Credit: © 2007 ACS

'Hydrogen bonding' is the name given to the weak non-covalent interaction between a slightly positively charged hydrogen atom and an electron donating atom such as oxygen or nitrogen. This phenomenon is not only of great importance in biological systems — where it governs, for example, which DNA bases pair with one another and how proteins fold — but it can play a significant role in determining the physical properties of synthetic materials.

To date, most studies of hydrogen-bond strength have considered interactions between molecules either in the gas phase or dissolved in a solvent. Now, Ludwig Bartels and co-workers1 at the University of California, Riverside in the USA have used a scanning tunnelling microscope to investigate hydrogen bonds formed between molecules adsorbed on a copper substrate. Two structurally similar molecules — anthraquinone and its sulphur analogue 9,10-dithioanthracene — are each known to form hydrogen-bonded chains on a copper surface. Bartels and colleagues observed individual molecules detaching from the ends of such chains and then calculated how the rate of the process varied with temperature.

It was found, as expected, that the oxygen-based hydrogen bonding present in the anthraquinone chains was stronger than the sulphur-based interactions holding the rows of 9,10-dithioanthracene moelcules together. The good correlation with values reported for similar systems studied in both gas and solution phases suggests that this is a viable experimental technique to measure hydrogen bonds between molecules on a surface.