- Scientists have set up and observed a single electron bond between two carbon molecules.
- A traditional “single bond” is actually two electrons—one from each atom. This is one electron.
- The fragile bond required outside-the-box thinking, with a complex molecule acting as a shell.
From the omnipresence and ubiquity of carbon—it has its own branch of chemistry!—you may already know that this element is special in its properties. Carbon can form long chains with only its own atoms, and it has a dozen or more allotropes (shapes of carbon-only molecules) compared to, say, just a few for oxygen. These include graphene, diamond, and the delightfully named buckyball. Carbon even pairs with itself in an unusual form called green carbon.
And now, carbon can put one more feather in its cap—the ability to form single electron bonds. In new research, scientists have realized a century-old dream of a single electron covalent bond between two carbon atoms observed in situ. They’ve previously observed this bond between atoms other than carbon, but other atoms are considered different with regards to the ease of forming the single electron bond. And it turns out scientists are very interested in this special bond when it comes to the elemental building block of all known life forms.
The new research appears in the peer reviewed journal Nature, and was completed by four scientists from Hokkaido University in Japan. It’s the culmination of a huge journey—in 1931, chemist Linus Pauling wrote in the Journal of the American Chemical Society that he believed it was possible to have a pair of carbon atoms with a bond of just one electron, rather than the standard single bond that contains two electrons. “These bonds,” Pauling wrote, “have not the importance of the electron-pair bond, for they occur in only a few compounds, which, however, are of especial interest on account of their unusual and previously puzzling properties.”
At the time, he and others believed a handful of compounds must have these bonds. Electron theory was just a few decades old, and scientists were chipping away at the nature of atoms in fits and starts. Things were discovered and changed during and after this time, especially as the new idea of quantum mechanics made waves (that were also particles?) in the scientific community.
Since that time period, we’ve learned a lot about how exactly building blocks like atoms and molecules work. For instance, the three major bonds between atoms are single, double, and triple. Each “single” bond has two electrons—one from each atom. To continue the line of thinking, double bonds have four total electrons, and triple bonds have six. The single bond is the weakest of these three, but not inherently weak. They’re a bit like a double-wrist-grab handshake, because there are two electrons to anchor even the single bond.
A single electron bond, then, is like a normal handshake. If you were trying to hold onto someone while you were jostled around a busy party, or if one of you were sliding off the side of a cliff, you wouldn’t settle for a single handshake hold. It’s not as well anchored because it has only one point of connection instead of two. So, the single electron bond is thought to be extremely fleeting when it does exist. It’s just too easy to break.
“We aim to clarify what a covalent bond is—specifically, at what point does a bond qualify as covalent, and at what point does it not?” corresponding author Takuya Shimajiri said in an article about their study (also in Nature). In other words, is something this fragile even a bond at all? Or is it just a blip or transition as the bond seeks a second electron?
This is why stabilizing the overall chemical reaction was so vital.
Shimajiri explained that the research team built an exacting molecule with structures to help support the very fragile bond they wanted to form. Specifically, they used derivatives of hexaphenylethane—a theorized carbon compound—to help structure their experiment. This complex molecule deteriorates in a way that creates a “shell” in which the carbon atoms will remain, allowing for a delicate bond that doesn’t have to hold itself together. From there, the researchers were able to use a specialized X-ray technique to observe the bond, which confirmed their calculated predictions.
When Linus Pauling wrote about this bond in 1931, he thought it already existed, and could theoretically be found. He lived another 53 years—during which time he won a Nobel Peace Prize and a Nobel Prize in Chemistry, making him the only person to win two solo Nobel Prizes—but never saw his prediction come to fruition.
Now, he would probably be delighted that scientists have finally isolated and observed such an early-career prediction in the ubiquitous carbon atom. Indeed, the ramifications for things like nanomaterials, which already heavily rely on carbon graphene in shapes like sheets and nanotubes, may take another 100 years to fully realize. Carbon continues to impress.
Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She’s also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.