Special relativity can warp chemical bonds – now we’ve seen it happen

Albert Einstein’s theory of special relativity can reshape chemical bonds within molecules, and researchers have just seen it happen for the first time.

The theory of special relativity describes how moving at speeds close to the speed of light would affect travellers’ experience of space and time. Because of this, it is usually associated with particle accelerators and spacefaring objects, but within some heavy atoms, electrons experience relativistic speeds too.

Lai-Sheng Wang at Brown University in Rhode Island and his colleagues have now managed to take an unprecedented look at how this breaks the standard notion of chemical bonds within a charged molecule made from bismuth and carbon.

Within the molecule, a bismuth atom and a carbon atom were connected by three bonds, one of which the researchers expected to be of “sigma” type and two of “pi” type. The difference between these two types stems from electrons’ quantum character – each electron is “smeared” across some region of space, instead of being a tight ball, and whether these regions overlap head on or side by side determines the type of chemical bond they create between the atoms.

In their experiment, Wang and his colleagues mapped the distribution of electrons throughout the molecule, effectively getting a look at its bonds. But instead of seeing electrons distributed in shapes associated with sigma and pi bonds, the team noticed that two of the bonds resembled two different mixes of sigma and pi shapes. “Their characters are different from our normal understanding,” says Wang. “You can’t really call it the sigma and pi.”

His team turned to Kirk Peterson at Washington State University, whose calculations ultimately showed that this mixing was a consequence of electrons near the bismuth nucleus feeling such a strong electromagnetic interaction that they moved at relativistic speeds. He says this effect hadn’t previously been captured in an experiment.

“The hardest thing about [studying] heavy elements is a lack of really good experimental data,” says Peterson. “To have such a beautiful experiment to be able to essentially compare very high-level theory to data is really a luxury.”

Wang says one important part of the new experiment is that he and his colleagues could make the molecule very cold before looking at its electrons. This dampened any jitters and excitations that would have made the final images imprecise.

“The methods they have used, both experimental and theoretical, are the best possible ones,” says Pekka Pyykkö at the University of Helsinki in Finland.

He says this relativistic reshaping of bismuth’s bonding with carbon could influence how organic bismuth compounds are used in chemical reactions. In fact, a recent study by researchers at the Max Planck Institute for Coal Research in Germany has already shown that relativistic effects help make this heavy metal a good catalyst of chemical processes.

Wang says the researchers now want to repeat their experiment but swap bismuth for elements close to it in the periodic table, so they can see when exactly special relativity makes the traditional chemical bond structure collapse.