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Acetone and Hydroperoxyl Radical Equilibrium Certainly Fascinating, But Is It Important To You?

Acetone and Hydroperoxyl Radical Equilibrium Certainly Fascinating, But Is It Important To You?. Fred Grieman, Aaron Noell, Stan Sander, Mitchio Okumura Funding: NASA Upper Atmospheric Research Program NASA Senior Post-Doctoral Fellowship

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Acetone and Hydroperoxyl Radical Equilibrium Certainly Fascinating, But Is It Important To You?

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  1. Acetone and Hydroperoxyl Radical EquilibriumCertainly Fascinating, But Is It Important To You? Fred Grieman, Aaron Noell, Stan Sander, Mitchio Okumura Funding: NASA Upper Atmospheric Research Program NASA Senior Post-Doctoral Fellowship NASA Summer Faculty Research Fellow Program

  2. Importance to you? HO2/OH Atmospheric Chemistry Laboratory Study of and Atmospheric Observation of HOx Radicals

  3. For example: Photochemical Ozone Production

  4. Simplified Tropospheric Chemistry Volatile Organic Compounds Oxygenated Volatile Organic Compounds

  5. Understanding Atmospheric Chemistry Overall Picture

  6. Acetone in the Upper Atmosphere • One of main OVOCs in the Upper Troposphere (UT) • Key source of OH and HO2 (HOx) from photolysis • Primary loss pathways in Upper Troposphere: Photolysis, Reaction with OH • Recent experiments by Blitz, Orr-Ewing, Heard, Pilling • suggest much lower photolysis yields at low T HO2 + Acetone  HO2Acetone  (CH3)2C(OH)OO? • An alternate oxidation pathway in the atmosphere? • Possible Reaction with HO2? • Hydrogen radicals in Upper Troposphere: HOx = OH, HO2 • In the atmosphere, [HO2] >> [OH] • HO2 is known to react rapidly with formaldehyde at room temperature So, YES!!! Determination of Acetone/Hydroperoxyl Radical Equilibrium IS Important to YOU! Literature?

  7. Int. J. Chem. Kinet. 32, 573 (2000).

  8. REACTANTS HO2 + Acetone  HO2Acetone  (CH3)2C(OH)OO? ADDUCT MOLECULAR COMPLEX COMPUTED STATIONARY POINTSB3LYP/cc-pVTZ GeometriesG2Mc/DFT Energies PEROXY RADICAL HO(iPr)OO

  9. Atmospheric Loss Process 1. HO2 + Acetone are in equilibrium with peroxy (H-bonded molecular complex is pre-equilibrium config) HO2 + CH3C(O)CH3→ HOC(CH3)2OO k(200K)= 6.9 10-12 cm3 s-1Kc(210K) = 6.0  10-13 cm3 2. Peroxy radical reacts with HO2 or NO, leading to loss of HO2 (then important to include in HO2 / OH budget) 3. Acetone sink: If Herman’s et al. calculation correct, HO2 removal on par with photolysis & greater than from OH

  10. Abstraction Higher Barrier – NO REACTION! Addition

  11. k+ HO2 + CH3C(O)CH3⇌ HOC(CH3)2OO k- Does this rxn occur at relevant atmospheric T? Because k- is so large, Keq is the quantity that determines effective rate of removal How?? Experimental Determination via Infrared Kinetics Spectroscopy (IRKS)

  12. Infrared Kinetic Spectroscopy Apparatus NIR {2ν(OH)} 6.8 MHz current modulator computer λ = 220 nm (near HO2 max) monochromator diode laser low pass filter 2x/ phase shifter UV D2 lamp Excimer laser 308 nm demodulated signal FM signal detector PD Herriott cell exit exit Cl2 + hν → 2 Cl Cl + CH3OH → CH2OH + HCl CH2OH + O2 → HO2 + CH2O gas entrance T-controlled FLOW CELL

  13. Herriot Cell Mirror

  14. FM Detection of HO2 NIR Lines by Diode Laser InGaAs/InP single-mode DFB Diode Lasers 1.4 and 1.5 m fabricated at JPL, Selectivity for HO2 Detection of single rotational lines Wavelength Modulation 2f detection at 7 MHz modulation Near shot-noise limited detection Herriott Cell 30 passes, Leff = 2000 cm Sensitivity(Minimum detectable absorption) 5x10-7Hz or 2. 5x10-10 cm-1Hz HO2 Detection Limit (6636 cm-1, 295K, 100 Torr): 1.0 x 1010 cm-31 Hz 3 x 1011 cm-3 10kHz, 1 shot

  15. Association Reaction HO2 + (CH3)2CO ⇄ (CH3)2CO---HO2 isomerization ↓ (CH3)2COH ←(CH3)2CO---H† O▬O O▬O MOLECULAR COMPLEX Measuring [HO2] decay upon adding Acetone Does not occur at room T, but may at lower T Measure with increasing [Acetone] 2-hydroxyisopropylperoxy (2-HIPP) Preliminary Result: HO2 NIR Decay Curves at Varying [Acetone] T = 221 K T = 297 K No HO2 + Acetone rxn !!! Must consider all chemistry Cl + Acetone  HCl + CH3C(O)CH2 Decreases HO2 made Slows at Low T {k(297) = 2.1E-12 ; k(221) = 1.0E-12)} Dramatic decrease in [HO2] at lower T & same [Acetone] HO2 Absorbance Time (msec) Time (msec) • Interpretation: 1) Complexation occurs at lower T • 2) Equilibrium reached quickly followed by HO2 rxns

  16. Fitting Rise and Fall of Short time decay not possible • Method Developed: • Fit Longer time decay with simple HO2 self-reaction • Determine [HO2] at time = 0, w/out & w/ [Acetone] • Correct for Cl + Acetone reaction • Determine Keq from equilibrium concentrations • Repeat for several [Acetone] at several T •  Keq(T)  ΔrH & ΔrS • First must determine Cl + Acetone reaction at T=298K

  17. T =297 K Cl + CH3C(O)CH3→ HCl + CH3C(O)CH2(~10 sec) O2 + CH3C(O)CH2 → CH3C(O)CH2OO (fast excess O2) HO2 + CH3C(O)CH2OO → Products (k12f) HO2 + HO2 → H2O2 + O2 (k1f) Fit with literature k12f and k1f from [Acetone] = 0 fit Agree w/ lit. (no HO2 + Acetone reaction at Room T) Fits of Cl chemistry with Acetone & O2

  18. Family of NIR HO2 decay curves at T = 221Kat varying acetone concentrations Cannot Fit Curves with Cl reactions Preliminary objective: Determine thermodynamics

  19. Initial analysis: find [HO2]o([Ace]) at t = 0 s to determine equilibrium concentration prior to subsequent kinetics 1) [HO2]o(0) determined from fit & corrected for Cl rxn with Acetone 2) [HO2]eq = [HO2]o([Ace]) determined from fit 3) [Complex] = [HO2]o(0) – [HO2]o([Ace]) [Complex] Keq = [Ace] [HO2]o([Ace]) (excess) Measure Keq at several atmospherically relevant temperatures

  20. Kc(T) (cm3 molec-1) Van’t Hoff Plot: Rln(Kp) vs. 1/T slope = -ΔrH°; intercept = ΔrS° ΔrH° = -31  1.7 kJ/mol ΔrS° = -70  7.2 J/mol/K ΔrG° = ΔrH° - T ΔrS° Keq(T) = exp (- ΔrG° /RT)

  21. Comparison of Equilibrium Constants Kc, cm3 molec-1

  22. More Comparisons Reaction Thermodynamics Compared to Calculated Values Aloisio product: Like complex!!!

  23. Reaction to Complex HO2 + (CH3)2CO ⇄ (CH3)2CO---HO2 ↓ (CH3)2CO---H† O▬O MOLECULAR COMPLEX Herman et al. Both Planar Aloisio et al. Calculat ions Cours et al. Perpendicular

  24. Comparison with Methanol and Water

  25. Atmospheric Implications (Just a taste.) Analysis by Hermans et al.: Acetone removal (keff) from UT  Keq At 190 K, keff = 5 x 10-6 s-1 which is greater than acetone photolysis (4 x 10-7 s-1) However, if our results are correct and 2-HIPP is product: Keq = 1.9 x 10-15 compared to Hermans et al. Keq = 2.0 x 10-11 keff = 4.3 x 10-10 s-1

  26. Summary • Discovered reaction between HO2 + Acetone • Developed Method to Determine Keq for HO2/Carbonyl Reactions • Able to Measure Keq Over Wide Temperature Range Including Atmospherically Relevant Temperatures • Thermodynamic Parameters Determined: Possible Clues to Reaction Product and Its Structure • Will Be Able to Determine Its Impact on the Atmosphere

  27. Future Work • Search for products (acetonylperoxy, 2-HIPP, Molecular Complex) • We have done some of this: T = 297 K acetonylperoxy: CH3C(O)CH2OO [Ace] = 2.05E16 [Ace] = 0 • For (CH3)2C(OH)OO and (CH3)2C(O)OOH • No spectrum observed in uv; Calculations underway to estimate OH stretching • frequency and A-X transition • 2) Measure forward rate constant • Very difficult work; has been accomplished for HO2 + methanol • Apply this method to many HO2 / Carbonyl systems: • MEK, Acetaldehyde, Formaldehyde

  28. Acknowledgements Mitchio Okumura Stan Sander Harry Kroto Aaron Noell

  29. The Future Kira Watson Aileen Hui 1st yr. Caltech Grad Student (not shown) Pomona Chem Majors Casey Davis- Van Atta The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract to the National Aeronautics and Space Administration *This research was supported by an appointment of Fred Grieman to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA.

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