Tracking an elusive molecule key to climate and combustion chemistry

For some years, chemists knew that a specific class of molecules important to understanding both climate change and combustion chemistry must exist. But they were not able to hunt it down and study it. Until now.

In a paper appearing in the journal Science, researchers from the U.S. Department of Energy's (DOE) Argonne National Laboratory and the University of Pennsylvania report they have directly observed and studied a prototypical version of this class of molecules for the first time. The official chemical name for it is carbon-centered hydroperoxyalkyl radical. Chemists normally just call it QOOH. It is an intermediate product during reactions important to climate change models and the design of more efficient combustion engines.

The results from this research could contribute to the design of higher-efficiency, lower-polluting engines and improved understanding of the oxidation reactions that polluting emissions undergo in the atmosphere. They are even applicable to understanding the atmospheric reactions of natural volatile organic compounds that occur worldwide.

While QOOH had been hypothesized for many years, it has been difficult to observe directly because it quickly degrades. "This intermediate product is a switchyard controlling various subsequent steps that are really important for the propagation of this chemistry," said Marsha I. Lester, Christopher H. Browne Distinguished Professor of Chemistry at the University of Pennsylvania. "But prototypical QOOH intermediates have not been directly observed, so there were critical pieces missing about how this network of chemical reactions occurs."

The QOOH molecule is an intermediate product of reactions of volatile organic compounds. They are commonly emitted by trees, as well as industrial processes, and are key components of gasoline fuel. These compounds are also emitted into the environment through use of household products and even building materials.

A similar pathway for the reaction of the volatile organic compounds with oxygen at low temperature occurs in both combustion and the atmosphere. The QOOH molecule is central to whether this oxidation process happens or not.

"There is always a competition between the QOOH splitting into smaller molecules or oxygen reacting with the QOOH," explained Stephen Klippenstein, Argonne Distinguished Fellow in the Chemical Sciences and Engineering division. "Understanding that competition is essential to much of what happens in atmospheric and combustion chemistry."

The team first isolated and then probed this elusive prototypical molecule. While earlier experimental studies observed only the final products of the oxidation reaction, the current study finally observed this crucial intermediate product.

The team also determined how the QOOH lifetime and decay rate change with its energy. "We have been making predictions of these quantities for years, but had no idea how good they were," said Klippenstein. "We found out they had some flaws we could fix."

Team members thus improved their theoretical model, and the prediction and experimental results now agreed with great precision. They will be using this valuable new knowledge in future studies of related molecules involved in environmental and combustion chemistry.

This research appeared in Science in an article entitled "Watching a hydroperoxyalkyl radical (•QOOH) dissociate." In addition to Klippenstein and Lester, authors include Anne S. Hansen, Trisha Bhagde, Kevin B. Moore III, Daniel R. Moberg, Ahren W. Jasper, Yuri Georgievskii and Michael F. Vansco.

Funding was received from the DOE Office of Basic Energy Sciences, the National Science Foundation and U.S. Army Research Office. The theoretical calculations made use of the computing resources provided on Bebop, a high-performance computing cluster at Argonne's Laboratory Computing Resource Center.