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Produced by the NASA Center for Aerospace Information (CASI)
(X&SA-T8-80743) PRESSURE
DEPBYDRAICE OF TbE
111 8 0-3 04 5 8
ABSOLUTE RATE CONSTANT !OR THE REACTION Od
0282 FROM 228 TO 413K (NASA) 25 p
HC AWAY A01
CSCL
07D
unclas
G3/25 31327
RVSA
Technical Memorandum 80743
Pressure Dependence of the
Absolute Rate Constant for the
Reaction OH + C
2
H
2
from
228 to 413 K
J. V. Michael, D. F. Nava,
R. P. Borkowski, W. A. Payne,
and L. J. Stief
JULY 1980
National Aeronautics and
Space Administration
Goddard
Space
F1191hit,Canter
Greenbelt, Maryland 20771
C^
'7
kp
.^S
PRESSURE DEPENDENCE OF THE ABSOLUTE RATE CONSTANT FOR TOE REACTION
OH + C
2
H
2
FROM
228 TO 413 K
J. V. Michael, a D. F.
Nava,
R. P. Borkowski ,
b
W. A.
Payne and L. J. Stiefa
Astrochemistry Branch
Laboratory for Extraterrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, Maryland
20771
a.
Visiting Professor of Chemistry, Catholic University of America.
Washington, D. C.
20064
b.
Participant NASA/ASEE Summer Faculty Fellowship Program; permanent address:
Chemistry Department, King's College, Wilkes-Barre, PA 18711
c.
Adjunct Professor of Chemistry, Catholic University of America. Washington,
D. C. 20064
ASS
Abstract
The
pressure dependence of absolute rate constants for the reaction of ON
+ C
2
H
2
+products
has
been examined at five temperatures ranging from 228 to
413 K.
fie experimental
technique which
was
used is flash photolysis-
resonance fluorescence (FP-RF).
ON
was
produced by water photolysis and
hydroxyl resonance fluorescent photons were measured by multiscaling
techniques.
The results indicate that the low pressure bimolecular rate constant is
s 4 x 10
-13
cm
3
molecule
-1
5
-1
over the temperature range studied. A
substantial increase in the bimolecular rate constant with an increase in
pressure was observed at all temperatures except 228 K. This indicates the
importance of initial adduct formation and subsequent stablization. The high
pressure results are well represented by the Arrhenius expression (kbi)•
(6.83 ± 1.19) x 10
-12
exp(-646
t
47/T) cm
3
molecule
-1
s-1.
The present results are compared to previous investigations and are
theoretically discussed. The implications of these results on modeling
of terrestrial and planetary atmospheres and also in combustion chemistry are
discussed.
2
E
Aj
m9r
ps
v
s
-• --- .
INTRODUCTION
The reaction between hydroxyl radicals and acetylene is important in
terrestrial
and planetary atmospheric chemistry
2.3
as
well
as
in combustion
chemistry.
4,5
Thus, for example, the
presence of CO in the reducing
atmosphere of Jupiter has been explained with chemical models involving
reactions of 0 and OH.
2.3
Which model contributes probably
depends
on
a
variety of atmospheric conditions. In the model of Prather, Logan and
McElroy, 3 one of the principal paths to CO formation is the reaction
sequence:
H 2
O + b
y
+ H
+
OH, OH + C
2
H
2 +
CH
2
CO + H. CH
2
CO + b
y
+ CH + CO. Both
H
2
O
and
C
2
H
2
have been identified in the Jovian atmosphere, the
latter being a
product
of CH chemistry
in
the atmosphere of that
planet.6
The role of the reaction
OH + C
2
H
2 + products
(1)
in atmospheric and combustion chemistry has prompted
several previous studies
of both the products of the reaction and the absolute rate constant. Kanofsky
et al.
7
showed in a crossed molecular beam-mass spectrometric experiment that
C
2
H
2
O was a product. Thus, there is an open reactive pathway at room
temperature and very low pressure. If this is the only pathway then it can be
argued that pressure dependence for the thermal rate constant may not
exist,
and the reaction yields C 2
H
2
C (probably ketene) and H atoms
exclusively under
any condition of temperature and pressure. On the other hand, if pressure
dependence is observed in the thermal rate constant, then adduct formation is
definitely indicated, and the product of the reaction becomes, at least in
part, the stabilized adduct radical. The ratio of reactive pathways then may
be both pressure and temperature dependent. The implications for modeling
applications is clear; thit is, more than one product can be formed which
depends on the conditions of the system whether that system be a flame, the
terrestrial troposphere, a polluted air mass, or a planetary atmosphere.
Several of the early rate constant determinations were carried out in a
low pressure (.- 1 torr) discharge flow apparatus,
8-10
and at least two
studies
9110
agreed on a room temperature value of s 2 x 10
-13
cm
3
molecule-1
3