BREAKING BONDS: Texas A&M Chemists Shed Light on Important Sunlight-Driven Atmospheric Reaction
COLLEGE STATION -- Chemists at Texas A&M University have discovered a unique and unexpected photochemical process occurring within the atmosphere that challenges century-old conventional models of chemical reactions.
The landmark finding was the result of an investigation of the nitrate radical NO3 by a team of scientists led by Texas A&M chemist Dr. Simon W. North and Japanese theorists from Kyoto University's Fukui Institute for Fundamental Chemistry and the Hakubi Center for Advanced Research. Their breakthrough results, published last spring in the journal Science, rattle the foundations of several longstanding molecular ideologies.
North says NO3 is an atmospheric oxidant that photodissociates into either NO2 and O or NO and O2 and notes that it was the molecule's transformation into NO and O2 that had puzzled researchers for years.
"Since about 1950, NO3 has been recognized as a very important atmospheric molecule, and what happens when it absorbs light has been the subject of a lot of interest," North says. "From an atmospheric point of view, I think we've finally put to rest a 60-year-old problem. From a fundamental chemistry point of view, we've shown that roaming can occur as the only way a reaction can occur."
The classical scientific explanation for molecular reactions has long been the transition state theory, which surmises that chemical reaction mechanisms with multiple-bond breaking endure a transition state in the form of a saddle point on the potential energy surface. However, a recently discovered mechanism called "roaming" involves straying from the reaction pathway and bypassing the saddle point entirely. An atom or bond cleavage breaks away from the molecule as if to dissociate entirely but instead orbits the remaining molecule fragment until encountering a reactive site to form a new product.
First identified in the formaldehyde molecule in 2004, roaming reactions had been noticed in other reactions in recent years, but in all previous cases, a traditional transition state was involved. North and his team are the first to confirm that NO3 yields NO and O2 entirely through the roaming mechanism, making it the only known reaction to occur in that manner. In addition, they found that the reaction occurs both in the ground state and excited state -- another crucial trait not previously found in other reactions.
Through experimentation and theoretical calculations, the group determined that when NO3 absorbs light and enters an excited state, its electrons will shift in such a way that it enters a state unattainable by light absorption, or a dark state. It's here, North says, that the reaction mostly occurs. An oxygen atom breaks free as if to form NO2 and O, but the low-energy threshold results in a weakened N-O bond. The oxygen atom is now only loosely bound to the rest of the molecule. Instead, it orbits or roams the molecule, where it will eventually encounter another oxygen atom to form the NO and O2 bond. Because the reaction occurs mostly in the dark state, North's team has labeled it "roaming in the dark."
"One of the nice things about this experiment is that it shows a beautiful collaboration between experiment and theory," North says. "In our original experiments, we showed there were multiple pathways, then theory came in and proposed a multistate pathway, and we were able to follow that up with experiments that clearly show what was happening. That back and forth between chemistry and theory made it a nice complete story."
North's team was able to chart the reaction using a technique known as velocity map imaging. The scientists created a supersonic beam of NO3 in a vacuum first by synthesizing a molecule and sending it through an 800-degree microfurnace, then by cooling it down to 20 kelvins. A laser that simulates sunlight is fired into the beam of NO3 to dissociate it into NO and O2. A little later, the team uses two more laser beams to ionize the NO. Upon detonation, the particles are sent in various directions which are accelerated to collide with a detector, at which point each ion that hits makes a light pulse.
The images are then consolidated to pinpoint where each trace of NO in a particular state is hidden. The farther out an NO ion is, the greater the impact it was subject to during dissociation. By taking detailed measurements of the resulting molecules, North's team could piece together evidence about how the reaction took place.
The findings are the culmination of a two-year study that began as the thesis work of Texas A&M chemistry graduate student Michael Grubb and expanded into an international collaboration. Grubb, a 2012 recipient of the Association of Former Students Distinguished Graduate Student Award for Excellence in Doctoral Research, called the NO3 reaction a benchmark molecule for studying the roaming process.
"Our work on the NO3 system has provided a lot of insight to how roaming reactions work, how to think about them and what makes them special compared to more typical dynamics," Grubb says. "The reaction was quite a challenging puzzle for us to decipher, and thus it is incredibly rewarding to now have finally solved the longstanding mystery and tell the whole story of NO3 photodissociation."
North says it is unlikely that a multistate roaming reaction is exclusive to only NO3, which opens the door for the exploration of roaming reactions in other molecules.
"It's great to be a part of a story that has a beginning, middle and end that is so self-contained," North says. "We peck away at things and think we know what's going on, but I've never been a part of something so clear. Now that we know what to look for and what it means, we can start to look for it in other systems."
To learn more about this project and other facets of North's research, visit http://www.chem.tamu.edu/faculty/north/.
Contact: Shana K. Hutchins, (979) 862-1237 or firstname.lastname@example.org or Dr. Simon North, (979) 845-4947 or email@example.com