4502 J. Phys. Chem., Vol. 100, No. 11, 1996
Misochko et al.
(3) The main product of reaction of F atoms (either hot or
thermalized) with isolated C2H4 molecules is the â-fluoroethyl
radical. In the absence of F atom diffusion, the fraction of
radicals in the product balance is about 0.1. However, at T
>20 K, reactions of diffusing thermal F atoms form C2H4F
radicals as the major product.
(4) Secondary reaction of a thermal F atom with the â-C2H4F
radical forms trans- and gauche-1,2-C2H4F2 but not C2H3F-
HF. The rate constant of this radical recombination reaction is
about 3 times larger than the rate constant of the primary F +
C2H4 reaction at 26 K.
(5) The difference between products of the UV photolysis
reaction of F2-C2H4 complexes and reaction of two thermalized
F atoms with isolated C2H4 is explained by the difference in
size of the reaction cages and excess energies of the vibrationally
excited intermediate (C2H4F2)*.
Acknowledgment. We thank Prof. V. A. Benderskii for
many a helpful discussion during this work. E.Ya.M. is grateful
to the University of Utah for the warm reception during his
stay in Salt Lake City. The project was supported by the
National Science Foundation (Grant CHE-9300367).
Figure 8. Energy diagram of the reactants, intermediates, and products
in the system F2 + C2H4.
initiated cage reaction of the complex F2-C2H4 can be as much
as 400 kJ/mol higher, depending on the relative rates of atomic
addition and energy transfer from the excited intermediates of
(4) to the matrix (see Figure 8). This also can explain the
predominant formation of the elimination product in the reaction
sequence (4-5). The energy diagram in Figure 8 also shows
the initial energy of the C2H4F2* formed in the F2-C2H4
complex by IR excitation at λ ) 5.27 µm, studied by Frei et
al.16 In spite of the large difference in the available excess
energies in IR- and UV-induced reactions, the elimination
channel (5a) is predominant in both cases. On the other hand,
the excess energy in the IR-induced reaction of the complex
(508 kJ/mol) is close to that in the thermal reaction (6), which
produces exclusively C2H4F2 molecules. This implies that the
energy factor is less important than the effect of the cage size
in competition between relaxation and elimination in reactions
5.
As demonstrated above, the observed difference in products
of reactions 5 and 6 clearly shows that decomposition of highly
excited intermediates is predominant when the reactants are
initially arranged as complexes. In particular, this can play an
important role in aggregates of several reactant molecules or in
their binary mixtures, leading to specific chain reactions
propagated by the decomposition products. The possibility of
such a two-step energetic chain in (F2‚‚‚CH4‚‚‚F2) was recently
demonstrated.30
References and Notes
(1) Feld, J.; Kunttu, H.; Apkarian, V. A. J. Chem. Phys. 1990, 93,
1009.
(2) Kunttu, H.; Apkarian, V. A. Chem. Phys. Lett. 1990, 171, 423.
(3) Alimi, R; Gerber, R. B.; Apkarian, V. A. J. Chem. Phys. 1990, 92,
3551.
(4) Alimi, R.; Gerber, R. B.; Apkarian, V. A. Phys. ReV. Lett. 1991,
66, 1295.
(5) Misochko, E. Ya.; Benderskii, V. A.; Goldschleger, A. U.; Akimov,
A. V. MendeleeV Commun. 1995, 5, 198.
(6) Misochko, E. Ya.; Benderskii, V. A.; Goldschleger, A. U.; Akimov,
A. V.; Shestakov, A. F. J. Am. Chem. Soc., submitted for publication.
(7) Benderskii, V. A.; Goldschleger, A. U.; Akimov, A. V.; Misochko,
E. Ya.; Wight, C. A. MendeleeV Commun. 1995, 6, 245.
(8) Parson, J. M.; Lee, Y. T. J. Chem. Phys. 1972, 56, 4658.
(9) Moehlmann, J. G.; Gleaves, J. T.; Hudgen, J. W.; McDonald, J. D.
J. Chem. Phys. 1974, 60, 4790.
(10) Farrar, J. M.; Lee, Y. T. J. Chem. Phys. 1976, 65, 1414.
(11) Bauer, S. H. Chem. ReV. 1978, 28, 147.
(12) Kato, S.; Morokuma, K. J. Chem. Phys. 1980, 72, 206.
(13) Kapralova, G. A.; Chaikin, A. M.; Shilov, A. E. Kinet. Katal. 1967,
8, 485.
(14) Jacox, M. E. Chem. Phys. 1981, 58, 289.
(15) Hauge, R. H.; Gransden, S.; Wang, J.; Margrave, J. L. Ber. Bunsen-
Ges. Phys. Chem. 1978, 82, 104; J. Am. Chem. Soc. 1979, 101, 6950.
(16) Frei, H.; Fredin, L.; Pimentel, G. C. J. Chem. Phys. 1981, 74, 397.
(17) Frei, H.; Fredin, L.; Pimentel, G. C. J. Chem. Phys. 1983, 78, 3698.
(18) Misochko, E. Ya.; Benderskii, V. A.; Goldschleger, A. U.; Akimov,
A. V. MendeleeV Commun. 1994, 5, 203.
(19) Johnson, G. L.; Andrews, L. J. Am. Chem. Soc. 1982, 104, 3043.
(20) Anderson, D. T.; Winn, J. S. Chem. Phys. 1994, 189, 171.
(21) Huber-Wa¨lchli, P.; Gu¨nthard, Hs. H. Chem. Phys. Lett. 1975, 30,
347.
CONCLUSIONS
(22) Harris, W. C.; Holtzclaw, J. R.; Kalasinsky, V. F. J. Chem. Phys.
1977, 67, 3330.
(1) The combination of kinetic IR and EPR data in experi-
ments at different temperatures allows assignment of the
vibrational bands of the â-fluoroethyl radical. Absolute absorp-
tion intensities of eight vibrational modes have been measured.
(2) Closed-shell products are formed when reactants are
initially arranged as F2-C2H4 complexes. The geometry of the
complex and surrounding Ar atoms is such that neither of the
F atoms formed by photodissociation of F2 escapes from the
cage. Closed-shell products C2H3F-HF complex and trans-
and gauche-1,2-C2H4F2 are formed with the relative yields 0.6:
0.2:0.2.
(23) Fredin, L.; Nelander, B. J. Mol. Struct. 1973, 16, 205.
(24) Wight, C. A.; Misochko, E. Ya.; Vetoshkin, E. V.; Goldanskii, V.
I. Chem. Phys. 1993, 170, 393.
(25) Dubs, M.; Ermanni, L.; Gu¨nthard, Hs. H. J. Mol. Spectrosc. 1982,
91, 458.
(26) Chen, Y.; Rauk, A.; Tschuikow-Roux, E. J. Chem. Phys. 1990,
93, 6620.
(27) Raff, L. M. J. Chem. Phys. 1990, 93, 3160 and references therein.
(28) Raff, L. M. J. Chem. Phys. 1990, 93, 3160; 1991, 95, 8901.
(29) Raff, L. M. J. Chem. Phys. 1992, 97, 7459.
(30) Misochko, E. Ya.; Benderskii, V. A.; Goldschleger, A. U. J. Phys.
Chem. 1995, 99, 13917.
JP953155F