At the smallest scale, chemistry isn’t all about ‘billiard ball’ reactions

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Scientists are now one step closer to understanding how to live in a “quantum” world — and not just by watching the character “Ant-Man” in the Marvel movie franchise.

Take, for example, a microscopic view of the delicate, protective barrier of the Earth’s atmosphere that creates the ozone layer and protects life as we know it. Within that layer of air, oxygen molecules are almost constantly attacked by ultraviolet (UV) sun rays that break down these molecules through a chemical process known as photodissociation. Although this process is invisible to the naked eye, scientists can observe these micro-interactions at the smallest scale: the quantum level.

In a recently published study in Science, scientists at the University of Missouri provide evidence of the effects of photodissociation at the quantum level for an air pollutant, formaldehyde, showing that photodissociation reactions cannot be treated classically, such as billiard balls coming together, colliding and reconnecting, Arthur Suits said. Curators Distinguished Professor of Chemistry in the MU College of Arts and Science, and a co-corresponding author of the study.

“By thinking only of chemical reactions in the classic sense of ‘billiard balls,’ a chemist will miss what a molecule really does,” Suits said. “It is well known that quantum effects are very important when a molecule gets very cold. What is surprising here is that strong quantum effects occur at the high energy of photodissociation. This new insight would not only change our view of how the molecule behaves, can change, but can also affect the overall chemical makeup, and that in turn could cause the chemistry to go in unexpected ways because of this added dimension of the quantum properties.”

The new study is about roaming, in which photodissociation breaks a molecule into pieces, but the pieces come back and react with each other. Until now, “billiard ball” models were able to fully emulate such experiments. The research shows that more detailed measurements cannot be handled in this way.

Instead, they must use a more complicated quantum model to confirm the unusual properties they observe. Suits believes their findings could one day help scientists gain a better theoretical understanding of the chemistry in the atmosphere, both in the stratosphere where ozone protects us, and at ground level where it is a dangerous pollutant.

“For example, if you want to understand the chemistry of the atmosphere, you first have to understand what happens when light is absorbed and a molecule starts to dissociate,” Suits said. “Chemists may think they don’t have to worry about what happens at the quantum level in photodissociation, and it’s just the classic billiard ball effect of atoms, but we show here that this isn’t always the case, and chemists have to use their intuition to refine it to some extent.”

“Source Resonances in Formaldehyde Reveal Coupling of Roaming, Radical and Molecular Channels,” was published in Science. Co-authors include Casey Foley of MU, and Hua Guo and Changjian Xie of the University of New Mexico. Xie also holds a dual tenure at Northwest University in China.

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More information:
Casey D. Foley et al, Orbiting resonances in formaldehyde reveal coupling of roaming, radical and molecular channels, Science (2021). DOI: 10.1126/science.abk0634

Provided by the University of Missouri

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