Atomic Force Microscopy Provides Astonishing Images Of Chemical Bonds

High-resolution atomic force microscopy has produced several molecular pinup pictures in recent years. The first memorable image showed the atoms and bonds in a single molecule of pentacene. This year, AFM enabled scientists to go well beyond that to illuminate hydrogen bonds between molecules as well as before-and-after images of a molecule undergoing a chemical reaction. A team led by Xiaohui Qiu and Zhihai Cheng of China’s National Center for Nanoscience & Technology and Wei Ji of Renmin University of China succeeded in visualizing hydrogen bonds by depositing 8-hydroxyquinoline on a copper surface (C&EN, Sept. 30, page 5; Science 2013, DOI: 10.1126/science.1242603). At temperatures near absolute zero, the researchers found that 8-hydroxyquinoline molecules formed hydrogen-bonded aggregates, with the electron density of the hydrogen bonds picked up by AFM. In separate work, a group led by Felix R. Fischer and Michael F. Crommie of the University of California, Berkeley, used AFM to image an enediyne compound and its heat-induced cyclization products on a silver surface (C&EN, June 3, page 7; Science 2013, DOI:10.1126/science.1238187). Enediynes undergo a variety of competing cyclization processes, producing a multitude of products that are challenging to identify. Fischer, Crommie, and their colleagues found that the molecules in their system transformed into fused-ring polycyclic aromatic structures.