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

HO2/OH Atmospheric ChemistryImportance to you?

Laboratory Study of and Atmospheric Observation of HOx Radicals

For example: Photochemical Ozone Production

Simplified Tropospheric Chemistry

Volatile Organic Compounds

Oxygenated Volatile Organic Compounds

Understanding Atmospheric Chemistry

Overall Picture

HO2 + Acetone HO2Acetone (CH3)2C(OH)OO?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

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

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 POINTSB3LYP/cc-pVTZ GeometriesG2Mc/DFT Energies

HO2 + Acetone HO2Acetone (CH3)2C(OH)OO?

MOLECULAR COMPLEX

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-1 Kc(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

Abstraction

Addition

Higher Barrier – NO REACTION!

Does this rxn occur at relevant atmospheric T?

Kc(T)

2.27E-172.32E-151.17E-133.05E-12

k+

4.50E-131.06E-122.20E-124.04E-12

k-

1.98E+044.58E+021.89E+011.33E+00

How?? Experimental Determination via Infrared Kinetics Spectroscopy (IRKS)

HO2 + CH3C(O)CH3 ⇌ HOC(CH3)2OOk+

k-

Because k- is so large, Keq is the quantity that determines effective rate of removal

Excimer laser308 nm

D2 lamp

diode laser

detector

low pass filter

monochromator

computer

6.8 MHz current modulator

2x/phase shifter

demodulated signal

FM signal

gas entranceexit exit

Herriott cellPD

Infrared Kinetic Spectroscopy Apparatus

UV

NIR{2ν(OH)}

T-controlled FLOW CELL

λ = 220 nm (near HO2 max)

Cl2 + hν → 2 ClCl + CH3OH → CH2OH + HClCH2OH + O2 → HO2 + CH2O

Herriot Cell Mirror

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 linesWavelength Modulation 2f detection at 7 MHz modulation Near shot-noise limited detectionHerriott Cell 30 passes, Leff = 2000 cmSensitivity (Minimum detectable absorption) 5x107 Hz or 2. 5x1010 cm1HzHO2 Detection Limit (6636 cm1, 295K, 100 Torr):

1.0 x 10 cm3 1 Hz 3 x 10 cm 10kHz, 1 shot

HO2 line6625.80 cm-1

-1.0

-0.5

0.0

0.5

1.0

1.5

-40 -30 -20 -10 0 10 20 30 40

Relative Frequency (milli-cm-1)

HO2 S

igna

l (m

icro

volt)

Association Reaction

HO2 + (CH3)2CO ⇄ (CH3)2CO---HO2

isomerization ↓ (CH3)2COH ← (CH3)2CO---H †

O▬O O▬O

MOLECULARCOMPLEX

2-hydroxyisopropylperoxy (2-HIPP)

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

1086420ms

IR05t [Ace] = 0 IR06t 2.24e15 IR09t 2.98e15 IR10t 3.50e15 IR13t 4.52e15 IR14t 5.62e15

HO2 NIR Decay Curves at Varying [Acetone]

T = 221 K T = 297 K

0.12

0.10

0.08

0.06

0.04

0.02

0.00

3020100ms

IR02t [Ace] = 0.0e15 IR03t 2.18 IR04t 4.06 IR05t 5.48 IR06t 5.41 IR07t 7.09 IR08t 9.93 IR09t 12.8 IR10t 17.5 IR11t 20.5

Time (msec)Time (msec)

HO

2 A

bsor

banc

e Dramatic decreasein [HO2] at lower T& same [Acetone]

Measuring [HO2] decay upon adding Acetone

Does not occur at room T, but may at lower T

Measure with increasing [Acetone]

Preliminary Result:

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)}

Interpretation: 1) Complexation occurs at lower T 2) Equilibrium reached quickly followed by HO2 rxns

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

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

1086420ms

IR05t [Ace] = 0 IR06t 2.24e15 IR09t 2.98e15 IR10t 3.50e15 IR13t 4.52e15 IR14t 5.62e15

0.12

0.10

0.08

0.06

0.04

0.02

0.00

3020100ms

IR02t [Ace] = 0.0e15 IR03t 2.18 IR04t 4.06 IR05t 5.48 IR06t 5.41 IR07t 7.09 IR08t 9.93 IR09t 12.8 IR10t 17.5 IR11t 20.5

80

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

210

-1-2

x10-3

0.10

0.08

0.06

0.04

0.02

0.00

IR s

igna

l/ (V

)

1086420ms

3210

-1-2

x10-3

0.10

0.08

0.06

0.04

0.02

0.00

IR s

igna

l/ (V

)

1086420ms

3210

-1-2

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

210

-1

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

210

-1-2

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

210

-1

x10-3

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

3210

-1-2

x10-3

70

60

50

40

30

20

10

0

IR s

igna

l/ (V

x10

-3 )

1086420ms

2

0

-2

x10-3

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)

T =297 K

Fits of Cl chemistry with Acetone & O2

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

IR s

igna

l/ (V

)

3.53.02.52.01.51.00.50.0ms

43210

-1-2

x10-3

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

IR s

igna

l/ (V

)

3.02.52.01.51.00.50.0ms

3210

-1-2

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10-3

)

3.53.02.52.01.51.00.50.0ms

210

-1

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10-3

)

3.53.02.52.01.51.00.50.0ms

1.0

0.0

-1.0

x10-3

80

60

40

20

0

IR s

igna

l/ (V

x10-3

)

3.02.52.01.51.00.50.0ms

1.00.50.0

-0.5-1.0

x10-3

60

40

20

0

IR s

igna

l/ (V

x10-3

)

3.53.02.52.01.51.00.50.0ms

1.0

0.0

-1.0

x10-3

Preliminary objective: Determine thermodynamics

Family of NIR HO2 decay curves at T = 221Kat varying acetone concentrations

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

1086420ms

IR05t [Ace] = 0 IR06t 2.24e15 IR09t 2.98e15 IR10t 3.50e15 IR13t 4.52e15 IR14t 5.62e15

Cannot Fit Curves with Cl reactions

Initial analysis: find [HO2]o([Ace]) at t = 0 s to determine equilibrium concentration prior to

subsequent kinetics

60

40

20

0

IR s

igna

l/ (V

x10

-3 )

3.53.02.52.01.51.00.50.0ms

1.0

0.0

-1.0

x10-3

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

T(K) (2 Kc(cm3/molec) (pph)

215.6 2.957E-16 12.7220.7 1.506E-16 6.9222.5 1.227E-16 9.9226.8 9.087E-17 13.7227.6 7.856E-17 17.3231.9 7.177E-17 22.5232.3 5.977E-17 9.5237.1 3.955E-17 5.8242.7 2.589E-17 12.6243.5 2.451E-17 17.4245.9 2.961E-17 1.6249.6 2.898E-17 5.2254.5 1.335E-17 16.9266.2 1.408E-17 23.7272.3 7.671E-18 18.3

Kc(T) (cm3 molec-1) Van’t Hoff Plot: Rln(Kp) vs. 1/T slope = -ΔrH°; intercept = ΔrS°

ΔrH° = -31 1.7 kJ/molΔrS° = -70 7.2 J/mol/K

ΔrG° = ΔrH° - T ΔrS° Keq(T) = exp (- ΔrG° /RT)

Van't Hoff Plot (not weighted)

y = 30.97x - 0.0700R2 = 0.9614

0.040

0.045

0.050

0.055

0.060

0.065

0.070

0.075

0.080

0.00360 0.00380 0.00400 0.00420 0.00440 0.004601/T(K-1)

Rln

Kp

Comparison of Equilibrium ConstantsKc, cm3 molec-1

More ComparisonsReaction Thermodynamics Compared to Calculated Values

Aloisio product:

Like complex!!!

Reaction to Complex

HO2 + (CH3)2CO ⇄ (CH3)2CO---HO2

↓ (CH3)2CO---H †

O▬O

MOLECULARCOMPLEX

Herman et al.

Cours et al.

Aloisio et al.

Both Planar

Perpendicular

Calculations

Comparison with Methanol and Water

Source ΔrHo (kJ/mol) Do (kJ/mol)

HO2 + Acetone(This Work)

-31

HO2 + Methanol(Christiensen et al., 2006)

-36.8

H2O + H2O(Curtiss et al., 1979)

-15.0

HO2Acetone(Aloisio et al., 2000)

37.3

HO2Methanol(Christiensen et al., 2006)

35.7

H2OH2O(Klopper et al., 1995)

21.0

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

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

Future Work

1) Search for products (acetonylperoxy, 2-HIPP, Molecular Complex)

We have done some of this: T = 297 K acetonylperoxy: CH3C(O)CH2OO

12

10

8

6

4

2

0

-2

-4

x10-3

2520151050ms

12

10

8

6

4

2

0

-2

-4

x10-3

2520151050ms

σ(cm2/molec) at λuv = 280 nm

2.07E-18 acetonylperoxy

0 HO2

2.00E-20 H2O2

[Ace] = 0 [Ace] = 2.05E16

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

3) Apply this method to many HO2 / Carbonyl systems: MEK, Acetaldehyde, Formaldehyde

Acknowledgements

HarryKroto

AaronNoell

StanSander

MitchioOkumura

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

KiraWatson

Casey Davis-Van Atta

AileenHui

1st yr.Caltech

GradStudent(not shown)

PomonaChem Majors