The Mysteries of Proton Tunneling

Understanding how wave-particle duality may lead to a greener world

Wave–particle duality may no longer only confuse intro chemistry students but also help solve climate change. Two weeks ago, Assistant Professor of Chemistry Miriam Bowring discussed their research into proton tunneling in organometallic catalysts and its potential in the utilization of methane as a liquid fuel.

Bowring began by explaining that chemical reactions require energy to happen. Typically, you have to add enough energy to overcome its activation energy and achieve a transition state before reaching completion. Catalysts can lower the energy required to achieve this transition state and allow reactions to go forward. There is, however, another possibility for changing how much energy is required for a reaction. Protons may be so small that they can act as a wave rather than as a particle and “tunnel” through the energy barrier instead of having to go over it. Bowring hopes to understand whether tunneling is truly happening, what causes it, and what moderates it. 

Proton tunneling, however, is so fast that it cannot be observed traditionally, so chemists assess the Kinetic Isotope Effect (KIE) of various reactions. The KIE is a ratio of rate constants between a reaction with normal protons (used interchangeably with hydrogen) and deuterium, a hydrogen nuclei with an extra neutron. If reactions are simplified to only involve electrons and not the nuclei, as often done in introductory chemistry classes, the KIE should be one. But because these molecules do affect the reaction, they may have different reaction rates. It is generally accepted that a reaction can have a KIE of up to seven under normal circumstances (assuming protons act as a particle). Some reactions, however, have an observed KIE larger than seven, and one possible explanation is proton tunneling.

Bowring hopes to spend their career researching these reactions with large isotope effects. They aim to understand what creates these large isotope effects and how they can be controlled. With the help of Postdoctoral Researcher and former Visiting Professor Phan Truong and a large group of chemistry thesis students (Leo Gartner ‘21, Mia Faulkner ‘21, Lexi Carlson ‘20, Jo Keller ‘20, Maryam Ahmad ‘19, Ellis Douma ‘19, Josh Tsang ‘18, and Zac Mathe ‘17), they began to explore reactions with large KIEs, first beginning with a bimetallic, ruthenium and iridium catalyst used to produce hydrogen gas that could eventually be used as a more convenient and reversible way to store hydrogen as a liquid fuel. This reaction was very complex and made isolating various variables difficult.

The mechanism they currently research is a platinum-methyl system, which, when treated with acid, acts as a catalyst in the production of methane. This reaction has a reported KIE of 18. This platinum system is especially exciting as it may be useful in figuring out how to use methane as a viable liquid fuel source. Currently, methane is often wasted as a byproduct of petroleum processes. Understanding this reaction could lead to cleaner and more readily available sources of energy than traditional fossil fuels. The platinum system is also promising because it appeared to be relatively simple. The literature does not agree on the mechanism of this reaction but has reached general consensus on two possible different pathways, each a single step reaction. 

Bowring found that changing the concentration of acid present affected the isotope effect. With larger concentrations of acid (greater than 100 times the amount of platinum), the reaction had a KIE of 14, but with acid concentrations of 0.5 to two, the reaction had a KIE of seven. If this reaction were a single step, this change in KIE should not be possible, suggesting that it is, at least, a two step reaction. 

After some further analysis, Bowring found this reaction to have an order of 1.7, meaning that 1.7 acid molecules are used for every one platinum complex molecule. This is, yet again, strange. In light of this finding, they hypothesized that the reaction was actually a two step reaction with a reversible first step.

With some help from their Chem 212 class, Bowring modeled the reaction showing hydrogen bonding between multiple acid molecules and the platinum complex, further supporting their hypothesis. 

They will continue working to understand why some reactions seem to involve proton tunneling, as well as exploring how it can be controlled, in order to create new and unimaginable catalysts and reactions to provide alternative energy sources. 

Bowring also mentioned two other projects. The first was a palladium reclamation project. This involves the capture of palladium, a rare metal that is an important catalyst in various reactions, from car pollution from the road as car catalytic converters contain palladium. This is also a predominantly undergraduate based project led by Sophia Miller ‘22. They also mentioned their groups most recently published paper, authored by Lexi Carlson ‘20, Ellis Douma ‘19, and Dan Primka ‘21, in which they quantitatively analyzed the effectiveness of air-free glassware for the first time. Air-free reactions are essential in highly volatile organometallic reactions.