The Energetic Adolescence of Carbon Dioxide
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) 颅鈥 a main contributor to air pollution and climate change.
But it鈥檚 more complicated than that. Before CO2听is formed, a short-lived, intermediate molecule, called the hydrocarboxyl radical (HOCO), is formed. The existence of HOCO was听, 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听and their collaborators听. 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. 鈥淭here were a lot of missing details in the reaction pathway鈥 said Thinh Bui, a postdoctoral researcher in the Ye group.
In their new paper,听, 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,听trans-DOCO and听cis-DOCO. The听cis- and听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听cis-DOCO resembling a boat, and听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听cis-DOCO rotates, vibrates, and generally evades detection. Remove this high energy with a molecular collision听and the听cis-DOCO calms down. Let听cis-DOCO run around however, and something more interesting happens:听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听trans-DOCO variation. To identify听cis-DOCO, the group had to wade through heaps of overlapping molecule signals.