Slamming molecules opens new reaction paths

February 02, 2021
  What happens to a molecule when you throw it at a surface? An international collaboration between scientists from the Max Planck Institute for Solid State Research, the Max Planck Institute of Colloids and Interfaces, and the University of Oxford have shown that the outcome depends on how fast the molecule approaches the surface. Slow-approaching molecules experience geometrical changes, useful in revealing the three-dimensional structures of biomolecules; while fast-approaching molecules experience selective chemical reactions, giving products that cannot be obtained by conventional thermal chemistry.  

The outcome can be largely demarcated by the kinetic energy of the molecule approaching the surface. Low-energy collision (0.5 – 5 eV) changes the three-dimensional structure (i.e. conformation) of incident molecule, while high-energy collision (5 – 50 eV) can initiate a mechanochemical reaction.

We demonstrate the utility of low-energy collisions by colliding a carbohydrate molecule with a surface to explore its ground and excited conformational states. The carbohydrate molecule used as an example here is a short chain of cellulose, the main structural molecule in plants, which are prepared by chemists at the Max Planck Institute of Colloids and Interfaces. Knowing the range of conformational states that a biomolecule can adopt is important in drug-discovery since the most bioactive conformation of a molecule is not necessarily the most stable or the most abundant one.

Soft molecule-surface collision (0.5 – 5 eV) changes the conformation of the incident molecule as seen from variously folded molecules on surface imaged by Scanning Tunnelling Microscopy (STM) (i.e. extended, partly coiled, coiled). Hard molecule-surface collision (5 – 50 eV) induces mechanochemical reactions of the incident molecule as seen from variously reacted molecular species on surface observed by STM (i.e. intact, cracked, split).

High-energy collisions, in contrast, are found to induce selective chemical reactions due to the compression of an incident molecule when it arrives at the surface. Experiments and calculations in collaboration with Dr. Stephan Rauschenbach at the University of Oxford reveal that the molecular compression causing the reaction originates from the fast-approaching molecule being brought to sudden halt when it encounters the surface. The reaction products generated from these high-energy collisions are those that cannot be obtained by conventional thermal chemistry, thereby opening new opportunities to synthesize new molecules on surfaces.

Kelvin Anggara

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