Substituent Effects on the Electronic Spectroscopy of Four-carbon Criegee Intermediates

Atmospheric ozonolysis of alkenes generates zwitterionic carbonyl oxide intermediates (R1R2C=O+O), known as Criegee intermediates (CIs). Since the size and structure of alkenes from anthropogenic and biogenic sources vary widely, ranging from ethene to large terpenes, CIs with different structural motifs are formed. Here, we report a systematic study of substituent effects on the electronic spectroscopy of four-carbon CIs, methyl ethyl substituted CI (MECI) and isopropyl substituted CI (IPCI), which are isomers generated in ozonolysis of asymmetric branched alkenes. Comparisons will be made with methyl vinyl ketone oxide (MVK-oxide)1 and methacrolein oxide (MACR-oxide)2 -carbon unsaturated CIs formed in isoprene ozonolysis, which have the same backbone connectivity as MECI and IPCI, but with extended conjugation across the vinyl and carbonyl groups. The strong UV-visible absorption associated with the * transition of the carbonyl oxide group of four-carbon CIs is characterized using tunable pulsed BBO-OPO radiation to induce a ground state depletion, which is detected by VUV photoionization (10.5 eV) with time-of-flight mass spectrometry. A remarkable 50 nm shift of the peak absorption to longer wavelength is observed for MVK-oxide / MACR-oxide compared to MECI / IPCI, respectively. Vertical excitation energies computed theoretically agree well with the experimental findings, confirming that the spectral shift is caused by extended conjugation in MVK-oxide and MACR-oxide. The strong UV-visible absorption provides a sensitive and distinctive means for detecting four-carbon Criegee intermediates with saturated and unsaturated substituents in unimolecular and bimolecular reaction studies. References M. F. Vansco, B. Marchetti, and M. I. Lester, J. Chem. Phys. 149, 244309 (2018). M. F. Vansco, B. Marchetti, N. Trongsiriwat, T. Bhagde, G. Wang, P. J. Walsh, S. J. Klippenstein, and M. I. Lester, J. Am. Chem. Soc. 141, 1505815069 (2019). Research supported by DOE Basic Energy Sciences DE-FG02-87ER13792.