double.... h-atom transfer, and of alkoxy radicals from isoprene prompt chemistry intramolecular......

Double . . . .H-atom Transfer, and of Alkoxy Radicals

from Isoprene

Prompt Chemistry

Intramolecular. . . Hydrogen Bonding,

T. S. DibbleChemistry Department, SUNY-ESF

OH O2

OH O2, NO O3

HOOH

Global Organic compound emissionsCH4 530 Tg/yearIsoprene 500 Tg/yearOther 650 Tg/year

Importance of Isoprene in the Atmosphere

(Oxidizes SO2 to sulfuric acid)

(Health Hazard)

(“Vacuum cleaner of the atmosphere”)

OH..

O2

OH

OO.OH.

OH

.OH

OH

OH

O.OHOH

.OH

OH

.O2

NO

.

OHOH

OO iso

*

(E)-1 (Z)-1

(Z)-V.

OHOH

ONO

Chemistry Forming Second Generation Radicals(Paulson and Seinfeld, 1992)

Note many branching pathways are not shown above!!

VIIO VIIOO VII

Second Generation Alkoxy Radicals Expected to Decompose(Paulson and Seinfeld, 1992)

(Dibble, 2002)

OH

O

+ CH2OHVIIO

OHOH

O.

.

But double hydrogen bonding of (Z) isomerinhibits decomposition, unlike single H-bond (Dibble 1999) in first generation alkoxy radicals.

0 Kcal/mole

(4)

-19 Kcal/mole

Unexpected Chemistry

Double H-atom Transfer

Unexpected ChemistryUnexpected ChemistryUnexpected Chemistry

OH

OH

O.

OH

O

OH

.

OH

O

OH

.

NomenclatureWe identify oxygen atoms by the number (in isoprene) of the

carbon atom to which it is bound. The O atom acting as ahydrogen bond donor is to the left of an arrow, the acceptor

is to the right.

(Z) VIIO(14, 4 3)

1

3

4

(Z) VII’OH (34, 4 1)

1

3

4

Multiple Hydrogen bonding Arrangements(zero, one, or two hydrogen bonds)

(Z)-VII’OH (41)

(Z)-VII’OH (43,31)

(Z)-VII’OH (43) (Z)-VII’OH (none)

(Z)-VII’OH (31) (Z) VII’OH (34, 4 1)

1

3

4

5

-10

-15

10

0

-5

-20

(Z)-VII’OH (none) (Z)-VII’OH

(3 4)(Z)-VII’OH (4 3)

(Z)-VII’OH (4 3,3 1)(Z)-VII’OH (3 4,4 1)

(Z)-VII’OH (4 1) (Z)-VII’OH (3 1)

(Z)-VIIO (1 4,4 3)

(Z)-VIIO (1 3)’

Kca

l/m

ol

Relative Energies, H-atom transfer, and H-bonding

5

-10

-15

10

0

-5

-20

OHO

+CH2OH

OH

O

+ CH2OH

(Z)-VII’OH (none) (Z)-VII’OH

(3 4)(Z)-VII’OH (4 3)

(Z)-VII’OH (4 3,3 1)(Z)-VII’OH (3 4,4 1)

(Z)-VII’OH (4 1) (Z)-VII’OH (3 1)

(Z)-VIIO (1 4,4 3)

(Z)-VIIO (1 3)’

Kca

l/m

ol

Part of the Potential Energy ProfileThermal Decomposition of VII’OH is slow...

0 5 10 15 20 250.00

0.25

0.50

0.75

1.00

F

ract

ion

Pro

mpt

Dec

ompo

tion

Initial Energy (Kcal/mol)

10 Torr

760 Torr

298 K

… But Prompt Decomposition of VII’OH is Extensive!

OOH

+ CH2OH

OHO

OH.

OHO

OH. M

*

* OHO

OH.

Consequences for Atmospheric Chemistry?

OH

O

+VIIO

OHOH

O.

CH2OH.

CH2OH.

OHO

OH.VII'OH

O

OH

+

OHO

OH.

O2,NO

OHO

OHO.

O

O.

OH

OH

+

MGLY

O2

OH

O

+ HO2VII'OH

*

Decomposition of VIIO same as Prompt Decomposition of VII’OH!

But quenched VII’OH results in different chemistry

Theodore S. Dibbletsdibble@syr.edu

Chemistry DepartmentState University of New York - Environmental Science and Forestry

Syracuse, NY 13210http://www.esf.edu/chemistry/faculty/dibble.htm

AcknowledgementsNSF-Atmospheric SciencesNCSA - University of KentuckyWei Deng, Katherine Kitney, Lei Zhang

J. Phys. Chem A, (2004) 108, 2199 and 2208.