For the first time, chemists have used mechanical power to access unconventional pathways on the potential energy surface of a reaction and create products that differ from those predicted by statistical thermodynamics. The researchers hope that the technique can be used to discover new reaction schemes and develop probes for reaction dynamics.
Mechanical force can reshape the potential energy surface of the reaction, changing the thermodynamically favorable competing reaction pathways, which in turn changes the product distribution. Chemists in the United States have now carried out ring opening reactions on cyclobutane derivatives attached to long polymer chains on either side. When ultrasonic shear was applied, the molecular weights of the polymer chains dramatically affected the product isomer ratio. With a higher molecular weight, more alkene products were formed, the geometry of which reflects the stereochemistry of the substituents in the cyclobutane starting ring despite the free rotation of the substituents – which should lead to the formation of Z and E alkenes with equal probability.
“The most obvious reason is that the potential energy surface has been altered by violence,” explains Jeffrey Moore of the University of Illinois at Urbana-Champaign, who co-led the study with Todd Martinez of Stanford University. “As the polymer chain gets longer and longer, the force exerted on the reacting molecule increases.”
Martinez found that her thermodynamic calculations did not agree with the experimental results. “It is true that the potential energy surface changes when a force is applied, and that can have an effect, but that is not enough to explain the isomer distribution shown here.” Postdoctoral fellow Yun Liu – who works with Moore – suggested that the force reaction could go through a high energy state of the intermediate directly from the initial to the final state without examining all possible exits from the intermediate. This violates a central dogma of statistical thermodynamics that as a reaction explores all paths, a lower energy path is always more likely. The researchers called these dynamic, statistic disobedient paths “flyby trajectories,” which describes how the trajectories move from an even higher energy state across the energy barriers.
When the researchers changed the functional groups of the cyclobutane from alkyl to carbonyl groups to increase the stability of the intermediate, they found that “the product selectivity becomes lower as the systems become more thermalized because the barriers are now higher and these trajectories are higher block, ”explains Liu. Dynamic effects have been identified earlier in mechanochemistry and other organic reactions, says Liu, but “what’s new here is that we’re in control”. By first quantifying and then modifying the dynamics of a system, “that really turns an idea into a handle that chemists can use to control reactions,” he says.
Mechanocheme Guillaume De Bo of the University of Manchester, UK, describes the results as “quite remarkable”. “Mechanochemistry allows you to uncover unexpected reaction pathways, either by modifying the potential energy surface … or really new ones, where you play with how atoms move within the molecule itself to lead to a particular product,” he says . “I suspect this is going to be something that is increasingly being studied, and that might be more general the more we get into it.”