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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
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
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
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)
____________________________
____________________________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
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
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
____________________________
Results and….
Failure!
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!
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.
Scheme C: Formation of and Competing Pathways for VHPs____________________________
Two possible reaction pathways, with resulting structures IX, or VI and VII
1,4‐Hydrogen Shift of CriegeeIntermediate to Form Vinyl
Hydroperoxide____________________________
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)
____________________________
1,3‐Hydroxyl Shift
(Non .OH formation pathway)
____________________________
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
____________________________
I
IX
VI + VII
V
TS IVTS VIII
III
TS II
III TS IV V VII + VII
Ozonolysis of Isoprene____________________________
Ozonolysis of Isoprene____________________________
TSIV
III
TS II
V
VI + VII
Ozonolysis of Isoprene____________________________
I
IX
TS VIII
III
TS II
Ozonolysis of Isoprene
NOTISOLATED
____________________________
TS IV
I
III
TS II
XIV
TS XIII
XII
TS XI
V
TS X
Relative Energy Summary
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
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)
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)
AcknowledgementsProfessor Keith T. Kuwata Professor Rebecca C. Hoye
Dan Yonker (Hope College)
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