Unexpected Epoxide Formation in the Gas-Phase Photooxidation of Isoprene
Fabien Paulot1,*,
John D. Crounse2,
Henrik G. Kjaergaard3,4,
Andreas Kürten1,
,
Jason M. St. Clair1,
John H. Seinfeld1,2 and
Paul O. Wennberg1,5
1 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA.
2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
3 Department of Chemistry, University of Otago, Dunedin, New Zealand.
4 Department of Chemistry, University of Copenhagen, Copenhagen, Denmark.
5 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
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Fig. 1. Consecutive formation of ISOPOOH and IEPOX in the photooxidation of isoprene. Following the time when the photolysis of H2O2 [initially 1.66 parts per million by volume (ppmv)] begins (t = 0), isoprene (black dotted line) decays quickly. ISOPOOH and then IEPOX are detected as major products of the oxidation of isoprene [because they are isobaric, they both are detected at m/z = 203 (red), the cluster of these compounds with CF3O–]. Tandem mass spectroscopy provides for separation of the m/z = 203 signal: ISOPOOH (green) is observed as the m/z = 63 daughter, whereas IEPOX (blue) is observed as the m/z = 183 daughter. The sum of IEPOX and ISOPOOH is indicated by the dashed black line.
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Fig. 2. Formation of light and heavy ISOPOOH and IEPOX in the oxidation of isoprene using H18O18OH as the OH source. Formation of ISOPOOH is monitored via the daughter m/z = 63 (circles) of m/z = 203 (red) and m/z = 205 (magenta). Formation of IEPOX is monitored via the loss of HF (squares) from m/z = 203, m/z = 205, and m/z = 207 (blue). Formation of isotopically light ISOPOOH and IEPOX reflects OH reformation. Solid lines represent the modeled mixing ratios for the different isomers. Isoprene initial concentration was 23.5 parts per billion by volume (ppbv), and 18OH was generated from the photolysis of H18O18OH (1.75 ppmv initial concentration, UV lights on at t = 0).
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Fig. 3. Relative energies for the formation of β-IEPOX from β4-ISOPOOH (Reaction Series 2A). The alkylradical resulting from the addition of OH onto β4-ISOPOOH double bond is formed with enough excess energy (~30 kcal/mol) that it quickly decomposes to the β-IEPOX + OH via the β4- transition state. Energies are calculated with the CCSD(T)-F12/VDZ-F12 explicitly correlated method at the B3LYP/cc-pVTZ optimized structures (13).
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Fig. 4. Simulated daily distribution of IEPOX in the planetary boundary layer during the Northern Hemisphere summer (A) and winter (B). IEPOX seasonal cycle mirrors the isoprene emissions. The mixing ratio of IEPOX is higher in the tropics than in other isoprene production regions in the northern mid-latitudes (e.g., the southeast United States). This reflects the reduction in the yield of IEPOX from isoprene due to anthropogenic emissions of NO.
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