A Love for alpha-Pinene That Sticks Around

By Hilary M. Chase

Our group really likes alpha-pinene. It’s a cool little molecule; it smells like pine trees, and it has an interesting rigid structure. It looks like a sphere, actually. Alpha-pinene falls under a class of compounds called terpenes. Strong smells that you associate with vegetation, fruits, etc. generally come from terpenes. We love alpha-pinene in particular because it is a feedstock for biorenewable polymers,¹ and it is the most abundant naturally emitted terpene. When it gets emitted by vegetation, it splits open and is oxidized in the atmosphere by ozone and other oxidants.² These oxidation products eventually form atmospheric aerosol particles. These particles are really complex and are difficult to understand. However, we hope that by understanding how alpha-pinene behaves, we can understand more about the atmospheric aerosols that derive from it.

Because we love alpha-pinene so much, and because it is so important to our atmosphere, we do a lot to poor alpha-pinene. We replace hydrogen atoms with deuterium atoms within the molecule.³ We also attack it with high-powered lasers to determine how alpha-pinene sits on a solid surface, and determine exactly how long it takes for the vibrations within the molecule to ring out.⁴

Yet, while we already know a lot about this molecule, we really wanted to know how alpha-pinene behaves on surfaces – does it stick to the solid surface and come off readily? How strong is that interaction? If it sticks, how will the stickiness play out in atmospheric aerosol chemistry? Well, I will tell you, because we just published a paper on it.⁵

In our work, we needed to not only use experiments but also computations. Computations help us to understand dynamic chemical processes that we can’t see with our eyes- it’s like watching a movie with the atoms that make up alpha-pinene in the starring roles. By using both laser spectroscopy, specifically sum frequency generation spectroscopy (which allows us to look at the vibrational signatures of pinene vapor selectively at a solid surface), and molecular dynamic simulations (a large calculation of how 130 alpha-pinene molecules move on a solid surface), we were able to see whether and how it sticks to surfaces.

When we started this project, we assumed initially that alpha-pinene would be only loosely bound to most surfaces and come off readily. This expectation was rooted in the spherical structure of alpha-pinene that would allow it to “roll off” a surface with much ease. Yet, we found this wasn’t necessarily the case. Alpha-pinene really likes to stick to solid surfaces. We tried a number of approaches to removing the alpha-pinene molecules, but some of the molecules just wouldn’t budge for days. It’s like putting marbles on a glass surface and tilting the glass but the marbles don’t roll off but instead stay in place, as if glued on! Indeed, our atomistic movies showed that, hey, some of the alpha-pinene molecules would rather nestle themselves onto the surface and not come off. We also discovered that while the molecule was stuck to the surface, it didn’t just stay in one position, but it would rotate, quite quickly, in place. Indeed, we estimated the population of molecules that would rotate quickly and would rotate a lot slower, due to being stuck more or less to the surface.⁶

There are a couple of reasons why these findings matters. The first reason is because alpha-pinene gets oxidized at a very specific location, at the carbon-carbon double bond, just like on a particularly colored spot on a marble. If alpha-pinene is on a surface and is constantly tumbling around, that also means that the availability of that double bond for oxidation is constantly in flux. This is very important in the atmosphere, where oxidation of that double bond is the necessary first step to the formation aerosols, which cool the region. The second reason why our finding of the “unanticipated stickiness of alpha-pinene” is important is because a number of researchers use alpha-pinene to make models of atmospheric aerosols to study them in a lab.⁷ These labs are often equipped with glass tubes where alpha-pinene is injected in the gas phase and reacts with oxidants to form atmospheric aerosols in a highly controlled setting. Some of this alpha-pinene may stick to the glass and other walls of the instrumentation used in the studies and that could impact how we use the models to understand aerosols in the natural environment.

So, as I mentioned in the beginning, we really like alpha-pinene. We do a lot to understand how this little molecule works (not only because it smells really good, but because it is scientifically interesting). Hopefully some of this information “sticks” with you!