Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Spectroscopy

Background:

• Criegee intermediates are key species in atmospheric oxidation processes.
• Understanding their unimolecular decay pathways is crucial for atmospheric modeling.
• Previous studies focused on bimolecular reactions, with less known about unimolecular decay of complex Criegee intermediates.

Purpose of the Study:

• To establish a novel allylic 1,6 hydrogen-atom-transfer mechanism for 2-butenal oxide Criegee intermediates.
• To investigate the role of infrared activation and conformational isomerization in the unimolecular decay.
• To quantify the rate of hydroxyl radical production from this pathway.

Main Methods:

• Synthesis of a new precursor (Z/E-1,3-diiodobut-1-ene) to generate a conjugated four-carbon Criegee intermediate.
• Infrared (IR) action spectroscopy combined with UV laser-induced fluorescence detection of OH radicals.
• Theoretical calculations including conformational analysis, anharmonic frequencies, and statistical RRKM theory.

Main Results:

• Experimental evidence for a rapid unimolecular decay of 2-butenal oxide via allylic 1,6 hydrogen-atom transfer upon IR activation.
• Identification of a low-lying (tZZ) conformer and a higher-energy (cZZ) conformer involved in the reaction pathway.
• Quantum mechanical tunneling significantly enhances the unimolecular decay rate, with an effective decay rate of ~10^8 s^-1 at ~3000 cm^-1.

Conclusions:

• A novel, rapid unimolecular decay pathway for 2-butenal oxide Criegee intermediates has been elucidated.
• This mechanism significantly contributes to hydroxyl radical production, especially under non-photolytic conditions.
• The findings are expected to improve atmospheric models of alkene ozonolysis and tropospheric OH radical concentrations.