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A Computational Study of Acid Catalyzed Aerosol Reactions of Atmospherically Relevant Epoxides
Citation:
Piletic, I., E. Edney, AND L. Bartolotti. A Computational Study of Acid Catalyzed Aerosol Reactions of Atmospherically Relevant Epoxides. PHYSICAL CHEMISTRY CHEMICAL PHYSICS. Royal Society of Chemistry, Cambridge, Uk, 15(41):17707-18302, (2013).
Impact/Purpose:
The National Exposure Research Laboratory′s (NERL′s) Human Exposure and Atmospheric Sciences Division (HEASD) conducts research in support of EPA′s mission to protect human health and the environment. HEASD′s research program supports Goal 1 (Clean Air) and Goal 4 (Healthy People) of EPA′s strategic plan. More specifically, our division conducts research to characterize the movement of pollutants from the source to contact with humans. Our multidisciplinary research program produces Methods, Measurements, and Models to identify relationships between and characterize processes that link source emissions, environmental concentrations, human exposures, and target-tissue dose. The impact of these tools is improved regulatory programs and policies for EPA.
Description:
Epoxides are important intermediates of atmospheric isoprene oxidation. Their subsequent reactions in the particle phase lead to the production of organic compounds detected in ambient aerosols. We apply density functional theory to determine the important kinetic factors that drive epoxide reactions in the particle phase. Specifically, the importance of acid catalysis and solvent polarity are investigated using a variety of epoxides and nucleophiles. The condensed phase is modeled using molecular clusters immersed in a dielectric continuum and a majority of the calculations are performed with the M062x density functional and 6-311++G** basis set. Calculations of acid catalyzed epoxide hydrolysis transition states for simple primary, secondary and tertiary epoxides are consistent with an A-2 mechanism where the nucleophile (water) interacts with an epoxide carbon in the transition state. By applying transition state theory to this mechanism, the overall rate constants of epoxide reactions such as hydrolysis, organosulfate formation, organonitrate formation and oligomerization are determined. The calculations indicate that the acid catalyzed hydrolysis rate constant of 2-methyl-2,3-epoxybutane-1,4-diol (β-IEPOX - an isoprene epoxide produced under low NOx conditions) is approximately 30 times greater than 2-methyl-2,3-epoxypropanoic acid (MAE - methacrylic acid epoxide derived from isoprene and produced at high NOx concentrations). Furthermore, acid catalyzed organosulfate formation and epoxide oligomerization reactions are competitive and appear kinetically favorable over the hydrolysis of IEPOX.
URLs/Downloads:
FINAL FINAL IRP_PCCP_EPOXIDECALCS_SUBMISSION_REVISED.PDF (PDF, NA pp, 983.459 KB, about PDF)FINAL FINAL SUPPORTING INFORMATION IRP_PCCP_EPOXIDECALCS_SI_SUBMISSION_REVISED.PDF (PDF, NA pp, 8319.843 KB, about PDF)
Physical Chemistry Chemical Physics
