Controlling Chemical Reactions with Electric Fields | Interview

Totally new way of thinking about chemistry

6:00 AM, 17th October 2016
Controlling Chemical Reactions with Electric Fields | Interview
Professor Michelle Coote, a member of the ARC Centre of Excellence for Electro Materials Science, Research School of Chemistry, Australian National University in Canberra

In an interaction Professor Michelle Coote, a member of the ARC Centre of Excellence for Electro Materials Science, Research School of Chemistry, Australian National University in Canberra with Chemical Today magazine speaks about her research related to electronic catalysts and how the ‘new way of thinking’ will benefit chemical manufacturers.

Explain your current research work?

We are interested in the idea of controlling chemical reactions with electric fields –using either externally applied fields, or the short-range electric fields generated by charged functional groups within catalysts or substrates.

You mentioned “We now have a totally new way of thinking about chemistry.” Kindly explain your inspiration and the research technology?

It is well known that redox reactions can be manipulated with electric fields because they involve transfer of electrons. However, few people thought that electric fields could be used to manipulate non-redox processes as well.

I became interested in studying the effects of electric fields on chemical reactions when my research group accidentally discovered that remote charged functional groups could dramatically alter the stability of nitroxide radicals. Follow-up theoretical research convinced us that the effects were electrostatic in origin and very general. Basically, most chemical species can be stabilized  to some extent by charge-separated resonance contributors.

It follows that an appropriately aligned electric field should be able to stabilize these contributors and in doing so enhance the overall resonance stabilization of the molecule. Since this stabilization will undoubtedly be different in reactants, products and transition states, the electric fields can alter the rate and equilibrium constants of chemical reactions.

In the course of our work we read an elegant paper by professor Sason Shaik (Hebrew University of Jerusalem) who had shown via theoretical calculations that external electric fields should be able to catalyze a Diels-Alder reaction. In collaboration with an amazing team of experimentalists we decided to see if this actually possible. My collaborators designed a surface model system that used scanning tunneling microscopy to both orient the reagents in an electric field, create a potential difference between them and measure the resulting reaction rate.

They showed that changing the strength and polarity of the electric field changed the rate of the Diels- Alder reaction. We showed that these experimental results were consistent with quantum-chemical predictions of the effect of electric field on that particular process, confirming it was the electric field that was responsible for the catalysis.

Now that we know that electrostatic effects are important, we can start to interpret the activity of existing catalysts in a new light, and design better ones that harness electrostatics more effectively. We can also think of implementing external electric fields as yet another tool to manipulate reagents on surfaces.

Explain importance of the chemical materials/ catalysts used for your research?

Our most recent work on external electric fields focused on the Diels-Alder reaction, which is a cornerstone of organic synthesis. We deliberately chose to work with a relatively non-polar non-reactive diene/dieneophile combination so we could measure the catalysis easily and be confident that any effects were due to the electric field. We also needed to control the orientation of the reagents in the field. We thus chose a rigid non-polar norbornylogous bridge with a terminal double bond, which was tethered to the plate of the scanning tunneling microscope.

This was reacted with a furan, which was tethered to the tip of the instrument. Although the system we chose was specific, our theoretical work, and the pioneering studies of professor Shaik, indicate that these electric field effects should be general and that much larger catalysis should be possible for more polar reagents.

Elaborate on how your research work will make a difference for the chemical manufacturing sector?

External electric fields are unlikely to be of use in catalyzing bulk chemical reactions due to the low field strengths achievable and the problem of controlling the orientation of molecules in the electric field. However, they could be useful in nanoscale applications such as surface patterning. We also envisage using electric field effects to trigger annealing in Diels-Alder based self-healing polymers.

However, where we have high hopes for electrostatic catalysis is in the use of conventional catalysts bearing charged functional groups (i.e. acids or bases). When charged, such groups will have electric fields associated with them that can be precisely oriented relative to the reaction centre. What is particularly attractive is that the charge on the acid or base can be easily altered by changing the pH, and hence the electrostatic effects can be switched on or off accordingly. This type of switching effect will be particularly useful in applications such as controlled radical polymerization, which is our first synthetic target.

Controlled radical polymerization is one of the most important developments in the polymer field of the last few decades, allowing one to combine the advantages of radical polymerization with the ability to control the molecular weight and architecture of the polymer formed. We have been harnessing electrostatic effects on nitroxide radical stability to develop pH switchable control agents for free-radical polymerization. When switched “on” we can dramatically stabilize the nitroxide and lower the temperature at which polymerization is possible; by altering the pH we can then stop polymerization altogether. This will allow us to work at lower temperatures than currently possible, and control the sequence of monomer addition to produce speciality materials.

Mention the commercialize aspects of your research and future plans?

We have a patent on pH switching nitroxides and we’re currently looking for commercial partners to develop the research.

What are the challenges you faced while carrying out the research?

In the Diels-Alder work, my experimental collaborators faced the most challenging problem, how to control the orientation of the approaching reagents in a dynamic chemical system-solving this is what took the concept out of the computer and into the real world. They did this by tethering one reagent on the plate of the STM and the other on the tip. They then used the STM to allow the reagents to approach each other react in a controlled way, whilst simultaneously controlling the field strength and bias, and using the “blinking” technique to measure the reaction rate.

In our pH-switching work, the biggest challenge has been choosing appropriate reaction conditions. The electric field effects of charged functional groups are greatest in low polarity solvents, but the solubility of charged species is greatest in aqueous solution. So we had to compromise with moderately polar solvents, and work on developing reagents with optimal solubility and switching effects.

Give us some of the advantages of working in the fast-growing field of computer-aided chemical design. Have you used the technology for your research?

Yes, computational quantum chemistry has been an integral part of our work-providing the initial discoveries that inspired us to explore these new areas of chemistry and the mechanistic understanding that is now allowing us optimize catalysis. Computational chemistry provides information that is difficult or impossible to obtain experimentally, effectively allowing you to “watch” reactions as they take place. It also allows you to test ideas-in this case measuring the effect of external electric fields and charged functional groups on chemical reactivity-relatively easily. At the same time, to be useful chemistry ultimately has to be done experimentally and the translating computational discoveries to lab environment is not always straightforward.

However, the combination of theory and experimental is powerful and is becoming increasingly important in chemical research.

© Chemical Today Magazine

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