Scientists measure reaction rates of second key atmospheric component (Update)

Researchers at Sandia National Laboratories' Combustion Research Facility, the University of Manchester, Bristol University, University of Southampton and Hong Kong Polytechnic have successfully measured reaction rates of a second Criegee intermediate, CH3CHOO, and proven that the reactivity of the atmospheric chemical depends strongly on which way the molecule is twisted.

The measurements will provide further insight into hydrocarbon combustion and atmospheric chemistry. A paper describing the research findings titled "Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO" is featured in the April 12 edition of Science magazine.

Criegee intermediates—carbonyl oxides—are considered to be pivotal atmospheric reactants, but only indirect knowledge of their reaction kinetics had previously been available. Last year, Sandia and its UK-based partners reported, for the first time, direct measurements of reactions of the smallest gas-phase Criegee intermediate using photoionization mass spectrometry. That research was featured in the January 13, 2012, edition of Science. A short video featuring two Sandia researchers describing the work can be seen below:

This video is not supported by your browser at this time.

New findings include confirmed fast reactions, first-time measurements with water

Sandia combustion chemist Craig Taatjes, the lead author on the Science papers, said there are several significant aspects about the new research findings.

In particular, the measurements show that the reaction rate depends dramatically on whether the CH3CHOO is bent, with the CH3– and –OO ends pointing toward the same side, a conformation called "syn–" or more straightened, with the CH3– and –OO ends pointing away from each other, called "anti–".

"Observing conformer-dependent reactivity represents the first direct experimental test of theoretical predictions," said Taatjes. "The work will be of tremendous importance in validating the theoretical methods that are needed to accurately predict the kinetics for reactions of Criegee intermediates that still cannot be measured directly."

In fact, said Taatjes, the latest results supply one of the most critical targets for such validation. Because of the large concentration of water in Earth's atmosphere, Criegee concentrations—and, hence, the tropospheric implications of all Criegee intermediate reactions—depend on knowing the rate constant for reaction with water.

Although the reactions for most Criegee intermediates, including the syn- conformer of CH3CHOO, with water may simply be too slow to be measured by the research team's methods, anti-CH3CHOO has been predicted to have a vastly enhanced reactivity with water. Taatjes and his colleagues confirmed this prediction and made the first experimental determination of the reaction rate of a Criegee intermediate with water. "A Criegee intermediate's reaction with water determines what the concentration of these intermediates in the atmosphere is going to be. This is a significant benchmark," he said.

Taatjes said one of the questions remaining after the first direct measurement of Criegee reactions was whether the remarkably fast reaction of CH2OO with SO2 was representative of other Criegee intermediates.

"This measurement of a second intermediate—which we found to react just about as fast with sulfur dioxide as the intermediate we measured last year—supports the notion that the reactions of all Criegee intermediates with SO2 will occur easily," said Taatjes "It also confirms that Criegee intermediate reactions are likely to make a contribution to sulfate and nitrate chemistry in the troposphere." This increase in reactivity, he said, provides additional evidence that Criegee intermediates will play a significant role in the oxidation of sulfur dioxide in the atmosphere.

Unraveling the mysteries, complexities of Criegee intermediates

Hydrocarbons that are emitted into Earth's troposphere, either naturally or by humans, are removed by many reactive atmospheric species. For unsaturated hydrocarbons—molecules with at least one C=C double bond—a prominent removal mechanism is reaction with ozone, called ozonolysis. It is accepted that ozonolysis produces other reactive species, including carbonyl oxides, which are known as Criegee intermediates. Rudolf Criegee, a German chemist, first proposed the mechanism of ozonolysis in the 1950s.

Because so much ozonolysis happens in the atmosphere, the reactions of Criegee intermediates are thought to be very important in a wide range of tropospheric processes like secondary organic aerosol formation and nighttime production of highly reactive OH radicals. As a result, the chemistry of these reactive Criegee intermediates has been the subject of intense investigation for decades, but without any direct measurement of their reaction rates until last year's published work by Sandia and its collaborators.

More information: "Infrared Absorption Spectrum of the Simplest Criegee Intermediate CH2OO," by Y.-T. Su, Science, 2013. www.sciencemag.org/content/335/6065/204

Journal reference: Science

Provided by Sandia National Laboratories

View the original article here

Chemists illuminate elusive mechanism of widely used click reaction

Scientists at The Scripps Research Institute (TSRI) have illuminated the mechanism at the heart of one of the most useful processes in modern chemistry. A reaction that is robust and easy to perform, it is widely employed to synthesize new pharmaceuticals, biological probes, new materials and other products. But precisely how it works had been unclear since its invention at TSRI more than a decade ago.

"These new findings allow us to exert greater control of the reaction and make it faster and more efficient under the most challenging conditions," said chemist Valery Fokin, an associate professor at TSRI, who was principal investigator for the new study. "The reaction-tracking techniques we developed here also can be applied to the study of other complex processes, both chemical and biological."

The report, which sheds light on the reaction known as copper-catalyzed azide–alkyne cycloaddition (CuAAC), on April 4 in Science Express, the advance online edition of the journal Science, and in the April 18, 2013 issue of the journal.

Classic Click Reaction

Fokin and his laboratory, and the laboratory of K. Barry Sharpless, a Nobel laureate and the W.M. Keck Professor of Chemistry at TSRI, reported the discovery of the CuAAC reaction in 2002. Danish researchers independently reported a similar reaction in the same year. The reaction involves the use of copper compounds to catalyze the linkage of two functional groups, a nitrogen-containing azide and a hydrocarbon alkyne, to make a stable five-membered heterocycle, 1,2,3-triazole. Azides and alkynes are small functional groups that can be easily introduced into a wide variety of structures using chemical or biological methods without interfering with normal biological processes.

The experimental simplicity and reliable performance of CuAAC under virtually all conditions, including in water and in the presence of oxygen, has made it a "go-to" method whenever covalent stitching of small man-made molecules or large biopolymers is needed, exemplified by protein and nucleic acid labeling, in vitro and in vivo imaging, drug synthesis and the forging of complex molecular architectures with surgical precision.

"Despite its many uses, the nature of the copper-containing reactive intermediates that are involved in the catalysis had not been well understood, in large part due to the promiscuous nature of copper, which rapidly engages in dynamic interactions with other molecules," said Fokin.

Previous studies had hinted that in the swirl of short-lived bondings and partings that occur during a given CuAAC reaction, not one but two copper-containing catalytic units—"copper centers"—are needed to help build the new triazole structure. To confirm this, Fokin and two of his graduate students, Brady Worrell and Jamal Malik, tried to reproduce key steps of the CuAAC catalytic cycle with either one or two copper atoms available. Analysis of the reaction course by tracking the heat given off by each reaction as well as product yield indicated whether it worked efficiently. "By monitoring the reaction in real time, we showed that both copper atoms are needed and established the involvement of copper-containing intermediates that could not be isolated or directly observed," said Worrell, who was the paper's first author.

In a second set of experiments, Worrell, Malik and Fokin introduced a pure isotope of copper—which differs slightly in mass from the isotope blend found in natural copper—as one of the two copper centers so that they could track their respective fates during the reaction. "We hypothesized that the two copper centers would have distinct roles, but found unexpectedly that their functions during key steps in the reaction are effectively interchangeable," said Malik.

New Linkages

The research reveals the popular CuAAC reaction in unprecedented detail. In addition to the fundamental insights into the chemistry of copper and its interactions with organic molecules, the techniques will lead to better understanding of many chemical and biological processes involving copper. The current study also enables development of new reactions that exploit weak interactions of copper catalysts with carbon-carbon triple bonds. In fact, based on the new findings, Fokin and his team have begun to devise new reactions in which one copper center can be replaced with a different element, to pursue complementary biocompatible and efficient techniques.

More information: "Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalyzed Azide–Alkyne Cycloadditions," Science Express, 2013.

Journal reference: Science Science Express

Provided by Scripps Research Institute

View the original article here