Longfellow et al.: Dissociation of two perfluorobutenes
7207
mate the ⌬Hrxn for CF3 loss from octafluoro-1-butene as the
⌬Hf of the perfluoroallyl radical is unknown. Because the
two other reaction pathways, ͑1͒ and ͑2͒, give similar trans-
lational energy distributions, the fluorine migration to form
the CF3CFCF2CF2 diradical in both compounds may be an
important process at higher laser fluences. The differences in
the branching ratios for the simple bond rupture reaction
indicate isomerization between octafluoro-1-butene and
octafluoro-2-butene, requiring a 1,3-fluorine migration, is not
as competitive as a 1,2-fluorine migration in octafluoro-2-
butene, otherwise the branching ratios would be identical.
FIG. 8. Energy level diagram for octafluoro-1-butene and octafluoro-2-
butene. The heats of formation for the perfluorobutenes are assumed to be
similar. The lowest accessible reaction is the formation of CF3 and the
perfluoroallyl radical. A diradical species reached by a 1,3-fluorine migra-
tion in octafluoro-2-butene and a 3,2-fluorine migration in octafluoro-1-
butene is suggested as a key intermediate. This diradical intermediate allows
both octafluoro-1-butene and octafluoro-2-butene to access pathways to all
three dissociation channels.
D. Question concerning CF4 and solid carbon
formation
As discussed earlier, one goal in these fluorocarbon ex-
periments was to explain the explosive decomposition of
C2F4 to the products CF4 and solid carbon. From the IRMPD
experiments, no evidence of the direct elimination of CF4
was observed. Rather, the predominant reaction process in
these compounds is CF3 loss. It is possible that CF4 can be
formed if CF3 abstracts a fluorine atom from another species
such as the fluorinated allyl radical. If the reaction of two
C2F4 molecules can produce CF4 and CF2CCF2 the subse-
quent reaction of C2F4 and CF2CCF2 could produce CF4 and
CF2CCCF2. In this manner a carbon chain or network could
be produced. In these IRMPD experiments direct evidence
for a fluorine migration was observed in the loss of CF2 from
octafluoro-2-butene. In addition, at high fluences the TOF
spectra for these two molecules begin to share similar char-
acteristics, indicating that the activation barrier for isomer-
ization from octafluoro-1-butene to octafluoro-2-butene is
greater than the activation energy for CF3 loss, but close to
the activation energy for CF2 loss.
reasonable. If two tetrafluoroethylene molecules are formed,
the repulsion between two closed-shell species would pre-
sumably result in a translational energy distribution peaked
further away from zero. In addition, this channel involves
simultaneous migration of two fluorine atoms and is prob-
ably unlikely. The most likely reaction mechanism for reac-
tion ͑1͒ is formation of one C2F4 molecule and one CFCF3
carbene. A 1,2-fluorine migration in octafluoro-2-butene and
a 3,2-fluorine migration in octafluoro-1-butene can produce
the CF3CFCF2CF2 diradical, which would be a common in-
termediate in the production of the C2F4 and CF3CF pair.
The similarity in the translational energy distributions sug-
gests that the intermediate is the same in both molecules.
This indicates that although migration of a fluorine atom to
the diradical intermediate followed by dissociation takes
more energy than CF3 elimination from octafluoro-1-butene,
it takes less energy than cleavage of the double bond in
octafluoro-2-butene.
V. CONCLUSIONS
The predominant reaction in octafluoro-1-butene at mod-
erate laser fluences is cleavage of a C–C single bond to give
the products CF3 and C3F5. These products were observed at
very low fluences owing to allylic resonance stabilization of
the C3F5 fragment. In octafluoro-2-butene and octafluoro-1-
butene at high fluences CF2 loss and formation of two equal
mass products, C2F4, from a diradical intermediate compete
with CF3 loss. No evidence for the loss of CF4 was observed
in either perfluorobutene.
The third channel present in the IRMPD of octafluoro-
2-butene and octafluoro-1-butene is reaction ͑2͒, which re-
sults in the loss of CF2. Again, the translational energy dis-
tributions are similar and both are peaked away from zero at
ϳ9 kcal/mol. In the case of octafluoro-2-butene a 1,2-
fluorine migration, which produces the CF3CFCF2CF2
diradical mentioned above, could also eliminate CF2 com-
petitively. The similarity of the translational energy distribu-
tions suggests that a 3,2-fluorine migration in octafluoro-1-
butene takes place rather than direct cleavage of the C–C
double bond. The repulsion between the closed-shell species,
ACKNOWLEDGMENTS
1
hexafluoropropene, and CF2 explains the observed transla-
tional energy distribution, which is peaked away from zero.
The octafluoro-1-butene was kindly supplied by Dr. M.
H. Hung at DuPont. This work was supported by the Direc-
tor, Office of Energy Research, Office of Basic Energy Sci-
ences, Chemical Sciences Division of the U.S. Department
of Energy under Contract No. DE-AC03-76SF00098. Addi-
tional funding for this project was provided by DuPont.
C. Overall energetics
From the reactions observed at the varying fluences in
octafluoro-1-butene and octafluoro-2-butene, a rough energy
level diagram can be sketched ͑Fig. 8͒. From the extensive
signal at low fluence, the simple bond rupture reaction of
octafluoro-1-butene must have a lower activation energy than
any of the other reaction pathways. It is not possible to esti-
1 P. A. Schulz, Aa. S. Sudbo, D. J. Krajnovich, H. S. Kwok, Y. R. Shen, and
Y. T. Lee, Annu. Rev. Phys. Chem. 30, 379 ͑1979͒.
2 C. A. Longfellow, L. A. Smoliar, Y. T. Lee, Y. R. Lee, C. Y. Yeh, and S.
M. Lin, J. Phys. Chem. 101, 338 ͑1997͒.
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
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