3600
JULIANHEICKLEN AND VESTERKNIGHT
--
The Mercury-Photosensitized Decomposition of Perfluoropropenel
by Julian Heicklen and Vester Knight Aerospace Corporation, El Segundo, California
(Received M a y 1.2, 1966)
The Hg-sensitized decomposition of C3F6 was studied at temperatures of 210, 291, and 370" and pressures from 0.6 to 20 mm. The only products were C2F4 and C4Fs-2; the former was more important, and in most cases the only product detected. The CzF4yield falls off with increasing pressure or duration of exposure, but it rises with rising temperature. Where data exist, @(-C3F6) is similar to a(CzF4) for small conversions. However, @(-C3F6)> unlike +(C2F4), does not depend on exposure time. The primary process is given by reactions a to f in the text. The activation energy for the limiting low-pressure dissociation is about 2.2 kcal./mole, which, when added to the energy of the absorbed radiation, yields a value of about 114.9 kcal./mole for the double-bond dissociation energy at absolute zero. The CF3CF species formed combines with another radical or rearranges to CzF4.
Introduction The mercury-sensitized photolysis of C2F4*as well a s its direct photolysisa have been studied and CF2 radicals were produced. I n the Hg-sensitized experiments, an electronically excited molecule has also been postulated. 2b Dalby3 has examined briefly the direct photolysis of C3F6, and his results suggest a split to CF2 and CF3CF radicals. We have examined the Hg-sensitized photolysis of C3F6 to see if the photochemistry is analogous to C2F4 and to gain more information about the reactive species produced. Experimental Section Hexafluoropropene from Peninsular Chem Research, Inc., was used without further purification except for degassing at -196' immediately before use. Gas chromatograms showed no impurity peaks. The phot#olysis vessel was a 10-cm. long, 5-cm. diameter quartz cell. It was encased in a wire-wound aluminum furnace that overlapped each end of the cell by 2.5 cm. The ends of the furnace were covered with quartz plates to minimize convectionlosses. Temperature measurements were made by a thermocouple and were constant to * 2 O . Irradiation was from two spiral Hanovia mercury-resonance lamps, one at each end of the cell. The light passed through Corning 9-54 glasses before enteringothe cell, to remove unwanted radiation below 2200 A. At the conclusion of a run, the gases were transferred in a glass vacuum system with Teflon-Vyton stopcocks The Journal of Physical Chemistry
and collected for analysis in an F & M Model 720 programmed dual-column chromatograph with a 3-m. silica gel column. The products found, C2F4 and C4Fs-2, as well as the C3F6,were calibrated so that chromatogram areas could be converted to pressures.
Results The results are listed in Table I. The absorbed intensity I. was estimated by photolyzing mixtures of 200 mm. of C3F6 with either 60 or 200 mm. of oxygen at room temperature and measuring the sum of CFzO and CF3CF0 produced. S a u n d e r ~ ,working ~ in this laboratory, has shown that under these conditions a(CF20) = a(CF3CFO) = 0.50. The C3F6 pressures listed in Table I are initial pressures at the appropriate temperatures, as obtained by direct measurement. During photolysis, C3F6 was depleted; for some low-pressure runs carried to extended conversions, as much as 60% of the C& was consumed. The final CIF6 pressure was computed from the chromatograms; thus the C3F6 consumption could be estimated. Experiments were performed for various exposures a t four temperatures and several pressures. At room (1) This work was supported by the U. S. Air Force under Contract NO. AF 04(695)-469. ( 2 ) . (a) B. Atkinson, J . Chem. SOC.,2684 (1952); (b) J. Heicklen, V. Knight, and 9. A. Greene, J . Chem. Phys., 42, 221 (1965). (3) F. W. Dalby, ibid., 41, 2297 (1964). (4) D. Saunders, unpublished work.
3601
MERCURY-PHOTOSENSITIZED DECOMPOSITION OF PERFLUOROPROPENE
Table I : Mercury-Sensitized Photolysis of CaFe (CsFd, mm.
0.64 0.61 0.66 0.66 2.15 2.13 2.13 2.13 2.36 2.10 6.6 6.3 6.2 6.4 6.0 6.0 6.2 20.0 20.0 20.0 22.0 22.0 19.0 18.5 20.5 60.0 199.0 0.72 0.63 0.64 0.64 0.67 0.67 0.59 0.59 0.64
0
Exposure time, min.
@(-
C~FE)
@(CzFd
T = 212 f 4" 2.50 ... 0.09 5.00 0.06 0.13 0.06 0.12 7.50 0.07 0,090 10.00 0.10 5.00 ... 0.066 1~1.00 ... 0.060 15.00 ... 0.050 20.00 ... 0.027 60.00 0.022 60.00 ... 0.05 5.00 0.034 10,OO 20.00 0.035 ... 0.037 30.00 0.031 30.00 ... 0.019 60.00 ... 0.0057 240.00 ... 0.024 15.00 ... 0,011 15.00 ... 0.012 30.00 30.00 ... 0,010 0.0087 6O.00 0.0044 90.00 ... 0.0011 9oc.00 ... 0.00048 93C.00 ... 0.0020 66.00 ... 60.00 ... Trace
2.50 5.00 5.00 7.50 -10.00 10.00 15 00 15 00 20 00
The 0 means rp
T = 291 f 2' 0.11 0.06 0.11 0.14 ... 0.13 0.10 0.084 0.10 0.095 ... 0.13 0.046 0.061 0.056 0.054 0.041 0.034
5 O.lO/time in min.
5
(I,= 5.4X lo1*quanta/cc.-sec.) Q(C4F8-2)
"
Exposure time, min.
@(-C3Fd
T O=
0 0 0 0 0 0 0 0.0033 0.0030 0 0 0 0 0.0069 0.0074 0.0042 Traceb Trace Trace
0.0076 0.0064 0.0036 0,0019 0.00064 0.0045 0,001
0 0 0 0 0 0 0 0 0
Trace means
N
0.72 0.61 2.24 2.27 2.27 2.19 2.13 2.24 6.2 6.3 6.2 6.3 6.3 6.3 6.3 20.0 20.0 21.5 22.0
20.00 55.00 2.50 5.00 7.50 10.00 15.00 21.00 3.00 6.00 9.00 12.00 15,OO 50.00 150.00 5.00 10.00 15.00 780.00
0.62 0.64 0.62 0.67 0.62 2.11 2.09 2.16 2.13 2.19 2.22 2.13 2.19 2.08 6.4 6.3 6.3 6.4 6.5 6.4
2.00 5.00 7.50 10.00 10.00 1.00 2.00 2.50 5.00 5.00 10.00 15.00 16.00 40.00 2.50 5.00 7.50 10.00 15.00 15.00
*(CzFd
291 i: 2' 0.071 0.072 0.039 0.0145 ... 0.17 ... 0.14 ... 0.10 ... 0.12 ... 0.077 ... 0.070 ... 0.09 ... 0.08 ... 0.085 ... 0.074 ... 0.062 ... 0.031 ... 0.0061 ... 0.050 ... 0.044 ... 0.034 ... 0.00074
=
0 0 0 0 0 0 0 Trace 0
0 0 0 0 Trace
0.00053 Trace Trace Trace
0.00061
T = 370 f 2' 0.1 0.14 0.12 0.15 0.11
... ...
... 0.17 0.29 0.18 0.10 0.16 0.12
...
...
... ... ... ...
0.24 0.24 0.155 0.140 0.114 0.34 0.29 0.39 0.21 0.27 0.16 0.12 0.12 0.038 0.20 0.18 0.15 0.15 0.10 0.11
0 0 0 0
0 0 0 0 0 0 Trace Trace Trace Trace Trace Trace Trace Trace Trace
0.025
0.3/time in min.
temperature, under no conditions were products found, even for extended exposures. The only products found a t the elevated t)emperatures were C2F4 and, in some cases, perfluorobutene-2. For a few runs at extended conversions, small amounts of c-CZFs were observed, too. Presumably it is a secondary product. The CZ8-2 yield rarely exceeded that of C2F4. It decreases in relative importance a t low pressures and high temDeratures. The CzF4yield, and probably also the C4Fs-2 yield, falls off with increasing pressure or dura~~
(C3Fd I mm.
tion of exposure. However, it rises with temperature. Where data exist, the quantum yield of C3F6 consumption iP(-c3F6) is similar to @(CzF4)for small conversions. However, @(-C3F6),unlike @(C2F4), does not depend on the exposure time.
Discussion If C2F4 and C4F8-2 are the only products, then massbalance considerations require that
I
@( -C3F6)
=
(2/3)@(CzFJ
+ (4/3)@(C4Fs-2)
Volume 69,Number 10
(1)
October 1966
JULIANHEICKLEN AND VESTERKNIGHT
3602
greater than 0.5 and independent of temperature, contrary to extrapolated values from Figure 1. The most likely mechanism that explains the primary process is
+ hv *Hg* Hg* + P +P,* Hg* + P +Po* P,* CF2 + CF3CFX P,* + P *Po* + P Hg
---t
0
8
4
( C s F6),"
Figure 1. Plot of [%(CZF4)]-l
US.
(CaFe,).
For short exposures, eq. 1 is obeyed reasonably well. However, for longer exposures, both a(CzF4) and 9(C4Fs-2) drop markedly, whereas @( -C3Fs) falls only slightly, if at all. Since @(-C3Fe) is fairly constant to changing exposure times, the primary process presumably is not being inhibited. Apparently, the products are formed in such a way that they can easily disappear by polymerization. Consequently, the discussion of results will concern only the short-time limiting values of product quantum yields, %(CZF4) and @o(C4Fs-2). The reciprocals of @O(CZF4) are plotted in Figure 1 vs. the CaF6pressure for the three temperatures. It is apparent that the decomposition is a function of temperature and is inhibited by increasing the pressure. A temperature-dependent process can arise either from the energy distribution in the ground electronic state before excitation or from the energy distribution of the thermally equilibrated upper electronic state. For this system, the second possibility can be eliminated for two reasons. First, the results with added oxygen5 show that the upper electronic state is not removed by collision with C3F6; thus if the decomposition had occurred from this state, no pressure effect should have been observed. Second, the results with added oxygen show that the quantum yield of formation of the upper electronic state is at least 0.5 and probably unity. Consequently, if decomposition had occurred from the thermally equilibrated upper electronic state, a0(CzF4)extrapolated to zero pressure should have been The Journal of'Physical Chemistry
(a> (b) (c)
(4 (e)
Po* +P (f) where P is C3Fe, the asterisk represents an electronically excited molecule (probably a triplet), the subscripts n and 0 refer, respectively, to vibrational levels sufficiently energetic or not sufficiently energetic to permit dissociation, and CF3CFXis some energetic state of CF3CF. The amount of excess energy in CF3CFX should increase with temperature. We have included the possibility that CF3CF contains some extra energy to explain the temperature and pressure dependence on the fate of the CRCF species. The secondary reactions that then will explain the results are 2CFz
*C2F4
(d
CF3CF* +CZF, CF3CF*
+ P +CF3CF + P
2CFXCF * C4Fs-2 CF3CF
+ CF2 +P
(h)
(9 (j ) (k)
The mechanism predicts that
[ + 2P]
[@o(C2F4)I-l = [ @ o o ( CI-'~ F ~1
(2)
where @m(CZF4)is the low-pressure limit of aO(CZF4). Figure 1 shows plots of [@o(CzF4)]-lus. P for the three temperatures. The data are badly scattered owing to the difficulty in analysis at extremely short conversions. Nevertheless, reasonable straight lines could be drawn that follow a regular trend; the intercepts give ao0(CzF4), and the ratios of slope to intercept give k,/kd. Even though there is some difficulty in determining the slopes and intercepts, the conclusions are hardly affected at all. For example, if the intercepts were altered by a factor of two, the estimated dissociation energy would change by less than 1kcal./mole. At low pressures, the only product is CzF4,and eq. 1reduces to (2/3) @oo (C2F4)
= 400
(3)
( 5 ) J. Heicklen and V. Knight, J . Phys. Chem., 69, 3641 (1965).
HEATSO F FORMATION AND POLYMERIZATION O F c302
where doois the short-duration primary quantum yield of C3Fs consumption extrapolated to zero pressures. The approximate values obtained for 400 and kd/k, are listed in Table 11. Now 400 is just kt,/k, and is related to the distribution of energy states in the ground electronic level through the Boltzmann function
900= exp(-Eo/RT)
3603
Table 11: Approximate Rate Constants (kd/ke)
x
Temp., ‘C.
@OD
Eo, kcal./mole
mole/l.
210 291
0.11
0.14
370
0.18
2.1 2.2 2.2
0.54 i.3 5.7
lo4,
(4)
where Eo is the energy needed by the ground electronic state to form Pn* upon collision with Hg*. The approximate values of Eo, computed at each temperature, are also listed in Table 11. The value is roughly independent of temperature, as indeed it should be. If EO is added to the energy of the absorbed radiation (112.7
kcal./mole), a value of about 114.9 kcal./mole is found for the dissociation energy of the double bond in C3Fsat absolute zero.
Acknowledgment. The authors wish to thank Mrs. Barbara Peer for assistance with the manuscript.
The Heats of Formation and Polymerization of Carbon Suboxide
by B. D. Kybett, G. K. Johnson, C. K. Barker, and J. L. Margrave Departments of Chemistry, Rice University, Houston, Texas, and University of Wisconsin, Madison, Wisconsin (Received M a y 18, 1966)
The heat of formation of carbon suboxide has been determined from its heat of combustion in oxygen. The bond energies are compared with those in allene and carbon dioxide. The heat of polymerization to form the thermal polymer of C302 and the heat of hydrolysis of this polymer have also been determined.
Introduction Marxl and Grauer2 have reviewed the large number of investigations of the reactions and properties of carbon suboxide* There has been increased interest in the thermochemistry of carbon suboxide since it is a possible intermediate in the radiolysis of carbon monoxide produced in carbon dioxide-cooled reactors. Redgrove3 has stated that Diels determined its heat of combustion but this work has not been published. There is serious disagreement among estimates which have been made. The “JANAF Tables”4 quote mfo(g) = -8.3 kcal./mole. Botter5 calcu-
’
lated that AHfo(g) -30 kcal’/mole from the pearance potentials Of its fragment ions, and E -25
kcal./mole from estimated bond energies. The old e ~ t i m a t e ,which ~ has recently been quoted,6 is 47.4 kcal./mole. The structure of the thermal polymer has recently been investigated by X-ray diffraction and infrared, visible, and ultraviolet absorption spectresc0py.~,8 This paper describes determinations of the (1) D. Marx, Thesis, Facult6 des Sciences de Paris, 1959. (2) R, Grauer, chimia (Aarau), 14, 11 (1960). (3) H. s. Redgrove, chm. 120, 209 (1920).
(4) JANAF Interim Thermochemical Tables,” D. R. Stull, Ed., The Dow Chemical Co., Midland, Mich., Dec. 31, 1960. (5) R. Botter, “Advances in Mass Spectrometry,“ Vol. 11, R. M . Elliot, Ed., Pergamon Press, Oxford, 1963. (6) 0. Glemser, “Handbook of Preparative Inorganic Chemistry,” Vol. I, G. Brauer, Ed., Academic Press, New York, N. Y., 1963. 11
Volume 69, Number 10 October 1966
