Competing pathways in the infrared multiphoton dissociation of hexafluoropropene

Article abstract of DOI:10.1021/jp960653v

The infrared multiphoton dissociation of hexafluoropropene was studied by photofragment translational spectroscopy. Two primary channels and one secondary channel were identified. The predominant primary channel produces CF₃CF or C₂F₄ and CF₂, with the heavier species undergoing further dissociation to two CF₂ fragments. A number of dissociation mechanisms are proposed for the elimination of CF₂, including direct cleavage of the carbon - carbon double bond. In the second primary channel, a simple bond rupture reaction produces CF₃ and C₂F₃. As expected, the translational energy distribution for this channel peaks near zero, indicating no exit barrier is present. The activation energy for this simple bond rupture is estimated to be 100-105 kcal/mol. The branching ratio, [CF₂]/[CF₃], between the two primary pathways is 4.0 ± 1.0.

Full text of DOI:10.1021/jp960653v

338
J. Phys. Chem. A 1997, 101, 338-344
ARTICLES
Competing Pathways in the Infrared Multiphoton Dissociation of Hexafluoropropene
C. A. Longfellow, L. A. Smoliar, and Y. T. Lee*
Chemical Sciences DiVision, Lawrence Berkeley Laboratory and Department of Chemistry,
UniVersity of California, Berkeley, California 94720
Y. R. Lee, C. Y. Yeh, and S. M. Lin
Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan
ReceiVed: February 29, 1996X
The infrared multiphoton dissociation of hexafluoropropene was studied by photofragment translational
spectroscopy. Two primary channels and one secondary channel were identified. The predominant primary
channel produces CF3CF or C2F4 and CF2, with the heavier species undergoing further dissociation to two
CF2 fragments. A number of dissociation mechanisms are proposed for the elimination of CF2, including
direct cleavage of the carbon-carbon double bond. In the second primary channel, a simple bond rupture
reaction produces CF3 and C2F3. As expected, the translational energy distribution for this channel peaks
near zero, indicating no exit barrier is present. The activation energy for this simple bond rupture is estimated
to be 100-105 kcal/mol. The branching ratio, [CF2]/[CF3], between the two primary pathways is 4.0 ( 1.0.
I. Introduction
Reaction 3 has also been suggested in mercury-sensitized
photolysis5 and flash photolysis6 studies of hexafluoropropene.
Nevertheless, the prediction of a barrierless reaction from the
adiabatic compression studies is somewhat surprising as ex-
perimental7 and theoretical8 studies on fluorine atom shifts in
CF3CH have found barriers greater than 20 kcal/mol.
With the widespread availability of high-power CO2 lasers,
IRMPD studies have become a practical alternative to thermal
studies. The possibility of exciting the C-F stretch on the
central carbon of hexafluoropropene at 1037 cm-1 makes this
compound a viable candidate for such infrared multiphoton
pumping.9,10 In a previous IRMPD experiment, the products
C2F4 and C2F6 were identified.10 The production of C2F4 is
postulated to result from reaction 1 as well as from the
recombination of CF2 radicals. One possible mechanism used
to explain the presence of C2F6 is a fluorine abstraction, reaction
4, followed by recombination (5).
The decomposition of hexafluoropropene has been previously
investigated using both thermal and infrared multiphoton
dissociation (IRMPD) techniques. A single primary reaction
(1) was proposed on the basis of a thermal decomposition
experiment, although neither of the products, tetrafluoroethylene
or difluorocarbene, was directly observed.1
C3F6 f C2F4 + CF2
(1)
An activation energy of 75 kcal/mol was estimated for reaction
1; however, because of the circuitous method used to obtain
this value, it is listed as highly questionable in a compilation
of gas kinetic data.2 In a later investigation, perfluoroisobutene,
perfluoro-1-butene, and perfluoro-2-butene were identified as
pyrolysis products of hexafluoropropene, highlighting the
extensive role of recombination in this reaction.3
In a more recent study, a free-piston adiabatic compression
setup was used to decompose hexafluoropropene.4 In the initial
compression stages the only product identified was tetrafluo-
roethylene, and an activation energy of 82.7 ( 1 kcal/mol was
obtained for reaction 1. From their experiment Buravtsev et
al. predict that the precursor to tetrafluoroethylene (C2F4) is
trifluoromethylfluorocarbene (CF3CF), which initially forms in
the dissociation of hexafluoropropene (2).
CF2 + CF2 f CF3 + CF
CF3 + CF3 f C2F6
(4)
(5)
In another IRMPD study, the major products, C2F4 and C2F6,
were identified by their infrared absorption spectra.11 The
fluence dependence of the yield of the products was further
probed as discussed in a subsequent paper.12 Higher fluence
favors formation of C2F6, but with prolonged irradiation C2F6
and C2F4 production decreases. The authors suggested that
reactions 1, 4, and 5 cannot completely describe the hexafluo-
ropropene dissociation mechanism. Upon further examination
of the infrared spectra, absorption lines attributable to poly-
(tetrafluoroethylene) were identified. It is hypothesized that the
incorporation of CF into poly(tetrafluoroethylene) (-CF2-CF2)
does not change the absorption spectra significantly and can
explain the eventual fate of the CF radicals from reaction 4.
As indicated by the experiments carried out so far, the results
of the decomposition experiments of hexafluoropropene are
C3F6 f CF3CF + CF2
(2)
A subsequent 1,2-fluorine atom shift (reaction 3) was suggested
to take place without a barrier, with the carbene species 17 (
1.5 kcal/mol higher in energy than tetrafluoroethylene.
CF3CF f C2F4
(3)
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5639(96)00653-6 CCC: $14.00 © 1997 American Chemical Society

IR Multiphoton Dissociation of Hexafluoropropene
J. Phys. Chem. A, Vol. 101, No. 4, 1997 339
difficult to interpret. This is because of the multiple collisions
that take place after the initial unimolecular decomposition,
obscuring the primary decomposition pathways. Besides the
primary reactions already discussed (1, 2), rupture of the
carbon-carbon single bond may be possible (6).
C3F6 f CF3 + C2F3
(6)
Because molecular beam techniques allow for the direct
detection of the primary products in a unimolecular reaction,
the present study using photofragment translational spectros-
copy13 coupled with IRMPD was undertaken.
In addition to the identification of the primary products,
photofragment translational spectroscopy yields insight into the
dissociation dynamics of a reaction through measurement of
the translational energy release of the products. The observed
translational energy distributions in hexafluoropropene decom-
position may facilitate understanding of the CF2 loss reaction.
Although cleavage of a carbon-carbon double bond seems
unusual, it is not unprecedented. In the 193 nm dissociation of
tetrafluoroethylene, formation of two CF2 fragments occurred
via double-bond cleavage (7).14
C2F4 f 2CF2
(7)
In that case, a large translational energy release, peaked well
away from zero, was observed as well as a polarization
dependence, indicating dissociation from an excited state. It
will be informative to compare the translational energy distribu-
tions and therefore the dynamics of these two systems, as the
IRMPD of hexafluoropropene results in rupture of the double
bond from the ground electronic state.
Figure 1. Experimental evidence for reaction 1. (a) Time-of-flight
spectrum for m/e ) 100 at 20°. The open circles represent data points,
and the solid line is the fit to the data using the forward convolution
method. (b) The center-of-mass translational energy distribution derived
from m/e ) 100 is represented by the open squares. Due to the
secondary dissociation of m/e ) 100, this distribution is biased toward
molecules with greater translational energy. The open diamonds
represent the translational energy distribution derived from the primary
m/e ) 50 fragment in Figure 4b, while the open circles are derived
from Figure 4c. See text for details.
II. Experiment
These experiments were carried out at the Institute of Atomic
and Molecular Sciences in Taiwan,15 and the features of the
rotating source molecular beam machine have been previously
described.16 Briefly, a mixture of 5% C3F6 in helium was passed
through a solenoid-type pulsed valve (General Valve, Series 9)
with a 0.020 in. nozzle, operating at room temperature with a
typical stagnation pressure of 1 atm. The supersonic expansion
of hexafluoropropene was characterized by standard time-of-
flight techniques with a spinning slotted wheel, and a mean
velocity of 900 m/s with a spread of ∼12% was found. The
molecular beam was collimated with two skimmers, resulting
in an angular divergence of slightly less than 3°. A Lambda
Physik EMG 202 pulsed CO2 laser was tuned to the P(26) line
of the 9.6 µm branch (1041 cm-1) and crossed the molecular
beam at right angles in the interaction region. The laser beam
was focused to a 1.5 × 2 mm2 spot using a 1 in. ZnSe lens
with a 25 cm focal length, resulting in a fluence of ∼10 J/cm2.
The fragments created by IRMPD traveled 36.7 cm to the
detector that consisted of an electron impact ionizer, quadrupole
mass filter, and Daly type ion detector.17 A multichannel scaler
triggered by the laser collected the ion counts as a function of
the flight time from the interaction region to the detector.
presence of a second primary channel (6). In addition, m/e )
50 shows evidence of a secondary dissociation channel.
The resulting data were analyzed using standard forward
convolution techniques.18 A center-of-mass translational energy
distribution is assumed for each channel, and the time-of-flight
spectrum generated is averaged over apparatus functions, such
as the ionizer width. This spectrum is then compared to the
experimental time-of-flight spectrum, and the translational
energy distribution is modified until the two match. In principle,
for each dissociation channel it is necessary to measure only
one of the dissociating fragments to obtain the center-of-mass
translational energy. In practice, the time-of-flight spectra of
all fragments are measured, and the conservation of linear
momentum requirement in the center-of-mass system is used
to ensure that the dissociation products are assigned to the
correct channel.
A. Primary and Secondary Reactions. 1. C3F6 f C2F4
+ CF2. The time-of-flight spectrum of m/e ) 100 is shown in
Figure 1a. This confirms the unimolecular dissociation of
hexafluoropropene by either reaction 1 or reaction 2 under
collisionless conditions. The corresponding momentum matched
partner, m/e ) 50, will be discussed later. The translational
energy distribution is derived from m/e ) 100 time-of-flight
spectra at 20°, 30°, and 40°. This distribution, which is peaked
away from zero, is shown in Figure 1b. The average transla-
tional energy release is 13.3 kcal/mol.
III. Results and Analysis
Measurements were taken at source to detector angles of 15°,
20°, 30°, and 40° and laser-correlated dissociation signal was
observed at m/e ratios 100 (C2F4+ or CFCF3+), 81 (C2F3+), 69
(CF3+), 62 (C2F2+), 50 (CF2+), and 31 (CF+). The signal at
m/e ) 100 is unambiguous evidence for the CF2 loss channel.
For the time being, this reaction will be referred to as reaction
1. As will be shown later, the laser-correlated signal at m/e )
81 is notably broader than that at m/e ) 100, indicating the
2. C3F6 f CF3 + C2F3. As mentioned above, the signal
observed at m/e ) 81 (C2F3+) could not be explained by
assuming the only contribution was fragmentation of m/e ) 100
in the electron impact ionizer. The discrepancy in the fit occurs
at longer times, indicating the contribution of another channel
with little translational energy. The differences between the

340 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Longfellow et al.
Figure 3. Time-of-flight spectra for m/e ) 69 and 62 at 20°. (a) The
trifluoromethyl fragment shows a large contribution from reaction 6
as indicated by the dashed line. It is the momentum matched partner
of m/e ) 81 shown in Figure 2a. A fast contribution, shown with the
solid line, from the fragmentation of m/e ) 100 in the electron impact
ionizer is possible but not significant. (b) At m/e ) 62 the dashed line
indicates fragmentation from m/e ) 81 while the solid line is
fragmentation from m/e ) 100. CFCF is not formed in any primary
processes.
Figure 2. Evidence for the simple bond rupture reaction. (a) Time-
of-flight spectrum for m/e ) 81 at 20°. The solid line represents
+
the m/e ) 100 species that fragments in the ionizer to C2F3 while
the dashed line represents the contribution of reaction 6 at m/e )
81. (b) The translational energy distribution of the products of reaction
6. The cross-hatched area represents the uncertainty associated with
this measurement. The solid and dotted lines are the result of the
IRMPD modeling calculations and are further explained in part B of
section III.
Figure 5 illustrates the range of these two secondary translational
energy distributions. The distributions are both peaked away
from zero, near 5 kcal/mol, while the average translational
energy release ranges from 5.6 to 7.2 kcal/mol, respectively.
As a consequence of the secondary dissociation of the C2F4/
CF3CF species, the primary translational energy distribution for
reaction 1 cannot be obtained from the m/e ) 100 time-of-flight
spectrum. The translational energy distribution derived from
m/e ) 100, shown in Figure 1b, is biased toward faster
molecules with less internal energy since they do not undergo
as much secondary decomposition. In other molecules, the
primary translational energy distribution could be obtained by
observing the corresponding momentum matched fragment.
However, in hexafluoropropene the signal from CF2 produced
in the primary process cannot be separated from the secondary
decomposition signal, which also results in CF2. A comparison
of the translational energy distributions obtained from the m/e
) 100 time-of-flight spectrum (Figure 1a) and those obtained
from the m/e ) 50 primary dissociation signal (Figure 4b,c) is
shown in Figure 1b. The difference between the m/e ) 100
and 50 distributions was used as the primary translational energy
distribution for the secondary dissociation products in both cases
as previously discussed.13a
The method for calculating the experimental branching ratio
between the two primary reactions has been discussed in detail
earlier.19 The branching ratio between reactions 1 and 6 was
only determined at the maximum attainable fluence, ∼10 J/cm2,
owing to limitations in detector sensitivity at lower fluences.
The relative contribution from each primary fragment, C2F4,
C2F3, CF3 and CF2, at each m/e ratio was determined. In the
case of secondary dissociation, the contribution at m/e ) 50
and 31 was included in the C2F4 yield, taking into account that
each m/e ) 100 fragment produces two CF2 fragments. The
m/e ) 81 and the m/e ) 100 spectra can be explained by
assuming a second primary channel involving CF3 loss. The
time-of-flight spectrum for m/e ) 81 at 20° is shown in Figure
2a along with the corresponding translational energy distribution
(Figure 2b). As expected, the translational energy distribution
for the simple bond rupture reaction peaks near zero with a low
average translational energy release. Further evidence of this
slow channel is apparent in the time-of-flight spectra at m/e )
69, CF3+, and m/e ) 62, CFCF+ (Figure 3). The signal
observed at these masses cannot be explained without consider-
ing reaction 6.
3. C2F4/CF3CF f 2CF2. As fluorocarbons readily fragment
in the electron impact ionizer, contributions from many higher
molecular weight products are found in the lower m/e spectra.
However, there is a portion of the m/e ) 50 and 31 time-of-
flight spectra that cannot be explained by the two primary
reactions discussed above. Since the time-of-flight spectrum
at m/e ) 31 results solely from fragmentation of m/e ) 50 giving
no new information, we will focus on the m/e ) 50 spectrum.
In Figure 4a it is evident that the contributions from fragmenta-
tion of m/e ) 100, 81, and 69 are not fast enough to fully explain
the time-of-flight spectrum observed. CF2 is also a primary
product from reaction 1, and its contribution is illustrated in
Figure 4a. It is constrained to be momentum matched to m/e
) 100, and it is too fast to explain the additional signal observed.
The secondary dissociation of the m/e ) 100 species to form
two CF2 fragments seems to be the only viable explanation.
Determining the extent of secondary dissociation in hexafluo-
ropropene is complicated by the overlapping signal of the
primary and secondary reactions at m/e ) 50. Fragment 4b
illustrates one limiting case in which the secondary dissociaiton
products have the minimum possible translational energy, while
Figure 4c is a fit with faster secondary dissociation products.

IR Multiphoton Dissociation of Hexafluoropropene
J. Phys. Chem. A, Vol. 101, No. 4, 1997 341
in our group to calculate dissociation barriers in situations where
a simple bond rupture and a concerted reaction compete.13 In
order to determine a dissociation barrier for reaction “A”, it is
necessary to know the activation energy for reaction “B”, the
experimental branching ratio, and the simple bond rupture
translational energy distribution. A program that models the
competition between absorption, stimulated emission, and
dissociation is used to obtain the population created by the laser
and the yield of each channel.22 RRKM translational energy
distributions at each energy level above dissociation are
weighted by the population distribution of the excited parent
and summed to create the overall translational energy distribu-
tion, which is compared to the experiment.23 This iterative
process entails modifying the quasi-continuum cross sections
using different barrier heights until the experimental branching
ratio and translational energy distribution are reproduced.
The dissociation rate constants and translational energy
distributions for hexafluoropropene were determined using a
readily available RRKM program.24 The ground state vibra-
tional frequencies necessary for the RRKM calculations were
obtained from the literature.9b The transition state frequencies
were assumed to be similar to the ground state and then varied
to reproduce the preexponential A factor. For reaction 1 an A
factor of 13.0 was utilized,1,2 while for reaction 6 a typical A
factor for fluorocarbons undergoing simple bond rupture of 16.1
was assumed.25 Table 1 lists the relevant RRKM parameters.
To predict the population created by infrared multiphonon
excitation, a laser pulse consisting of a 100 ns spike followed
by a 1 µs tail was used.26 We assumed the spike contained
70% of the total available energy as has been reported for CO2
laser pulses.27
Two values have been measured for the activation energy of
reaction 1 (751,2 and 82.74 kcal/mol). In Figure 2b, the dotted
line represents the best fit using an activation energy of 75 kcal/
mol for reaction 1; a barrier height of 100 kcal/mol was obtained
for reaction 6. The solid line represents a second calculation
using 82.7 kcal/mol as the activation energy for CF2 elimination.
In this instance a barrier height of at least 105 kcal/mol is
necessary to reproduce the experimental translational energy
distribution. There is a large uncertainty in assigning an
activation barrier to reaction 6 owing to the uncertainty in the
value of the activation energy for reaction 1. In addition, the
range of translational energy distributions that can be used to
fit reaction 6 is large, as seen by the cross-hatched area in Figure
2b. At best, we can estimate that the barrier height for simple
bond rupture of hexafluoropropene is 100-105 kcal/mol.
Figure 4. Time-of-flight spectra for m/e ) 50 at 20°. (a) Contributions
from m/e ) 100 (solid line), m/e ) 81 (long dashed line), m/e ) 69
(short dashed line), and m/e ) 50 (dotted line) cannot completely
explain the signal observed at this mass. (b) The dash-dot-dash line
represents the slowest possible contribution from secondary decomposi-
tion. The corresponding secondary translational energy distribution is
shown in Figure 5. (c) In this representation the secondary dissociation
(dash-dot-dash line) is as fast as possible while retaining a significant
contribution of primary m/e ) 50. The translational energy distribution
in this case is also shown in Figure 5.
IV. Discussion
There is clear experimental evidence for reactions 1 and 6 as
well as a secondary dissociation reaction in the IRMPD of
hexafluoropropene. The formation of CF3 from reaction 6 can
explain the presence of C2F6 in earlier IRMPD studies10-12 as
recombination of the trifluoromethyl radicals is possible. The
secondary dissociation reaction highlights the reactivity of C3F6,
which may explain the extensive polymerization seen in previous
experiments.1,3,4,10-12 In the following paragraphs, we discuss
possible reaction mechanisms for reaction 1 and the identity of
the heavier species which undergoes secondary decomposition.
Difluorocarbene Loss. The mechanism by which CF2 is
formed in the dissociation of hexafluoropropene is not well
understood. Although tetrafluoroethylene has been detected in
a number of IRMPD and thermal experiments, there is still
uncertainty as to whether it is formed in the primary decomposi-
tion step of hexafluoropropene. The adiabatic compression
studies suggest that CFCF3 is formed intially and then isomerizes
to tetrafluoroethylene.4 The consensus of the IRMPD studies
Figure 5. Limiting translational energy distributions for the secondary
dissociation of m/e ) 100. These distributions are derived from the
dash-dot-dash lines in Figure 4b,c.
contributions of fragmentation at lower masses could not be
quantified, which adds to the overall uncertainty. The branching
ratio between reactions 1 and 6, [CF2]/[CF3], was found to be
4.0 ( 1.0.
B. Using RRKM Theory To Obtain a Simple Bond
Rupture Activation Energy. RRKM theory is often used to
calculate dissociation rate constants for unimolecular reactions.20
In the case of a simple bond rupture reaction without an exit
barrier, one can predict the translational energy distribution
based on the total available energy.21 The resulting translational
energy distribution is typically peaked at zero and decays
exponentially. An extension of RRKM theory has been used

342 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Longfellow et al.
TABLE 1: Parameters Used in the RRKM Calculations
vib freq (cm-1
critical configurationa
)
vib freq (cm-1
)
description
molecule
CF3 loss
CF2 loss
1797
1399
1333
1211
1179
1122
1037
767
CdC stretch
C-F stretch, CF3
assym CF2
C-F stretch, CF3
C-F stretch, CF3
C-F stretch
C-F stretch
C-C stretch
CF2 def
1797
1399
1333
1211
1179
1122
1037
767
1797
1399
1333
1211
1179
1122
1037
rxn coor
655
rxn coor
1399
1333
1211
1179
1122
1037
767
655
655
655
609
sym CF3 def
asym CF3 def
CF2 rock
609
100
609
559 × 2
559 × 2
100 × 2
559 × 2
513
513
513
513
462
370
CF2 wag
CF wag
462
370
462
370
462
370
364
C-C-C def
364
100
364
250 × 2
C-F rock
250 × 2
250 × 2
250 × 2
171
CF2 twist
171
171
700
134
94
CF3 twist
CF3 rock
not used
94
not used
94
not used
94
reduced moment of inertia for internal
rotations (amu Å2)b
79
79
79
external moments of inertia (amu Å2)c
energy threshold (kcal/mol)
calculated value log A
198, 403, 512
varied
16.1
75, 82.7
13.0
a The transition state frequencies in bold were modified to reproduce the preexponential A factor. b The CF3 twist was treated as an internal
rotation. See, for example: Gordy, W.; Cook, R. L. MicrowaVe Molecular Spectra, 3rd ed.; Wiley: New York, 1984; p 574. c The external
rotations were obtained from the rotational constants in: Jacob, E. J.; Lide, D. R. J. Chem. Phys. 1973, 59, 5877.
Figure 6. Energy level diagram for hexafluoropropene illustrating the
observed dissociation pathways. The heats of formation at 298 K were
obtained from the following sources: C3F6, -268.9 ( 2 kcal/mol, ref
29; C2F4, -157.4 ( .7 kcal/mol, J. Phys. Chem. Ref. Data 1985, 14,
(Suppl. 1), 655; CFCF3, -140.4 ( 2 kcal/mol, ref 4; 1CF2, -44.2 ( 1
kcal/mol, ref 29. The dashed line illustrates the three competing
pathways. No barrier in the formation of C2F4 from CFCF3 is shown
as suggested from ref 4. The activation energy for reaction 6, CF3 +
C2F3, is an estimate from IRMPD modeling.
Figure 7. Four dissociation mechanisms for the elimination of CF2
are illustrated: (a) isomerization to hexafluorocyclopropane occurs prior
to dissociation; (b) a diradical, CF2CF2CF2, is formed by fluorine
migration; (c) a concerted mechanism in which fluorine migration and
tetrafluoroethylene formation occur simultaneously; (d) direct cleavage
of the double bond occurs as the carbon-carbon double bond elongates.
is that tetrafluoroethylene is generated directly from hexafluo-
ropropene; however, no mechanism is given.10-12 Benson
suggests that an intermediate, cyclohexafluoropropane, proceeds
tetrafluoroethylene formation.2 Another intermediate that could
be involved is the diradical CF2CF2CF2; the presence of its
hydrocarbon analog, trimethylene, has been predicted in the
isomerization from cyclopropane to propene.28
By examining the possible dissociation pathways and the
reaction dynamics, it may be possible to eliminate some
mechanisms based on the observed translational energy distribu-
tion. The formation of the cyclic isomer, cyclohexafluoropro-
pane, is energetically possible as it lies only 35.1 kcal/mol above
the ground state of hexafluoropropene.29 Direct dissociation
of cyclohexafluoropropane should result in the expulsion of CF2
as two single bonds are broken while a double-bond closed-
shell species (tetrafluoroethylene) is formed (Figure 7a). This
repulsion would result in a translational energy distribution
peaked away from zero. If dissociation occurred from the
diradical species, CF2CF2CF2, the transition state might be
expected to look like that of a simple bond rupture with one of
the carbon-carbon single bonds stretching until two distinct
species are formed (Figure 7b). Although some electronic
rearrangement would be necessary to form tetrafluoroethylene,
the translational energy distribution should peak at or near zero.
An important caveat is that the CF2CF2CF2 diradical may not
be a distinct transition state; a concerted mechanism whereby a
fluorine migrates as tetrafluoroethylene forms is plausible
(Figure 7c). This concerted reaction might be expected to have
an exit barrier which would result in a translational energy
distribution peaked away from zero. On the other hand, if the
products CF2 and CFCF3 are formed, they could result from
direct cleavage of the carbon-carbon double bond (Figure 7d).
One might initially think that the stretching of this bond to form
two carbenes would result in a translational energy distribution
similar to that for a simple bond rupture reaction. However, if
we assume that C3F6 behaves in a manner similar to C2F4, the
application of the Woodward-Hoffman rules predicts a barrier
to the formation of the parent from two singlet species.30 The
unusual stability of singlet CF2, owing to the large electroneg-
ativity and lone pairs on the fluorine atom,31 results in a singlet-

IR Multiphoton Dissociation of Hexafluoropropene
J. Phys. Chem. A, Vol. 101, No. 4, 1997 343
triplet splitting of 56.6 kcal/mol.32 On the other hand, the
singlet-triplet splitting for CFCF3 has been calculated to be
only 9.2 kcal/mol.33 These ground state singlet species, CF2
and CFCF3, have no open-shell electrons and therefore require
energy for the excitation of each species in order to form
covalent bonds.31 The energy released from electron pairing
to form the two singlet species in the reverse reaction of C3F6
dissociation would result in a translational energy distribution
peaked away from zero.
activation energy for a 1,2 shifts increases in the following
manner: Cl < H < alkyl < F.7 Although calculations8 and
experiments7 on 1CF3CH indicate a barrier greater than 20 kcal/
mol for fluorine migration, it could occur as suggested by
Buravtsev et al.4 As discussed earlier, the formation of two
singlet species in the cleavage of a double bond is likely to
result in a translational energy distribution peaked away from
zero. Preliminary results from the IRMPD of octafluorocy-
clobutane show that the C2F4 produced in the primary reaction
dissociates further to CF2.38 The translational energy distribution
for CF2 formation is peaked away from zero at ∼4.5 kcal/mol
and extends to 20 kcal/mol, which is similar to both our
experiment and that of Yokoyama and co-workers. Although
the same species appears to be undergoing secondary dissocia-
tion in all three experiments, it remains unknown whether
dissociation occurs from C2F4, CFCF3, or an intermediate
species. It is not possible in this situation to determine the
identity of the dissociation product based solely on the observed
translational energy distribution.
The range of the primary translational energy distribution for
the formation of CF2 and its momentum matched partner is
represnted in Figure 1b. The uncertainty in this distribution,
as discussed earlier, lies in our inability to separate CF2 formed
in the primary step from that produced in the secondary
dissociation reaction. This distribution does peak away from
zero to a maximum of 10 kcal/mol and extends to 30 kcal/mol,
which eliminates the involvement of the diradical (Figure 7b)
as an intermediate. The isomerization of hexafluoropropene to
perfluorocyclopropane, the concerted fluorine migration, or
cleavage of the double bond could all result in the observed
primary translational energy distribution. Although the barrier
from hexafluoropropene to perfluorocyclopropane is estimated
to be greater than 90 kcal/mol,34 the IRMPD/RRKM calculations
suggest that the excited fluorocarbon contains at least 100-
105 kcal/mol, which may be enough for this isomerization to
take place.
V. Conclusion
Two primary pathways, CF3 loss and CF2 loss, have been
observed in the IRMPD of hexafluoropropene. The loss of CF3
has not been previously observed in the unimolecular decom-
position of this molecule and may explain the observation of
C2F6 in bulk experiments. Modeling the dissociation with a
well-known RRKM/IRMPD model gives an activation energy
of 100-105 kcal/mol for this simple bond rupture reaction. CF2
loss was seen to be the predominant channel, accounting for
80% of the products, with significant secondary dissociation of
the heavier fragment producing additional CF2.
The possibility of isomerizations (Figure 7a) or fluorine
migrations (Figure 7c) cannot be definitively ruled out in the
IRMPD of hexafluoropropene. In both the IRMPD of hexafluo-
ropropene and the UV photolysis of tetrafluoroethylene,14 the
translational energy distributions peak away from zero in the
reaction which destroys the double bond, but the dynamics are
not similar. In the case of tetrafluoroethylene photodissociation
at 193 nm, the cleavage of the carbon-carbon double bond
occurs on a short (picosecond) time scale as indicated by the
slight polarization dependence (â ) -0.2). In the IRMPD of
hexafluoropropene, where the dissociation occurs on the nano-
second or longer time scale, it is unclear whether direct cleavage
of the double bond is the mechanism that takes place.
Acknowledgment. C.A.L. thanks Dr. T. T. Miau and Dr.
A. G. Suits for many helpful discussions. The hexafluoropro-
pene was kindly supplied by Dr. M. H. Hung at Du Pont. This
work was supported by the Director, Office of Energy Research,
Office of Basic Energy Sciences, Chemical Sciences Division
of the U.S. Department of Energy, under Contract DE-AC03-
76SF00098. Additional funding for this project was provided
by Du Pont.
Secondary Dissociation. The primary product, CFCF3 or
C2F4, undergoes further dissociation to produce two difluoro-
carbene species. The translational energy distribution from the
secondary dissociation of hexafluoropropene (Figure 5) peaks
near 5 kcal/mol and extends to ∼16 kcal/mol. A similar
translational energy distribution is observed in the IRMPD of
2-chloro-1,1,1,2-tetrafluoroethane.35 The complementary frag-
ment in the elimination of HCl is CF3CF, and the secondary
dissociation of this fragment results in a translational energy
distribution peaked at 3 kcal/mol and extending to ∼20 kcal/
mol. These two very similar distributions indicate that the same
dissociation mechanisms occur in both hexafluoropropene and
2-chloro-1,1,1,2-tetrafluoroethane. One pathway, suggested by
Yokoyama and co-workers, is that trifluoromethylfluorocarbene
directly undergoes a three-centered concerted dissociation
reaction to form two CF2 carbenes. This is the reverse reaction
of CF2 insertion into the CF bond of CF2, and typically insertion
reactions of carbenes with singlet ground states such as CF2
will have barriers.36 This entrance barrier translates into an exit
barrier for CFCF3 dissociation and will lead to a translational
energy distribution peaked away from zero as observed.
References and Notes
(1) Atkinson, B.; Trenwith, A. B. J. Chem. Soc. 1957, 2082.
(2) Benson, S. W.; O’Neal, H. E. Kinetic Data on Gas Phase
Unimolecular Reactions; National Bureau of Standards, U.S. Government
Printing Office: Washington, DC 1970; p 493.
(3) Matula, R. A. J. Phys. Chem. 1968, 72, 3054.
(4) Buravtsev, N. N.; Grigor’ev, A. S.; Kolbanovskii, Y. A. Kinet.
Catal. 1989, 30, 13.
(5) Heicklen, J.; Knight, V. J. Phys. Chem. 1965, 69, 3600.
(6) Dalby, F. W. J. Chem. Phys. 1964, 41, 2297.
(7) Holmes, B. E.; Rakestraw, D. J. J. Phys. Chem. 1992, 96, 2210.
(8) So, S. P. J. Phys. Chem. 1993, 97, 11908.
(9) (a) Edgell, W. F. J. Am. Chem. Soc. 1948, 70, 2816. (b) Nielsen,
J. R.; Classen, H. H.; Smith, D. C. J. Chem. Phys. 1952, 20, 1916.
(10) Nip, W. S.; Drouin, M.; Hackett, P. A.; Willis, C. J. Phys. Chem.
1980, 84, 932.
(11) Santos, M.; Siguenza, C.; Torresano, J. A.; Gonzalez-Diaz, P. F.
Spectrochim. Acta 1990, 46A, 455. Siguenza, C.; Santos, M.; Torresano,
J. A.; Pino, A.; Gonzalez-Diaz, P. F. Spectrochim. Acta 1990, 46A, 1499.
(12) Torresano, J. A.; Santos, M.; Gonzalez-Diaz, P. F. Laser Chem.
1994, 14, 217.
(13) (a) Hintsa, E. J.; Wodtke, A. M.; Lee, Y. T. J. Phys. Chem. 1988,
92, 5379. (b) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. J. Phys. Chem.
1986, 90, 3549. (c) Wodtke, A. M.; Lee, Y. T. In AdVances in Gas Phase
Photochemistry and Kinetics; Ashfold, M. N. R., Baggot, J. E., Eds.; Royal
Society of Chemistry: London, 1987.
(14) Minton, T. K.; Felder, P.; Scales, R. C.; Huber, J. R. Chem. Phys.
Lett. 1989, 164, 113.
A 1,2-fluorine atom shift to tetrafluoroethylene followed by
dissociation could also produce the CF2 observed in the
secondary dissociation of hexafluoropropene and 2-chloro-
1,1,1,2-tetrafluoroethane. In the analogous hydrocarbon system,
1CH3CH is predicted to have only a 0.6 kcal/mol barrier to the
formation of ethylene via a 1,2 H shift.37 In general, the
(15) Lee, Y. R.; Chen, F. T.; Hsueh, C. C.; Chou, H. M.; Chen, H. C.;
Yang, Y. J.; Lin, S. M. Proc. Natl. Sci. Counc., Repub. China, Part A 1994,
18, 55.
(16) Wodtke, A. M.; Lee, Y. T. J. Phys. Chem. 1985, 89, 4744.

344 J. Phys. Chem. A, Vol. 101, No. 4, 1997
Longfellow et al.
(17) Lee, Y. T.; McDonald, J. D.; LeBreton, P. R.; Herschbach, D. R.
ReV. Sci. Instrum. 1969, 40, 1402.
(18) Wodtke, A. M. Ph.D. Thesis, University of California at Berkeley,
1986. Zhao, X. Ph.D. Thesis, University of California at Berkeley, 1988.
(19) Myers, J. D. Ph.D. Thesis, University of California at Berkeley,
1993.
(20) Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley:
London, 1972. Forst, W. Theory of Unimolecular Reactions; Academic
Press: New York, 1973.
(21) Safron, S. A.; Weinstein, N. D.; Herschbach, D. R.; Tully, J. C.
Chem. Phys. Lett. 1972, 12, 564.
(22) This program was first reported by: Schulz, P. A.; Sudbo, As. S.;
Grant, E. R.; Shen, Y. R.; Lee, Y. T. J. Chem. Phys. 1980, 72, 4495.
(23) Butler, L. J.; Buss, R. J.; Brudzynski, R. J.; Lee, Y. T. J. Phys.
Chem. 1983, 87, 5106.
(24) Hase, W. L.; Bunker, D. L. Quantum Chemistry Program Exchange,
Indiana University, Prog. QCPE 234.
(25) Tschuikow-Roux, E. J. Chem. Phys. 1965, 43, 2251. Kato, S.;
Makide, Y.; Tominaga, T.; Takeuchi, K. J. Phys. Chem. 1987, 91, 4278.
(26) Chowdhury, P. K.; Rama Rao, D. V. S.; Mittal, J. P. J. Phys. Chem.
1988, 92, 102.
(27) Grimley, A. J.; Stephenson, J. C. J. Chem. Phys. 1981, 74, 447.
Stephenson, J. C.; King, D. S. J. Chem. Phys. 1978, 69, 1485. Beaulieu,
J. A. Proc. IEEE 1971, 59, 667.
(28) Bergmen, R. J. In Free Radicals; Kochi, J. K., Ed.; Wiley: New
York, 1973; Vol. 1, Chapter 5.
(29) Bomse, D. S.; Berman, D. W.; Beauchamp, J. L. J. Am. Chem.
Soc. 1981, 103, 3967.
(30) The orbital and state correlation diagrams for the formation of C2F4
from two singlet species can be drawn and indicate a barrier exists between
reactants and products. Woodward, R. B.; Hoffmann, R. The ConserVation
of Orbital Symmetry; Academic Press: New York, 1970. Atkins, P. W.
Molecular Quantum Mechanics, 2nd ed.; Oxford University Press: New
York, 1983; pp 336-345.
(31) Carter, E. A.; Goddard, W. A. J. Phys. Chem. 1986, 90, 998.
(32) Koda, S. Chem. Phys. Lett. 1978, 55, 353; Chem. Phys. 1982, 66,
383.
(33) Dixon, D. A. J. Phys. Chem. 1986, 90, 54.
(34) Berman, D. W.; Bomse, D. S.; Beauchamp, J. L. Int. J. Mass
Spectrom. Ion Phys. 1981, 39, 263.
(35) Yokoyama, A.; Yokoyama, K.; Fujisawa, G. J. Chem. Phys. 1994,
100, 6487.
(36) Bach, R. D.; Su, M.-D.; Aldabbagh, E.; Andres, J. L.; Schlegel, H.
B. J. Am. Chem. Soc. 1993, 115, 10237. Haszeldine, R. N.; Parkinson,
C.; Robinson, P. J.; Williams, W. J. J. Chem. Soc., Perkin Trans. 2 1979,
954.
(37) Evanseck, K. D.; Houk, K. N. J. Phys. Chem. 1990, 94, 5518.
(38) Lee, Y. R.; Lin, S. M. To be published.