Computational Studies of the Atmospheric Impact of ...discus/muccc/muccc6/MUCCC6-Kujala.pdfAtmospheric Impact of Cycloalkene Ozonolysis Brianna Kujala Macalester College 2 Outline

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Computational Studies of the Atmospheric Impact of Cycloalkene

Ozonolysis

Brianna KujalaMacalester College

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Outline

Introduction– OH & O3– Alkenes, cycloalkenes – Natural alkenes & isoprene ozonolysis– Carbonyl oxide unimolecular reactions– Aerosols

Computational MethodsResults for Cyclopropene OzonolysisFuture Work

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Introduction

Ozonolysis of Alkenes & Its Impact on the Atmosphere

-Thompson, 19924

Oxidizing Capacity of Earth’s Atmosphere

OH radical, O3, NO3and H2O2 as principal oxidants in troposphere (lower atmosphere)Reactions with each other & with key trace gasesPhotochemical reactions:

-Thompson, 1992 and Heard & Pilling, 20035

The Hydroxyl Radical (OH)

Most important oxidizing agent in the atmosphere – Important in its reaction-it’s the fastest

Therefore hard to measure-due to low [ ] at any timeNegative effects w/too little or too much– Controls pollutant buildup; removal of VOCs– Some regional counterbalance of climate

effects from green house gases– Can lead to more rapid acid formation & thus

deposition on Earth’s surface

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“Nighttime” Hydroxyl Radical Formation

Modeled OH values from daytime chemistry analysis found to be lower than measured OH valuesObservation of high [OH] in winter & thru summer nights, despite lack of light intensityMore complex chemistry in polluted urban environments, due to VOC emission

* Therefore, work done by Heard & Pilling!

-Heard & Pilling, 20037

“Nighttime” Hydroxyl Radical Formation cont’d

Conclusions from Heard & Pilling:– O3 reaction

with alkenes provide major source of winter OH!!

-Thompson, 19928

Ozone (O3)

NMHC, CO, NO & CH4as primary precursors of O3 in troposphere– Half natural, half

artificial in source– More with/from

pollution– Energy consumption,

deforestation, biomass combustion, aircraft, influx from stratosphere, etc.

-Heard & Pilling, 2003 and Zhang & Zhang, 20029

Alkene Ozonolysis

Most alkenes w/natural sources-emitted by plants– Isoprenes & terpenes!

Particularly important in urban (w/a more artificial source of alkenes) & forested areas (natural source of alkenes)Can produce both OH radicals & aerosols

-Zhang & Zhang, 200210

Isoprene

One of the most abundant naturally emitted hydrocarbons in the lower atmosphere; regionally distributedIts ozonolysis an important, natural source of nighttime OH radicals, in regions of abundance

-Zhang & Zhang, 2002 and Kuwata, et. al, 200011

Mechanism for Isoprene Ozonolysis

Concerted cycloaddition of O3 to C=C formation of vibrationally excited 1º ozonide– An exothermic reaction!– 1,2 addition can yield OH

OO

OO

OO

-Zhang & Zhang, 200212

Mechanism cont’d

Unimolecular decomposition of excited 1ºozonide formation of chemically activated carbonyl oxide=Criegee intermediate (CI) & an aldehyde

OO

O

OO

HCHO

-Zhang & Zhang, 200213

Hydroperoxide and OH Formation

1,4 H-shift, for migration pathwayFor general isoprene ozonolysis: experimentally measured OH yields range from ~0.19 to 0.27 near atmospheric pressure

OO

OO

HO

HOO

OHH

HH

-Luke Valin's Honors Thesis, 2005 and Cremer, et. al., 199614

OO

OO

OO

Dioxirane

Formed from the CI resonance contributor that puts the + at the carbonyl carbon atom & the – at the terminal oxygen atomIncrease in stability of carbocationic character dioxirane formation more competitive w/1,4 H-shiftMore stable than CI, but excess energy leads it to rapid ring opening

-Chuong, et. al., 200415

Secondary Ozonide Formation

Seen w/cyclohexenes, in soln.Seen w/specific conformers of collisionally stabilized syn CI– Has to easily be able to cyclize into SOZ– Endothermic, but small en. barrier

Dominant at thermal energies, atm. pressureIncrease in pressure or C atomscollisional stability in syn-CI then stability in POZ increased likelihood of SOZ formation

-Hasson, et. al., 2003 and Luke Valin's Honors Thesis, 200516

Collisional Stabilization of CI

Excited carbonyl oxides can lose energy via collisions w/bath gas molecules. become energetically stabilized may form OH radicals, but on a much slower time scaleAccounts for fate of ~10-50% of CIs at atmospheric pressureReaction w/H2O is thought to be important sink for stabilized CIsMostly affects dioxirane formation pathway, due to strong pressure dependence

-Luke Valin's Honors Thesis, 200517

Dioxole Formation

Formation of 5-membered ringFrom CI resonance structure w/primary carbocationiccharacterImplications for possible stable products such as oxoepoxides & dicarbonyls

OO O

OO

O OO

OO

-Ramanathan, et. al., 200118

Aerosols

Very small particles or liquid dropletsNatural or anthropogenic in sourceIncrease the reflection of solar radiation to spaceLarge spatial & temporal variabilityShortly lived: 1 week/lessAn overall global cooling effectSpin down of hydrological cycle?

-Ziemann, 200219

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Specifics of My Study

GoalsMethods

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General Goals

Locate all reaction pathways involved in ozonolysis of cyclopropeneLook at reaction barriers to rotation & overall reaction energiesLocate different conformers of molecules and their various transition states to formationPredict the rates of reactions & their energy dependencePredict potential yields of products coming from various reaction pathways

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Computational Methods

Geometries, energies & frequencies of TSs& minima determined w/use of Density Functional Theory (DFT)B3LYP functional & 6-31G(d,p) basis set– Quicker!

CBS-QB3 for more accurate energiesDensum and MultiWell calculations for determination of reaction rates and yields

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Results

The Mechanism for Ozonolysis of Cyclopropene

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Mechanism for Cyclopropene Ozonolysis

OO

O

O O

O

O

O

OO O

O

O

O

O

Exo 1o OzonideEndo 1o Ozonide

-76.08 kcal/mol

Cyclopropene Ozone

-6.53 kcal/mol for exo TS

-6.90 kcal/mol for endo TS

0.0 kcal/mol*

-75.76 kcal/mol -70.06 kcal/mol

* Energies shown are Relative Energies, from CBS-QB3 calculations

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Mechanism cont’d

O

O

OO

O

OOO

O

O O

O

O O

O

OOO

Exo 1o

Ozonide

-75.76 kcal/mol

Anti carbonyl oxide TS opens up to:

-112.01 kcal/mol

Syn carbonyl oxide TS opens up to:

-115.82 kcal/mol

Endo 1o

Ozonide

-76.08 kcal/mol

-68.76 kcal/mol

-72.55 kcal/mol

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Conformers of Anti-Carbonyl Oxide

τ1= -132.3o

τ2= -2.9o

-112.01 kcal/mol

τ1= 18.1o

τ2= -136.1o

-110.74 kcal/mol

τ1= 100.3o

τ2= 119.0o

-111.20 kcal/mol

TS

τ1= -122.2o

τ2= -61.5o

-110.76 kcal/mol

TS

τ1= -17.2o

τ2= -138.6o

-110.39 kcal/molτ1=(O2-C1-C2-C3)

τ2=(C1-C2-C3-O3)

O2

O3

O2 O3

O2 O3

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Conformers of Syn-Carbonyl Oxideτ1= 180.0o

τ2= 0.0o

-117.28 kcal/mol

τ1= -75.3o

τ2= -123.4o

-115.65 kcal/mol

τ1= 53.6o

τ2= -163.4o

-115.82 kcal/mol

τ1= 172.3o

τ2= 137.7o

-114.52 kcal/mol

τ1= 50.5o

τ2= 65.5o

-117.33 kcal/mol

τ1=(O2-C1-C2-C3)

τ2=(C1-C2-C3-O3)

O2

O3

O2

O3

O2

O3

O3O2

O2

O3

TS

-112.82 kcal/mol

TS

-114.32 kcal/mol

TS

-113.09 kcal/mol

TS

-115.86 kcal/mol

TS

-114.21 kcal/mol

TS

-113.09 kcal/mol

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Fate of CI-Hydroperoxide

OOO

OO

O

H

H

OO

O

H

H

H H

O

OH

HO

H

Syn carbonyl

oxideVinyl hydroperoxide

Eact = 9-11 kcal/mol for isomerization of syn carbonyl oxides

Eact = 29-31 kcal/mol for isomerization of anti carbonyl oxides

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Fate of CI-Dioxirane

OOO

OO

O

H

H

OO

O

H

H O

H

H

O O

DioxiraneAnti carbonyl oxide

Eact for isomerization of the anti and syn carbonyl oxides to dioxiranes range from 16 to 23 kcal/mol

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Fate of CI-Secondary Ozonide

Syn Carbonyl Oxideτ1= 50.5o

τ2= 65.5o

-117.33 kcal/mol

TS to Secondary Ozonideτ1 = 56.22o

τ2=52.67o

-101.59 kcal/mol

Secondary Ozonide

-127.08 kcal/mol

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Future Work

Completion of Mechanism

Long-term Goals

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Fate of Dioxirane

O

H

H

O O

O

H

H

O O

O

H

O

OH

O

H

CO2

O

HO

HO O

H O

O

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Fate of Vinyl Hydroperoxide

Weak O-O bond—therefore homolysis & thus OH readily breaks offAbility to delocalize vinoxy radical

O

OH

HO

H

O

OH

HOH

O

OH

H

O

OH

H

Vinyl hydroperoxide

Vinoxy radical

*most stable of vinoxyradicals!

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Long-term Goals

Use of RRKM Theory/Master Equation Simulations– To determine the rate constants of the

reaction pathways for isomerization & decomposition, particularly of CI

– To determine the relative yields of all possible products formed, particularly OH radical

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Acknowledgements

Special thanks to:– Professor Keith Kuwata– Macalester College Chemistry Department– Violet Olson Beltmann Fund– My parents!

Thanks to you!

Questions??

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References

Chuong, Bao, Zhang, Jieyuan, and Neil M. Donahue. “Cycloalkene Ozonolysis: Collisionally Mediated Mechanistic Branching”. JACS, 126, 12363-12373, 2004.Cremer, Dieter, Kraka, Elfi, and Peter G. Szalay. “Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane—a CCSD(T), MR-AQCC and DFT investigation”. Chemical Physical Letters, 292, 97-109, 1998.Fenske, Jill D., Kuwata, Keith T., Houk, K.N. and Suzanne E. Paulson. “OH Radical Yields from the Ozone Reaction with Cycloalkenes”. J. Phys. Chem. A, 104, 7246-7254, 2000.Gutbrod, Roland, Schindler, Ralph N., Kraka, Elfi, and Dieter Cremer. “Formation of OH radicals in the gas phase ozonolysis of alkenes: the unexpected role of carbonyl oxides”. Chemical Physics Letters, 252, 221-229, 1996.Hasson, Alam S., Chung, Myeong Y., Kuwata, Keith T., Converse, Amber D., Krohn, Debra, and Suzanne E. Paulson. “Reaction of Criegee Intermediates with Water Vapor—An Additional Source of OH Radicals in Alkene Ozonolysis?”. J. Phys. Chem., 107, 6176-6182, 2003.Heard, Dwayne and Michael J. Pilling. “Measurement of OH and HO2 in the Troposphere”. Chem. Rev., 103, 5163-5198, 2003.

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References cont’d

Kanakidou, M., et. al. “Organic aerosol and global climate modeling: a review”. Atmospheric Chemistry and Physics, 5, 1053-1123, 2005.Ramanathan, V., Crutzen, P. J., Kiehl, J. T., D. Rosenfeld. “Aerosols, Climate, and the Hydrological Cycle”. Science, 294, 2119-2124, 2001.Thompson, Anne M. “The Oxidizing Capacity of the Earth’s Atmosphere: Probable Past and Future Changes”. Science, 256, 1157-1165, 1992.Valin, Luke. “Mechanistic Study on the Gas-phase Ozonolysis of Isoprene and a Prediction of Hydroxyl Radical Yield”. Macalester College, 2005.Zhang, Dan, Lei, Wenfang and Renyi Zhang. “Mechanism of OH formation from ozonolysis of isoprene: kinetics and product yields”. Chemical Physics Letters, 358, 171-179, 2002.Zhang, Dan and Renyi Zhang. “Mechanism of OH Formation from Ozonolysis of Isoprene: A Quantum-Chemical Study”. JACS, 124, 2692-2703, 2002.Ziemann, Paul J. “Evidence for Low-Volatility Diacyl Peroxides as a Nucleating Agent and Major Component of Aerosol Formed from Reactions of O3 with Cyclohexene and Homologous Compounds”. J. Phys. Chem. A, 106, 4390-4402, 2002.