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Current Research in the Atmospheric Degradation of Isoprene
Theodore S. Dibble
Chemistry DepartmentSUNY-Environmental Science and Forestry
Syracuse, NY 13210
http://www.esf.edu/chemistry
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OH O2
OH O2, NO O3
HOOH
Global Organic compound emissionsCH4 530 Tg/yearIsoprene 500 Tg/yearOther 650 Tg/year
Isoprene in the Air
(Oxidizes SO2 to sulfuric acid)
(Health Hazard)
(“Vacuum cleaner of the atmosphere”)
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Multiple Degradation Pathways!
(E)-1
OH
OH
.
OH.
OH
.
(Z)-4OH
.OH.OH
.
60% 5% 5% 30% SAR ‘96
56% 2% 5% 37% CVTST ‘00
41% S. Paulson 59% ‘92
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I(E)-V
IVIIIII (E)-VI
or or
O2 O2 O2 O2
OH OO.
.
OH
OO
OHOO. OH
OO.
OO
OH
. OO
OH
.
Multiple Degradation Pathways!
(E)-1
OH
OH
.
OH.
OH
.
(Z)-4OH
.OH.OH
.
Next: R• + O2 ROO•
34% 22% 2% 5% 29% 8% R. Zhang and S. W. North CVTST ‘01
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Structure-Reactivity Relationships
-10
20
10
CH3.+
O
HOCH2
HOCH2. +
O
+
HOCH2 O
CH2=CH.
OHOCH2 .
0
Kca
l/m
ole
Favor decomposition of RO• making RCHOHrather than R• : leavinggroup effect. Paulson ‘92and Dibble ‘99 (DFT)
•
Endothermic productionof vinyl radical unlikely
ROO• + NO RO• + NO2
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H-bonds and Barton (1,5 H-shift)
k = A e-Ea/RT A = (ekBT/h) eS/R
S = S(TS)-S(Reactant) usually negative
S usually -3R; here S is -R A is unusually large Dibble ‘02
(Z) V
1.920.97
1.35
1.211.39
0.98 1.710.96
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Tunneling in the 1,5 H-shift
Reaction coordinate
ReactantProduct
Tunneling rate 10-200 times the classical rate!
Dibble ‘02
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Chemically Activated Reactions
-10
-15
0
-5
RR’CHOO• + NO
Kca
l/m
ole
RR’CHO• + NO2 R• + R’CH=O
RR’CHO•*
quenching
decomposition
Prompt (chemically activated) decomposition / isomerization dominates fate of RO• from isoprene in 1 atm of air. R. Zhang and S. W. North ‘03
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Double H-Bond and Double H-shift
0
4 Kcal/ mole
-19 Kcal/mole
OH
OH
O.
OH
O
OH
.
OH
O
OH
.
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Traditional Atmospheric Chemistry- large vessel, initiate chemistry. FTIR/GC/HPLC of products- OH fluorescence for OH + isoprene kinetics
Newer Methods- Chemical Ionization-MS in flow tube for OH and O2 kinetics- large vessel, Atmospheric Pressure Ionization MS, MS/MS
for products-OH cycling for kinetics of alkoxy radical reactions
Theory- Density Functional Theory, basis set additivity for ab initio - Canonical Variational Transition State Theory (CVTST)- Master Equation (thermal and activated processes)
Methods
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Conclusions-1
Isoprene from human breath(1,3-butadiene and isoprene from outside air)
OH from O3 + alkenes (including isoprene)NO brought indoors by ventilation (not necessary)
All the chemistry reviewed here is occurring in this room right now.
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Conclusions-2
Fundamental interest and atmospheric importance chemical activation source of O3
tunneling sink for OH structure-reactivity source for HOOH H-bonds
Great example for lecture and homework!
AcknowledgementsNSF-ATM Simon W. North
http://www.esf.edu/chemistry
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Atmospheric Chemistry of Organic Compounds•OH + CH4 HOH + •CH3
•CH3 + O2 CH3OO•
Unpolluted Air (very low [NO])2 CH3OO• 2CH3O• + O2 2[ CH3O• + O2 CH2=O + HOO• ]2 HOO• HOOH + O2
Polluted Air (NO)CH3OO• + NO CH3O• + NO2
CH3O• + O2 CH2=O + HOO•HOO• + NO •OH + NO2
2[ NO2 + h NO + O ]2[ O + O2 O3 ]
net : CH4 + 4 O2 2 O3 + HOH + CH2=O