World’s first: Chemists rearrange atomic bonds in a molecule

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An international team led by researchers from the IBM research laboratory in Zurich (Switzerland) has developed a method for controlling the selectivity of chemical reactions. Their approach is based on the reversible and selective formation and dissociation of atomic bonds induced by the tip of a scanning tunneling microscope. This breakthrough could not only enable the initiation of chemical reactions with unprecedented spatial and temporal resolution, but also open up entirely new reactions.

In order to create molecules or complex molecular machines, a set of atoms must be precisely combined and connected. It consists of mixing the reactants, possibly stimulating the reaction with a catalyst, and then extracting the products in more or less interesting yields. What if you could control simple molecules and chemical bonds at will? This is the ultimate goal of the MolDAM (Single Molecular Devices by Atom Manipulation) project launched by IBM Research two years ago.

Specifically, we are talking about the construction of matter “to order” from individual atoms by controlling chemical reactions using the tip of a scanning probe microscope. Using ultra-fast pulses of light, the scientists observed how bonds are formed and how atoms rearrange themselves during a chemical reaction. Now, by applying a certain tension to atomic bonds, Dr. Florian Albrecht of IBM Research Europe and his co-authors have managed to create three different products from the same molecule.

Three new molecules created from one

The selectivity and yield of chemical reactions can be improved by adjusting certain parameters (temperature, pH, etc.) or by adjusting the available proton donors, thus influencing how atoms exchange electrons to form bonds. “However, by these means, the reaction conditions are altered to such an extent that the underlying mechanisms governing selectivity often remain elusive,” the researchers note in their study.

So they decided to control the formation of atomic bonds in a completely different way. They first synthesized a molecule of 5,6,11,12-tetrachlorotetracene (C18H8Cl4), a molecule made up of a series of four carbon rings to which four chlorine atoms are attached, and then absorbed it in a thin layer of copper-grown salt. Using voltage pulses generated by the tip of a scanning tunneling microscope (STM), they removed chlorine atoms, thereby breaking four C-Cl bonds and the central CC bond.

Molecular rearrangement caused by voltage pulses generated by the tip of a scanning tunneling microscope. © I. Alabugin et al.

This intervention led to the formation of an intermediate molecule containing a diradical – a central ring of 10 carbon atoms, six of which have an unpaired electron (and therefore are able to form new bonds). By applying a relatively high voltage, they forced the creation of a new DC bond, which resulted in the formation of a new ring called a bent alkyne; the original molecule then turns into a new molecule consisting of four rings.

Note that the system is designed in such a way that the remaining unpaired electrons cannot reconnect to create another DC bond, as they normally would. Applying a lower voltage to the intermediate molecule resulted in the formation of a new four-carbon ring within the molecule, called cyclobutadiene. Getting these different carbon networks would be impossible with conventional chemistry.

More selective and reversible chemistry

So the team converted the original molecule into three different products. “The ability to interact with a different set of partners makes this polymorphic molecular system a Swiss army knife with three distinct and useful chemical tools,” write Igor Alabugin and Chaowei Hu in an accompanying paper. Each of the resulting products is capable of performing various chemical functions, for example, serving as a binding site for transition metals or participating in redox reactions. All three can even be used as logic gates in molecular electronics.

The great advantage of the method is that these reactions can be reversed by impulses of opposite polarity; each product could indeed return to its original state with the help of a new impulse of electrons. Thus, by forcing a single molecule into various forms (or isomers) by applying precise voltages and currents, researchers can directly observe the behavior of electrons and determine the optimal configurations of organic compounds according to the desired result.

Note that this experiment was carried out under cryogenic conditions (at -268°C), a temperature at which atoms and molecules are practically immobile. Reproducing the method near room temperature can be more difficult. Also, some atomic bonds can be much harder to break.

However, this work paves the way for more precise control of chemical reactions – at least in some cases – and sheds light on the mechanisms underlying redox reactions. This could, for example, lead to the development of catalysts capable of directing the reaction in a very specific direction. “Single-molecule selectivity reactions induced by pulses improve our understanding of redox chemistry and could lead to new molecular machines,” the researchers conclude.

F. Albrecht et al., Science.

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