2
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According to the transition state theory k P exp([*Gt/RT ),
thus the free-energy of activation determines the magnitude of
the rate constant at a given temperature. The more positive
*Gt is, the slower is the reaction, as can be seen in the Ðgure.
The standard free-energy change is largest for the CCl ] Cl
reaction. The linear free-energy relationship obtained is good
evidence that these exothermic reactions proceed via a similar
Cl atom abstraction mechanism, where inductive e†ects of
substituents at the radical site determine the value for the free-
energy of activation of the reaction.
3
4
5
3
2
6
7
8
9
Summary
10 I. R. Slagle and D. Gutman, J. Am. Chem. Soc., 1985, 107, 5342.
11 J. A. Seetula, Ann. Acad. Sci. Fenn., Ser. A2, 1991, 234.
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on, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G.
Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Ste-
fanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y.
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The kinetics of four R ] Cl reactions were measured as a
2
function of temperature. The rate constants of three of these
reactions showed nonlinear temperature dependency and were
Ðtted with a Kooij rate expression. The kinetic trend of the
reactions studied with four similar reactions was explained
using a free-energy scale, where electronegativity di†erence
values of the radicals were used as a measure of the inductive
e†ect on the radicals. The transition states of the R ] Cl
2
reactions were localized and optimized by ab initio methods.
Thermochemical and activation parameters of these reactions
were determined. It seems that the kinetics of the R ] Cl
2
reactions are controlled by electronic e†ects of the substit-
uents. Alkyl groups enhance electron density at the radical site
and facilitate reaction, while diminished electron density is
caused by halogen atom substituents, increasing the free-
energy of activation of the reaction and thereby slowing the
overall reaction.
The kinetic results demonstrate that the reactions of a-
halogenated radicals with molecular chlorine become more
important at elevated temperature. This is especially the case
for a-chlorinated radicals. These radicals can also withstand
higher temperatures better than alkyl radicals.
17 J. B. Foresman and þ. Frisch, Exploring Chemistry with Elec-
tronic Structure Methods, Gaussian, Inc., Pittsburgh, 2nd edn.,
1996.
18 J. A. Seetula, J. J. Russell and D. Gutman, J. Am. Chem. Soc.,
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19 G. R. de Mare and G. Huybrechts, T rans. Faraday Soc., 1968, 64,
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20 A. Fontijn and W. Felder, J. Chem. Phys., 1977, 67, 1561.
21 H-H. Grotheer, G. Riekert, D. Walter and T. Just, T wenty-
Second Symposium (International) on Combustion, The Com-
bustion Institute, 1988, 963.
22 G. S. Hammond, J. Am. Chem. Soc., 1955, 77, 334.
23 M. M. Tirtowidjojo, B. T. Colegrove and J. L. Durant, Jr., Ind.
Eng. Chem. Res., 1995, 34, 4202.
24 J. A. Seetula and D. Gutman, J. Phys. Chem., 1991, 95, 3626.
25 J. W. Moore and R. G. Pearson, Kinetics and Mechanism, Wiley,
New York, 2nd edn., 1981.
26 P. J. Robinson, J. Chem. Edu., 1978, 55, 509.
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Initio Molecular Orbital T heory, Wiley, New York, 1986.
28 L. Pauling, Nature of the Chemical Bond, 3rd edn., Cornell Uni-
versity Press, Ithaca, NY, 1960.
This research was supported by the National Science Founda-
tion, Chemistry Division (USA) by the University of Helsinki,
by the Finnish Combustion Research Program LIEKKI and
by the Center for ScientiÐc Computing at Espoo (all in
Finland). I wish to thank Prof. Irene R. Slagle for kindly
lending me the experimental apparatus used for the experi-
mental part of this study.
CAS registry no.: CH Cl, 6806-86-6; CHBrCl, 56732-36-6;
2
CCl , 3170-80-7; CH CCl , 19468-97-4; Cl , 7782-50-5.
3
3
2
2
References
1
G. C. Fettis and J. H. Knox, in Progress in Reaction Kinetics, ed.
G. Porter, Pergamon, New York, 1964, vol. 2, p. 1.
Paper 8/04223C
J. Chem. Soc., Faraday T rans., 1998, 94, 3561È3567
3567