B. A. DEGRAFFAND G. B. KISTIAKOWSKY
3984
Photolysis of Ketene in the Presence of Carbon Monoxide
by B. A. DeGraff and G. B. Kistiakowsky Department of Chemistry, Haruard University, CambrtZge, hfa88aChU8ett8 OB198
(Received May 88, 1967)
The reaction of photochemically generated singlet and triplet methylene with carbon monoxide to form ketene has been studied a t several wavelengths. It is found that triplet methylene reacts more readily with carbon monoxide than with ketene or butene, whereas the reverse is true for singlet methylene. The relative rates for the reactions
+ CH2C0 products ‘CH2 + CO CHZCO aCH2 + CHzCO products %H2 + CO +CH2CO lCH2 + RiI TH2+ M ‘CH2
--f
--j
ki k2
kg kd
k5
are presented. The ratio kd/k3 for the triplet is 3.6 a t 3160 A and might be somewhat wavelength dependent. The ratio k& for the singlet is 0.14 ii 0.02 over the wavelength region 2900-3340 A. The ratio k5/kl is approximately 0.01 when M is nitrogen and/or carbon monoxide. When methylene reacts with trans-butene, the ratio of rates for the singlet is substantially the same as with ketene (0.10 f 0.02), but the corresponding ratio for the triplet is somewhat lower (1.3 ii 0.3). Using the reaction with carbon monoxide as the diagnostic, the percentages of singlet methylene were found to be 67% a t 2900 A, 59% a t 3160 A, 39% a t 3340 A, and much less than 40% a t 3660 A.
Introduction Since Herzberg and Shoosmith showed that methylene exists in two long-lived electronic states of different multiplicity, there has been considerable interest in the relative abundance of these spin states produced by the common sources of methylene (e.g., ketene, diazomethane, and diaxirine) . The relative population of singlet and triplet states is important in understanding the extensively studied reactions of methylene with hydrocarbons and is relevant to the problem of spin inversion leading to the formation of triplet methylene from its precursors.2 It appears that each form is capable of distinct reactions, and hence quantitative interpretation of reaction mechanisms is difficult without fairly accurate estimates of the initial amounts of singlet and triplet methylene formed from the precursor. As the singlet can “relax” to the triplet state, an additional complication is introduced in inferring initial populations by estimating the relative amounts of methylene that ultimately react as singlet or triplet.* The Journal of Phu8ical Chemistry
Several workers have made estimates as to the singlet and triplet yields in the photolysis of ketene a t various wavelength^.^ The technique used was to allow methylene to react with a subst,ituted olefin and follow the product distribution as a function of various experimental parameters. Using the Doering-Skell hyp o t h e s i ~ ,mechanistic ~ arguments can be made that certain products arise only from reactions of triplet G. Heraberg and J. Shoosmith, Nature, 183, 1801 (1969); (b) G.Herzberg, Proc. Roy. SOC.(London), A262, 291 (1961). (2) (a) For recent reviews see J. A. Bell, Progr. Phys. Org, Chem., 2 , 1 (1964);(b) W. B. DeMore and S. W. Benson, Advan. Photochem., 2 . 219 (1964): (c) 1%.M. Frev. Proor. Reaction Kinetice. 2 . 131 (1964); see also (d) J. W. Simins a n d B . S. Rabinovitch, j,Phya. Chem., 68, 1322 (1964). (3) (a) H.M. Frey, J . Am. Chem. Soc., 82, 5947 (1960); (b) R. F. W. Bader and J. I. Generosa, Can. J . Chem., 43, 1631 (1965). (4) (a) S. Ho, I. Unger, and W. A. Noyes, Jr., J . Am. Chem. SOC., 87, 2297 (1965); (b) R. W.Carr, Jr., and G . B. Kistiakowsky, J. Phys. Chem., 70, 118 (1966). (5) P.S. Skell and R. C. Woodworth, J . Am. Chem. SOC.,78, 4496 (1956);81, 3383 (1959). (1) (a)
PHOTOLYSIS OF KETENEIN
THE
PRESENCE OF CARBON MONOXIDE
3985
Table I : Effect of Added Carbon Monoxide in Ketene-trans-Butene-2 Mixtures on the CSProduct Yields a t 3160 A Pressures, mm
r
ICOl/
Q
Ketene
Butene
Other
[butene]
TDMCa
CDMCO
5.0 3 .0 5.0 5.0 5.0 5 .0 5.0
4.i .i) 43.0 45.0 45.0 45.0 45.0 45.0
... 65.0 (CO) 125.0 $0) 250.0 (CO) 338.0 (CO) 485.0 (CO) 10.0 (0%)
1.45 2.78 5.55 7.50 10.8
1.94 1.49 1.23 1.11 1.00 0.84 1.63
0.208 0.074 0.046 0.020 0.010 0.00 0.020
...
Normalized yields trans-22-Methylpentene butene-2
1.51 1.29 1.09 1.02 0.92 0.78 1.48
0.339 0.288 0.244 0.228 0.206 0.177 0.314
3-Methylbutene-I
0.106 0.038 0.023 0.011
...
... ...
The letters TDMC and CDMC denote trans- and cis-dimethylcyclopropane, respectively.
methylene, while other products must have their origin in singlet reactions. Unfortunately, the product mixtures from such studies are rather complex. Thus, while there seems to be general agreement on the trends, there are differences on the quantitative aspects. Earlier, it had been shown that methylene reacts with carbon monoxide to re-form ketene.6 It seemed worthwhile to reinvestigate the photolysis of the ketenecarbon monoxide system in the hope that like the isoelectronic species, 0 atoms, singlet methylene would be more reactive toward CO, and thus a direct measure of the rate of triplet production could be obtained.' The results of this study are described below.
Experimental Section The photolysis apparatus consisted of a cylindrical, I-shaped quartz cell connected in series with a magnetically operated mixing pump and trap, and through a bakable metal valve to a high-vacuum line. The cell and related acceswries were enclosed in a thermostat whose temperature could be controlled to h0.5'. The cell pressure was measured by means of a Wallace and Tiernan 0-50-torr differential gauge in conjunction with a standard mercury manometer. The optical train was composed of an Osram HBOdOO high-pressure mercury arc whose output was isolated into various bands centered at 3660, 3340, 3160, and 2900 A by means of a Bausch and Lomb high-intensity monochromator. The band width was 64 A in each region. The emergent beam was passed through a Corning CS7-54 filter to reniove scattered visible light, was collimated by a series of lenses and stops, was passed through the cell, and was focused on the cathode of an RCA 935 phototube. The signal from the phototube was displayed on a Moseley 680 recorder. The light fluxes for the various spectral regions were frequently measured using ferrioxalate actinometry as described by Hatchard and Parker with suitable reflection cor-
rections.* Extinction coefficients were determined a t each temperature and wavelength region using the described photometer. The hydrocarbon products were analyzed using a Perkin-Elmer flame ionization detector mounted in a Model 154 vapor fractometer. Standard analyses were done with a 2-m silica gel column (30-60 mesh) operated at 70". Products from the reaction of methylene with butene were analyzed using a 6-m column packed with 60-80 mesh firebrick coated with 33% by weight of dimethyl sulfolane, operated at room temperature. The carrier gas was helium a t a flow rate of 60 cc/min in each case. The response of the detector was calibrated before each analysis by use of a known gas mixture which closely approximated in both size and coniposition the experimental sample. In a typical experiment the cell was brought to the desired temperature and pressurized with condensable gases; then, after condensing these a t liquid nitrogen temperature, the cell was pressurized with the noncondensable gases, and finally the condensed material was allowed to vaporize. The gases were then stirred for 1.5-2 hr. The completeness of mixing had previously been checked by following the absorption as a function of mixing time. Aft,er irradiation, the mixture was passed through a packed trap a t solid nitrogen temperature. When an analysis for carbon monoxide was made, the noncondensable fraction was circulated over CuO at 220' and the resultant C0, mas condensed and then measured in a calibrated gas buret. The hydrocarbon fraction was transferred to a sample bulb which contained a small amount of acetic anhydride to destroy the excess ketene. The sample mas then (6) T. Wilson and G. B. Kistiakonskq, J A m . C'hem. Soc , 80, 2934 (1958). (7) (a) 0. F. Raper and W. B. DeMore, J . Chem. PhyY., 40, 1053 (1964); (b) It. Klein, Trans. Faraday S o c , 6 2 , 3135 (1966); (c) M. Clerc and F. Barat, J . Chem. P h y s , 46, 107 (1967). (8) C. G. Hatchard and C . A. Parker, Proc. Roy. Soc. (London), A235, 518 (1956).
Volume 71, ,Vumber 18 hTovember 1967
3986
B. A. DEGRAFFAND G. B. KISTIAKOWSKY
Table 11: Quantum Yields of Ethylene, Ethane, and Acetylene from Ketene-Carbon Monoxide Mixtures a t 50" Pressures, mm Ketene
25.0 10.0 10.0 10.0 10.0 10.0 10.0
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 50.0 25.0 20.0 13.2 8.0 5.0 5.0 50.0 50.0 49.0 50.5 50.0 40.0 20.0 19.1 10.0 5.0 21.6
co
5.0 10.6 20.0 50.0 70.0 107.5 107.8 2.6 5.0 10.6 21.0 20.8 54.0 75.0 100.0 200.0 200.0 200.0 337.4 200.0 242.0 200.0 200.0 10.0 19.5 26.0 49.5 105.2 20.0 10.0 9.6 5.0 2.5 8.4
Other
185.4 (Nz) 323.4 (Nz) 100.0 (Nz) 94.4 (Nz) 86.0 (Nz) 5 0 . 0 (Nz) 30.5 (Nz)
211.3 (Nz) 207.5 (Nz) 182.6 (Nz) 184.2 (Nz) 184.5 (Nz) 158.6 (Na) 134.6 (Nz) 101.2 (Nz)
212.4 (Nz) 100.7 (NI) 71.1 (Nz) 53.0 (NP) 18.5 (Nz)
[COI/ [ketene]
Part 1. 3160A 0.90 0.87 0.83 0.5 0.58 1.1 0.49 0.41 2.0 5.0 0.33 0.28 7.0 0.22 10.8 0.23 10.8 0.63 0.26 0.51 0.50 1.06 0.46 2.1 0.34 0.35 2.1 0.27 5.4 0.23 7.5 0.20 10.0 0.14 20.0 0.33 4.0 0.25 8.0 0.16 16.9 0.17 15.2 30.2 0.10 40.0 0.06 40.0 0.06 0.20 0.75 0.39 0.69 0.53 0.66 0.98 0.61 0.52 2.1 0.50 0.62 0.50 0.61 0.50 0.59 0.50 0.61 0.50 0.66 0.39 0.77 Part 2.
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 5.0
2.1 5.3 10.6 21.2 50.0 80.0 107.0 112.2 104.5
107.8 (Nz) 103.9 (Nz) 100.5 (Nz) 95.4 (Nz) 84.8 (Nz) 50.0 (Nz) 25.2 (Nz)
0.21 0.53 1.06 2.1 5.0 8.0 10.7 11.2 20.9
analyzed by vpc. All portions of the apparatus in contact with the products were equipped with grease-free stopcocks. The Journal of Physical Chemietry
CZH4
3340 A 0.61 0.62 0.61 0.41 0.27 0.21 0.19 0.14 0.11 0.092 0.091 0.056
CzHs
0.015 0.036 0.044 0.008 0.003 0.001
tra tr tr tr 0,006 0.005 0.002 0,001 0.001 0.001
tr tr tr nm* nm nm nm nm nm nm 0.003 0.002 0.002 tr
tr tr 0.001 0.002
0.004 0.006 0.005
0.067 0.062 0.062 0.016 0.003 tr
tr tr tr
tr tr tr
Quantum yields CzHz
... 0,046 0.044 0.007 0.005 0.003 0.002 tr tr tr 0.012 0.010 0.007 0.004 0.003 0,002
Other
.
2.02 (CO) 1.90 (CO) 1.80 (co)
tr tr tr nm nm nm nm nm nm nm 0.005 nm 0.005 0.004 0.002 0.004 0.004 0,005 nm 0,009 nm 0.067 0.061 0.062 0.022 0.007 0.006 0.003 0,001 tr tr
1.30 (co) 1 . 3 5 (CO)
tr tr
To test the procedure for handling and isolating the products, several known mixtures which approximated a typical reaction mixture were processed using the
PHOTOLYSIS OF KETENEIN
THE PRESENCE OF CARBON MONOXIDE
3987
Table I1 (Continued) Ke tene
Other
CO
KO]/ [ketene]
Part 3. 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 50.0 50.0 50.0 50.0 (1
4.2 18.5 25.0 38.7 77.6 106.5 206.8
106.2 (Nz) 103.3 (Nz) 88.5 (Nz) 81.5 (N2) 68.8 (Nz) 29.4 (N2)
51.0 (Nz) 52.4 25.2
t r indicates trace amounts.
b
0.42 1.85 2.5 3.87 7.8 10.6 20.6
Quantum yieldsCzHz
c
CZH4
CzHs
2900 A 0.93 0.66 0.49 0.45 0.40 0.29 0.26 0.16
0.01g 0,010 0.004 tr
0.020 0,020 nm 0.010 0.008 tr tr tr
tr tr tr tr
Part 4. 3660 A 0.021 0.021 1.05 0.003 0.50 0.005
5 4 3 5
x x x x
10-4 10-4 1010-6
5 4
x x
Other
2.0 (CO)
10-4 10-4
nm nm
nm indicates that peak of product was not measurable,
Table I11 : Special Experiments
Ketene
10.0 10.0 25.0 25.0 50.0 50.0 10.0 10.0 10.0
10.0
Pressures, mm Other
186.7 (CO) 180.0 (CO) 106.2 (Nz) 106.5 (Nz) 104.4 (CO) 106.0 (CO) 2.0 (02) 5.0 ( 0 2 ) 2.0 (02) 5.0 (02)
Other
A, A
Cell temp, OC
C1H4
CZH6’
3160 3160 3160 3160 3160 3160 3340 3340 2900 2900
50 50 50 50 22 85 50 50 50 50
0.13 0.13 0.64 0.65 0.53 0.50 0.21 0.17 0.67 0.66
tr tr tr tr nm nm tr tr tr tr
routine procedures. I n each case the values determined by this method agreed to within 201, of the known values. Ketene was prepared by the pyrolysis of acetic anhydride, using t~hemethod described by Jenkins.9 After purification, a ketene sample treated with acetic anhydride showed a light hydrocarbon (C,-C,) impurity level of less than 100 ppm. Nitrogen and carbon monoxide with less than 1 ppm of oxygen were supplied in 1-1. Pyrex flasks by J. T. Baker. The hydrocarbon gases were Phillips research grade. About 0.2% propane was added t o the trans-butene-2 to act as an “internal standard.” As the usual conversion was about 1%, impurity levels were critical. To check the possibility of trace oxygen impurity, the quantum yields in a ketenenitrogen-carbon monoxide mixture were determined a t 0.5, 1.0, and 2.0% conversion with no apparent trend
Quantum yields CzHz
tr tr 0.005 0.004 nm nm 0.003 0.004 tr tr
Other
2.95 (CO) 3.00 (CO)
(co) 3.02 (co) 2.41
2.97 (CO)
in the values. Several samples of less pure CO were used in preliminary experiments with no change in the results.
Results Table I shows the effect of added carbon monoxide on the Cs products for the methylene-trans-butene reaction a t 3160 A and 50”. The yields are normalized in the sense that they are relative to the internal standard and corrections have been made for the differences in the percentage decomposition. Since carbon monoxide production could not be used to measure the rate of decomposition, the latter was determined by multiplying the absorbed intensity by 0.5&0 for a ketene-nitrogen mixture of equivalent composition (Le., one in which the ketene pressure was assumed to be the (9) A. D. Jenkins, J. Chem. SOC.,2563 (1962).
Volume 71,hlumber 18 November 1967
3988
sum of the ketene and butene pressures while the nitrogen pressure was the same as that of carbon monoxide). With added oxygen the primary decomposition rate was assumed to be the same as that in the absence of oxygen. The values shown in this table are subject to some error owing to the fact that the large excess of butene, unavoidably injected along with the sample into the vpc column, made estimation of the base line very difficult. To reduce the error, several chromatograms were usually made for a given sample. The peaks were cut out and weighed, with the average value being used for subsequent calculations. cis-Pentene-2 is not shown in the table since the column used for this study could not isolate it. Table 11, parts 1-4, is a collection of the quantum yields of the observed products in other experiments. S o products higher than Cs were observed in mixtures containing carbon monoxide. In ketene-nitrogen mixtures small amounts of propane, cyclopropane, and propylene were found a t higher conversions. The experimental variables included total pressure, mole fraction of ketene, carbon monoxide, and nitrogen, temperature, and wavelength. Illuminetion times varied frorn 15 min for high ketene pressures a t 3160 A to 5 hr itt 3GGO A. Several blank experiments were performed which showed that thermal decomposition was negligible at the temperatures of these experiments (ie., 20-80’). Product quantum yields which are less than of the ethylene quan tum yield are not quantitatively reliable and are shown as trace amounts. The estimated error in the ethylene and carbon monoxide For the oOher C2products the error is values is *5%. likely to be as high as =k 10%. These error limits should not be confused with reproducibility, as individual values were reproducible to better than * 5 % . The major sources of systematic error were in determining the light flux and vpc response. Table I11 is a compilation of results from experiments with added oxygen a t various wavelengths. I n addition t o the products shown, carbon dioxide was found when oxygen was added, but no attempt was made to measure it,. Also included are the results of a temperature-dependence study done at 3160 A.
Discussion Since the decomposition quantum yield for ketene has been shown to be pressure dependent for X >2700 A, it was necessary to determine the pressure effect on the primary decomposition yield for our conditions so that collisional relaxation effects could be distinguished from recombination quenching.‘O As nitrogen would be expected to be similar to carbon monoxide in its The Journal of Physical Chemistry
B. A. DEGRAFFAND G. B. KISTIAKOWSKY
ability to deactivate excited ketene, experiments were performed in which the decomposition quantum yield of ketene was measured as a function of increasing nitrogen pressure a t various wavelengths. It was found (see Table 11) that addition of 100 torr of Nz caused essentially no change in the quantum yield at 2900, 3160, and 3340 A, while 50 torr produced no change at 3660 A. Further, -200 torr of Nz lowered the yield a t 3160 A only 4-50/, as compared to a 17-180/, decrease for a comparable pressure of pure ketene.1° These results are consistent with the findings of others that nitrogen is quite inefficient in deactivating excited ketene.6 Thus, no corrections for the “pressure effect” of added GO and Nzwere necessary except for the series of experiments done a t 3160 A with a total GO and Sz pressure of 200 torr or greater. For these experiments the correction was determined by measuring 4(C2H4) with a pressure of nitrogen equal to the combined pressures of nitrogen and carbon monoxide used. The ratio of the ethylene yield with added nitrogen to the value found in the absence of any added gas was used as the pressure correction. Preliminary experiments showed that the two forms of methylene did indeed react with GO a t different rates and that a definite trend with wavelength was present which was contrary to that expected if the singlet were the more reactive species.4b To obtain additional information as to the spin multiplicity of the more reactive methylene, the effect of carbon monoxide on the methylene-trans-butene reaction was studied. Previous work indicated that cis-dimethylcyclopropane and 3-methylbutene-1 could be used as triplet diagnostics, while trans-pentene-2 could be used for the singlet.4b Figure 1 shows the yields of these products relative to the GO-free value as a function of the [carbon monoxide]! [butene] ratio. The rapid removal of the triplet products compared to those of the singlet indicates that relative to butene, triplet methylene is more reactive toward CO than is singlet. We conclude that this is also the case in the ketene-carbon monoxide system. Additional evidence for this assignment is obtained from the observation that even a t small [CO]/ [ketene] ratios the quantum yields of ethane and acetylene are dramatically reduced. Both ethane and acetylene are reported to arise as a result of triplet methylene abstraction from ketene.” That these C2’sare quenched by CO more rapidly than ethylene, which can come from both singlet and triplet methylene, gives credence to (10) B. T. Connelly and G. B. Porter, Can. J . Chem., 36, 1640 (1958). (11) K. H. Sauer, Ph.D. Thesis, Harvard University, 1957.
PHOTOLYSIS OF KETENEIN
THE
PRESENCE OF CARBON MONOXIDE
3989
0.8) I
I
I
3650
I
1
I
I
3450
I
I
I
I
3250
I
I
I
I
3050
I
I
I
2850
A (8, Figure 1. Rate of selected Cg hydrocarbon formation a t 3160 A in a ketene-trans-butene-2 mixture with added carbon monoxide relative to the CO-free rate: 0 , trans-pentene-2; 0, CDMC 2-methylbutene-1.
+
the greater reactivity of triplet methylene toward carbon monoxide. This same effect is found in the ketene-butene system. That is, with no added carbon monoxide the Cz fraction is about 60% ethylene and 40% ethane, whereas at a [CO]/[butene] ratio of 10.8 the Cofraction is 100% ethylene. Oxygen has been used as a means of selectively removing triplet species. However, the ethylene quenching action of Ozin the photolysis of ketene is not well understood. As shown in Figure 2, the relative quenching curve of Oz as a function of exciting wavelength has two distinct regions. They differ not only in the magnitude of the quenching effect but also in that, a t the wavelengths (ie., 3660 and 3340 A) where Oz is particularly effect#ive,the ethylene quantum yield is dependent on Oz pressure, whereas in the region of less complete quenching the Oz pressure may be varied within wide limits with little effect on the yield of ethylene. Because of the oxygen pressure effect, it has been suggested that Oz reacts with excited ketene a t 3660 AI2 and this seems to be the case at 3340 A as well. Whether only triplet excited molecules are scavenged a t these wavelengths is not clear. Recent isotopic studies on the mechanism of oxygen quenching in methylene-olefin systems shows that oxygen reacts with the secondary radicals rather than with the meth~1ene.l~Consequently, the quantitative aspects of oxygen quenching as a triplet diagnostic are open t o question; however, it does provide a qualitative, indirect means of ascertaining triplet participation. II1 and Table I1i Part 1, shows of that, within experimental error, oxygen has no effect
Figure 2. Quantum yield of ethylene with added oxygen relative to the Orfree value: 0, this work; A, G. B. Porter (1957); 0, A. N. Strachan and W. A. Noyes, Jr. (1954).
on the ethylene quantum yield at high IC0 I/ [ketene] ratios and 3130-A radiation. This is in contrast to the 25-30ojO reduction in the ethylene yield by oxygen in the absence of CO (see Figure 2 ) and is further evidence that the triplet methylene has been preferentially removed by the CO before it react8 with the ketene. The remainder of the discussion will be more explicit if the following mechanism is considered CHzCO
+ h~ ----ticHz
\
3CH2
+ CO
+Js
+ CO
+Ja
+ CHzCO +CzH4 + CO 'CHZ + CO +CHzCO 3CHz+ CHzCO +CzH4+ CO 3CH2+ CO -+ CH2C0 'CHz + 11 + 3CHz + 11 'CH2
ki
kz
k3 164
ks
where & and +t denote the quantum yields of singlet and triplet methylene, respectively. This simplified mechanism, which omits the details of the primary processes in ketene, tacitly assumes that the pressure is high enough to stabilize all re-formed ketene. The validity of this assumption will be shown later. All hydrocarbon products other than ethylene are ignored. Additionally, the assumption is made that CO and NZ are equally effective in promoting reaction 5. Using the conventional steady-state treatment., the above yields (12) G . B. Porter, J. Am. Chem. Soc., 79, 1878 (1957). (13) c.McKnight and F. S. Rowland, ibid., 88, 3179 (1966).
Volume 71. Number 1.$ Noaember 1967
3990
B. A. DEGRAFFAND G. B. KISTIAKOWSKY
kz [CO1 + kl[ketene]
'(CzH4) =
''
1
-
+
5.0 -
kl[ketene]
4s (l/ks [MI)(kl[ketene] k4 [COI -t- kl[ketene]
4.0
+ kz [COI)
I
I
I
I
I
I
I
I
I
I
-
-
-
3.0
This expression is cumbersome, but under some conditions simplifying approximations can be made. For experiments a t [COI/ [ketene] ratios such that essentially all of the triplet methylene reacts with carbon [MI, the above monoxide and further that [CO] expression simplifies to
4)'
-
I
0
I
0.2
I
,
I
0.4
I
03
I
1
I
0.8
I
1.o
[COl /[Ketene1
Figure 4. Determination of the ratio kd/kn a t 3160 A.
Thus a plot of 1/+(CzH4) vs. [CO]/[ketene] should be linear if the triplet has been removed. Figure 3 shows such a plot for data taken a t 3160 A and 50'. The least-squares intercept is 1.88 f 0.1 which corresponds to (bs = 0.53 f 0.03. The slope was found to be 0.266 f 0.01, whence (kz k5)/kl = 0.141 f 0.007. A similar plot for data taken a t 3340 A gives a slope of 0.62 f 0.03 and an intercept of 4.1 f 0.2. Data at 2900 A when plot,ted in the foregoing manner give an intercept of 1.7 f 0.1 arid the slope as 0.22 i0.01. If ICl > k, and, as one may assume from previous work, kg
