Transcript KINETICS OF THE SELF-REACTION OF NEOPENTYL RADICALS Ksenia A. Loginova and Vadim D.

KINETICS OF THE SELF-REACTION OF NEOPENTYL RADICALS
Ksenia A. Loginova and Vadim D. Knyazev
Department of Chemistry, The Catholic University of America, Washington, DC, USA
neo-C5H11 + neo-C5H11 → C10H22 (1)
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In spite of the importance of these reactions, experimental information on them
is rather sparse and, in many cases, controversial.
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This lack of data is primarily due to the difficulties encountered in experimental
studies of reactions between radicals.
Typical experimental difficulties encountered in studies of radical-radical reactions:
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10
12
14
0
2
4
6
8
10
12
1 / Signal
0
10
20
C5H11+
a)
420 K
T = 300 K
C5H11+
+
k4 = 13.7 s
0
10
20
-1
-1
20
-1
10
100
t / ms
Often, radical sources create reactive byproducts that interfere with the
kinetics of the reaction under study.
Exact knowledge of radical concentrations is needed for determination of rate
constants. Errors in calibration of radical signal directly translate into errors in
rate constants.
6
300 K
C5H11+
k4 = -0.4 ± 2.6 s-1
C5H11
4
k1[neo-C5H11]0 (from curve fitting) / s
In addition, radical-radical reactions represent pathways of molecular mass
growth. Certain members of this class of reactions have been linked to
formation of aromatic rings and polyaromatic hydrocarbons (PAH), which
leads, in turn, to production of soot in combustion systems.
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2
200
k1[neo-C5H11]0 (from curve fitting) / s
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Usually, although not without exceptions, these reactions serve as chain
termination pathways.
0
30
t / ms
1 / Signal
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20
+
Radical-radical reactions are among the very important elementary processes
occurring in the oxidation and pyrolysis of hydrocarbons and substituted
hydrocarbons.
neo-C5H11 signal / arb. units
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-10
C10H22
Introduction
t / ms
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10
100
0
200
50
0
150
350 K
500 K
100
100
t / ms
50
b)
T = 500 K
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Experimental Method
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Kinetics of radical decay and growth of products are monitored in real time
using photoionization mass spectrometry.
Initial concentrations of radicals are obtained directly from real-time HCl signal
profiles.
neo-C5H11 wall decay
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14
0
-3
molecule cm (from HCl production)
5
10
12
[neo-C5H11]0 / 10
15
-3
molecule cm (from HCl production)
Circles: bath gas density of 12.01016 molecule cm-3, squares: bath gas density of 3.01016 molecule cm-3.
Experimental Conditions: T = 300 – 500 K; [He] = (3 – 12)1016 ; [(CClO)2] = (2.5 – 18)1014 molecule cm-3;
[neo-C5H12] = (2.4 – 8.7)1014 molecule cm-3; [neo-C5H11]0 = (2 – 14)1012 molecule cm-3.
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Kinetics of neo-C5H11 decay is studied at varied initial radical concentrations.
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Values of the k1[neo-C5H11]0 product are obtained from fitting the [neo-C5H11]
vs time profiles.
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Initial concentrations of neo-C5H11 ([neo-C5H11]0) are determined from the
real-time HCl profiles.
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(kw)
Fitted values of the k1[neo-C5H11]0 product are plotted as a function of
[neo-C5H11]0 to ensure the absence of systematic deviations from linearity.
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The values of k1 are obtained from the slopes of the k1[neo-C5H11]0 vs
[neo-C5H11]0 dependences obtained at different temperatures.
Analytical solution for neopentyl radical signal:
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The k1[neo-C5H11]0 vs [neo-C5H11]0 dependences obtained at different temperatures.
(k1)
2k1 neo  C5 H11 0
S0
exp kwt   1
 exp k wt  
S
kw
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Data Analysis
Kinetic mechanism in the experimental system:
neo-C5H11 + neo-C5H11  C10H22 (bineopentyl)
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[neo-C5H11]0 / 10
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Results
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The room-temperature value of the rate constant ((1.63  0.36)10-11 cm3
molecule-1 s-1) is in agreement with the value of (2.1  0.3)10-11 cm3
molecule-1 s-1 obtained by Nielsen et al.2
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The rate constant demonstrates strong negative temperature dependence.
6
5
4
cm molecule s
No photolysis byproducts interfere with the kinetics of the reaction under study.
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3
CH3 + CH3
C2H5 + C2H5
2
k1 / 10
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4
-1
 HCl + neo-C5H11
2
-1
(CClO)2 (oxalyl chloride)  2 Cl + 2 CO
Typical ion signal (SC5H11), the corresponding
reciprocal signal (1/SC5H11), and product (C10H22)
profiles. Curvature of the 1/SC5H11 plot indicates
the presence of heterogeneous loss of radicals
with the first-order constant kw. Values of the
k1[C5H11]0 product and kw are obtained from fitting
the SC5H11 vs time profiles.
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3
h
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30
-11
Radicals are created via 193 nm laser photolysis of oxalyl chloride1 with
subsequent fast conversion of the Cl atoms into neo-C5H11 radicals:
Cl + neo-C5H12
20
t / ms
Laser Photolysis / Photoionization Mass Spectrometry
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10
0
c-C6H11 + c-C6H11
1
neo-C5H11 + neo-C5H11
Comparison with data on other alkyl radical self-reactions:
Laser Photolysis / Photoionization Mass
Spectrometry Apparatus
Signal
Processing
PMT
Target
Daly Detector
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Exit Lens
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Computer
There are the only three alkyl radical self-reactions for which directly
determined temperature-dependent rate constants are available in the
literature: self-reactions of methyl, ethyl, and cyclohexyl radicals.
Within the 300 – 500 K temperature range, the rate constant of neopentyl
self-reaction decreases with temperature much faster than the rates of selfreactions of the two smaller radicals (CH3 and C2H5) but with a manner
comparable with that of cyclohexyl radicals.
The difference between the temperature dependences of the rate constants
of the self reactions of neopentyl and cyclehexyl radocals and those of
methyl and ethyl radical is in general agreement with the observation by
Klippenstein et al.3 that additional substituents at the radical center and
increasing steric bulk of radicals result in stronger negative temperature
dependences of their theoretically calculated recombination rate constants.
1.0
1.5
2.0
2.5
3.0
3.5
1000 K / T
k1(T) = 3.110-12 exp(+506 K/T) ( 17 %)
cm3 molecule-1 s-1
References
Acknowledgment
This research was supported by U.S. National Science Foundation, Combustion,
Fire, and Plasma Systems Program under Grant No CBET-0853706.
Diffusion Pump
Diffusion Pump
1. Baklanov, A. V.; Krasnoperov, L. N. J. Phys. Chem. A 2001, 105, 97.
2. Nielsen, O. J.; Ellermann, T.; Wallington, T. J. Chem. Physl. Lett., 1993, 203, 302.
3. Klippenstein, S. J.; Georgievskii, Y.; Harding, L. B. Phys. Chem. Chem. Phys. 2006, 8,
1133.