Chemical reactions are traditionally performed in flasks, where vast quantities of molecules undergo the same transformation at the same time. Such processes are described in terms of the collective properties of the system, and information on how any given molecule behaves is lost because the behavior of all molecules is averaged.
With a scanning tunneling microscope (STM), however, it is possible to 'see' individual atoms and molecules sitting on a surface by mapping its electron density. Moreover, the atomically sharp tip of an STM can also be used to manipulate molecules - they can be moved, their shape can be changed, and chemical bonds can be either made or broken.
Chemical reactions induced with STM tips are usually irreversible and not very selective for a particular bond. Now, a team led by Yousoo Kim and Maki Kawai from RIKEN's Discovery Research Institute in Wako has performed a highly specific and reversible reaction on a single molecule. "This is the first time anyone has been able to break and reform a single molecular bond," comments Kim.
Writing in the journal Science (1), Kim and co-workers describe how methylisocyanide (CH_3 NC) molecules adsorb to so-called 'on-top' sites on a platinum surface, binding to a single metal atom. When exposed to hydrogen gas (H_2), the CH_3NC molecules react to form methylaminocarbyne (CH_3NHC) molecules that, because of their different electronic and geometric structure, shift on the surface to adopt bridging positions between two platinum atoms (Fig. 1).
Bright spots - corresponding to the molecules - were observed in STM images of the surface taken before and after exposure to H_2, and a decrease in their height profile following the reaction confirmed the formation of CNHCH_3. The most significant finding, however, was that an individual CH_3NHC molecule could be converted back into a CH_3NC molecule by injecting tunneling electrons from an STM tip positioned directly above it.
This process specifically removes the hydrogen atom bonded to the nitrogen - rather than any of those in the CH_3 group - and the reformed CH_3NC molecule reverts back to its original 'on-top' site on the platinum surface. This reaction cycle was monitored with inelastic electron tunneling spectroscopy using the STM, a technique that looks at the vibrations of single molecules and can be used to confirm their chemical identity.
Controlling interactions between single molecules and a metal surface may prove useful for molecular electronics applications and this study could also offer a greater understanding of chemical reactions occurring on surfaces.
Reference
(1). Katano, S., Kim, Y., Hori, M., Trenary, M. & Kawai, M. Reversible control of hydrogenation of a single molecule. Science 316, 1883–1886 (2007).
