Chemistry Letters Vol.32, No.12 (2003)
1113
Table 1. Reaction mechanism for thermaldecomposition of
C6H5OCF3
dissociates to C3H3 and C2H2. And once C3H3 is produced,
C3H3 dissociates to produce C3H2 and H; C3H3 ! C3H2 + H.
Since the rate constant for this decomposition reaction has al-
ready been determined as k ¼ 5:2 Â 1012 exp (À328 kJ/RT) sÀ1
by measuring H atom behind the reflected shock of C3H3I/Ar
mixtures,8 the time profiles of H atom produced by the decom-
position of C3H2 radical can be simulated using these literature
values. The reaction scheme used for these simulations is shown
in Table 1. Since the rate constant for the decomposition of
C6H5OCF3 has not been determined yet, we assumed the rate
constant was similar to that for C6H5OCH3 decompotion. It
was also checked that the calculated time profiles of H atom
were not influenced by this rate constant because the decompo-
sition of C6H5OCF3 is much faster than the formation of H atom.
The rate constants for reactions (4) and (5) were taken from lit-
eratures. The result of the calculated time profile of H atom with-
out reaction (3) is shown in Figure 2. As shown in Figure 2, H
atom was formed much faster than the simulated profile of H
atom calculated from the literature values mentioned above.
No other source of H atom than the reaction of c-C5H5 + c-C5H5
! c-C10H8 + 2H could be found in our experimental conditions.
Therefore we concluded that H atom was produced by the reac-
tion of c-C5H5 + c-C5H5 ! c-C10H8 + 2H as was proposed by
Melius et al.5 By varying the rate constant for the reaction of c-
C5H5 + c-C5H5 ! c-C10H8 + 2H in Table 1, attempts to fit the
experimentally obtained profiles of H atom were performed. The
results of the kinetic simulations are also shown in Figure 2. As
shown in this figure, good agreements were obtained using the
reaction scheme tablated in Table 1. The Arrhenius plots of
the rate constant for c-C5H5 + c-C5H5 ! c-C10H8 + 2H are
shown in Figure 3. By the least square fits to these plots, the rate
constant was determined to be 3:3 Â 1017 exp(À135 kJ/
RT) cm3 molÀ1 sÀ1. Miyoshi et al.9 determined the rate constant
to be around 3 Â 1012 cm3 molÀ1 sÀ1 for this reaction by measur-
ing H atom behind the reflected shock of C6H5OCH3/Ar mix-
Reactions
A
n
E
Ref.
1
2
3
C6H5OCF3!C6H5O+CF3 1:2 Â 1016
0
0
0
270 Assumed
184 [10]
135 This
work
0
0
323 [7]
125 [7]
0 [3]
328 [8]
C6H5O!C5H5+CO
2C5H5!C10H8+2H
2:5 Â 1011
3:3 Â 1017
4
5
6
7
8
9
2CF3!C2F6
1:4 Â 1013
2:0 Â 1014
3:9 Â 1011
1:4 Â 1011
1:3 Â 1013
5:2 Â 1012
0
0
1
1
0
0
[11]
[12]
CF3+H!CF2+HF
C5H5!C5H5(l)
C5H5(l)!C3H3+C2H2
2C3H3!C6H5+H
C3H3!C3H2+H
A: cm3molÀ1sÀ1 E: kJ molÀ1
k ¼ ATn expðÀE/RTÞ cm3molÀ1sÀ1
tures9 and in spite of the difference of the activation energies
good agreements for the absolute value were obtained between
Miyoshi et al. and us. The difference of the activation energies
may be caused by the difference between the reactivity of CH3
radicaland that of CF radical.
3
References
1
M. Frenklach, D. Clary, W. C. Gardiner, and S. E. Stein,
‘‘20th Symposium (International) on Combustion,’’ The
Combustion Institute, Pittsburgh (1984), p 887.
H. Wang and M. Frenklach, Combust. Flame, 110, 173
(1997).
J. A. Miller and C. F. Melius, Combust. Flame, 91, 21
(1992).
S. Sherer, Th. Just, and P. Frank, ‘‘28th Symposium (Interna-
tional) on Combustion,’’ The Combustion Institute (2000),
p 1511.
C. F. Melius, M. Colvin, N. Marinov, W. Pitz, and S. Seakan,
‘‘26th Symposium (International) on Combustion,’’ The
Combustion Institute (1996), p 685.
S. Kumaran, M. Su, K. Lim, and J. V. Michael, ‘‘26th
Symposium (International) on Combustion,’’ Combustion
Institute (1996), p 605.
K. Roy, C. Horn, P. Frank, V. G. Slutsky, and Th. Just, ‘‘27th
Symposium (International) on Combustion,’’ The Combus-
tion Institute (1998), p 329.
S. Scherer, Th. Just and P. Frank, ‘‘28th Symposium (inter-
national) of Combustion,’’ The Combustion Institute,
Pttsburgh (2000), p 1511.
2
3
4
5
6
7
8
9
14.0
C6H5OCF3 /Ar = 4 ppm
C6H5OCF3 /Ar = 5 ppm
C6H5OCF3 /Ar = 8 ppm
13.5
13.0
12.5
12.0
11.5
Miyoshi et al.
This work
A. Miyoshi et al., private communications.
10 C. Y. Lin and M. C. Lin, J. Phys. Chem., 90, 425 (1986).
11 C. J. Cobbs and J. Troe, Chem. Phys. Lett., 113, 419 (1985).
12 J. Biordi, C. Lazzzara, and J. Papp, J. Phys. Chem., 80, 1042
(1976).
5.5
6.0
6.5
7.0
7.5
8.0
8.5
104 T -1 / K-1
Figure 3. Arrhenius plots for the reaction of c-C5H5+c-C5H5.
Published on the web (Advance View) November 10, 2003; DOI 10.1246/cl.2003.1112