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Photochemical transformation reactions

Direct photolysis and light absorption. Types of orbitals:bonding: ? (single) or ? (double)non-bonding: n (often lone pairs on hetero atoms such as N, O)anti-bonding: ?* (single) or ?* (double)Absorption of light causes electronic transitions:important transitions are usually n to ?* or

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Photochemical transformation reactions

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    1. Photochemical transformation reactions Direct photolysis = transformation of a compound due to its absorption of UV light Indirect photolysis = transformation of a compound due to its interaction with a reactant generated by the influence of UV light (photosensitizer or reactive oxygen species)

    2. Direct photolysis and light absorption

    3. HOMO and LUMO

    4. Ethylene

    5. Light Energy Energy E = hv = h(c/l) where h = Plank’s constant l = wavelength c = speed of light Longer wavelengths = less energy Bond E (kJ/mol) l (nm) O-H 465 257 C-H 415 288 N-H 390 307 C-O 360 332 C-C 348 344 C-Cl 339 353 Br-Br 193 620 O-O 146 820

    6. Light absorption

    7. Fate of excited species

    8. Chemical processes

    9. Reaction rates for direct photolysis in water:

    10. Example

    11. Rate constant from light absorption rate

    12. Indirect photolysis

    13. Important reactants (electrophiles)

    14. Hydroxyl radicals

    15. Reactions of OH in solution

    16. Tropospheric photochemistry: Ozone Ozone in the upper atmosphere or stratosphere acts as a protective layer screening harmful ultraviolet light (UV). Ozone found in the lower atmosphere or troposphere does not act as an essential screen but as a pollutant. About 8 % of the total column ozone is in the troposphere. Ozone is a green house gas and possibly contributes to the global warming. Ozone is harmful for human being and crops in the troposphere. Ozone oxidizes many chemical substances in the troposphere. Ozone is continually monitored at many urban locations.

    17. Tropospheric chemistry NO2 + h? ? NO + O O + O2 ? O3 O3 + h? ? O (1D) + O2 h? < 310 nm O (1D) + H2O ? 2HO• OH concentrations highest during the day (max at noon)

    18. Troposperic chemistry of pollutants Reactivity is always a function of reaction rate and concentration of reactant Reactive species include OH, NO3, O3, sometimes HNO3 and Cl OH almost always dominates, despite low concentrations (106 molecules/cm3), it is very reactive. “tropospheric vacuum cleaner”

    19. Other Tropospheric Reactants Reactions with NO3 important for compounds containing (non-aromatic) double bonds, fused rings (PAHs), and S atoms. NO3 concentrations peak at night. O3 reacts with (non-aromatic) double bonds. O3 concentrations are higher during the day but can still be substantial at night. Cl atoms can be generated in marine environments at conc’s up to 104 molecules/cm3. May be important reactants in some situations.

    20. Mechanisms of reaction with OH in the troposphere H abstraction: RH + •OH ? R• + H2O Addition to double bonds or aromatic rings (favored):

    21. Rate constants for reactions of OH

    22. Fate of species in troposphere

    23. Fate of radicals in troposphere

    24. Reactions of PCBs with OH during atmospheric transport Laboratory-measured rate constants between 5.0 - 0.4 ? 1012 cm3s-1 (Anderson and Hites, 1996) Half-lives for gas-phase PCBs of 0.5 to 7 days at relevant OH concentrations Single most important sink for PCBs on global scale (?) Reactivity decreases as number of chlorines increases.

    25. Approach Examine data for daytime depletion of PCBs. Derive environmental rate constants and compare with laboratory measurements. Examine relative reaction rates (relationship between rate constant and number of chlorine substituents).

    26. Diurnal variation in PCB concentrations

    27. Environmental rate constants: kobs = ke[OH] Assume OH = 3 ? 106 molecules/cm3

    28. Slope of log ke vs. #Cl:

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