Picking identical molecules out of chemical soup

Conformers, molecules that differ only by rotation around single bonds, pose a persistent problem in chemical kinetics. While they have different reaction rates, their identical masses and overlapping spectra make it difficult to measure them individually.

However, knowing individual reaction rates for conformers such as Criegee intermediates (CIs) is key to understanding their role in oxidizing atmospheric reactions that affect climate change.

To isolate specific conformers, Jiang et al. developed a high-repetition laser-induced instrument that measures the fluorescent fragments of decomposition products in real time.

The researchers tested the system using CH₃CHOO — a CI where only one of its conformers produced OH radicals upon decomposition.

The instrument has three subsystems: a component that introduces reactants, a photolysis system that initiates reactions and detects OH radicals, and a precise temperature and pressure control system.

The system collects data a thousand times faster than conventional methods with an improved signal-to-noise ratio. In doing so, it virtually eliminates experimental errors caused by long-term system drift and unstable reactants.

“Because only specific CIs produce OH, this technique allows us to ‘pick them out’ of a complex chemical soup without interference from coexisting conformers,” said author Wenrui Dong. “It serves as a highly specific complement to broad-spectrum detection methods, rather than a total replacement.”

The team hopes to measure other OH-producing CIs, particularly those with slower, rate-limiting reactions, before moving to reactive intermediates that decompose into other radicals.

“By providing precise, temperature- and pressure-dependent rate coefficients for these specific isomers, we can help modelers more accurately predict the formation of secondary organic aerosols and the oxidation capacity of the atmosphere, leading to better climate and air quality predictions,” said Dong.

Source: “A 10 kHz laser-induced fluorescence system for time-resolved detection of reactive intermediates via proxy measurements,” by Haotian Jiang, Yue Liu, Miaoqing Li, Chunlei Xiao, Weiqing Zhang, Hongwei Li, Wenrui Dong, and Xueming Yang, Review of Scientific Instruments (2025). The article can be accessed at https://doi.org/10.1063/5.0302605 .