Kinetic Studies on the Reactions of CF3 with O(3P) and H Atoms
J. Phys. Chem. A, Vol. 102, No. 43, 1998 8347
have directly and exactly been measured by using a shock tube-
ARAS technique by our group,28 these rate coefficients cannot
create a large error in the determination of k2c. The sensitivity
coefficient for reaction 14 is also not 0 but is very small. For
example, an uncertainty of (50% for k14 only causes an error
of 3-5% to k2c. Therefore, we can conclude that k2c has little
serious error due to the uncertainty of kinetic data for these
side reactions.
The rate coefficients of collisions between CF3 and O(3P)
and between CF3 and H were calculated with electronic
degeneracy factors g’s under the L-J interaction potentials.33
(b) The rate coefficients for the reactions CF3 + O(3P) f
CF2O + F (1b) and CF3 + H f CF2(1A1) + HF (2c) were
experimentally determined to be k1b ) (2.55(0.23) × 10-11
and k2c ) (8.86(0.32) × 10-11 cm3 molecule-1 s-1 over the
temperature ranges of 1900-2330 and 1150-1380 K, respec-
tively. These results were in good agreement with some room-
temperature data reported previously.
(c) From comparison of the rate coefficients, the dominant
channel of CF3 consumption in the combustion is estimated to
be the reaction with H atoms, but not O(3P) atoms, differing
from CH3 consumption.
1/2
g‡
g1g2
Acknowledgment. The authors wish to express our gratitude
to Professor Frank Scott Howell S. J. for his valuable comments.
8πkBT
(2,2)/
12
kLJ
)
σ2
Ω
12
(
)
(
)
µ12
References and Notes
where µ12 is the reduced mass between CF3 radical and O or H
(1) Takahashi, K.; Yamamori, Y.; Inomata, T. J. Phys. Chem. A 1997,
101, 9105.
(2,2)/
atom. The reduced collision integral Ω
was given by
12
Troe’s approximation.11 The parameters σLJ and ꢀLJ/kB were
taken from the NASA technical report13 to be 4.320 Å and 121.0
K for CF3 radical, to be 3.050 Å and 106.7 K for O atom, and
to be 2.708 Å and 37.0 K for H atom, respectively. The
calculation gives kLJ(CF3 + O) ) (1.0-1.4) × 10-10 and kLJ-
(CF3 + H) ) (2.4-4.7) × 10-10 cm3 molecule-1 s-1 over the
temperature range of 300-2500 K, which are much larger than
the experimental results. The variational transition state theory34,35
must be applied to discuss in more detail, but the relation
between CF3 + O(3P) and CF3 + H systems in the L-J collision
rate coefficients is kLJ(CF3 + H) > kLJ(CF3 + O) in agreement
(2) Ryan, K. R.; Plumb, I. C. J. Phys. Chem. 1982, 86, 4678.
(3) Ryan, K. R.; Plumb, I. C. Plasma Chem. Plasma Proc. 1984, 4,
141.
(4) Tsai, C.; Belanger, S. M.; Kim, J. T.; Lord, J. R.; McFadden, D.
L. J. Phys. Chem. 1989, 93, 1916.
(5) Tsai, C.; McFadden, D. L. J. Phys. Chem. 1989, 93, 2471.
(6) Biordi, J. C.; Lazzara, C. P.; Papp, J. F. J. Phys. Chem. 1976, 80,
1042.
(7) Takahashi, K.; Inoue, A.; Inomata, T. Twentieth Symposium
(International) on Shock WaVes; World Scientific: New Jersey, 1995; p
959.
(8) Gaussian 94W, Revision D.3; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.
Gaussian, Inc.: Pittsburgh, PA, 1995.
(9) Whitten, G. Z.; Rabinovitch, B. S. J. Chem. Phys. 1963, 38, 2466.
(10) Whitten, G. Z.; Rabinovitch, B. S. J. Chem. Phys. 1964, 41, 1883.
(11) Troe, J. J. Chem. Phys. 1977, 66, 4758.
(12) Gardiner, W. C., Jr.; Troe, J. In Combustion Chemistry; Gardiner,
W. C. Jr., Ed.; Springer-Verlag: New York, 1984; p 173.
(13) Svehla, R. A. NASA Technical Report R-132, 1962.
(14) Cambi, R.; Cappelletti, G.; Liuti, G.; Pirani, F. J. Chem. Phys. 1991,
95, 1852.
(15) Foresman, J. B.; Frisch, M. J. Exploring Chemistry with Electronic
Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996; p 64.
(16) Sudbø, Aa. S.; Schulz, P. A.; Grant, E. R.; Shen, Y. R.; Lee, Y. T.
J. Chem. Phys. 1979, 70, 912.
with the relation of the experimental rate coefficients, k2c
k1b.
>
The rate coefficients for the high-temperature CH3 + O(3P)
and CH3 + H reactions have already been determined by
numerous workers. The rate coefficients recommended by
Tsang and Hampson36 are shown with broken lines in Figures
8 and 9. Both O and H atoms have higher reactivities with
CH3 radicals than with CF3 radicals. We are currently studying
the kinetics of the high-temperature reactions of CHF2 and CH2F
with O(3P) and H atoms to discuss the relation between the
number of fluorine atoms in fluoromethyl radicals and the
reactivities with O and H atoms.
In this work, we found the results that CF3 radicals could
react with H atoms more easily than with O(3P) atoms, differing
from CH3 radicals. In the practical combustion, the dominant
channel of CF3 consumption is estimated to be the reaction with
H atoms but not O(3P) atoms. Such difference in relative
reactivity between CF3 and CH3 radicals can be explained as
follows. For the CF3 + H reaction, the three-centered elimina-
tion of HF from CHF3* is the main channel. In contrast, the
elimination of H2 from CH4* cannot energetically occur for the
CH3 + H reaction, so that the possible products of CH4* are
either CH3 + H or CH4. As the former channel means the
reverse reaction, it decreases the apparent rate coefficient for
the CH3 + H reaction, leading to the result that k(CH3 + O) >
k(CH3 + H). This consideration is confirmed from the fact
that the relation of the L-J collision rate coefficients is kLJ-
(CH3 + H) > kLJ(CH3 + O) against the experimental data.
(17) Curtiss, L. A.; Raghavachari, K.; Pople, J. A. J. Chem. Phys. 1993,
98, 1293.
(18) Klatt, M.; Rohrig, M.; Wagner, H. Gg. Ber. Bunsenges Phys. Chem.
1991, 95, 1163.
(19) Thielen, K.; Roth, P. Combust. Flame 1987, 69, 141.
(20) Just, Th. In Shock WaVes in Chemistry; Lifshitz, A. Ed.; Marcel
Dekker: New York, 1981; p 279.
(21) Zaslonko, I. S.; Mukoseev, Yu. K.; Skorobogatov, G. A.; Khripun,
V. K. Kinet. Catal. 1990, 31, 912.
(22) Dobychin, S. L.; Mashendzhinov, V. I.; Mishin, V. I.; Semenov,
V. N.; Shpak, V. S. Dokl. Phys. Chem. 1990, 312, 494.
(23) Glanzer, K.; Maier, M.; Troe, J. J. Phys. Chem. 1980, 84, 1681.
(24) Westbrook C. K. Nineteenth Symposium (International) on Com-
bustion; The Combustion Institute: Pittsburgh, PA, 1982; p 127.
(25) Hanson, R. K.; Salimian, S. In Combustion Chemistry; Gardiner,
W. C., Jr., Ed.; Springer-Verlag: New York, 1984; p 361.
(26) Baulch, D. L.; Drysdale, D. D.; Duxbury, J.; Grant, S. J. EValuated
Kinetic Data for High Temperature Reactions. Volume 3 Homogeneous
Gas Phase Reactions of the O2-O3 System, the CO-O2-H2 System, and of
Sulphur-containing Species; Butterworths: London, 1976; p 33.
(27) Morris, R. A.; Donohue, K.; McFadden, D. L. J. Phys. Chem. 1989,
93, 1358.
Conclusions
This study on kinetics of the high-temperature reactions of
CF3 with O(3P) and H atoms can be summarized as follows:
(a) The CF3 + O(3P) and CF3 + H reactions proceed through
F dissociation and HF elimination after their electrophilic
additions, respectively.
(28) Okada, K. Master Thesis; Sophia University, 1996.
(29) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989,
15, 287.
(30) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran
Chemical Kinetics Package for the Analysis of Gas-Phase Chemical