A new study demonstrates that it is possible to deliberately alter chemical reactions and increase chemical selectivity using mechanical force – a long-standing challenge in the field.
The study, led by Jeffrey Moore of the University of Illinois Urbana-Champaign and Todd Martinez of Stanford University, demonstrates how external mechanical forces alter atomic motions to manipulate reaction outcomes. The findings of the study were published in the journal Science.
“We think of chemical reactions as molecules moving on a surface of potential energy in the way hikers follow the contour map of mountains and valleys along a trail,” said lead author Yun Liu, a post-doctoral researcher in Moore’s research group. “A mountain along a reaction path is a barrier that needs to be traversed before the molecules can descend into their final product. Therefore, the relative height of barriers control which path the molecules will most likely choose, allowing chemists to make predictions about what a particular chemical reaction will produce — an outcome called selectivity.”
Chemists have long assumed that the jiggling of molecules – referred to as “molecular dynamics” – is controlled by a potential energy surface. Molecules undergo transformations via chemical reactions that seek the least energy-consuming path. However, emerging evidence indicates that molecules frequently lack the time necessary to sample the surface, resulting in what the researchers refer to as nonstatistical dynamic effects.
Nonstatistical dynamic effects are observed in a number of common reactions, including benzene nitration and dehydration,” Liu explained. “Despite these examples, NDEs have not fully captured the attention of chemists because they are difficult to quantify and cannot be controlled to alter the reaction outcomes – the fundamental goal of chemistry.”
Liu devised an experimental design based on a carbon-13-labeled ring molecule attached to two polymer chains. Liu placed the polymers in a reaction vessel and used sonication to rip the ring into two distinct groups.
“After being ripped apart, the ring molecule can convert to one of three different products, making it an excellent model for studying NDEs,” Liu explained. “The 13-C label enables us to monitor and quantify the chemical changes occurring to the ring, which distinguishes it from the polymer’s thousands of other chemical bonds.”
Liu hypothesised that when mechanical force is applied, the atoms heat up in reaction directions rather than the directions defined by the potential energy surface. The researchers coined the term “flyby trajectory” to refer to this departure from conventional understanding of chemical reactions.
“Taking hiking as an example, the hypothesis is equivalent to asserting that the hiker simply chose not to follow the map,” Liu explained. “Rather than that, the hiker was enthused enough to hop on a hang glider and fly through the hills on their descent. As a result, the direction in which molecules move is determined by their initial jump rather than the subsequent barrier height.”
Liu conducted a series of experiments demonstrating the tunability of the flyby trajectory by increasing the mechanical force applied to the reaction, allowing it to overcome barriers with increasing ease. In an ideal world, researchers would be able to convert an unselective reaction into a highly selective one with undetectable side products.
To substantiate the experimental finding, Stanford University graduate student Soren Holm collected ten million computed geometries and used them to construct a theoretical model of the potential energy surface, from which he extracted the speed of reaction trajectories in the presence of mechanical force.
“We discovered that initial trajectories do not slow down as they pass through barriers,” Liu explained.
In other words, the researchers said, barriers are flown past rather than overcome, which should have slowed the chemical reaction rate. The molecules cool down over time, and subsequent trajectories follow the predicted minimum energy path.
“Our findings will provide a better understanding of how force can be used to alter the course of chemical reactions in order to increase production efficiency,” Moore said. “It’s another tool in our toolbox for creating the items we use on a daily basis.”
This research was supported by the National Science Foundation, the Army Research Office, the Dr Leni Schoninger Foundation, and the Deutsche Forschungsgemeinschaft.
Moore is the director of the Beckman Institute for Advanced Science and Technology, a professor of chemistry and materials sciences and engineering, and a member of the Center for Advanced Study, the Materials Research Laboratory, the Carle Illinois College of Medicine, the Carl R. Woese Institute for Genomic Biology, and the Center for Social and Behavioral Science at the University of Illinois.
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