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Einstein's relativity rules chemical bonds in heavy elements, new research shows
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Chemists at Brown University have provided direct experimental evidence that Einstein's theory of relativity alters the nature of triple chemical bonds in heavy elements, challenging textbook explanations. In a study published in Science, the researchers demonstrated that when atomic nuclei are sufficiently heavy, the traditional distinction between sigma and pi bonds blurs due to relativistic effects. Using photoelectron spectroscopy, the team analyzed molecules made from carbon and bismuth, a heavy element adjacent to lead on the periodic table. Their findings reveal that the triple bonds in these molecules do not conform to the conventional sigma-plus-two-pi bond model.
The textbook picture of triple bonding involves one strong sigma bond formed by head-on electron sharing and two weaker pi bonds formed side-by-side. This model works well for lighter elements, but for heavy elements, electrons orbiting the nucleus accelerate to significant fractions of the speed of light. At such velocities, relativistic effects become important, causing spin-orbit couplingâwhere an electron's spin and orbital motion are no longer independent. This coupling disrupts the strict separation between sigma and pi bonds, leading to hybrid bonds.
Lai-Sheng Wang, a professor of chemistry at Brown and the study's corresponding author, explained that the idea of relativistic effects in heavy elements has existed since the 1970s, but direct spectroscopic evidence was lacking. "We show direct spectroscopic evidence that what we learned in high school about chemical bonding isnât true in heavy elements," Wang said. The team formed bismuth-carbon molecules, cooled them near absolute zero, and used lasers to knock out electrons, measuring their binding energies. The resulting photoelectron spectrum showed one pi bond and two hybrid sigma-pi bonds, rather than the classic arrangement.
These findings have significant implications for chemistry, particularly for industries exploring heavy elements. Bismuth is gaining attention as a non-toxic alternative to lead in next-generation solar cells and for applications in quantum materials and quantum computing. Understanding the true nature of its chemical bonds is crucial for designing new materials.
Wang suggested that this discovery could lead to a rewriting of chemistry textbooks. "Maybe this will become the new textbook idea as we are dealing with more and more heavy chemistry of the heavy elements," he said. The research was funded by the U.S. National Science Foundation and the U.S. Department of Energy.
Looking ahead, the team plans to explore other heavy elements to see if similar relativistic bonding patterns emerge. As research into heavy-element chemistry expands, this work provides a fundamental shift in understanding chemical bonding, potentially impacting fields from materials science to quantum computing.
