exploration of energetic pathways of hydroperoxides in the...
TRANSCRIPT
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Exploration of Energetic Pathways of Vinyl Hydroperoxides in the Troposphere:Applied to Ozonolysis of Single‐Pi‐Bond Alkenes and Isoprene
Lina Luu and Alexander Weberg
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Hydroxyl Radical in the Atmosphere____________________________
• Most important oxidant in troposphere• Plays key role in initiating oxidation reactions• Responsible for removing trace gases
• Greenhouse gases• Carbon monoxide
• Most often produced through photolysis of ozone in the presence of water vapor
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Alkene OzonolysisMechanism to Hydroxyl Radical
“Dark” Hydroxyl Radical Formation Mechanism:
Is there a saddle point in the dissociation of vinyl hydroperoxide (VHP)?
____________________________
CriegeeIntermediate
Vinyl hydroperoxide
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There is a Saddle Point!• Decomposition of VHP:
• Found by Kurten and Donahue using very high‐level computational study
• MRCISD(4,4)+Q/cc‐pVTZ
Kurten, T.; Donahue, N. M. “MRCISD Studies of Dissociation of Vinylhydroperoxide, CH2CHOOH: There Is a Saddle Point.” J. Phys. Chem. A 2014, 116, 6823‐6830
1. VHP goes through transition structure (IV)2. O—O homolysis forms two moieties, held by hydrogen bond (V)3. Two moieties separate (VI + VII)
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____________________________Research Overview
I. Find a smaller yet accurate level of theory and basis set
I. Adding R groups to VHP and exploration of new VHP pathways
I. Exploration of other possible reactions of the VHP formed in Isoprene Ozonolysis
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Finding an Accurate, Smaller Level of Theory
and Basis Set Combination ____________________________• Level of theories: B3LYP, BP86, M06L, and M062X
• Basis Sets: 6‐31+G(d,p) and aug‐cc‐pVTZ
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Procedure• Isolation of structure III
• Subsequent coordinate scans by increasing O—O bond lengths to find max energy TS IV
• Additional lengthening coordinate scans to find min. energy V
• Finding energy for VI + VII
• Important keywords for diradical character• “U” before theory name• guess=mix and scf=xqc in the input file• opt(addredun) for all coordinate scans
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Results and….
Failure!
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NOT TS IV TS IV
Not so Rad…
We had been using the wrong structure for TS IV in our theory/basis set analysis, putting us back at square 1!
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BP86/6‐31+G(d,p) Justification____________________________
Our BP86/6‐31+G(d,p) combination gives close results to literature values reported by Kurten and Donahue, so we decided to stay the course and use this level of theory and basis set.
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Scheme C: Formation of and Competing Pathways for VHPs____________________________
Two possible reaction pathways, with resulting structures IX, or VI and VII
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1,4‐Hydrogen Shift of CriegeeIntermediate to Form Vinyl
Hydroperoxide____________________________
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Adding R Groups to VHP
• BP86 6‐31+G(d,p)
• R1, R2, and R3 are either H or CH3
• 8 possible combinations
• Calculating zero‐point corrected relative energies with proper scaling factor of 1.007
*Proper scaling factor taken from Computational Chemistry Department at the University of Minnesota, using BP86 6‐31G(d)
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1,3‐Hydroxyl Shift
(Non .OH formation pathway)
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Scheme C Energies with all Possible R Combinations
• Adding more methyl groups, TS more stabilized electron density (few deviations)
• R1 – CH3, difficult to find possible hydrogen interaction and steric effects
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I
IX
VI + VII
V
TS IVTS VIII
III
TS II
III TS IV V VII + VII
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Ozonolysis of Isoprene____________________________
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Ozonolysis of Isoprene____________________________
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TSIV
III
TS II
V
VI + VII
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Ozonolysis of Isoprene____________________________
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I
IX
TS VIII
III
TS II
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Ozonolysis of Isoprene
NOTISOLATED
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TS IV
I
III
TS II
XIV
TS XIII
XII
TS XI
V
TS X
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Relative Energy Summary
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TS XI• Acquired this structure
while trying to scan from V to XII
• Appears that an H2O molecule is leaving(Bond angle 109°)
• Future research to look into whether or not this is actually viable (seems unlikely)More likely a result of scanning bond distances unrealistically
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Summary
• The BP86/6‐31+G(d,p) theory/basis set combination gives relatively accurate results in a much faster and less demanding manner than the high level computation applied by Kurten and Donahue
• Increasing methyl groups increases stability of molecules (lowers energy), particularly in radical and diradical species– Electron donating groups stabilize radicals
• In isoprene ozonolysis, there is indeed a pathway straight from III to XIV via TS X (previously unconfirmed)
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Future Work
– Check to see if there is a better level of theory/basis set combination that we can use to analyze these reaction pathways
• It should not be too hard to use our existing geometries to then isolate structures using a different combination
– Find transition states we were unable to find (may need larger theory/basis set combination)
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AcknowledgementsProfessor Keith T. Kuwata Professor Rebecca C. Hoye
Dan Yonker (Hope College)
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References
Kurten, T.; Donahue, N. M. "MRCISD Studies of the Dissociation of Vinylhydroperoxide, CH2CHOOH: There Is a Saddle Point." J. Phys. Chem. A 2012, 116, 6823‐6830
Kuwata, K. T.; Hermes, M. R.; Carlson, M. J.; Zogg, C. K. "Computational Studies of the Isomerization and Hydration Reactions of Acetaldehyde Oxide and Methyl Vinyl Carbonyl Oxide." J. Phys. Chem. A 2010, 114, 9192‐9204
Kuwata, K. T.; Templeton, K. L.; Hasson, A. S. "Computational Studies of the Chemistry of Syn Acetaldehyde Oxide." J. Phys. Chem. A 2003, 107, 11525‐11532