8372 J. Phys. Chem. A, Vol. 106, No. 36, 2002
Rajakumar et al.
preexponential factors were calculated by using the partition
function for internal rotation48 instead of the torsional motion.
In the free rotor model, the reaction path degeneracy for EC is
3 and that for 1,2-DCE is still 4. Within the temperature range
of our study, using the free rotor model reduces the partition
functions by a factor of 9 for 1,2-DCE and 5 for EC, compared
to the torsional model. Thus, the A factors calculated by the
free rotor model are in excellent agreement with the experiment.
Accurate treatment of the torsional mode as a hindered rotor
may increase the preexponential factor, but it is clear that the
experimental and theoretical A factors would be in reasonable
agreement. Thus, we conclude that even with longer C-Cl bond
(“looser” TS), the experimental and theoretical preexponential
factors could be reconciled with appropriate treatment of the
torsional mode.
sundar of National Chemical Laboratory, (Pune, India) for
fruitful discussions in running Gaussian 94 and Mr. Saravanan
and Mr. Anandraj for help in the experiments. Mr. Nagaraja
helped in running calculations on EC. L. Gangadharaih and
Harikrishna are acknowledged for the workshop support in the
establishment of the shock tube facility.
Supporting Information Available: Ten tables containing
the optimized structures of gauche- and trans-1,2-DCE, EC,
and TS for HCl elimination from both molecules and normal
mode vibrational frequencies of all the five ground and transition
state species. Results are included for HF, MP2(Full), and
DFT(B3LYP) calculations with 6-31G*, 6-31G**, and
6-311++G** basis sets. This material is available free of charge
In our calculations, the frequencies have not been scaled as
it was found not to have a serious effect on the preexponential
factor during our work on CH3COCl.17 It is understandable given
the fact that the rate depends on the ratio of partition functions
for the TS and reactant. Moreover, scaling is useful for high-
frequency stretching modes and when one considers the H/D
isotopic effect. There are three (two) modes (CCCl deformation)
with less than 500 cm-1 for 1,2-DCE (EC) ground state and
four (three) for the TS. There is no reliable scaling procedure
for these low-frequency modes, which contribute significantly
to the preexponential factor. Moreover, harmonic approximation
is certainly not valid in treating these low-frequency modes.
References and Notes
(1) Barton, D. H. R.; Howlett, K. E. J. Chem. Soc. 1949, 155 and 165.
(2) Howlett, K. E. Discuss. Faraday Soc. 1952, 48, 25.
(3) Holbrook, K. A.; Walker, R. W.; Watson, W. R. J. Chem. Soc. B
1971, 577.
(4) Huybrechts, G.; Katihabwas, J.; Martens, G. J.; Nejszaten, M.;
Olbregts, J. Bull. Chem. Soc. Chim. Belg. 1972, 81, 65.
(5) (a) Hassler, J. C.; Setser, D. W.; Johnson, R. L. J. Chem. Phys.
1966, 45, 3231; (b) Hassler, J. C.; Setser, D. W. J. Chem. Phys. 1966, 45,
3246.
(6) Dees, K.; Setser, D. W. J. Chem. Phys. 1968, 49, 1193.
(7) Roussel, P. B.; Lightfoot, P. D.; Carlap, F.; Catoire, V.; Lesclaux,
R.; Forst, W. J. Chem. Soc., Faraday Trans. 1991, 87, 2367.
(8) Dyer, P. E.; Mathews, M.; Holbrook, K. A.; Oldershaw, G. A. J.
Chem. Soc., Faraday Trans. 1991, 87, 2151.
(9) Ma, P.; Liu, J.; Chen, G.; Chu, M.; Jing, Y.; Wu, B. J. Chem. Soc.,
Faraday Trans. 1993, 89, 4171.
(10) Ashmore, P. G.; Owen, A. J. J. Chem. Soc., Faraday Trans. 1 1982,
78, 677.
(11) Incavo, J. A. Ind. Eng. Chem. Res. 1996, 35, 931.
(12) Borsa, A. G.; Herring, A. M.; McKinnon, J. T.; McCormick, R.
L.; Yamamoto, S.; Teraoka, Y.; Natori, Y. Ind. Eng. Chem. Res. 1999, 38,
4259.
(13) Tsang, W.; Lifshitz, A. In Handbook of Shock WaVes, Vol. 3.
Chemical Reactions in Shock WaVes and Detonations; Ben-Dor, G., Igra,
O., Elperin, T., Lifshitz, A., Eds.; Academic Press: San Diego, 2001; and
references therein.
(14) Clough, P. N.; Polanyi, J. C.; Taguchi, R. T. Can. J. Chem. 1970,
48, 2919.
(15) Sudbo, Aa. S.; Schulz, P. A.; Shen, Y. R.; Lee, Y. T. J. Chem.
Phys. 1978, 69, 2312.
(16) Quick, C. R.; Wittig, C. J. Chem. Phys. 1980, 72, 1694.
(17) Srivatsa, A.; Arunan, E.; Manke, G., II; Setser, D. W.; Sumathi,
R. J. Phys. Chem. 1998, 102, 6412.
(18) Seakins, P. W.; Woodbridge, E. L.; Leone, S. R. J. Phys. Chem.
1993, 97, 5633.
(19) Tsang, W. J. Chem. Phys. 1964, 41, 2487.
(20) Milward, G. E.; Tschuikow-Roux, E. Int. J. Chem. Kinet. 1972, 4,
559.
(21) Cadman, P.; Day, M.; Kirk, A. W.; Trotman-Dickenson, A. F.
Chem. Commun. 1970, 203.
(22) Kato, S.; Morokuma, K. J. Chem. Phys. 1980, 73, 3900.
(23) Benito, R. M.; Santamaria, J. J. Phys. Chem. 1988, 92, 5028.
(24) Raff, L. M.; Graham, R. W. J. Phys. Chem. 1988, 92, 5111.
(25) Toto, J. L.; Pritchard, G. O.; Kirtman, B. J. Phys. Chem. 1994, 98,
8359, and references therein.
(26) (a) Rakestraw, D. J.; Holmes, B. E. J. Phys. Chem. 1991, 95, 3968.
(b) Jones, Y.; Duke, D. W.; Tipton, D. L.; Holmes, B. E. J. Phys. Chem.
1990, 94, 4957.
VI. Conclusions
The unimolecular HCl elimination from 1,2-DCE has been
reported both experimentally and theoretically. A single pulse
shock tube has been used for the experimental studies. Both ab
initio and DFT methods have been employed to characterize
the TS for HCl elimination from EC and 1,2-DCE. From the
experimental results, the rate constant for the HCl elimination
from 1,2-DCE is estimated to be 1013.98(0.80 exp (-57.8 ( 2.0/
RT) s-1. The activation energy for HCl elimination is nearly
the same as that for EC.
The activation energies calculated for 1,2-DCE at HF, MP2,
and DFT methods with the 6-311++G** basis set, differ from
the experimental value by +12.0, +12.5, and 0.1 kcal mol-1
.
The excellent agreement between DFT and experiment may be
fortuitous as it underestimates the barrier for HCl elimination
from EC by 4.5 kcal mol-1. For EC also, HF and MP2 level
calculations overestimate the barrier for HCl elimination by 4
and 8.5 kcal mol-1, respectively. All three levels of theory
predict the HCl elimination barrier to increase on â-Cl substitu-
tion of EC. The increase predicted by both MP2 and DFT
calculations, which include electron correlation, is about 4 kcal
mol-1. This is in modest agreement with our experimental
finding of no increase, given the 2 kcal mol-1 uncertainty in
the experiments. The preexponential factors calculated using a
free-rotor model for the torsional degree of freedom are in good
agreement with experimental values.
Acknowledgment. We acknowledge the financial support
from IISc-ISRO Space Technology Cell and the Director, Indian
Institute of Science for establishing the high temperature
chemical kinetics laboratory. E.A. thanks Prof. A. Lifshitz, Dr.
G. O. Thomas, and Dr. J. V. Michael for helpful discussions in
person, and Dr. W. Tsang for discussions through e-mail. Profs.
D. W. Setser and K. L. Sebastian are acknowledged for
stimulating discussions on TST. B.R. thanks Dr. R. B. Sunoj
of The Ohio State University (U.S.A.) and Mr. K. R. Shama-
(27) Chuchani, G.; Martin, I.; Rotinov, A.; Herna´ndez, J. A.; Reikonnen,
N. J. Phys. Chem. 1984, 88, 1563.
(28) Ho, W. P.; Barat, R. B.; Bozzelli, J. W. Combust. Flame 1992, 88,
265.
(29) Seetula, J. A. J. Chem. Soc., Faraday Trans. 1998, 94, 1933.
(30) Cardy, H.; Larrieu. C.; Chaillet, M.; Ollivier, J. Chem. Phys. 1993,
169, 305.
(31) Weissman, M.; Benson, S. W. Int. J. Chem. Kinet. 1984, 16, 307.
(32) Reddy, N. M.; Jagadish, G.; Nagashetty, K.; Reddy, K. P. J.
Sadhana-Proc. Ind. Acad. Sci. 1996, 741.