Tracking an elusive molecule key to climate and combustion chemistry


Date:
October 12, 2021
Source:
DOE/Argonne National Laboratory
Summary:
Researchers report they have directly observed a prototypical
version of a class of molecules central to environmental and
combustion chemistry.

This new knowledge is important to climate change models and the
design of more efficient combustion engines.



FULL STORY ========================================================================== Researchers make the most direct-ever observation of a key intermediate
product formed during breakdown of hydrocarbons important to understanding climate change and combustion.


==========================================================================
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 ( o 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.

========================================================================== Story Source: Materials provided by
DOE/Argonne_National_Laboratory. Original written by Joseph
E. Harmon. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Anne S. Hansen, Trisha Bhagde, Kevin B. Moore, Daniel R. Moberg,
Ahren W.

Jasper, Yuri Georgievskii, Michael F. Vansco, Stephen
J. Klippenstein, Marsha I. Lester. Watching a hydroperoxyalkyl
radical ( o QOOH) dissociate. Science, 2021; 373 (6555): 679 DOI:
10.1126/science.abj0412 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012130723.htm

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