms went.

With their light-based method, the researchers can reduce a range of organic molecules under mild conditions without any metals present. They demonstrated the technique on highly functionalized alkenes and halogen compounds—molecules that are difficult to modify using conventional chemistry. "For organic chemistry, the discovery of new chemical intermediates opens up new possibilities," says Peter H. Seeberger, director of the Department of Biomolecular Systems at the Max Planck Institute. "With this work, atomic hydrogen becomes a practical tool for synthetic chemists."

The team views this as a beginning rather than an endpoint. The unusual reactivity of a single hydrogen atom could prove useful in reduction reactions, but also in other areas of organic chemistry and potentially in biological applications. The work suggests that what has long been a liability—the extreme reactivity of atomic hydrogen—might become an asset if chemists can learn to harness it under the right conditions. For now, the field has a new tool, and the smallest atom in the periodic table has become a little less difficult to tame.