3679
NOTES
70
c 0 0
4
8
12
% CSOl. 0
1 2 Time, min.
3
Figure 1. Pressure variation a t 620°, showing inhibition period and explosion. Initial partial pressures: 40 mm of eo, 20 mm of 0 2 , 4 mm of c&.
Furby, and Wilkinson. Its infrared spectrum, checked against that published by Diallo and bar chew it^,^ revealed no impurities. Matheson C P grade CO and Ohio Medical O2 were used. A fresh mixture of CO, 02,and C302was prepared before each series of runs. The reaction took place in a Vycor cylinder 30 cm long and 4 cm in diameter, the temperature of which was uniform by 1" or better. The pressure was recorded by means of a Pace transducer. When cold 2CO: 1 0 2 mixtures were rushed into the reaction vessel at pressures above 30 mm and temperatures above 560", explosion took place within less than 1sec. However, in the presence of GO2,the same mixtures exploded only after considerable delay. Figure 1 shows a typical pressure record obtained at 620' with a mixture in which the initial partial pressures were 40 mm of CO, 20 mm of 0 2 , and 4 mm of C302. After introduction of the gases, a slow pressure rise precedes the explosion. I n all our experiments the pressure increased during the inhibition period by an amount equal to the partial pressure of C302initially present in the mixture. Such a pressure increase corresponds to 0 2 = COz 2C0, after the over-all change C3Oz completion of which explosion occurs. Figure 2 shows how the inhibition period varies with the initial c302concentration, expressed in volume per cent, in a mixture of 40 mm of CO and 20 mm of 0 2 at 620". Under these conditions, a threshold concentration of about 1% ( 2 3 0 2 was necessary to cause measurable inhibition. The shape of the curve in Figure 2 can be accounted for provided the rate of disappearance of C302 follows an order higher than zero. If so, the inhibition period, i.e., the time it takes 'for the C302 concentration to fall from the initial to the threshold value, will increase less rapidly than the initial concentration. To test the hypothesis that the slow combustion at CO-rich mixtures might be self-inhibited by formation of some C302, we did several runs with 3CO: lo2mixtures in the ranges 560-590' and 40-180 mm and withdrew samples after about 30% conversion. No C302 was detected by infrared analysis, showing at least that its concentration was well below the value of 1% nec-
+
+
Figure 2. Inhibition periods a t 620' for various percentages of C302 added. Partial pressures: 40 mm of CO, 20 mm of 0 2 .
essary to inhibit explosion in our experiments with stoichiometric mixtures at 620". The reactions of 0 and 0 2 with C302 and with the C20 radical have been the object of a number of investigations, especially those of von Weyssenhoff, Dondes, and Harteck,S of Williarnson and Bayes,6 of Kunz, Dondes, and Harteck,' and of Liuti, Kunz, and Dondes.8 There is little doubt that C302 competes with CO for the 0 atoms (and perhaps for some excited species) responsible for the branching reaction that leads to explosion. This is not necessarily linked to the main mode of destruction of GO2, which may begin with pyrolysis followed by reaction of C 2 0 with 02. This, in our view, is the reaction responsible for the pressure increase during the inhibition period. Acknowledgment. This work was supported by National Research Council of Canada Grant A-113 and by a Canadian Government Scholarship to P. M. (3) J. 8. P. Batchelor, E. Furby, and K. L. Wilkinson, Atomic Energy Research Establishment Report R-3942, Harwell, Great Britain, 1962. (4) A. 0. Diallo and P. Barchewitz, J . Chim. Phys., 61, 1296 (1964). (5) H. yon Weyssenhoff, S. Dondes, and P. Harteck, J . Amer. Chem. Soc., 84, 1526 (1962). ( 6 ) D. G. Williamson and R. D. Bayes, ibid., 89, 3390 (1967); 90, 1957 (1968). (7) C. Kunz, 8. Dondes, and P. Harteck, J . Chem. Phys., 46, 4157 (1967). (8) G. Liuti, C. Kunz, and 8. Dondes, J . Amer. Chem. Soc., 89, 5542 (1967).
The Photochemical Decomposition of Methanethiol.
Hot Hydrogen Atom
Reaction with Deuterium'
by G. P. Sturm, Jr:,2 and John M. White Department of Chemistry, The University of Texas at Austin, Austin, Texas 78719 (Received May 8, 1968)
The gas-phase photochemical decomposition of methanethiol has been investigated extensively by Volume 72, Number 10 October 1968
3680
NOTES
conventional photochemical techniques.a-6 It is well accepted that the main primary process is the production of hydrogen atoms and methylthiyl radicals. Some workers suggest the production of CH3 and HS radicals as a minor primary process accounting for about 10% of the light absorbedS6 To our knowledge, the steady-state distribution of hydrogen atom energies arising as a result of the main primary process has not been studied previously. Hot H atom effects were recently shown to be important in the photochemistry of HzS.' The purpose of this note is to report our results which confirm the importance of hot H atoms in the photochemistry of methanethiol. Experimental Section Methanethiol obtained from the Matheson Co. was used after two bulb-to-bulb distillations. Deuterium (99.5%), neon (99.995%), and helium (99.9957J purchased from J. T. Baker were used without further purification. The photolysis cell (5 cm in diameter and 15 cm long) was constructed of fused quartz with Suprasil windows. All gas handling was done with a mercury-free vacuum system (normal pressure about torr). A mercury manometer, separated from the mercury-free system via a transducer, was used for pressure measurements. Monochromatic light from a low-pressure mercury resonance lamp with a Suprasil window was used in all experiments. A Corning No. 7910 filter was used to remove the 1849-A mercury line for the 2537-A experiments. Irradiated lithium fluoride filters yere used in the 1849-8 experiments to eliminate 2537-A radiation. Products were analyzed with a Consolidated Engineering Corp. Model 21-401 mass spectrometer. Decomposition of methanethiol was 401, or less in all experiments. Results and Discussion Table I lists the experimental reactant pressures and the resulting groduct ratios for the 1849-A experiments. I n the 2537-A experiments, the methanethiol partial pressures were in the range 39-52 torr. All experiments were conducted at room temperature.
*"
1.0
CHaSH
DB
He
pressure, torr
pressure, torr
pressure, torr
The results of photolysis of CHaSH with 2537-i light in the presence of various amounts of Dz are shown in Figure 1. The quantity 2Hz: H D is found to be a linear function of the ratio CHaSH:Dz. Addition of rare gas (He or Ne) increases the 2Hz: H D ratio in the products. Figure 1 shows that if He:D2 or Ne:D2 is fixed and CHaSH:Dz is varied, 2Hz:HD is a linear function of CHaSH:Dz with qualitatively the same slope as the experiments in which no rare gas was added. However, the addition of a rare gas has a pronounced effect on the intercept. Figure 1 also shows the corresponding results for photolysis with 1849-A light. Table I1 summarizes the results. The positive intercepts in Figure 1 suggest that a hot atom effect is present.* Furthermore, the lower intercept obtained in the 1849-8 results implies enhanced H D production and suggests that the H atoms produced with 1849-8 light %remore energetic than those produced with 2537-A light. Finally, the increase in 2Hz: H D observed in the rare gas experiments is consistent with thermalization of the H atoms by the rare gas. The results are consistent with the mechanism
9.92 12.34 13.14 4.95 6.12 16.07 15.03 16.39 10.98 11.09
20.72 39.27 8.74 7.69 50.84 4.89 7.81 4.73 12.83 4.95
...
The Journal of Physical Chemistry
...
... ...
...
...
...
12.89 35.01 13.51
H*
2Ha:HD
4.24 3.41 7.97 4.55 2.87 14.3 9.11 21.7 9.43 16.0
4.0
Figure 1. Variation of product ratio with retctant ratio in the photolysis of C H ~ S F - D Zmixtures: 0 , 1849 A with no rare gas; m, 1849 A with He:D2 =o 2.73; 0, 2537 with no rare gas; A, 2537 A with Ne:D2 = 1.86; 0, 2537 b with He:Dz = 2.73.
CHaSH Table I : Experimental Data for Photolyses at 1849 A
CHJSH/O,
+ hv + CHaS + H*
+ CHaSH + Hz + CHaS
(1) (2)
(1) This work was supported in part by the Petroleum Research Fund of the American Chemical Society. (2) NASA Trainee. (3) N. P. Skerrett and H. W. Thompson, Trans. Faraday SOC.,37, 81 (1941) (4) T. Inaba and B. deB. Darwent, J. Phys. Chem., 64, 1431 (1960). (6) R. R. Kuntz, ibid., 71, 3343 (1967). (6) R. P. Steer, B. L. Kalra, and A. R. Knight, ibid., 71, 783 (1967). (7) B. deB. Darwent, R. L. Wadlinger, and Sr. M. J. Allard, ibid., 71, 2346 (19G7). (8) R. J. Carter, W. H. Hamill, and R. R. Williams, Jr., J . Anzer. Chem. SOC.,7 7 , 6467 (1965).
3681
NOTES
+ Dz -+ H D + D H* + CHaSH -+ H + CHaSH H*
H*
+ Dz
-+
H
(3) (4)
+ Dz
(5)
H*+M-+H+M
(6)
+ CHaSH Hz + CHaS D + CHaSH H D + CH3S CHsS + CHaS CHaSSCHa H + Dz + H D + D H
(7)
CHaSH/DZ, (2) a lower intercept was obtained for 1849-;i light than for 2537-;i light, (3) added rare gas increased the Hz:HD ratio, and (4) thermalization by He was much more effective than thermalization by Ne. (9) W. R. Schulz and D. J. LeRoy, Can. J . Chem., 42, 2480 (1964). (10) K. Yang, J . Amer. Chern. Soc., 84, 3795 (1962).
(8)
--+
(9)
4
(10)
where H* denotes translationally hot H atoms and M denotes He or Ne. Reaction 10 is insignificant in comparison with reaction 7 in our experiments. At the lowest CH3SH:D2 ratios used in these experiments, the rate of H2 production by reaction 7 is at least 1000 times
Spectroscopic Studies of Isotopically Substituted 4-Pyridones'
by Robert A. Coburn Army Materials and Mechanics Research Center, Watertown, Massachusetts OR178
and Gerald 0. Dudek
Table 11: Summary of CHaSH-Dg Photolyses at 1849 and 2537 A
Department of Chemistry, Harvard University, Cambridge, Massachusetts OR138 (Received M a y 9, 1968)
Wavelength,
A
M:Dz
2537 2537 2537 1849 1849
0 2.73 (He) 1 . 8 6 (Ne) 0 2 . 7 3 (He)
Slope
6.90 5.37 7.09 3.60 4.71
f 0.23 =t 0.90 =t 1 . 9 8
f 0.06 =k 0.02
Intercept
5.30 21.27 8.91 2.37 5.41
=k 0.47 f 1.53
f 1.38 =t 0.10 =k 0 . 0 5
that of reaction 10, based on thermal experiments near 300°K.4~9~10 This value appears conservative in the light of recent work.' Making a steady-state approximation for H and D concentrations, we find that
Recent spectroscopic studies employing nitrogen-15 and oxygen-18 substitution have produced significant impact upon present concepts of the molecular structure and properties of 2-pyridonesa2 In the infrared spectrum, unsuspected anomalous solvent and concentration effects were discovered, and revisions were made to previous vibrational assignments. Pmr studies have shown the intermolecular exchange of the enolizable proton to be unusually facile. We wish to report here analogous studies of the corresponding. isotopically substituted 4-pyridones.
The first term predicts the observed linearity in Figure 1. The intercepts are given by k6/k3 and (ks/k3) (Ic,3/k3)([M]/[Dz]) for no rare gas and for constant [MI/ [Dz],respectively. It is apparent that the slopes Experimental Section in Figure 1 are all of the same order of magnitude. Furthermore, it is evident that He is much more effi4-Pyridone was obtained from the Aldrich Chemical cient than Ne in thermalizing the hot H atoms, thereby Co. and was purified by repetitive sublimation and was reducing the HD yield. By comparing the intercepts dried over anhydrous phosphorus pentoxide. 4in Figure 1, we calculate ( k a ) H e / ( k s ) N e to be 3.0. AsPyridone-l&N was prepared from y-pyrone and amsuming the hard-sphere collision model with the rare monium chloride containing >95 atom % nitrogen-15 gas atom at rest, the fraction of energy lost per collision (Bio Rad Laboratories) using a procedure analogous between H and M is fM = 2 r n ~ r n ~ / ( m ~r n ~ ) ~ .to that of Stetten and Schoenheimer3for the preparaThe ratio fHe/fNe is calculated to be 3.5, predicting a tion of 2-pyridone-15N. 4-Pyridone-180 was prepared more rapid deactivation for helium, in agreement with by the diazotization of 4-aminopyridine followed by experiment. This confirms the interpretation that refluxing in a strongly acidified solution of 180-enriched hot hydrogen atoms are being thermalized. I n conclusion, the following evidence is presented (1) Presented in part a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968. indicating the importance of hot H atom! in the photo(2) R. A. Coburn and G. 0. Dudek, J . Phys. Chem., 72, 1177 (1968). chemistry of CHaSH at 1849 and 2537 A: (1) positive (3) M. R. Stetten and R. Schoenheimer, J . Biol. Chem., 153, 113 intercepts were obtained in plots of 2Hz/HD vs. (1944).
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Volume 78, Number 10 October 1968
