CM – Team measures the breaking of a single chemical bond


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October 4, 2021

by Molly Sharlach, Princeton University

The team used a high resolution atomic force microscope (AFM) operated in a controlled environment at Princeton’s Imaging and Analysis Center. The AFM probe, the tip of which ends in a single copper atom, was gradually brought closer to the iron-carbon bond until it broke. The researchers measured the mechanical forces exerted at the moment of the fracture, which were visible in an image taken by the microscope. A team from Princeton University, the University of Texas-Austin, and ExxonMobil reported the results in a paper published September 24 in Nature Communications.

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« It’s an incredible picture – to actually see a single tiny molecule on a surface that another is bound to is amazing, » said co-author Craig Arnold, Susan Dod Brown professor of mechanical engineering and aerospace Aerospace Engineering and Director of the Princeton Institute for the Science and Technology of Materials (PRISM).

“The fact that we have been able to characterize this particular bond by both pulling and pushing on it enables us to do a lot to understand more about the nature of these types of bonds – their strength, how they interact – and that has all sorts of implications, especially for catalysis, where you have a molecule on a surface and then you interact with something and it breaks, « said Arnold.

Nan Yao, a lead researcher on the study and director of the Imaging and Analysis Center at Princeton, noted that the experiments also provided insight into the effects of attachment bro chs on the interactions of a catalyst with the surface on which it is adsorbed. Improving the design of chemical catalysts is important for biochemistry, materials science and energy technologies, added Yao, who is also a professor of practice and senior research scholar at PRISM.

In the experiments, the carbon atom was part of a carbon monoxide molecule and the iron atom was derived from iron phthalocyanine, a common pigment and chemical catalyst. Iron phthalocyanine is structured like a symmetrical cross, with a single iron atom at the center of a complex of nitrogen- and carbon-based linked rings. The iron atom interacts with the carbon of carbon monoxide, and the iron and carbon share a pair of electrons in a type of covalent bond known as a dative bond.

Yao and his colleagues used the atomic-scale probe tip of the AFM- Instruments to break the iron-carbon bond by controlling the distance between the tip and the bound molecules to within 5 picometers (5 billionths of a millimeter). The break occurred when the tip was 30 picometers above the molecules – a distance about one sixth the width of a carbon atom. At this level, half of the iron phthalocyanine molecule in the AFM image became more blurry, indicating the point where the chemical bond broke. The researchers used a type of AFM known as non-contact, where the tip of the microscope is the The researchers did not touch the molecules directly, but instead used changes in the frequency of fine vibrations to create an image of the surface of the molecules.

By measuring these frequency shifts, the researchers were also able to calculate the force required to break the bond. A standard copper probe tip broke the iron-carbon bond with an attractive force of 150 piconewtons. With another carbon monoxide molecule attached to the tip, the bond was broken by a repulsive force of 220 piconewtons. To investigate the basis for these differences, the team used quantum simulation methods to model changes in electron density during chemical reactions.

The work uses AFM technology, which was first developed further in 2009, to make single chemical bonds visible. The controlled breaking of a chemical bond with an AFM system was more difficult than similar studies of bond formation.

“It is a great challenge to improve our understanding of how chemical reactions are carried out by atom manipulation, that is, with the tip of a scanning probe microscope « says Leo Gross, head of the Atom and Molecule Manipulation research group at IBM Research in Zurich, and was the main author of the study from 2009, which for the first time clarified the chemical structure of a molecule using AFM.

By breaking a particular one Binding with different tips that use two different mechanisms, the new study helps to « improve our understanding and control of bond cleavage through atomic manipulation molecules of increasing complexity, » adds Gross, who was not involved in the study.

The experiments are acutely sensitive to external vibrations and other disruptive factors. The Imaging and Analysis Center’s specialized AFM instrument is housed in a high vacuum environment and the materials are cooled with liquid helium to a temperature of 4 Kelvin, just a few degrees above absolute zero. These controlled conditions lead to precise measurements by ensuring that the energy states and interactions of the molecules are only affected by the experimental manipulations.

« You need a very good, clean system because this reaction can be very complicated – at with so many atoms involved, you may not know which bond you are breaking on such a small scale, ”said Yao. « The design of this system simplified the whole process and cleared the unknown » in breaking a chemical bond, he said. The study’s lead authors were Pengcheng Chen, Associate Research Fellow at PRISM, and Dingxin Fan, Ph.D. Student at the University of Texas-Austin. In addition to Yao, Yunlong Zhang from ExxonMobil Research and Engineering Company in Annandale, New Jersey, and James R. Chelikowsky, professor at UT Austin, were other corresponding authors. In addition to Arnold, other Princeton co-authors were Annabella Selloni, the David B. Jones Professor of Chemistry, and Emily Carter, Gerhard R. Andlinger ’52 Professor in Energy and the Environment. Other ExxonMobil co-authors were David Dankworth and Steven Rucker.

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