STM And AFM Zoom In On Aryne Structure

Researchers have captured the first-ever images of arynes, intermediates whose high reactivity and often fleeting existence make them difficult to characterize. Arynes are obtained from benzene and other arenes by removing two hydrogen atoms or substituents from the ring. Niko Pavliček of IBM Research in Zurich; Diego Peña Gil of the University of Santiago de Compostela, in Spain; and coworkers used low-temperature scanning tunneling microscopy and atomic force microscopy to characterize and image individual polycyclic ortho-aryne molecules they generated on ultrathin insulating films (Nat. Chem. 2015, DOI: 10.1038/nchem.2300). The study helps resolve debate and uncertainty about the electronic structure of arynes—whether they are best represented as alkynes with one arene double bond replaced by a triple bond or as cumulenes with three consecutive double bonds. The team’s evaluation of AFM-imaged bond lengths shows that cumulene resonance structures are dominant under the experimental conditions used. The finding could aid syntheses of polycyclic compounds and modifications of graphene sheets and carbon nanotubes, which are processes that can be carried out with reactions involving aryne intermediates.