Dramatic unimolecular decay of an unsaturated Criegee intermediate via allylic 1,6 H-atom transfer Anne S. Hansen 1 , Yujie Qian 1 , Stephen J. Klippenstein 2 and Marsha I. Lester 1 1 Department of Chemistry, University of Pennsylvania, USA, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, USA Very rapid unimolecular decay of an unsaturated Criegee intermediate is shown to proceed via a novel allylic 1,6 H-atom transfer mechanism, which is orders of magnitude faster than typical decay via 1,4 H-atom transfer. A new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups, 2-butenal oxide (CH 3 CH=CHCHOO), is generated and shown to facilitate rapid allylic 1,6 H-atom transfer, resulting in hydroxyl (OH) radical products. 1 A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower energy ( tZZ ) conformer to a higher energy ( cZZ ) conformer followed by 1,6 hydrogen transfer via a 7-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying ( tZZ ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its eight conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying ( tZZ ) conformer at 2996 cm -1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate k eff ( E ) on the order of 10 8 s -1 at ca. 3000 cm -1 in good accord with experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 10 6 s -1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the non-photolytic production of OH radicals upon alkene ozonolysis in the troposphere. References 1. A. S. Hansen, Y. Qian, C. A. Sojdak, M. C. Kozlowski, V. J. Esposito, J. S. Francisco, S. J. Klippenstein, and M. I. Lester, Rapid allylic 1,6 H-atom transfer in an unsaturated Criegee intermediate, J. Am. Chem. Soc. , in press (2022).
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