G Model
FLUOR-8173; No. of Pages 4
2
R.J. Waltman / Journal of Fluorine Chemistry xxx (2013) xxx–xxx
to any possible side reactions in the gas phase or is possibly due to
the accuracy of the experimentally determined G values. The gas
phase irradiation of COF has been observed to produce CO with a
G value of 0.2 [7]. We also note that the CF H impurity (1147 cm
2
2
ꢀ1
3
)
decomposes more efficiently in the gas phase than at low
temperature, perhaps contributing to the overall electron beam
chemistry.
An approximately 1:1 concentration ratio is also observed for
the formation of COF
argon matrix. In Fig. 3 the change in the number of CF
molecules decomposed and COF molecules formed per matrix
2
when CF
3
OCF
3
is decomposed in a solid
3
OCF
3
2
film thickness is plotted as a function of incident charge density.
Due to the number of overlapping absorption bands in the CF
(ꢁ1250–1230 cmꢀ ), the band
1
stretching region, including COF
due to CF
2
4
could not be similarly plotted. The results that are
shown in Fig. 3 indicate that for each molecule of CF
decomposed, approximately one molecule of COF is formed.
The energetics for the decomposition of PFDME to COF and CF
3 3
OCF that is
2
2
4
are considered below. As discussed in a previous publication on
high energy electron beam irradiation of matrix isolated systems,
the incident electron beam almost exclusively excites rare gas
atoms to most likely produce Wannier excitons that migrate in the
argon matrix [10]. This is a direct consequence of the high
concentration of rare gas matrix atoms to molecular guests (700:1,
respectively, in Fig. 1). If the excitons in the solid argon matrix
transfer their energy to a matrix isolated molecule, about 12 eV is
3 3
Fig. 1. The infrared spectrum of PFDME (CF OCF ) isolated in Ar (1:700) at T = 10 K
2
before and after 25 kV electron beam exposure: (top) 0
m
C/cm ; (middle) 38
cm ; and (bottom) difference spectrum. The bands attributed to the gas impurities
CF H and CO are identified in the top figure. The difference spectrum identifies the
mC/
2
3
2
bands which grow and decay as a result of the irradiation.
3 3 3 3
deposited into CF OCF . Since CF OCF is not capable of transfer-
ring energy back to argon, it must subsequently shed the energy by
radiative, non-radiative decay and/or decomposition. The events
we expect to dominate are formation of fluorine atoms (and their
diffuson in the matrix), unimolecular rearrangements within the
argon matrix cavity, and bond cleavage to form free radicals that
will recombine and disproportionate in the argon matrix. Any
bimolecular reactions are negligible because of the matrix isolated
characteristic irradiation products of many PFPEs [9]. The
decomposition of the CF
3
H impurity at 20 K is apparently very
inefficient as seen by the almost nondetectable changes in the
ꢀ
1
1
147 and 1379 cm infrared absorption bands, up to the doses
applied here. We therefore expect the impact of the CF H impurity
, to be negligible
3
on the radiation products of interest, COF
2 4
and CF
under these conditions.
3 3
nature of CF OCF in solid argon.
The PFDME gas was also exposed to an electron beam at room
temperature in an argon atmosphere (Fig. 2). The products
observed in the gas phase irradiation are identical to the 10 K
We have used the power of ab initio calculations to provide us
with dependable spectra of expected radiation-induced products
to prove or disprove the various reaction paths described above. If
ꢀ
1
ꢀ1
exposures. Thus COF
2
(1956, 1928 cm ), and CF
4
(1281 cm )
C–F bonds are being broken, we would expect to form CF
radicals. The computed spectrum of the CF OCF radical is shown
in Fig. 4; we do not observe these bands in the CF OCF sample
after irradiation at T = 10 K. Also shown in Fig. 4 are the spectra for
CF and CF O radicals formed by scission of a C–O bond. The
authentic CF and CF O radicals are well-characterized with
absorption bands at 1250 and 1084 cm for CF
3 2
OCF
appear to be exclusively produced. From the infrared data, the G
values for the decomposition of PFDME and product formation of
3
2
3
3
2 4
COF and CF are computed. The G value is defined as the number of
molecules transformed per 100 eV of absorbed energy. The
methodology by which the G values are computed from the
infrared data have been previously disclosed in detail [7,9]. From
3
3
3
3
ꢀ
1
3
and 1260 and
O, respectively [11,12]. Again, we do not observe
their IR absorptions in the irradiated samples. Consequently, the
products appear to be COF and CF . Two reasonable mechanisms
ꢀ1
Fig. 2, the G value for decomposing CF
3
OCF
3
is 5.9. COF
2
and CF
4
are
1207 cm for CF
3
produced with G values of 5.0 and 4.9, respectively. Thus, for every
molecule of CF
molecule each of COF
COF and CF
and that competing side reactions are minimal. However a 15%
difference between CF OCF decomposition and product yield is
3
OCF
3
that is decomposed, approximately one
2
4
2
and CF are produced. The nearly 1:1 ratio of
4
which explain the product formation are a unimolecular rear-
rangement or a cleavage of the C–O bond with a subsequent
2
4
products is indicative that stoichiometry is preserved
3 3
disproportionation reaction of the CF and CF O radicals. The
3
3
energy required to produce both reaction intermediates are
computed in Table 2. They are 91 and 114 kcal/mol, respectively
observed. We do not know at this time if the 15% difference is due
Table 1
ꢀ
1
Experimental and HF/6-31G* computed infrared frequencies (cm ) and potential energy distribution for perfluorodimethylether (CF
3 3
OCF ). y is defined as a bond stretch,
and and are valence angle bends.
a
b
Matrix isolated
Gas phase
Computed (scaled 0.87)
Potential energy distribution
1
323
253
1329
1256
1323
1264
0.80
0.78
0.50
0.70
0.62
0.92
0.40
y
y
y
y
y
y
b
(CF
(CF
3
)
)
1
3
1261
(CO) +0.34
(CF ) + 0.14
(CO) + 0.32
(CF
(FCO) +0.34
y
y
y
(CF
(CO)
(CF
3
)
1
239
163
1249
1188
937
3
1
1173
971
3
)
9
69
95
3
)
6
696
653
3
a (CF )
Waltman
