The Very First Image of a Hydrogen Bond

Image: AFM images of 8-hydroxyquinoline on a copper surface show hydrogen-bonding interactions at low temperature; C = gray, H = white, O = red, N = blue, Cu = orange. Credit: Science.

Using a technique called high-resolution atomic force microscopy (AFM), researchers in China have visualized the molecular structure of a hydrogen bond. Hydrogen bonds are incredibly useful and they're all over nature. Most famously, they're responsible for holding the two strands of the double helix of DNA together. They also give water its unique properties. Chemists describe a hydrogen bond as the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Chemists are very familiar with the way it looks — but only at a theoretical level. No one has ever actually seen a hydrogen bond, at least until now. The visualization breakthrough now appears in Science: “Real-Space Identification of Intermolecular Bonding with Atomic Force Microscopy.”

Image: AFM images of 8-hydroxyquinoline on a copper surface show hydrogen-bonding interactions at room temperature; C = gray, H = white, O = red, N = blue, Cu = orange. Credit: Science.

To create these remarkable images, a team led by Xiaohui Qiu and Zhihai Cheng of China’s National Center for Nanoscience & Technology, along with Wei Ji of Renmin University of China, used noncontact AFM to scan for the forces that rest between molecules in variety of compounds. The winning combination was 8-hydroxyquinoline (an organic compound) deposited on a copper surface.

As some of you may remember, this technique is similar to the one used by Felix Fischer and his team who earlier this year created the first image of molecules breaking and reforming chemical bonds. Specifically, they visualized the positions of individual atoms and bonds in a molecule having 26 carbon atoms and 14 hydrogen atoms structured as three connected benzene rings.

But for the new study, the scientists took the process to the next level by using the same technique to sniff-out weaker interactions rather than just the covalent bonds. Working at temperatures near absolute zero, the scientists watched as 8-hydroxyquinoline formed hydrogen-bonded aggregates, with the electron density of the hydrogen bonds made visible via the atomic force microscope. The researchers also produced images of hydrogen bonds at room temperature.

Interestingly, some scientists speculate that hydrogen bonds contain some covalent aspects. Breakthroughs like this could shed some light on the issue.