The hydrogen atom: the raw material of the universe barely exists on Earth – now we’ve made it in a flask

What do you see when you look up at the night sky? While everyone may have a different answer, the truth is that almost everything out there is hydrogen. The first element in the periodic table is by far the most abundant substance in the cosmos. It’s the fuel of stars and the raw material from which the heavier elements were forged over billions of years. Even the Sun is, at its core, a furnace for fusing hydrogen atoms together. Despite this abundance, the hydrogen atom has been almost completely inaccessible in the chemistry lab for over a century. It’s a strange irony; physicists have explored the hydrogen atom for as long as modern science has existed. In fact, the hydrogen atom is the most studied and well understood system in the history of physics. Take Niels Bohr’s famous model of atomic structure – it was built on the hydrogen atom. Erwin Schrödinger solved his famous quantum wave equation for it first. And when Paul Dirac merged quantum mechanics, the theory of particles and atoms, with Albert Einstein’s special relativity, it revealed a lot of the fine structure of the element. For synthetic chemists, who build chemical compounds and molecules, it has been almost out of reach – until now. To be clear, I’m not talking about the hydrogen molecule that comes in a gas canister, which consists of two hydrogen atoms bound together. And I’m not talking about the hydrogen ion (which has an extra electron) or the proton (a particle which makes up the atomic nucleus of the hydrogen atom). What I am talking about is the lone, simple hydrogen atom: a single electron orbiting a single proton. Two hydrogen atoms combine to form a hydrogen molecule through a covalent bond, sharing electrons to achieve stability and create the simplest diatomic molecule in nature. Watthana Tirahimonchan/Shutterstock This single, unpaired electron makes it what chemists call a radical; the most fundamental reactive species in all of chemistry. Until recently, generating it required conditions so extreme that it was simply not a tool synthetic chemists could use. The Insights section is committed to high-quality longform journalism. Our editors work with academics from many different backgrounds who are tackling a wide range of societal and scientific challenges. How we got here In 1912, Irving Langmuir, an American chemist at General Electric, who would win the Nobel Prize in Chemistry in 1932, became the first scientist to temporarily make atomic hydrogen. He did this by heating molecular hydrogen over a tungsten filament to above 1,700°C, splitting hydrogen gas into individual hydrogen atoms. When these atoms combine together at a cooler metal surface, they release enormous amounts of energy, producing temperatures of 3,400°C to 4,000°C. In fact, this heat proved sufficient to weld tungsten, the most heat-resistant of all metals. For decades, this method was one of the most potent industrial welding processes. James Frank in 1925. wikipedia And progress continued. In 1922, James Franck, a German-American physicist who would later win the Nobel Prize in Physics, discovered that mercury atoms exposed to ultraviolet light at 254 nanometres could absorb that energy and then transfer it to hydrogen molecules, breaking them apart into atoms. This process, called mercury photosensitisation, was based on fundamental physics, however. It wasn’t intended to be used to do chemistry. And while it worked, it required toxic mercury vapour, the use of which is heavily regulated, and specialised ultraviolet lamps. The first chemist to recognise that hydrogen atoms might be genuinely useful as synthetic reagents (artificial substances that can cause, control or assist chemical reactions) was Robert Crabtree at Yale University. In a 1991 paper in the Journal of the American Chemical Society, he demonstrated that James’ mercury photosensitisation could be adapted. The hydrogen atoms that were created in the process could be used to break double carbon bonds (where two carbon atoms are bound together by four bonding electrons instead of two) in organic compounds such as olefins, which are a building block of plastics. This was an important work, as it showed that the hydrogen atom could be successfully used to do chemistry, such as discovering new drugs. But his method still required toxic mercury vapour and a specialised UV lamp. Critically, it also failed to work in liquid solutions, requiring vapour instead. Synthetic chemistry, however, is done in solutions (it’s easier to mix components in a liquid). So even after Crabtree, the hydrogen atom remained out of reach to synthetic chemists. A flask and a lamp My laboratory at University College London set out to close that gap. I wanted to know if it was possible to create atomic hydrogen at room temperature, in a liquid solution, using ordinary laboratory equipment. Surprisingly, after all these years, the answer appears to be yes. We’ve now published our results in the Journal of the American Chemical Society. I began this work alone in my laboratory. But I was lucky that Nils Flodén, a postdoctoral researcher at the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany, soon joined me as a collaborator. Our method uses two inexpensive, commercially available chemicals: hydrazine (a molecule consisting of two nitrogen atoms and four hydrogen atoms) and thiophenol (consisting of a mix of carbon, hydrogen and sulphur). When mixed together and exposed to near-ultraviolet light from a standard LED lamp, these generate atomic hydrogen at only 30°C. The result truly amazed me. We used no toxic mercury vapour, no extreme temperatures. All we needed was a flask, a lamp and two reagents available from any chemical supplier. When hydrazine and thiophenol are mixed, they form an intermediate substance that absorbs near-ultraviolet light. This triggers an electron to move places, producing a short-lived, highly unstable species that rapidly falls apart, releasing a hydrogen atom in the process. But our goal was not creating a fleeting hydrogen atom that quickly disappears – we wanted to use it to do chemistry. And amazingly, we discovered that the hydrogen atom we’d created was free to react with organic molecules in ways that are crucial in many chemical experiments, such as adding it onto double bonds of carbon to form less reactive (more stable) single bond, and removing halogens (such as flourine, iodine and chlorine). The precise nature of the unstable intermediate is still being investigated, but our current evidence points toward a species physicists call a Rydberg radical, in which an electron occupies an unusually large and diffuse orbit around the molecule before the whole thing breaks apart. In fact, our hypothesis is inspired by the work of Nobel Laureate Gerhard Herzerbeg on something called a Rydberg radical of ammonia (ammonium radical), which was published in 1981. How do we know we have made a hydrogen atom? We used something called EPR spin-trapping, consisting of a chemical net that can ultimately catch short-lived radicals that we can identify by their magnetic fingerprint. And this revealed that hydrogen atoms had formed at the end. When we used a version of hydrazine where the hydrogen atoms were replaced by the heavier isotope deuterium (a hydrogen atom with a neutron added to its nucleus), we caught deuterium atoms instead, exactly as predicted. This EPR spectroscopy was performed with Alberto Collauto at Imperial College London. An EPR spectrometer used in the spin-trapping technique. wikipedia/Przemyslaw , CC BY-NC-SA A molecular stopwatch called a radical clock further supported the mechanism. Every experiment pointed in the same direction. What can you do with it? Having atomic hydrogen available as a practical reagent in a liquid solution could open possibilities that were not there before. The most immediate application is hydrogenation: adding hydrogen atoms across a carbon-carbon double bond to break them up. This is one of the most important reactions in pharmaceutical manufacturing, used to make drugs ranging from cholesterol treatments to antivirals. Currently, most industrial hydrogenation requires metals such as palladium, rhodium or iridium to act as catalysts, often under high-pressure hydrogen gas. But there’s a limited global supply of such metals, making them expensive and strictly regulated in drug products. Our hydrogen atom method requires none of them. It is metal-free, operates at atmospheric pressure and close to room temperature. But could it actually be used in drug manufacturing? We managed to use our method, without disturbing other chemical groups, to reduce double bonds in a derivative of fluoxetine (an SSRI), in terpenoids (compounds found in fragrances and flavours) and in an amino acid. This can help make drugs more stable in the body and less reactive. We also demonstrated hydrodehalogenation: the selective removal of halogen atoms from organic molecules. This is often done in medicinal chemistry for fine-tuning drug candidates. As for other applications, atomic hydrogen is already used industrially for cleaning semiconductor surfaces in microchip manufacturing, where it removes naturally occurring oxide layers that reduce device performance. Those processes currently require plasma equipment and ultra-high vacuum conditions. Whether our mild solution could eventually offer an alternative is currently unknown. But for the first time, it is an actual alternative worth exploring. A long obsession How did I come up with the idea in the first place? It grew from two parallel interests. I have long been fascinated by hydrogen as a clean fuel and the challenge of storing it conveniently. After all, hydrogen has been the fuel of the universe for 13 billion years. At the same time, I was working on developing new synthetic methods in organic chemistry. About half a decade ago, it struck me that, despite decades of exploration, the reactive hydrogen atom itself has remained almost entirely unexplored as a synthetic tool. Finding a practical way to generate it could be useful for hydrogen storage and beyond. This was the starting point. This was ultimately a dream project. The more I thought about it, the harder it was to let it go. At some point, it practically consumed me. It’s been a long journey for me. In 2010, as a student from a small village in northern India, I had the opportunity to travel to Berlin and work in the lab of Hans Ulrich Reissig of Freie Universität Berlin, the first research laboratory I had ever set foot in. The paper is dedicated to him on the occasion of his 77th birthday. It was where I first understood, with absolute certainty, that chemistry was my calling. It may have taken a century to bring atomic hydrogen into the synthetic chemist’s laboratory, but we hope it will prove to be worth the wait. For you: more from our Insights series: End of humanity? 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