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The DOCO molecule interconverts between isomers trans-DOCO (left) and cis-DOCO (right) through rotation of the single bond (center). The multiple colors of the frequency comb enabled Bui et. al to not just identify cis-DOCO, but all of the molecules in the reaction simultaneously. Image credit: Steven Burrows / JILA

The reaction, at first glance, seems simple. Combustion engines, such as those in your car, form carbon monoxide (CO). Sunlight converts atmospheric water into a highly reactive hydroxyl radical (OH). And when CO and OH meet, one byproduct is carbon dioxide (CO₂) ­– a main contributor to air pollution and climate change.

But it’s more complicated than that. Before CO₂��is formed, a short-lived, intermediate molecule, called the hydrocarboxyl radical (HOCO), is formed. The existence of HOCO was��, but the unstable nature of the molecule made it difficult, nearly impossible, to observe. The Ye group of JILA, however, has been closing in on the impossible.

The deuterated version of HOCO, called DOCO, was observed by��and their collaborators��. Deuteration, or substituting a heavier version of the hydrogen atom, called deuterium, reduced signal contamination from the atmospheric water in their system.

Yet their understanding of the reaction was incomplete. The version of DOCO that the Ye group observed did not dissociate into carbon dioxide. “There were a lot of missing details in the reaction pathway” said Thinh Bui, a postdoctoral researcher in the Ye group.

In their new paper,��, every step of the reaction, starting from the reactants, OD and CO, all the way to the final product of CO₂, and every intermediate in between, is finally accounted for.

Specifically, the group detected the two variations of DOCO,��trans-DOCO and��cis-DOCO. The��cis- and��trans- prefixes denote different geometric isomers, or arrangements of the atoms within the molecule. The two isomers differ by only a single bond rotation, with��cis-DOCO resembling a boat, and��trans-DOCO resembling a chair.

Cis-DOCO is the more elusive of the two isomers because it is more energetic. Like a child running around, the high energy��cis-DOCO rotates, vibrates, and generally evades detection. Remove this high energy with a molecular collision��and the��cis-DOCO calms down. Let��cis-DOCO run around however, and something more interesting happens:��cis-DOCO will dissociate into deuterium and CO₂, a trick that the calmer trans-DOCO cannot do.

The original work by the Ye group identified only the��trans-DOCO variation. To identify��cis-DOCO, the group had to wade through heaps of overlapping molecule signals.