Science http://doi.org/pbf (2013)

Credit: AAAS

A hydrogen bond is the attractive interaction between a hydrogen atom and a highly electronegative atom — such as fluorine, oxygen or nitrogen — in the same or another molecule. Weaker than covalent bonds but stronger than van der Waals interactions, hydrogen bonds act both inter- and intramolecularly — stabilizing ice, for example, or giving proteins their complicated three-dimensional structure.

Hydrogen bonds are, however, notoriously difficult to 'capture' in experiments or in first-principles calculations. Yet Jun Zhang and colleagues have now succeeded in directly visualizing the hydrogen bonds formed in assemblies of 8-hydroxyquilonine molecules on a Cu(111) surface. The 8-hydroxyquilonine molecule comprises, among several carbon and hydrogen atoms, one oxygen and one nitrogen atom — perfect conditions for hydrogen bonds to develop.

Zhang et al. used atomic force microscopy to obtain cluster images such as the one pictured here. The imaging process is far from trivial — what you see is not always what you get — however the backbones of the molecules are resolved, as are the hydrogen bonds; there are three of them around the dark hexagonal 'void' in the centre of this image. The technique should also enable more quantitative analyses, such as measuring hydrogen bond lengths for different types of substrate or different temperatures.