Controlling Chemistry with the Tools of Physics

UConn physicists explain how individual atoms and molecules react in an ultracold environment, providing new insight into the forces at work in chemical reactions.

Ionel Simbotin, left, a postdoctoral fellow, Robin Côté, associate dean and professor of physics, and John Montgomery Jr., research professor. (Peter Morenus/UConn Photo)

Ionel Simbotin, left, a postdoctoral fellow, Robin Côté, associate dean and professor of physics, and John Montgomery Jr., research professor. (Peter Morenus/UConn Photo)

Scientists from three major research universities successfully manipulated the outcome of a chemical reaction and, in doing so, created a rare molecular ion.

Through a process known as “controlling chemistry,” the researchers bonded an oxygen atom to two different metal atoms, creating the barium-oxygen-calcium molecular ion or BaOCa

<a href=""><em>Watch an animation of barium ions inside a Paul Trap</em></a><em>.</em>
Watch an animation of barium ions inside a Paul Trap.

The same process could lead to the creation of other exotic new materials and the design of novel chemical compounds, according to the team from the University of Connecticut, University of California-Los Angeles, and University of Missouri, whose work appeared in the Sept. 29 online issue of Science.

“It’s doing chemistry with the tools of physics,” says Robin Côté, UConn physics professor and associate dean for physical sciences who co-authored the study. Côté is a leading theorist in the field of ultracold molecular ion reactions.

“Usually in chemistry you can’t see molecules react,” says Côté. “You do experiments and you get a reaction with a range of products, which can be analyzed with mass spectrometry and other tools. What our team did allowed us to image ions directly to get a much clearer picture of what is happening within the chemical reaction itself. We were able to prepare a system in an ultracold environment that was very pristine and that allowed us to see exactly how the molecules and atoms were behaving.”

Previous work had observed hypermetallic alkaline earth oxides consisting of an oxygen atom sandwiched between two identical alkaline earth atoms, such as barium or calcium.

But for the first time in this study, researchers observed compounds consisting of an oxygen atom bonded to two different metal atoms. Such compounds are believed to have unique properties resulting from the broken symmetry.

It’s nice to see that a phenomenon you envisioned at one time as too difficult to do, is indeed possible. — Robin Côté

The researchers used a host of equipment to accomplish the breakthrough – quantum chemistry computations, lasers, a hybrid ultracold atom-ion trap, and something called a “radially-coupled time-of-fight mass spectrometer.”

By trapping atoms and ions in an ultracold hybrid magneto-optical trap and then cooling them down to a temperature one one-thousandth degree above absolute zero, the scientists were able to observe a reaction between atoms and molecules that hadn’t been seen before.

See a real time image of crystal ions.

They used lasers to manipulate calcium atoms into a specific quantum state, which allowed the chemical reaction to proceed and the new molecules to form when they normally shouldn’t. To confirm their results, the researchers released the ions and molecules from their ultracold suspension, where they would then “fly” into the mass spectrometer to be analyzed and measured further.

The process could serve as a platform for gaining greater insight into chemical reaction dynamics. Armed with such knowledge, chemists and physicists could engineer specific chemical reactions and synthesize new products currently unavailable to us, such as more advanced chemical sensors, quantum computer processors, and medications with fewer side effects.

Joining Côté on the project were UConn research professor John Montgomery Jr., an expert in computational quantum chemistry, and postdoctoral researcher Ionel Simbotin. UCLA physics professor Eric Hudson, an expert in hybrid atom-ion trapping, served as the project’s principal investigator; and UCLA graduate students Prateek Puri (lead author) and Michael Mills, together with postdoctoral researcher Christian Schneider, performed the measurements. University of Missouri chemistry professor Arthur Suits rounded out the team.

Côté was the first to propose studying ultracold atom-ion systems in 2000, shortly after arriving at UConn. And, UConn physics professor emeritus Winthrop Smith, in 2001, was the first to propose using a hybrid ultracold ion-atom trap to conduct the observations in order to learn more about the fundamental processes of chemical reactions.

Experimental and theoretical physicists and chemists have been working in the field ever since. Lately, new advances in technology, such as the hybrid trap used in this study, are providing scientists the tools they need to progress.

To some extent, the study confirmed Côté’s and Smith’s proposal that hybrid atom-ion traps could be valuable tools for exploring the fundamental dynamics underlying chemical reactions.

“It’s nice to see that a phenomenon you envisioned at one time as too difficult to do, is indeed possible,” says Côté. “We did calculations showing this behavior should take place, and it is great to see that now confirmed in the lab.”

In the current study, the researchers spent more than 18 months trying to figure out what was driving the chemical reaction creating the new molecule. They analyzed experimental data, performed quantum chemistry calculations, and tested various theories in the lab. In many ways, it was like solving a mystery. The scientists knew the molecule could be formed because their UCLA peers had seen them in the hybrid ion trap. The challenge was trying to explain how it was happening.

The team ultimately determined the precise mechanisms that allowed the mixed hypermetallic earth oxide to form, which involved the amount of energy expended in the reaction and a certain spin of electrons.

“Most chemical reactions take place in the ground state, which is to say, a system’s lowest-energy state,” says Montgomery. “The use of quantum chemical calculations enabled us to understand that this particular reaction could only take place on the excited triplet state, which is a higher-energy state. This was then verified by experiments.”

That kind of precise knowledge of what exactly is occurring during a chemical reaction goes to the very core of controlling chemistry.

“When something reacts in chemistry, you can compute the outcome or products, but you can rarely control the process,” says Côté. “But if you are able to know exactly what is happening in that reaction, what components are rotating, what is spinning, what’s vibrating with this energy or that energy, and if you’re able to control those quantities, then it is feasible to design a reaction or a device to ensure a particular outcome most of the time.”

The research was funded by the National Science Foundation (PHY-1205311, PHY-1415560, and DGE-1650604) and Army Research Office (W911NF-15-1-0121, W911NF-14-1-0378, and W911NF-13-1-0213).

UConn is one of seven universities working in this area under a multimillion-dollar research grant awarded by the Department of Defense. The MURI award, which stands for Multidisciplinary University Research Initiative, also includes researchers from the physics departments at UCLA, Northwestern, and Temple, and from the chemistry departments at Georgia Tech, Emory, and Missouri.