Intramolecular. . . Hydrogen Bonding,
Double . . . .H-atom Transfer, and
Prompt Chemistry of Alkoxy Radicals
from Isoprene
T. S. Dibble
Chemistry Department, SUNY-ESF
Importance of Isoprene in the Atmosphere
OH O2
HOOH
OH O2, NO O3
(Oxidizes SO2 to
sulfuric acid)
(Health Hazard)
(“Vacuum cleaner of
the atmosphere”)
Global Organic compound emissions
CH4
530 Tg/year
Isoprene
500 Tg/year
Other
650 Tg/year
Chemistry Forming Second Generation Radicals
(Paulson and Seinfeld, 1992)
*
.
.
OH
.
(E)-1
OH
OH
O
.
VIIO
NO
.
VIIOO
O2
(Z)-1
OH
OH
OO
OH
.
OH
OH
OH
OH
OH
O2
OH
.
OH
.
OO
NO
OH
OH
iso
VII
Note many branching pathways are not shown above!!
O
(Z)-V
.
.
Second Generation Alkoxy Radicals Expected to Decompose
(Paulson and Seinfeld, 1992)
(Dibble, 2002)
OH
OH HO
+ .CH 2OH
VIIO
O.
O
But double hydrogen bonding of (Z) isomer
inhibits decomposition, unlike single H-bond
(Dibble 1999) in first generation alkoxy
radicals.
Unexpected Chemistry
(4)
Double H-atom Transfer
0 Kcal/mole
-19 Kcal/mole
Nomenclature
We identify oxygen atoms by the number (in isoprene) of the
carbon atom to which it is bound. The O atom acting as a
hydrogen bond donor is to the left of an arrow, the acceptor
3
is to the right.
3
4
4
1
1
(Z) VII’OH (34, 4 1)
(Z) VIIO(14, 4 3)
HO
HO
O
.
HO
.O
OH
OH
HO
.
O
Multiple Hydrogen bonding Arrangements
(zero, one, or two hydrogen bonds)
3
4
1
(Z) VII’OH (34, 4 1)
(Z)-VII’OH (43,31)
(Z)-VII’OH (41)
(Z)-VII’OH (43)
(Z)-VII’OH (31)
(Z)-VII’OH (none)
Relative Energies, H-atom transfer, and H-bonding
10
5
Kcal/mol
0
(Z)-VIIO (1  3)’
(Z)-VIIO
(1  4,4 3)
-5
-10
-15
(Z)-VII’OH
(none)
(Z)-VII’OH (4 1)
-20
(Z)-VII’OH
(4  3)
(Z)-VII’OH
(3  4)
(Z)-VII’OH (3  4,4 1)
(Z)-VII’OH (3  1)
(Z)-VII’OH (4  3,3 1)
Part of the Potential Energy Profile
10
Thermal Decomposition of VII’OH is slow...
OH
O
5
Kcal/mol
0
+
(Z)-VIIO (1  3)’
CH 2OH
(Z)-VIIO
(1  4,4 3)
-5
OH
O
+
-10
-15
CH2OH
(Z)-VII’OH
(none)
(Z)-VII’OH (4 1)
-20
(Z)-VII’OH
(4  3)
(Z)-VII’OH
(3  4)
(Z)-VII’OH (3  4,4 1)
(Z)-VII’OH (3  1)
(Z)-VII’OH (4  3,3 1)
… But Prompt Decomposition of VII’OH is Extensive!
1.00
Fraction Prompt Decompotion
298 K
O
*
HO
O
OH
+
.
0.75
CH2 OH
OH
O
*
HO
.
OH
0.50
O
M
HO
.
OH
10 Torr
760 Torr
0.25
0.00
0
5
10
15
Initial Energy (Kcal/mol)
20
25
Consequences for Atmospheric Chemistry?
Decomposition of VIIO same as Prompt Decomposition of VII’OH!
OH
OH H O
VIIO
+
O
O.
O
VII'OH
HO
*
O
+ .CH 2OH
.
OH
O
VII'OH
.CH 2OH
OH
HO
O
HO
+
.
OH
OH
O2,NO
HO
HO
O
O
.O
+
O2
.
OH
MGLY
But quenched VII’OH results in different chemistry
O
HO2
Theodore S. Dibble
tsdibble@syr.edu
Chemistry Department
State University of New York - Environmental Science and Forestry
Syracuse, NY 13210
http://www.esf.edu/chemistry/faculty/dibble.htm
J. Phys. Chem A, (2004) 108, 2199 and 2208.
Acknowledgements
NSF-Atmospheric Sciences
NCSA - University of Kentucky
Wei Deng, Katherine Kitney, Lei Zhang