1950
DENNISSAUNDERS AND JULIAN HEICKLEN
Some Reactions of Oxygen Atoms.
I.
C,F,, C,F,, C,H,, C,H,, C,H,,
l-C,H,, C,H,, c-C,H,, and C3H2a
by Dennis Saunders and Julian Heicklenlb Aerospace Corporation, El Segundo, California
(Received J a n u a r y 3,1966)
Oxygen atoms 0 (") were generated from the mercury-sensitized decomposition of NzO in the presence of CZF4 or C3F6 or hydrocarbon-perfluoroolefin mixtures a t 25, 70, and 125'. With CZF4, CFzO is produced with a quantum yield of 1.0 for all conditions. With C3F6, @(CF&FO) is about 0.14 for all conditions, but @(CFzO)drops from 0.86 a t 25' to 0.64 a t 125". The competitive studies with hydrocarbons gave rate parameters, which are tabulated. The hydrocarbons studied are CzH2, CZH4, C3H6, l-CkHs, CZH6, c-C3H6, and C3Hs. Our results agree reasonably well with previous work, where comparisons could be made.
I. Introduction The reactions of oxygen atoms are extremely important in combustion and reentry. Consequently, it is useful to know the rates of these reactions. One of the most useful techniques for generating oxygen atoms, O(3P), is the mercury-sensitized photolysis of NzO. If two reactive species (to oxygen atoms) are also present, then the oxygen atoms will attack these species. The relative amounts of products from each species will give the relative rates of reactivity. Many ratios of rate constants have been found in this way.2 However, often two species give the same products or interfere with one another; even if they do not, it may be tedious to make a complete analysis. Recently we have investigated the room-temperature reaction of oxygen atoms with CzFd3" and C3F6.3b I n the former case, the only oxygen-containing product was CFzO, and, in the latter, small amounts of CF3CFO were also produced. These products are easy to follow by in situ infrared analysis, they are formed directly by oxygen atom attack, and they cannot be produced from oxygen atom-hydrocarbon reactions. Thus, C2F4 and C3F6 are ideal for competitive studies with hydrocarbons. Furthermore, the C3F6 reaction rate constant is one-thirtieth as large as that for CZF4; by the use of both fluoroolefins, a large range of hydrocarbon reactivities can be monitored. I n this paper, we extended the fluoroolefin studies to 125". Then we used these results to study the comT h e Journal of Physical Chemistry
petitive reactions with CZHZ,CZH4, C3H6, 1-C4He, CZH6, c-C~HG, and C3H8.
11. Experimental Section A . Chemicals. Matheson Co. NzO, 2-C4F8, CzH2, CzH4, C3H6, 1-C4Hs,CZH6, C3H8, and c-C3H6were used after degassing by pumping through a spiral trap a t -196". Acetone, which is a preservative in C2Hz, was separated from CzHz before degassing by distillation at - 126". At worst, no more than a trace of acetone was observed in the infrared spectrum even at 100 mm of CzHz. Peninsular ChemResearch Co. C3F6 was used after degassing, and Matheson Co. Hz, 02,N2, and CH4 were used without purification. CZF4 was prepared by the debromination of the vicinal dibromide CzF4Brz(Du Pont de Nemours, Freon 114-B-2). The liquid Freon was added dropwise to a warm (50") slurry of zinc dust and methanol containing some ZnClz. The rate of addition was adjusted to keep the solvent gently refluxing, and the effluent CzF4 was subsequently purified by passing it through water and Drierite, and finally degassing it as above. analysis of the above conipounds was performed using an F & M Model 720, (1) (a) This work was supported by the C. S. Air Force under Contract No. AF 04(695)-669; (b) t o whom requests for reprints should be sent. (2) For a review, see R. J. Cvetanovib, Advan. Photochem., 1, 115
(1963). (3) (a) D. Sanders and J. Heicklen, (1965); (b) ibid., 87, 4062 (1965).
J. Am. Chem.
Soc., 87, 2088
SOMEREACTIONS OF OXYGEN ATOMS
1951
Table I: Reaction of O(8P)with C ~ F ((NzO) G = 515 f 65 mm; A 2537 A) (C3F6)v mm
Exposure time, min
RWd,
Q(CFz0)
dmln
T 5 65 15 5 49 5 152
4 10 26 35
08 00 00 00
110 106 110 102
10 30 45 103
6.00 16.00 14.00 10.00
268 238 169 228
0 0 0 0
T
a
=
Q.(CFaCFO)
Q( CFz0)/ Q(CF3CFO)
+(cr20) + Q(CFsCF0)
25Oa 85 85 92 83
125' 0.67 0.66 0.64 0.53
0 0 0 0
149 151 152 132
5 5 6 6
7 6 1 3
1 00 1 00 1 07 0 96
4.5 4.8 4.4 4.7
0.82 0.80 0.78 0.64
=
0.147 0.138 0.145 0.114
Data from ref 3b corrected.
programmed-temperature, dual-column gas chromatograph, utilizing a 5 ft X 0.26 in. diameter silica gel column. All compounds indicated less than 0.5% contamination except the 2-C4Fs, which has about 2.5% impurity of a C4 or Cs compound. B. Apparatus. The vacuum manifold, X-shaped cell, and optical arrangement, which have been previously described, 3a were used with the following modifications. The windows were fixed to the cell using epoxy cement and Viton 0 rings; sodium chloride windows were used in the infrared axis. I n addition, the whole cell was surrounded by an aluminum heat sink, which was wrapped with heating tape and asbestos paper. Temperature was controlled by a Variac variable transformer and measured with a chromelalumel thermocouple placed on the cell at the center of the block. After correction for ambient reference potential, the temperature measured in this manner varied by no more than 1 O from the reported value. C. Procedure. Various mixtures of hydrocarbon and fluorocarbon were prepared in the cell such that their total pressure did not exceed 150 mm and such that the ratio of their rates of reaction with oxygen atoms would be between 0.5 and 5.0. I n addition, 450 to 580 mm of N20 was added. The mixture was allowed to stand for several minutes to permit complete diffusion. Then an initial infrared spectrum from 2 to 16 p was taken. After the lamp had warmed, the mixture was irradiated. After illumination had been terminated for a t least 3 min, a final spectrum was recorded. The 5.12-and 5.31-p bands were used for the analysis of CFzO and CFZCFO, respectively. I n some C3F6 runs the 5.31-p absorption was partially obscured by other bands. Thus, CF3CF0 was determined by calculation from the CFzO pressure and the known value of the CFZOto CF3CF0 ratio. The contents of
the cell were expanded through a trap at -196" to a McLeod gauge, and the pressure of the noncondensable gas (Nz) was measured. By use of suitable expansion factors, which were previously determined, the rate of nitrogen formation was obtained. To reduce secondary reactions, exposures were limited so that no more than 10% of any reactant was consumed. I n addition, the absorbances in some runs were followed during exposure, and the fluorocarbonyl compounds grew linearly with time. To convert absorbance to pressure, it is necessary to know extinction coefficients. Previous calibrations in our laboratory gave extinction coefficients (to base 10) a t 24" of 0.013 and 0.056 (mm-cm)-l for the 5.12-p band of CFzO and the 5.3-p band of CF3CF0, respectively. These calibrations have now been repeated in our laboratory for six different systems on three different infrared instruments; our best values are now 0.011 and 0.048,respectively, for the two bands. To obtain calibrations at elevated temperatures, the CFZO or CF3CFO was prepared at room temperature (the CF20 from Hg sensitization of NzO in the presence of C2F4and CF3CF0 from Hg sensitization of 2-C4F8 in the presence of 02)and the absorption was measured. The cell was then quickly (to minimize decomposition) heated to the desired temperature and the absorbance was measured again. Separate experiments showed that the amount of decomposition was insignificant for the CF3CFO for the heating times involved. For the 5.31-p band of CF3CFO, the heating experiments gave an extinction coefficient (to base 10) of 770 M-I cm-I a t 125", which can be compared to the value of 880 cm-I at room temperature. Because the values were so similar, the 70" value was taken to be 830 AII-l cm-' by interpolation. Volume 70, Number 6
June 1966
1952
DENNIS SAUNDERS AND JULIAN HEICKLEN
Table 11: Relative Rates of O(aP) Reactions with Hydrocarbons and CzF4 ((N10)
=
Hydrocarbon
C2H2
Run no.
36 38 39 40
Temp, OK
297 297 297 297
(CZFI), mm
15.0b 13.5 5.40 1.95
(Hydrocarbon), mm
0 49.5 44.6 38.0
515
&
65 mm)
R(Nz), a/min
59.6 51.8 48.9 47.6
...
...
32.4 18.1 9.29
0.164 0.205 0.211
Av 0.193 + 0.020 69 74 72 75 73 76
343 343 343 343 343 343
40.0' 19.5 15.5 15.0 11.5 16.0
0 23.5 33.5 65.0 87.5 0
129 132 130 131
398 398 398 398
89. Od 32.0 36.0 11.0
0 35.0 109.0 93.0
297 297 297 297 297 297 297 297 297
31.5 11.0 29.0 3.33 3.50 12.0 3.08 13.0 3.40
47.6 22.9 33.0 16.0 22.6 18.0
16.5 21.3 6.53 7.15
0,321 0,254 0.334 0.284
...
...
Av 0.296 5 0.03
C2H4
59" 62" 57" 65" 58" 61" 66" 60" 67"
10.5 13.0 36.0 5.27 88.5 32.5 10.0 89.0 34.5
273 147 169 146 127 115 133 124 119 125 128 134 130
...
...
91.0 60.9 26.0
0.564 0.587 0.547
Av 0.566 =k 0.014 101 0.78 48.9 1.14 67.8 0.77 53.3 0.84 30.7 1.13 34.2 0.98 32.9 0.89 22.6 0.72 12.6 0.92 Av 0.91 + 0.12 37.8 17.1 19.9
1.15 1.25 1.11
199 56 200
343 343 343
37.0 26.5 18.0
18.5 30.0 43.0
59.5 41.2 72.8
122 121 123
398 398 398
62.0 28.0 20.0
21.0 29.0 63.0
266 337.5 323
Av 1.17 + 0.05 189 1.20 133.5 1.47 59.2 1.42
149 126 137 121 132
Av 1.36 31 0.11 103 5.66 92.5 3.39 59.0 4.53 5.15 4.11 29.8 3.65
~
59 62 63 201 64 202 61 65 203
297 297 297 297 297
37.0 103.0 30.5 93.5 31.0
2.9 11.0 9.10 30.5 29.0
343 343 343 343 343 343 343 343 343
101.0 61.0 53.5 41.0 25.5 48.0 42.5 17.0 19.0
12.0 11.0 11.5 13.0 10.5 23.0 26.0 18.0 22.0
33.8 25.2 28.4 54.9 59.8 50.5 30.5 57.5 50.1
Av 4.27 23.8 16.3 16.4 21.7 27.4 18.8 10.8 11.1
9.91 Av 3.47
The Journal of Physical Chemistry
&
0.66 3.53 3.03 3.40 4.51 2.86 3.53 2.94 3.94 3.50
+ 0.28
SOMEREACTIONS OF OXYGENATOMS
1953
Table I1 (Continued) Hydrocarbon
Run no.
Temp, OK
C3Hs
126 125 124
398 398 398
mm
(Hydrocarbon), mm
98.0 69.0 30.0
11.0 20.0 35.0
(CzF4,
WNd, p/min
260 274 201
R(CFzO), p/min
k,/kd'
173 116 38.1
4.46 4.68 3.63 ~
Av 4.26 3~ 0 . 4 1 1-CIHa
100" 95" 97" 101"
297 297 297 297
105 33.0 30.0 73.0
10.0 4.0 10.0 31.0
76.9 78.2 90.9 78.1
59.4 54.7 39.7 33.5
3.10 3.55 3.87 3.13
Av 3.41 f 0.30 68 67 69 66
343 343 343 343
11.0 15.0 16.0 15.0
99.0 48.5 40.0 14.0
49.7 52.6 47.6 49.7
35.0 24.7 17.8 8.3
3.78 3.66 4.18 4.61
Av 4.06 f 0.34 129 128 127
398 398 398
11.0 23.0 31.0
89.0 68.0 36.0
274 226 228
173 102 43.3
4.69 3.61 4.96
Av 4.42 f 0.51 a Corrected from ref 3a. present. e From eq 1.
* Perfluoroolefin was C3F6 rather
than C2Fd.
However, for the CF20 (half-life of decomposition at 125' of about 6 hr), a small loss would be expected during heating (heating time about 2 hr). Ignoring the loss gave an extinction coefficient (to base 10) for the 5.12-p band of 177 M-l cm-l at 125" as compared to 205 M-l cm-' at 24". Using the value of 177 gave quantum yields of CF20 production in the N20-CzF4 experiments slightly in excess of unity. Assuming that the quantum yields are indeed unity gives an extinction coefficient of 191, the value used. At 70°, the extinction coefficient is 201. The small drop in the extinction coefficient at a peak is to be anticipated from temperature broadening.
with Perfluoroolehs Previously, oxygen atoms have been allowed to react with CzF43aat room temperature; the only products found were CFzO and C-CsF6. Since then, the absorption coefficient for the 5.12-p band of CF2O has been slightly modified (see Experimental Section). However, within experimental error, @(CF20) is still unity, independent of C2F4pressure or the absorbed intensity. For this work, this study was repeated at 125" with the same results. However, with extended exposures, SiFl was also observed as the result of secondary reac111. Reactions O(")
16 mm of 1-GHa also present.
11 mm of 1-C4Ha also
tions. Even though it would have been easily detected, tetrafluoroethylene oxide was not found. Earlier had shown that at room temperature the oxygen-containing products of the O(3P) reaction with C3FBwere CF20 and CFsCFO. A few of the recalculated values from that study are shown in Table I. From a large number of runs the ratio of @(CF20)to @(CFsCFO) was found to be 6.5 and the sum 1.00. Both quantities were invariant to absorbed intensity and C3FB pressure. The data at 125" are also shown in Table I. Both @(CF20)and @(CFsCFO) are unaffected by changes in the C3F6 pressure. Furthermore, G(CF3CFO) is the same at 125" as at 25". However, @(CF20)has dropped somewhat. Thus, the ratio of @(CF20)and G(CF,CFO) is about 4.6 and their sum is about 0.78. It is not clear what has become of the extra oxygen atoms. A photolysis to 30% conversion gave as products CF20, CF3CF0, C5Fl0, and unidentified infrared bands at 11.9, 12.3, and 14.9 p . However, these latter bands might belong to CjRo, as its spectrum is not known at wavelengths in excess of 11 p. Possibly, the oxygen deficiency might be attributed to the formation of a polymeric oxide, though we have no definite evidence for such a reaction. By interpolation, the sum of Volume 70,Number 6 June 1966
1954
DENNISSAUNDERS AND JULIAN HEICKLEN
Table I11 : Rate Constant Data for O( ") Eo - Ed,a kcal/mole
Hydro-
carbon
CzHz
Eo
Reactions
lo-%, M-1
- Ed,=
k/k(CzHd
kcal/mole
sec-1 a t 2 5 O
at 2 5 O
...
0.090 0.0186 0.054 i: 0,017' 0.6Zd
*
0.21"
,..
5.ai 4.7"
2.6
C2H4
0.92
-0.65
C3H6
0.00
...
1-CdHg
0.62
...
...
CZH6
...
2.1
...
5.84 3.8i 5.50 0 . 0044a
C&
... ...
1.2 1.6
... ...
0.00115 0.016"
...
... ...
...
1.10' 0. 036k
l.lE
3.0' 3.1'
CzF4 C3F6
...
0.011orh 0.021
lO-gA,
M-1 sec -1
... 26' 8.8d 16"' 6.9' 6.5' 13Q 12" 3.51 5.4" 80" 0.19' 5.31 llO* 1.58' 0.77'
E,
kcal/mole
