Hydroxyl radicals are simple two-atom molecules consisting of an oxygen atom bound to a hydrogen atom. They are an important part of the chemistry of our living world.
In living organisms, hydroxyls are among those DNA-damaging “free radicals” that have been linked to a number of degenerative diseases, but from the perspective of atmospheric physical chemists such as Nathan Kidwell, hydroxyl radicals can be a good thing.
“You can think of hydroxyl radicals as the atmosphere’s detergent,” he explained. “They’re really good at cleaning the atmosphere. When hydroxyl radicals interact with pollutants — hydrocarbons, greenhouse gases and things like that —in the atmosphere, these radicals break them down. So, from that perspective, they’re good players.”
Kidwell is an assistant professor in William & Mary’s Department of Chemistry. He recently received a CAREER award from the National Science Foundation that will fund his lab’s continuing research into a deeper understanding of the molecular scrubbing action of hydroxyl radicals. He said the chemical rates of the hydroxyl detergent action are understood, but the mechanisms are not well characterized.
“We know that when hydroxyl radicals collide with other molecules in the atmosphere, you have two main outcomes that can happen,” Kidwell explained. “One outcome is similar to billiard balls: They collide with each other, and they can exchange energy.”
The billiard-ball scenario is a non-reactive event, Kidwell explained: There’s an energy exchange but each molecule goes away chemically intact. The second type of outcome results in a chemical reaction — the molecules interact and create new products.
“And so, we’re exploring the extent to which each of those happens, those nonreactive and reactive collisions in the atmosphere,” he said. “What variables control the end result? The basis of my CAREER grant is to try to figure that out.”
To figure out the intricacies of the nonreactive and reactive collisions of hydroxyl radicals in the atmosphere, Kidwell must first carefully define the collision conditions in his lab.
“When you make a collision, say a collision of molecule A to molecule B, there’s a certain amount of time when they’re actually stuck to each other,” he explained. “And we call that a collision complex.”
Kidwell points out that molecules aren’t spheres, indeed they tend to be tangly, spiky assemblies of tail-like appendages and functional groups. The shape of the collision complex, he said, dictates the nature of the resulting reaction — or lack thereof. Accordingly, the lab’s strategy is to start with the collision complex.
“That allows us some degree of control,” he said. “So, we create these complexes that may have different shapes. We want to control these conditions really well to be able to steer or understand the outcomes.”
The lab can steer those outcomes by using lasers. Kidwell explained that they use infrared lasers to collect a spectrum (or take a “fingerprint”) of the collision complex and then excite a certain vibrational mode to selectively access a reactive or nonreactive pathway. Then, they can compare the results to other collision complexes with different shapes.
“By controlling which vibration or infrared transitions we excite, we can really explore different scenarios, because there is a probability that these molecules can collide with different amounts of energy in different collision geometries,” he explained.
Kidwell said that the fundamental chemistry aspects will be supplemented by the work of a group of collaborating researchers. The collaborators are Laura McCaslin, at Sandia National Laboratories; Daniel Tabor, at Texas A&M University; and Andrew Petit, at California State University – Fullerton.
“When hydroxyl radicals interact with pollution — hydrocarbons and things like that —in the atmosphere, they make them into compounds that are relatively benign. So, from that perspective, they’re good players.”
“We’ve all known each other for some time,” he said. “So, it’s great not only to work with your colleagues, but your friends as well.”
Kidwell explained that one of the collaborators will address the problem using high-level theory dynamics. Another will employ machine learning, and the third is a specialist in electronic structure.
“As a result, we will have three different points of reference for comparison,” he said. “And that allows us to really unpack what’s going on fundamentally. By changing one small, initial condition, we can see how it might impact the outcome of that reaction. Taking a step back, we can then gain some understanding of the atmospheric chemistry that happens all around us.”
The CAREER grant also provides for inclusion of both undergraduates and high school students in the research. Kidwell explained that one of the aspects involving students will be flying drones bearing sensor-laden Raspberry Pi computers.
“We call it the ‘Raspberry Sky,’” he said. “The long-term goal is to use a fleet of drones that have atmospheric sensor payloads. The drones will allow student researchers to vertically profile the atmosphere in different environments. I’m a firm believer in using accessible technology to engage students in the process of discovery. It’s exciting to have students see themselves as scientists by coding, prototyping designs, collecting atmospheric data and communicating their results.”
The work has a wide range of potential applications. At the chemists’ level, Kidwell said that the field has a good grasp of the chemistry of three-atom systems, but there is a lot left to understand about chemical reactions involving hydroxyl radicals with large molecules.
At the real-world level, Kidwell points out that not only are hydroxyl radicals the detergent of the atmosphere, but the molecules are also important factors in combustion engines and exoplanets.
“This grant will enable us to explore the very fundamental reaction chemistry of hydroxyl radicals and why these reactions happen,” Kidwell said. “And there are different mechanisms at play, where by simply changing the collision orientation or varying where the energy is placed, the reaction will be significantly impacted. So, we will explore how two-collision partners interact with each other so that we can fundamentally understand and make better predictions for what’s happening in the atmosphere.”