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

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”)

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

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

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

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

Tunneling in the 1,5 H-shift

Reaction coordinate

ReactantProduct

Tunneling rate 10-200 times the classical rate!

Dibble ‘02

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

Double H-Bond and Double H-shift

0

4 Kcal/ mole

-19 Kcal/mole

OH

OH

O.

OH

O

OH

.

OH

O

OH

.

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

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.

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

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