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 NASA Summer Faculty Research Fellow Program

HO 2 /OH Atmospheric Chemistry Importance to you? Laboratory Study of and Atmospheric Observation of HO x Radicals

For example: Photochemical Ozone Production

Simplified Tropospheric Chemistry Volatile Organic Compounds Oxygenated Volatile Organic Compounds

Understanding Atmospheric Chemistry Overall Picture

HO 2 + Acetone HO 2 Acetone (CH 3 ) 2 C(OH)OO? Acetone in the Upper Atmosphere One of main OVOCs in the Upper Troposphere (UT) Key source of OH and HO 2 (HO x ) 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 An alternate oxidation pathway in the atmosphere? Possible Reaction with HO 2 ? Hydrogen radicals in Upper Troposphere: HO x = OH, HO 2 In the atmosphere, [HO 2 ] >> [OH] HO 2 is known to react rapidly with formaldehyde at room temperature Literature? So, YES!!! Determination of Acetone/Hydroperoxyl Radical Equilibrium IS Important to YOU!

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

ADDUCT PEROXY RADICAL HO(iPr)OO REACTANTS COMPUTED STATIONARY POINTS B3LYP/cc-pVTZ Geometries G2Mc/DFT Energies HO 2 + Acetone HO 2 Acetone (CH 3 ) 2 C(OH)OO? MOLECULAR COMPLEX

Atmospheric Loss Process 1. HO 2 + Acetone are in equilibrium with peroxy (H-bonded molecular complex is pre-equilibrium config) HO 2 + CH 3 C(O)CH 3 HOC(CH 3 ) 2 OO k(200K) = cm 3 s -1 K c (210K) = cm 3 2. Peroxy radical reacts with HO 2 or NO, leading to loss of HO 2 (then important to include in HO 2 / OH budget) 3. Acetone sink: If Hermans et al. calculation correct, HO 2 removal on par with photolysis & greater than from OH

Abstraction Addition Higher Barrier – NO REACTION!

Does this rxn occur at relevant atmospheric T? K c (T) 2.27E E E E-12 k+k+ 4.50E E E E-12 k-k- 1.98E E E E+00 How?? Experimental Determination via Infrared Kinetics Spectroscopy (IRKS) HO 2 + CH 3 C(O)CH 3 HOC(CH 3 ) 2 OO k+k+ k-k- Because k - is so large, K eq is the quantity that determines effective rate of removal

Excimer laser 308 nm D 2 lamp diode laser detector low pass filter monochromator computer 6.8 MHz current modulator 2x/ phase shifter demodulated signal FM signal gas entrance exit Herriott cell PD Infrared Kinetic Spectroscopy Apparatus UV NIR {2ν(OH)} T-controlled FLOW CELL λ = 220 nm (near HO 2 max) Cl 2 + h ν 2 Cl Cl + CH 3 OH CH 2 OH + HCl CH 2 OH + O 2 HO 2 + CH 2 O

Herriot Cell Mirror

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

Association Reaction HO 2 + (CH 3 ) 2 CO (CH 3 ) 2 CO---HO 2 isomerization (CH 3 ) 2 COH (CH 3 ) 2 CO---H OO O O MOLECULAR COMPLEX 2-hydroxyisopropylperoxy (2-HIPP) HO 2 NIR Decay Curves at Varying [Acetone] T = 221 KT = 297 K Time (msec) HO 2 Absorbance Dramatic decrease in [HO 2 ] at lower T & same [Acetone ] Measuring [HO 2 ] decay upon adding Acetone Does not occur at room T, but may at lower T Measure with increasing [Acetone] Preliminary Result: No HO 2 + Acetone rxn !!! Must consider all chemistry Cl + Acetone HCl + CH 3 C(O)CH 2 Decreases HO 2 made Slows at Low T {k(297) = 2.1E-12 ; k(221) = 1.0E-12)} Interpretation: 1) Complexation occurs at lower T 2) Equilibrium reached quickly followed by HO 2 rxns

Fitting Rise and Fall of Short time decay not possible Method Developed: Fit Longer time decay with simple HO 2 self-reaction Determine [HO 2 ] at time = 0, w/out & w/ [Acetone] Correct for Cl + Acetone reaction Determine K eq from equilibrium concentrations Repeat for several [Acetone] at several T K eq (T) Δ r H & Δ r S First must determine Cl + Acetone reaction at T=298K

Cl + CH 3 C(O)CH 3 HCl + CH 3 C(O)CH 2 (~10 sec) O 2 + CH 3 C(O)CH 2 CH 3 C(O)CH 2 OO (fast excess O 2 ) HO 2 + CH 3 C(O)CH 2 OO Products (k 12f ) HO 2 + HO 2 H 2 O 2 + O 2 (k 1f ) Fit with literature k 12f and k 1f from [Acetone] = 0 fit Agree w/ lit. (no HO 2 + Acetone reaction at Room T) T =297 K Fits of Cl chemistry with Acetone & O 2

Preliminary objective: Determine thermodynamics Family of NIR HO 2 decay curves at T = 221K at varying acetone concentrations Cannot Fit Curves with Cl reactions

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

T(K) ( 2 K c (cm 3 /molec) (pph) E E E E E E E E E E E E E E E K c (T) (cm 3 molec -1) Vant Hoff Plot: Rln(K p ) vs. 1/T slope = -Δ r H°; intercept = Δ r S° Δ r H° = kJ/mol Δ r S° = J/mol/K Δ r G° = Δ r H° - T Δ r S° K eq (T) = exp (- Δ r G° /RT)

Comparison of Equilibrium Constants K c, cm 3 molec -1

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

Reaction to Complex HO 2 + (CH 3 ) 2 CO (CH 3 ) 2 CO---HO 2 (CH 3 ) 2 CO---H O O MOLECULAR COMPLEX Herman et al. Cours et al. Aloisio et al. Both Planar Perpendicular CalculationsCalculations

Comparison with Methanol and Water Source Δ r H o (kJ/mol) D o (kJ/mol) HO 2 + Acetone (This Work) -31 HO 2 + Methanol (Christiensen et al., 2006) H 2 O + H 2 O (Curtiss et al., 1979) HO 2 Acetone (Aloisio et al., 2000) 37.3 HO 2 Methanol (Christiensen et al., 2006) 35.7 H 2 O (Klopper et al., 1995) 21.0

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

Summary Discovered reaction between HO 2 + Acetone Developed Method to Determine K eq for HO 2 /Carbonyl Reactions Able to Measure K eq 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

Future Work 1)Search for products (acetonylperoxy, 2-HIPP, Molecular Complex) We have done some of this: T = 297 K acetonylperoxy: CH 3 C(O)CH 2 OO σ ( cm 2 /molec ) at λ uv = 280 nm 2.07E-18acetonylperoxy 0HO E-20H2O2H2O2 [Ace] = 0 [Ace] = 2.05E16 For (CH 3 ) 2 C(OH)OO and (CH 3 ) 2 C(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 HO 2 + methanol 3)Apply this method to many HO 2 / Carbonyl systems: MEK, Acetaldehyde, Formaldehyde

Acknowledgements Harry Kroto Aaron Noell Stan Sander Mitchio Okumura

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. The Future Kira Watson Casey Davis- Van Atta Aileen Hui 1 st yr. Caltech Grad Student (not shown) Pomona Chem Majors