REACTION OF OXYGEN ATOMSWITH TETRAFLUOROETHYLENE
3893
Reaction of Oxygen Atoms with Tetrafluoroethylene in the Presence of Molecular Oxygen1
by Julian Heicklen and Vester Knight Aerospace Corporation, El Segundo, California (Receiaed June 20, 1966)
~
Oxygen atoms were produced in the presence of CzF4 and 0 2 from the mercury-photosensitized decomposition of NzO. Besides Nz, the products were CFZO, C2F40 (tetrafluoroethylene oxide), and cyclo-C3F6. CF,CFO was not found. The mechanism involves a short chain which produces CFzO: CF202 C2F4 + 2CF20 CFZ % CF202. The chain carrier CF202 is removed by either 2CF202 --t 2CF20 0 2 or CF202 C2F4 + CFzO C2F40. At room temperature, O2 suppresses cyc10-C3F6formation, but at 125' its presence enhances cyclo-CaF6 production. The detailed mechanism is given by reactions a through p in the text. A number of rate constant ratios were determined, and they are included. The mechanism and rate constant ratios are compared to those for C3F6and found to be similar.
+
+
I. Introduction Earlier com~nunications~~~ from our laboratory have discussed the reactions of O(3P) (prepared from the Hgsensitized photolysis of KzO) with C2F4. The reaction scheme between room temperature and 150" seems reasonably well established. The detailed mechanism and pertinent rate constant ratio^,^ as well as the C2F4 absolute rate constant parameters for the 0 reaction,zb are known. Briefly, the results are: first, the only products are CFZO and cyclo-C3F6; tetrafluoroethylene oxide C2F40as well as CF3CFO is definitely absent for all conditions studied; second, e(CF20) = 1.0 for all conditions; and third, @(cycleC3F6) can vary from zero to 1.0 and is dependent on the temperature, the C2F4 pressure, and the absorbed intensity. A few preliminary runs at room temperature in the presence of molecular oxygen have been reported, za and @(CF20)was roughly doubled by the addition of oxygen. I n the present work, we have extended this investigation.
+
11. Experimental Section Matheson Clo. NzO and O2 were used. The NzO was degassed a t - 196' before use, but the 0 2 was used without further purification. CzF4 was prepared by the debromination of the vicinal dibromide CzF4Brz
+
+
+
(E. I. du Pont de Nemours, Freon 114-B-2). Liquid Freon was added dropwise to a warm (50") slurry of zinc dust and methanol containing some ZnC12. The rate of addition was adjusted to keep the solvent gently refluxing, and the effluent C2F4 was subsequently purified by passing it through water, then Drierite, and finally through a trap at -126" to separate any heavy ends. Before use, the CzF4was degassed a t - 196". The general analytical procedure has been described previously.z At room temperature, mixtures of reactants were prepared in a cylindrical Pyrex cell, 10 em long and 5 cm in diameter, with sodium chloride windows. Irradiation was from two Hanovia lowpressure spiral mercury lamps, one a t each window. After irradiation an infrared spectrum of the cell contents was taken on a Perkin-Elmer Model 21 infrared spectrometer. At 125", a T-shaped cell was used. Both the stem and cross were 10 cm long and 5 cm in diameter. Irradiation was through a Corning 9-54 filter (to remove radiation below 2200 A) and a quartz window on the stem of the cell. The cross of (I) This work was supported by the U. S. Air Force under Contract NO. AF 04(695)-669. (2) (a) D. Saunders and J. Heicklen, J. Am. Chem. Soc,, 8 7 , 2088 (1965); (b) D. Saunders and J. Heicklen, J . P h p . Chem., 70, 1950 (1966). (3) N. Cohen and J. Heicklen, ihid., 70, 3082 (1966).
Volume 70, rumher 12 December 1966
3894
JULIAN HEICKLEN AND VESTER KNIGHT
the T had NaCl windows and was situated in the sample beam of a Beckman IR-4 infrared spectrometer. During any irradiation, only one product band was followed, and it was followed continuously. The absorbed intensity I , was measured from the CFzO production for runs with oxygen absent, where @(CF20) = 1.O.
111. Results The products of the Hg-photosensitized decomposition of S z O in the presence of both CZF4 and Oz are Nz, which was not monitored, and CF20, cyclo-CaF6, and CzF40, which were easily identified and analyzed by infrared spectroscopy. CRCFO was never found, though we looked for it specifically and would have detected it easily if it were present. At room temperature, a small amount of polymer was also formed, but it did not interfere. However, at 125" the polymer formation was appreciable, and it was necessary to clean the cell occasionally during the series of runs. The infrared bands used for analysis were a t 5.12, 6.22, and 11.66 p, respectively, for CFZO,CzF40, and cyc1o-C3F6. The absolute absorption coefficients are known for CFzO from previous work.2b The absorption coefficientsfor cyc1o-CsF6were found by performing photolyses in the absence of O2 and using as absolute quantum yields those predicted from earlier results. The value for the absorption coefficient (to base 10) is 530 I./mole em at both 23 and 125". For CzF40, we used a value (to base 10) of 123 ]./mole cm for the extinction coefficient at 6.22 p based on the infrared spectrum reported by Caglioti, Lenzi, and Mele.5 Spot checks showed that a t room temperature the products grew linearly with time. However, at 125" this was definitely not the case. The rates of growth of CF20 and cyclo-CsF,jwere markedly enhanced as the reaction proceeded. On the other hand, the rate of growth of CzF40 was retarded. These effects were most pronounced at the lowest intensities where longer times were needed to obtain measurable amounts of products. Under these conditions, it was extremely difficult to determine accurately initial rates of growth, and the initial quantum yields we report for cycloC3F6 and C2F40easily may be in error by 50% at the lowest intensities at 125'. Because more CFzO is formed, its analysis is much more reliable. After the irradiation was discontinued at 125", both the CFzO and cyclo-C3F6 continued to grow, whereas the C2F40 decayed until it eventually vanished. When the C2F40 was consumed, the CFzO stopped growing, but the cyc10-C3F6 continued to grow indefinitely. (We sometimes monitored it for about 1 hr after the CF,O had stabilized.) Lenzi and bIele6 314
The Journal of Physical Chemistry
found that CzF40 rapidly decomposes at 125" to CF20 and CF, (which could show up as either CFzO or cycloC3F6 in our system). Thus some of these results are not unexpected. For some experiments, we measured the increase in CFZO and found it to be roughly about two or three times the CzF40 consumed. Consequently, some of the CFZO growth, but not all of it, can be explained as due to CzF40decomposition. On the other hand, the continual growth of cyclo-C3Fe, even after the CzF40 is exhausted, is a complete mystery. The results at 23" are reported in Table I, and the initial quantum yields at 125" are reported in Table 11. At room temperature and at low C2F4 pressures and highvalues of I,, @(CF20)rises slightlyas O2increases from 5.0 mm. Eventually a plateau is reached, and then further increases in 0 2 markedly reduce @(CF20). As (C2F4) increases or I , falls, the plateau area becomes more important; eventually @(CFzO) is essentially independent of (Oz) and approaches a value of 3.0. For the latter region, @(cyc10-C3F~)is independent of I , but rises with the ratio (C2F4)/(02). @(CZF~O) is independent of (Oz) but is enlarged as either (CZF4) is enhanced or I , is diminished. Its absolute value varies between zero and unity. At 125" under our experimental conditions, all initial quantum yields are essentially independent of ( 0 2 ) though they are all larger than in the absence of 02. All of the yields increase with increasing (CZF,) or declining I,. @(CFzO) varies from about 3 to 10; @(cyclo-CsF6),from about 0.1 to about 2.0; and @(C2F40), from about 0.6 to about 4.
IV. Discussion In the absence of
02,
Hg Hg*
the reaction mechanism is3
+ hv +Hg*
+ N20 +Hg + N2 + O(3P)
0
+ CZF4 0 + CzF4
CFZO
+ CFZ
4CzF40*
2CFz +CzF4
CFz CzF40*
+ CzF4 --+ CYCIO-C~FB
+ CzF4 +CFz0 + cyclo-CaFa
(4 (b)
(c) (4 (4 (f) (g)
where the asterisk denotes an excited state. For the time being, the multiplicity of the CF2 species will not be specified, though we believe it to be triplet. (4) N. Cohen and J. Heicklen, J . Chem. Phys., 43, 871 (1965). (5) V. Caglioti, hl. Lenzi, and A. Mele, Nature, 201, 610 (1964). (6) M.Lenzi and A. hlele, J . Chem. Phys., 43, 1974 (1965).
REACTION OF OXYGENATOMS WITH TETRAFLUOROETHYLENE
Mixtures [(N20) Table I : Results at 23" for N~O-C~FI
-
3595
500 mm, A = 2537 AI
Exposure (Oz), mm
(C2F4), mm
time, min
1, = 64 X 6.0 4.6 5.2 5.0 5.0 5.1 5.3 5.0 5.2 18.5 16 17 17 15 64 59 55 47 50 167 160 159
6.0 8.0 15 16 54 56 106 150 152 4.5 5.0 18 62 147 5.0 7.0 15 58 103 6.0 17 55
5.0 7.0 5.0 6.0 15 16 15 16 55 53 52 50 139 146 139
6.0 14 55 147 5.0 22 52 147 6.0 16 67 173 6.5 17 49
1.
5.0 2.0 2.0 2.0 2.0 3.0 5.0 6.0 13.0 3.0 2.0 2.0 2.0 3.0 1.0 1.5 1.5 1.5 1.5 1.0 2.0 1.5 =
Exposure *(cyolo-
*(CFzO)
C8FO)
quanta/cc see 0.83 0,009 0 1.10 0 1.07 0 1.07 0.58 0 0.49 0 0.31 0 0.22 0 0 0.113 1.27 0.025 0.019 1.31 1.04 0 0 0.88 0.35 0 2.39 0.103 1.59 0.065 2.06 -0.03 1.64 0.008 1.08 0.008 1.81 0.128 0.09 1.77 1.02 N O
*( C2F4O)
22 X 1013 quantalcc 8.0 0.83 6.0 1.08 10.0 0.48 25.0 0.22 5.0 1.62 5.0 1.55 1.00 6.0 10.0 0.38 5.0 1.60 5.0 1.80 4.0 1.55 5.0 0.82 3.0 1.93 6.0 1.82 5.0 1.07
see 0
... 0 0 0.016 0,008 0 0 0.048 0.045 0.016 0.01 0.121 0.093 0.016
0 0 0 0 0 0 0 0 0 0.08 0.09 0 0 0 0.24 0.24 0.29 0.20 0.20 0.58 0.47 -0.25 0 0 0
0 2
mm
mm
0
0 0 0 0 0.24 0.30 0.21
+Hg
time, min
*(cycle-
Q(CFz0)
CaFa)
6.4 6.4 6.2 6.8 17.5 16 16 16 48 56 51 49 146 162 163
I , = 4 . 0 X 1013quanta/cc sec 0.93 0 5.6 31.0 1.00 0 12.6 20.0 30.0 0.54 0 63 90.0 0.22 0 168 5.0 15.0 2.00 0.06 15.0 1.75 0 19 20.0 1.37 0 51 30.0 0.56 0 154 0.105 15.0 2.46 5.5 0,052 17.0 1.77 17 2.25 0 15.0 61 140 15.0 1.75 0.01 0.144 5 10.0 2.12 0,097 17 20.0 2.74 0.118 51 15.0 2.75
7.7 16 14 51 50 141 192
I, = 0.98 X 1013quanta/cc see 0 60.0 1.58 5.3 0 30.0 1.84 16 0 60.0 1.25 141 0.03 50.0 2.50 57 0.03 45.0 2.34 155 3.4 0.22 15 60.0 2.56 0.15 49 45.0
I, 14 51 137
19 60 49
=
0.24 X 10'3 quanta/cc sec 90.0 2.7 0.03 150.0 2.3 0.03 155.0 2.8 0.17
WCZFIO)
0 0 0 0 0 0 ...
0 0.66 0.50 0.56 0.41 0.63 0.91 0.92 0 0 0 0.83 0.74 1.35 1.11 0 0.92 1.08
... 0.55 0.52 0.37
+
+
(Od,
lOI3
Actually, C2F40* also can decompose in a first-order step, but this is only important for C2F4 pressures less than 1 mm and need not be considered here. The ratio of k d / ( k , k d ) is 0.15 at 23" and -0.10 at 125". It is also necessary to consider the reaction Hg*
(CaFd,
+ Oz*
0 2 is about as efficient as N20 in quenching Hg*. Thus for ( 0 2 ) = 150 mm, about 25% of the Hg* reacts with 0 2 . We will ignore this reaction in the subsequent analysis, though we realize that to do so introduces some error. Fortunately, the 0 2 * reacts rapidly with C2F4 to give results very similar' to that of the 0
+
CzF4 reaction in the presence of 0 2 . Thus the importance of this complicating competition is masked. For smaller O2 pressures the removal of Hg* by 0 2 is not sufficientlyimportant to influence the results. At 23". At room temperature, in the presence of molecular oxygen, @(cyclo-CsFe) falls as (CzF4)/(02) is diminished, even in the regions where the other product yields are independent of the 02 pressure, and even at low intensities and high C2F4 pressures where reaction f should be important. In fact, with 50 mm of C2F4, 60 mm of 02,and I , = 0.24 X 1013quanta/cc (7) J. Heicklen, V. Knight, and S. A. Greene, J. Chem. Phys., 42,221 (1965).
Volume 70,Number 1.3 December 1966
3896
JULIANHEICKLEN AND VESTER KNIGHT
Table 11: Initial Results a t 125' for NZO-C2F4Mixtures [(NzO)
Q(cyclo-CaFa)
Z, 16 55 52 53 147
18 15 56 151 50
3.3 3.5 3.8 3.4 5.1
0.12 0.29 0.32 0.26 -0.75
16 52 50 51 150
16 14 56 147 49
5.0 6.4 6.6 10.4 9.8
0.22 0.68 0.64 0.76 1.6
16 51 52 52 149
18 16 53 151 52
6.5 6.8 7.4 8.4 9.2
0.44 1.15 1.35
16 53 54 54 149
17 17 55 153 51
9.1 9.0 11.2 6.2 9.2
I,
I,
a
=
From eq 9.
=
=
1.4 1.8
-
500 mm, X = 2537 A]
*(C%F40)
[Q(CZFIO)-
Q(cyClo-CaF6)/ L(cyclo-CaF6)
1 j/*(cyclo-
CaFd
?a
km/knb
11.2 X 1013 quanta/cc sec 0.64 0.87 1.17 1.32 1.10 1.48 1.00 1.18 1.67 -1.8 1.65 X 1013 quanta/cc sec 0.92 1.44 1.84 2.1 1.51 2.0 1.60 2.4 3.1 2.8
...
...
*..
1.2 0.8 0.8 1.3
2.2 3.0 2.7 1.5
1.0 1.3 2.4 0.9
0.31 X 10'3 quanta/cc sec 1.0 2.2 1.8 2.4 2.4 2.8 1.5 3.4 2.6 3.0
...
...
...
0.7 1.0 0.3 0.9
2.2 1.7 3.1 1.6
1.1
0.8 2.0 1.1
1.1 1.3 1.6
2.0 1.5 1.5 1.6 1.3
1.5 0.9 1.3 0.5 0.5
I, = 0.061 X 1013quanta/cc sec 0.88 -2.0 0.7 1.45 2.9 1.9 1.2 2.9 1.3 ... 2.6 ... 2.0 4.2 2.0
...
...
...
... ... ...
... ... ...
...
0.9
2.5
0.7
... 1.6
...
...
From eq 10.
sec, @(cyclo-C3F6) is 0.04, whereas in the absence of O2 it would be 0.11 from reaction f alone (ignoring any contribution from g). With CZF4 = 150 mm and the same ( 0 2 ) and I,, +(cyclo-CiFs) is 0.17, which can be accounted for completely by reaction g plus the CF2 produced from Hg*
+ CzFr +Hg + 2CFz'
(h)
where the superscript 1 denotes the singlet state. The latter reaction gives about 20% as many CF2 radicals as reaction c when C2F4 = 150 mm,2*and these singlet radicals are not scavenged by 02.' @(cyclo-CaFs) predicted from reaction f alone would be 0.28. Consequently, it seems clear that either the CF2 radical produced in reaction c or its precursor must be removed by 02. There appears to be n o way of escaping this conclusion. Alternatively, we can examine the chain-terminating step. CzF40 can be formed only in a chain-terminating step. If there are no branched chains a t 23", then when @(CZFdO) = 1.0, every reaction of an oxygen atom with C2F4 must lead ultimately to C2F40. Again The Journal of Physical Chemistry
the conclusion is reached that either the CF2 radicals from reaction c or their precursors are scavenged by 02.
Now we consider the precursor to C2F40 formation. I n the absence of 02,this molecule is not formed;2 thus the 0 C2F4reaction is too energetic to yield a stable C2F40molecule. Another intermediate must be formed when 0 2 is present which can donate an oxygen atom to C2F4 in a less exothermic reaction. The indicated intermediate is the CF20z radical, and it is most easily formed in the reaction
+
CFz3
+
0 2
+CF202
(9
where the superscript 3 denotes a triplet state. The molecular oxygen can also react with 0 atoms and the CzF40*intermediate
+ + C2F40* + 0
0 2
+ NZO CF2O + CF9.02
n.20 40 3
0 2
---f
(j )
(1)
Reaction j is well known, and reaction 1 must occur to account for the diminution in @(cyclo-CsFe)in the pres-
3897
REACTION OF OXYGEN ATOMSWITH TETRAFLUOROETHYLENE
ence of 0 2 . (In the absence of 02,most of the cycleC3F6 comes from reaction g.) The intermediate CF20zcan react with CzF4
-
+ C2F4-+ CFz02 + CzF4 CFzOz
+
CFZ3 2CF20
( 4
CFzO 3. CzF40
(4
where reaction m is the chain-propagating step and reaction n is a chain-terminating step. Since @(C2F,O) is often less than unity and there is an intensity effect, another chain-terminating step must be 2CF202 42CF20
+ Oz
(0)
There is an alternative mechanism that can explain the results. Instead of reactions c and m producing CFZ3radicals, they might form some other intermediate that is scavenged by O2 but does not react with CZF4. However, it would seem odd that such an intermediate could exist in lieu of the fact that CZF40* reacts with both C2F4 and 0 2 . On the other hand, the complication introduced by reaction i concerns the fact that CFz radicals produced in three other systems are not scavenged by 02.7-9 Presumably, those radicals were singlets. Spin conthe data; yet it is easy to draw a straight line of unit servation would require the CF2 produced in this slope through them. The value of k,/kl is about 0.10. system to be triplets, and we have reported this to When reaction n is small compared to reaction o [Le., be the case.2a,10Our results here seem to confirm this when @(CzF40)is small], a condition that prevails at hypothesis. However, if this is so, then kf/ke1I2aphigh I, and low C2F4, then the mechanism requires that pears to be the same for both singlet and triplet CF2 radical^,^ a result that would be too fortuitous to be true. Presumably, the initially formed triplet CF2 radicals pass over to the singlet state in the absence as long as all the CFZ3radicals and CzF40*are scavenged of O2before reaction. by Oz. Since (NzO) is constant, [@(CF20)]-' can be Except at low I , and high (C2F4), the cyclo-C~F6 plotted against (02)/(C2F4),and a straight line should comes essentially from reaction g. Thus, the mechresult. This is done in Figure 2 for the runs where anism predicts that @(C2F40)is small. The data fit a straight-line plot @(c~c~o-C~F~) which we have forced to pass through an intercept of 0.5. It can be seen that the high-intensity points at @o(CyClo-C$6) (1 CY)-' - @(CYC~O-C~FG) low (Oz)/(C2F4)lie about a factor of 2 high, a result that could be caused by the removal of CFz by reaction e rather than reaction i. Roughly, ki/kel" can be if this explanaestimated to be about 0.13 (mm min) where Q0(cyclo-CsFe) is the quantum yield of cyclotion is correct. From the slope, a value of 95 l./mole C3F6 in the absence of 02,and is obtained for k j / ( k o kd). Now kj is about 5.5 X lo91.2/mole2sec with N20 as a third body," and k, kd is 0.6 X 109 l./mole sec at room temperature.28 Under the conditions of interest, CY can be neglected, Consequently, the observed ratio is 10 times too large. and log { @(cycle-C3Fs)/ [@~(cyclo-C,Fe)- @(cycleC3Fe)] is plotted us. log { (CzF4)/(02)] in Figure 1. (8) F. W. Dalby, J . Chem. Phys., 41, 2297 (1964). The few data points of Saunders and HeicklenZa are (9) J. P. Simons and A. J. Yamood, Nature, 192, 943 (1961). included. (It should be pointed out that Saunders (10) J. Heicklen, N. Cohen, and D. Saunders, J. Phys. Chem., 69, and Heicklen misinterpreted their results concerning 1774 (1965). this competition.) There is considerable scatter in (11) F. Kaufman, private communication.
t
+
+
+
1
Volume 70, Number 1% December 10613
3898
7,
JULIAN HEICKLEN AND VESTERKNIGHT
I
I
I I,,quonta/cc-rac
40 x IO'' 22 x 10'1 40 1013
8
I
I
i C z G l , mm
our experimental error would outweigh any minor trends in y. The factor 1 CY can be estimated from the results of Figure 2 . Consequently, the left-hand sides of eq 4 through 7 can be computed under all conditions, and they are plotted us. (C2F4)/Ia"'in Figure 3. For low values of (C2F4)/Ia"', R(o) is greater than R(n) and @(CF20){ l a ) is slightly less than 2.0, as indicated by eq 4, and @(C2F40)(1 a ) "* rises linearly with (CzFd)/I,'/' (unit slope on the log-log plot). The value of kn/ko'"2 is found to be 5.6 X (mm min) -'/'. For high values of (C2F4)/ Ia'", R(o) is less than R(n) and @(CF20){1 a) rises to a value of about 3.0, yielding a value of 0.5 for k,/k,. Under these conditions, { l a ] is essentially unity and @(C2F40)(1 a)'" becomes 1.0, as predicted by eq 7. An alternate but less accurate method of computing lc,/ko is from the break point of the @(CF20){ 1 a ] curve. When @(CF20)is about R(n) and halfway between its limiting values, R(o) kn/k,'/z can be estimated to be about 5 X (mm min) -'/', in good agreement with that found from eq 6. At 125". At the elevated temperature, both kJk1 and k,/L,"' are probably similar to their room-temperature values. On the other hand, k , / ( k c kd) is smaller, both because k , kd has a small positive activation energy2b and because IC, has a negative activation energy.14 Therefore, we could design our experiments with (oz)/(CzF4) ratios to minimize the importance of reactions e, f, g, and j. Thus, these reactions can be ignored under our conditions a t 125'. The simplified mechanism explains the trends in the data. AS expected, the quantum yields are independent of ( 0 2 ) and rise with increasing (C2F.I) and falling I,. However, there are two marked differences from the room temperature data. In the first place, the addition of 0 2 does not inhibit cyclo-C3F6 production but actually promotes it; in many cases @(cyclo-CsFs) is measurably greater than unity. I n the second place, both @(cyc10-C3F6)and @(C2F40)often exceed 1.0. Since these molecules must be formed in chain-terminating steps, a branched-chain mechanism must be operative. An examination of the data in Table I1 shows that both @(CYC~O-C~FG) and @(C2F40)rise with (C2F4)/Ia1". Consequently, chain branching is more important when termination occurs by reaction n rather
+
e
15 16
and R ( x ) is the rate of reaction x. Except at very high I, and low ( 0 2 ) where reaction e can be important, y varies from 0.85 to 1.00. We set it equal to unity as
+
+
[02)/( c$41
Figure 2. Plot of [+(CFzO)]-l us. (Oz)/(C2F4) for low values of ( C Z F ~ ) ~at , ' /23". ~
+
From the limited data of ref 2a, a more favorable comparison can be made. In a comparable study of the reactions of 0 atoms with C3Fs in the presence of 0 2 , l 2 the value for ki was found to be too high by a factor of 2 . Furthermore, an odd feature of both systems is that product formation is suppressed by reaction j in spite of the fact that O3 reacts rapidly with perfluoro01efins.l~ The O3 must either react with the Hg or absorb the radiation, or both, in a manner to suppress product formation, -and the amount of suppression may be characteristic of the system. I n other systems we have found HgO when O3is produced. Thus, the value kd) obtained in this study is not reliable. of k j / ( k , Nevertheless, it is useful in the subsequent analysis to correct for reaction j. However, because of the uncertainty we shall limit our analysis of the data to the region where (Oz)/(CzF4) is less than about 3, so as to minimize any complications resulting from reaction j. Finally, the room-temperature mechanism predicts
+
+ a] = 1 + 2rkm @(CFzO)(1 + a ] = 2 + k,
@(CFz0)(1
Y
[ N o ) > R b ) l (4)
< R(n>l
(5)
[ N o )< R b ) l
(7)
W(o)
and
+(Cd?40){ 1
+ a ) '" = { 1 +Y a]"'
where y is the fraction of the CF2 plus CzF40* produced from reactions c and d which react with 0 2 , The J O U Tof~Physical Chemistry
+
+
+
+
-
+
(12) D. Saunders and J. Heicklen, J . Am. C h m . Soc., 87, 4062 (1965). (13) J. Heicklen, J . Phys. Chem., 70, 477 (1966). (14) W. M. Jones and N. Davidson, J. Chem. Phus.,84, 2868 (1962).
REACTION OF OXYGEN ATOMSWITH TETRAFLUOROETHYLENE
10
I
ip (CzFq), mm.
I50
50
10-1
15
ICzFqI, mm. 150 50
15
I
I
I I
I
I
+
I I
:
+
a)'/z
CzF4
+
0 2
+
and
X
than by reaction 0. Thus, the chain-branching step can be associated with reactions m and n. CZF4 is thermally stable up to temperatures of about 600". In the presence of 02,however, CF,O and cycloC3F6 are formed, even at 200" in both static and flow experiments. The process accelerates with time and a branched-step mechanism undoubtedly is involved. l 6 At 200", there are only two thermodynamically possible chain-initiating steps, and these are CF2'
during but not prior to illumination at 125" and how it is related to reactions m and n. The answer can be related to the energetics of the reaction scheme. Reactions m and n are the most exothermic of all the reactions, the heats of reaction being about 200 kcal/ mole. Clearly, the product molecules will be extremely energetic when first formed. Because the bonds formed are carbonyl bonds, the energy can be expected to reside mainly in the CF2O product. These energetic molecules will transfer energy to C2F4 or O2 or both by collision, and thus initiate reaction p. If reaction p is the chain-branching step, then at large (CzF4) and small I,, all the CF2radicals from p and only those from p and h will appear as cyclo-C3F6. Also, reaction o will be unimportant. The mechanism would then require that
r I l l
I
Figure 3. Log-log plots of { 1 a}@(CF*O)and { 1 @(CzF&) us. (CzFa)/l,'/'for (CZF~)/(OZ) 2 0.3 at 23".
2CzF4
3899
+c~c~o-C~F~ CF2' + CF202 (P)
The first reaction can be discarded for two reasons. First, such a reaction would occur in the absence of (0,) and some cyclo-C3Fe should be observed, even in spite of the fact that it readily decomposes back to CzF4 at this temperature. A more severe objection results from a consideration of the type of CF2formed. Surely such a reaction would produce the less energetic, spinallowed, singlet CF2 which at 125" does not oxidize. Thus, there would be no way for @(C2F40)to rise above unity. On the other hand, reaction p adequately explains our observed results. Again, for energetic reasons, the CF2 must be singlet and does not enter the oxidation scheme. It ultimately results in either C2F4 or cycloC3F6. However, the CF202 is the additional radical that enters the oxidation mechanism. The remaining problem is why reaction p occurs
(9) where q is the ratio of rates of reaction n to p, Le., the number of times reaction n must take place for reaction p to be induced. Unfortunately, a t high C2F4 and low I,, our data are the most unreliable. However, the left-hand sides of eq 8 and 9 are listed in Table I1 for the appropriate runs. Considering the uncertainty in the data, we see that eq 8 is well obeyed. Also, q is reasonably constant and averages to 2.0. Furthermore, at high C2F4 and low I , where reaction o is unimportant, the mechanism predicts that
The right-hand side of (10) is listed in Table I1 for the appropriate runs, and k,/k, is about 1.1, or approximately twice as large as a t 23". The enhancement is in the expected direction, as the less exothermic reaction should be more favored by raising the temperature. It should be recognized that if q had been less than unity, an unstable situation would have resulted, and the oxidation would have become self-sustaining. An evaluation of k,,/k,,'/' can be made by considering the situation at low (C2F4) and highI,, i.e., when R(o) > R(n). The expressions for @(CF20)and @(C2F40)are given by eq 4 and 6, respectively. Unfortunately, our results are never entirely in this region, as can be seen from an examination of Table 11. @(CF20) never (15)
Unpublished results of the Aerospace Carp. Laboratories.
Volume 70,Number 12 December 1966
3900
JULIAN HEICKLEN AND VESTERKNIGHT
Table III: Summary of Rate Constant Data value
0.10 95 -0.13 -2.3
5.6 x 10-3 0.098 -5 x 10-3 -0.09 125
-1
x
10-2
-0.2 k,/kn 7
23 125 125
0.5 1.1 2.0
Eq 1, Figure 1 Eq 3, Figure 2
(mm min)-''a (I./mole sec)l/p
Figure 2
(mm min)-'/~ (l./mole sec)'/z (mm min)-'/z (l./mole sec)'/2
Eq 6, Figure 3 Figure 3
(mfn min)-'/z (I./mole sec)'/z
Table I1
None None None
Eq 5, Figure 3 Eq 5, Table I1 Eq 9, Table I1
drops to 2 and a(CzF40) is not proportional to (C2F4)/Ia1/'. For example, at our highest intensities, a factor of 9 change in (C2F4) gives less than a factor of 3 change in $(C2F40). Thus a good computation for k,,/lco'/a cannot be performed. However, an approximate value can be found by realizing that when +(CF20) is halfway between its limits of 2 and 9, then R(o) = R(n). Thus k,/ko'/z can be estimated (mm min) -"*. crudely to be about 1 X
V. Summary The products of the reaction of oxygen atoms with C2F4 in the presence of 0 2 are CF20, C2F40,and cycloC3F6; CFBCFO is not produced, The mechanism involves a simple but small chain at room temperature, and a larger, branched chain at 125". The mechanism is given by reactions c-f, which occur in the absence of 02,and reactions i-p which occur in the presence of 0 2 . An important result is that the reaction of O(3P) with C2F4 is so exothermic that the intermediate C2F40 molecule is so energetic that it cannot be stabilized and thus is not found in the absence of 0 2 . However, the triplet CF2 radical produced can add to 02,if it is present; the resulting CF202 radical can donate an oxygen atom to C2F4 in a less exothermic reaction than direct addition of oxygen atoms. Thus, the C2F40 intermediate is more easily stabilized and appears as a product. A number of rate constant ratios were estimated, The Journal of Physical Chemistry
Source
Units
None l./mole
Comments
... Ten times too large; see Discussion Competition not well established. Result very approximate and only valid if (e) actually competes with (i)
... Based on break point in a(CF20)approximate value Based on break point in %(CFgO)approximate value
...
and they are tabulated in Table 111. Both k,,/kO'I2 and k,/k, rise very slightly from 23 to 125". Thus both kn and k , have small, but measurable, activation energies (-2 kcal/mole), that of k , being slightly larger, as might be expected, since reaction m is less exothermic than reaction n. It should be noted that if both L, and k , have normal frequency factors (Le., about 1O1O l./mole sec), then the activation energy difference E n - '/2E0 should be about 7 kcal/mole, somewhat larger than found. However, our hightemperature value for kn/ko'Iz is sufficiently inaccurate to accommodate this discrepancy. It is interesting to compare the results with those of the C3F6 system where the oxidation is principally of the C R C F radical.12 In that system, the reaction of oxygen atoms with C3Fe also led to two sets of products. About 85% of the time (at 23O), CF20 and CFlCF were produced in an analogous fashion to reaction c. About 15% of the time, CF3CFO was produced, perhaps in a reaction sequence similar to reactions d and g (though this has not been ascertained). If so, then in the presence of 02,a reaction analogous to reaction 1 could occur also. In the C3F6 system, reactions analogous to m, i, and o were shown to be important. At 2 3 O , the only temperature for which data exist for the C3F6 system, reaction p is unimportant in both the C2F4 and C3F6 systems. Thus the only reported difference between the systems is that reaction n produces CzF40in the C2F4 system, but no analogous reaction produces perfluoropropylene oxide in the C3F6
MERCURY-PHOTOSENSITIZED OXIDATION OF TETRAFLUOROETHYLENE
system. Recent work in our laboratory has now established the formation of the epoxide in the O-C3Fr02 system. The rate constant ratio k,,,/k,,l'q is 0.05 (l./ mole sec)l/' for the C2F4 system compared to 0.068 (l./mole see)'/' for the analogous ratio in the CaFe system.
3901
Acknowledgment. The authors wish to thank A h . Dennis Saunders for preparation of the C2F4 and Drs. Caglioti, Lenzi, and Mele for access to their original infrared spectrum of C2F40. They also wish to thank Mrs. Barbara Peer and Miss Jeanne Kiley for assistance with the manuscript.
A Reexamination of the Mercury-Photosensitized Oxidation of Tetrafluoroethylene
by Julian Heicklen and Vester Knight Aerospace Corporation, El Segundo, California (Received June BO, 1966)
The mercury-photosensitized oxidation of C2F4 was studied at 29 and 127'. The absorbed intensity was varied by a factor of 1000, and the 0 2 and C2F4pressures by a factor of 30. The products of the reaction were cyclo-CaFs,CF20, and C2F40 (tetrafluoroethylene oxide). Important intermediates in the oxidation are an electronically excited C2F4 molecule and the CF202 radical. In addition, both singlet and triplet CF2 radicals are involved. A detailed reaction mechanism is presented, and several rate constant ratios are obtained. Where comparisons with literature values could be made, agreement is good. The important oxidation step that generates the CFzOz radicals is Eo* 0 2 ---t CFz02 CF2', where Eo* is a vibrationally equilibrated electronically excited C2F4 molecule, and CF2' is the singlet CF2 radical.
+
I. Introduction The mercury-sensitized photolysis of C2F4 has been studied previously, both in the absence of 0214 and in the presence of 0 2 . 2 In the absence of 0 2 , the mechanism has been reasonably well established to be
+ hv +Hg* Hg + E,* (112.7 kcal/mole)
Hg Hg*
+ E -+
E,* +2CF2' E,* E +Eo* E Eo* E +2E
+
CF2'
+
+
+ CzF4 +CYCIO-C~F~ 2CF2'
CzF4
(a) (b)
(c> (4 (4 (f) (g)
+
where E is CzF4,the asterisk represents an electronically excited molecule (surely a triplet), the subscript n represenk vibrational excitation, and the subscript 0 represents vibrationally unexcited molecules. The CF2 radicals formed are in the singlet, state, which is shown by the superscript 1. That the singlet CF2 is formed was indicated in the oxidation studies where the results excluded the possibility of CF2 radicals reacting with 02. However, with oxygen present, oxidation products were formed from the oxidation of the electronically excited C2F4. In the absence of 0 2 , (1) B. Atkinson, J . Chem. SOC.,2684 (1952). (2) J. Heicklen, V. Knight, and S. A. Greene, J . Chem. Phys., 42, 221 (1965).
(3) N. Cohen and J. Heicklen, ibid., 43, 871 (1965).
Volume 70, Number 12 December 1966
