A dance of two atoms reveals chemical bonds forming and breaking

Two rhenium atoms approach and retreat from one another in an electron microscope video

Scientists spied on two atoms forming chemical bonds and breaking them by studying molecules made of two rhenium atoms (illustrated in green) inside a carbon nanotube (gray).

University of Nottingham

By Emily Conover

January 17, 2020 at 2:00 pm

Scientists have now captured video of the intimate dance of two atoms as they bond with one another, break apart and come back together again.

In a sequence of images from an electron microscope, two atoms of the metal rhenium, bound together to create a molecule, shimmied around one another, moving closer and then farther apart. In videos of such molecules, this atomic do-si-do revealed the bond order, or the number of chemical bonds between the two atoms, and how that bond order changed over time. The closer the atoms were to one another, the greater the number of bonds. At their closest approach, the atoms had four bonds tethering them together.

To make imaging easier, scientists trapped the molecules inside carbon nanotubes. But then, in a fortuitous accident, one molecule escaped its confinement and nestled into a gap between two nanotubes. There, the bond between the atoms completely broke before soon reforming, the team reports January 17 in Science Advances.

A molecule made of two rhenium atoms (dark spots) travels around two carbon nanotubes (lighter lattice of spots), settling into the gap between the nanotubes. When the atoms separate by a larger distance, the bond between the atoms is broken, and then later reforms.

Researchers had previously coaxed two atoms to bond (SN: 4/12/18). But directly observing how chemical bonds change in number “was not done before,” says physicist Ute Kaiser of Ulm University in Germany.

Kaiser and colleagues made the images with a transmission electron microscope specially designed to operate at low voltages, so that its beam of electrons wouldn’t damage the carbon nanotubes or send the rhenium atoms flying. That electron beam was useful for imaging, but also served another purpose: It gently jostled the atoms, causing them to dance rather than sitting still.