B. C. ROQUITTE
1204 tional difficulty is avoided by simply introducing an arbitrary parameter to start the area change at an infinitesimal distance behind the shock front. Calculations show that the results are virtually unaffected by this parameter for values less than 0.0001 of the chemical relaxation distance. The computer program is general in that the inputs to the calculation are the shock velocity, state of the gas
ahead of the shock wave, and the chemical species and reactions to be considered. The thermodynamic properties of the species used in the calculation are automatically furnished by a computer tape library containing almost every molecule which may be of interest for shock tube study. The computer program is capable of solving chemistry problems with up to 100 reactions and 200 species.
Flash Photolysis of Hydrocarbons in the Far-Ultraviolet. I. Ethane’“ by B. C. Roquitte’b Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania 16.918 (Received July 14, 1969)
The flash photolysis ofoethane in the far-uv has been studied with a newly developed flash lamp of loz1quanta/sec intensity below 1650 A. The products of decomposition were primarily hydrogen, methane, and ethylene with propane and wbutane amounting to 2-5% of the total product. Both ethane-l,l,l-daand an equimolar mixture of ethane-do-ethane-dewere photolyned, and the isotopic compositions of hydrogen and methane in the product were measured. The result shows that the molecular detachment of hydrogen is the major primary process hu in the flash decomposition of ethane: C Z H+ ~ CZH, Hz. Also, the methane-forming reactions appear to be one-third as important as the hydrogen-producing processes. The effect of different rare gas continuum as a flash light source on the decomposition of ethane has been investigated.
+
Introduction I n recent years, attention has been focused on the mechanism of the decomposition of hydrocarbons in the far-uva2 I n particular, the decomposition of ethane has been the subject of several investigation^.^ In most of these studies a low-intensity rare gas resonance lamp4 was used to initiate photodecomposition. It appears that, in the decomposition of ethane, the molecular detachment of hydrogen from one carbon atom CHa.CHa
CH3.CH
+ H2
(a>
constitutes the major primary step and that the elimination of one hydrogen atom from each carbon atom
+ H2
CHa.CH3-%- CH2:CH2
(b)
is a much less important process. In these studies it appears that the primary formation of ethyl radicals and hydrogen atoms as originally proposed by Wijnen6 prove to have little significance. However, in the CHa-CH,
CzH6
+H
(c)
mercury *PI sensitized decomposition of ethane16primary step (c) appears to be the important reaction. T h e JOUTnal of Physical Chemistry
In the continuous irradiation technique which has previously been used in the investigation of the ethane decomposition, it is possible that secondary reaction of the products might obscure the importance of (c). The end product analysis in flash photolysis, where the combination reaction of radicals may become more significant than in the low-intensity photolysis, is performed to reexamine the direct decomposition of ethane.
Experimental Section The sketch and the detailed construction of the flash lamp used in this investigation have been described elsewhere.’ Argon, krypton, or xenon at a pressure of
(1) (a) Supported in part by the U.5 . Atomic Energy Commission. Presented a t the 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 10-14, 1967. (b) Department of Chemistry, University of Minnesota, Morris, Minn. 56267. (2) “Advances in Photochemistry,” W. A. Noyes, Jr., G. 8. Hammond, and J. N. Pitts, Jr., Ed., John Wiley & Sons, New York, N. Y.,Vol. 3,1964,p 204. (3) (a) See ref 2, page 210; (b) J. R . McNesby, et al., J. Chem. Phys., 40, 1099 (1964); (c) T.Tanaka et al., ibid., 42, 3864 (1965); (d) D.R . Crosley, ibid., 47,1351(1967). (4) H.Okabe, J. O p t . SOC. Amer., 54,478 (1964). (5) M. H.J. Wijnen, J . Chem. Phys.,24,851 (1956). (6) S.Bywater and E. S. R. Steacie, ibid., 19, 326 (1951). (7) B. C.Roquitte, J. A p p l . Opt., 6,415(1967).
1205
FLASH PHOTOLYSIS OF HYDROCARBONS IN THE FAR-UV ~~~~
Table I : Flash Photolysis of Equimolar Mixture of C Z H ~ - C Using ~ D ~ Different Rare Gases as Flash Discharge Media4 Pressure, mm
111.0 132.5 128.0 111.0 7-96 15.0b 35.0b
Rare gas in flash tube, 15 m m
Argon Argon Krypton Xenon 1067 A 1236 A 1470 A
Hydrogen,
HydrogenHD
Methane,
7-%
pmol
Ha
0.94 0.90 1.07 0.48
64.3 65.0 63.9 67.9 43.0 53.0 65.0
7.3 7.1 7.8 6.2 19.0 12.0 3.0
%
r - Methanec-
Da
pmol
CH4
CHaD
CHaDa
CDaH
CD4
28.4 27.9 28.3 25.8 38.0 35.0 32.0
0.27 0.23 0.34 0.15
38.5 39.6 36.6 41.3 42.0 40.0 65.0
10.8 11.4 9.4 8.7 4.0 4.0 3.0
8.9 6.3 11.3 12.3
15.5 15.6 15.2 14.7 9.0 5.0 7.0
26.3 27.1 27.4 24.0 45.0 50.0 25.0
...
...
...
All experiments were carried out with 10 flashes of 1215 J per flash. Data taken from J. R. McNesby, et al., J. Chem. Phys., 42, 3329 (1965). c For analysis of methanes, the mass spectrometer was calibrated with standard mixed methanes. (1
Table 11: Flash Photolysis of CHsCD8 Using Different Rare Gases as Flash Discharge Media4
mm
Rare gas in flash tube, 15 m m
111.0 162.5 126.0 194.0 128.0 172.0 10.Ob 25.Ob
Argon Argon Krypton Krypton Xenon Xenon 1236 A 1470 A
Pressure,
Hydrogen,
-%
pmol
Ha
HydrogenHD
DB
#mol
CHI
CHaD
1.04 1.01 0.79 1.26 0.76
50.2 51.2 51.3 51.8 52.3 52.2 44.6 51.6
24.7 23.9 24.5 24.0 23.2 23.5 30.3 19.5
25.1 24.9 24.2 24.2 24.5 24.3 25.1 28.9
0.99 0.68 0.25 0.35 0.62 0.48
4.8 6.0 11.3 10.6 4.8 7.6 2.0
4.2 4.9 18.6 16.8 4.0 9.9 22.4
1.00
Methane,
All experiments were carried out with 10 flashes of 1215 J per flash. (1965),and J. R.McNesby, et al., ibid., 40,1099 (1964).
15 mm has been used in the lamp as a discharge medium. I n most of the experiments reported here, a 120-pF capacitor bank charged up to 6.0 kV was discharged through the lamp to produce the photochemically effective light. The absorbed intensity was -loz1 quanta/ sec. The effective light was a rare gas continuum which covers the wavelength region 1600-1040 A. I n the case of argon, krypton, and xenon thefe continua cover the regions 1680-1040 A, 1880-1236 A, and 2250-1470 A, respectively.8 The reaction cell (-200 cc), which was separated from the lamp by means of an 8 cm2and 4 mm thick LiF window, was permanently connected through a mercury cutoff to the analytical system. The latter consisted of two LeRoy traps, a solid nitrogen trap, a small mercury diffusion pump, and a toepler pump-gas buret. After the reaction, the noncondensable fraction at -210" was toepled into a sample tube for mass spectrometric analysis. The condensable fraction was analyzed by gas chromatographic equipment provided with a flame ionization detector and temperature programming arrangement. A 2-m silica gel column was used for complete analysis of the condensable products. Research grade ethane from Phillips Petroleum Co. was purified by cold temperature distillation through a silica gel trap a t -78 ". Gas chromatographic analysis
-% Methane CHaDz CDaH
12.8 12.0 13.1 14.9 11.0 14.0 5.6
48.9 48.5 44.7 44.5 49.7 46.6 65.8
CD4
29.3 28.6 12.2 13.2 30.5 21.9 4.2
Data taken from I. Tanaka, et a?., J. Chem. Phys., 42,3864
of this sample indicated the presence of only one constituent. Ethane-l,l,l-d3 was supplied by Volk Radiochemical Co. with stated purity of 99 atom per cent of deuterium, while the sample of C2Ds, also from Volk, contained 3.5% of CzD6H. These deuterated samples were degassed prior to use.
Results The main products of the decomposition of ethane were €I2, CHI, and CZH,. Propane and n-butane were detected to the extent of 2-5% of the total products. I n all experiments conversion was less than 0.01% per flash. The results of the analysis of these products from an equimolar mixture of C2H6-C2D6and CH3D3with different rare gas continuum and at different pressures are summarized in Tables I and 11. For comparison, the results of the isotopic analysis of hydrogen and methane from continuous photolysis experiments are included in the same table. The plot of hydrogen yield per ten flashes as a function of the applied voltage squared is shown in Figure 1. Three experiments were carried out with different incident intensities. All the products were analyzed quantitatively in these runs. Since propane and n(8) Y. Tanaka, A. S.Jursa, and F.J. LaBlanc, J.O p t . SOC.Amer., 48, 304 (1958).
Volume 74, Number 6 March 19,1970
B. C. ROQUITTE
1206
I n the present investigation, it is rather difficult to prove which of the two hydrogen atom forming primary steps is more important. Nevertheless, it appears that reaction (d) occurs to a certain extent as n-butane is a minor product CzHs
+ CzHi
+C4H10
(f)
The results given in Table I indicate that a change of rare gas continuum does not change the isotopic hydrogen distributions within the limit of experimental error, However, the distribution of isotopic methane is changed. It is interesting to note that CHzDzis a product of continuous photolysis of CHaCDa but not of Cd&-C2D6 mixture. This is in contrast to the flash photolysis results where CHZDZis a product of the decomposition of CH&Da and CZHO-CZD, mixture as well. The exact source of this difference is not well understood a t this time. However, we believe that in our system methylene radical is the precursor of CHzDzand it most probably reacts as follows to produce CH2Dz.
Figure 1. Hydrogen yield as a function of flash intensity (kV2).
butane were present in the reaction mixture in very small amounts, their analyses were not very reliable. Thus, in the material balance calculation given in Table 111, propane and n-butane were not taken into
A CH4 + CHz or CD4 + CDz CHz + CzD6 +CHzD + CzDs CHzD + D CHzDz
Cd% or CZD6
(g)
--j
CDz Table 111: Material Balance in the Flash Photolysis of C2He“ Voltage, kV
7 - P r o d u c t a , pMol-
Ratio H/C
Hz
CHI
CZH4
0.01 0.12 0.29
0 * 09b
3.05
0.49
3.03 3.10
1.5
0.09
3.0
0.45
4.5
0.86
0.90
Initial pressure 171.0 mm. Products obtained per 10 flashes. Minor products were not taken into account for carbon-hydrogen calculation. b Assumed to be equal to hydrogen yield.
+ CzHs +CDzH + CzHs
CDzH
+H
--j
CDzHz
The abstraction of a hydrogen by methylene from hydrocarbon to form methyl, which recombine to give ethane, was observed by F r e ~ .Thus ~ the proposed mechanism for the formation of CHzDzdoes not appear to be unreasonable. The isotopic analysis of methane also indicates that about 38% of the total methane species is formed by the molecular detachment process compared to 90% in the low intensity system. The CDaH) in amounts of about mixed methane (CH3D 25% appears to have been produced according to the reaction sequence
+
account. The average ratio, H/C = 3.06, is in good agreement with the theoretical ratio H/C = 3.00 considering the difficulties involved in the analysis of ethylene in the presence of the large excess of ethane.
Discussion As it has been mentioned previously, the primary processes (a) and (b) in the vacuum-uv photolysis of ethane appear to be well e s t a b l i ~ h e d . ~Our * ~ results on the flash photolysis of ethane indicate that molecular detachment of hydrogen is the major primary step. From the isotopic analysis of hydrogen (Table I), assuming complete random mixing of hydrogen atoms, it may be shown that about 87% of the hydrogen is formed by a molecular process. The remaining 13% appears to form from atomic processes such as
+ H or CH3sCH3 + CzH4 + 2H
CH3.CHs ---t CzHs
The Journal of Physical Chemistry
CZH6 or CZD6 -% 2
+D CD3 + H
CHI
~ or ~2
3~
~
3 (h)
---t
CH3D
(i>
---f
CD3H
(j )
I n their investigation of the continuous photolysis of ethane, McNesby, et aL13have shown that excited ethylene is produced by reaction (k),which either decomposes to acetylene and hydrogen or deactivates to normal ethylene CZH6 5 CZH4* 3. HZ
(k)
CZH4* +Ci”
(1)
CzH4*
+ HZ
+ &I----) CzH4
(m)
(dl (e)
(9) H.M.Frey, J . Amer. Chem. Soc., 79,1259 (1957); 80,5005 (1958); Proc. Roy. Soc., A250, 409 (1959); Proc. Chem. Soc., 318 (1959).
PROPERTIES OF TRAPPED H AND D ATOMS I n our system, we are unable to detect acetylene.1° Since the initial pressure of ethane is >lo0 mm, it is very likely that if excited ethylene is formed, it is quickly deexcited by collision to normal ethylene. Experimental results on the CHaCDa photolysis (Table 11) indicate that the molecular detachment of hydrogen (-75%) takes place from the same carbon ~ from CH3CDa atom. The formation of ~ 2 5 of7 HD may occur by a four-center reaction mechanism involving hydrogen atoms from both the carbon atoms. It is noted from the data of the low intensity photolysis that elimination of molecular hydrogen from the terminal carbon atom was less favorable with a decrease of wave-
1207 length. However, in flash photolysis this appears to be invariant with a change in rare gas continuum. In brief it may be pointed out that the mechanism of the photodecomposition of ethane, both by continuous and flash irradiation, appears to be generally similar, with, however, a few minor differences. These differences we believe are due to different secondary reactions of products in the continuous and flash photolysis. (10) I n order t o be sure that small amounts of acetylene in the reaction products did not get absorbed on the “0”rings, an artificial mixture of small amount of acetylene and large excess of ethane was introduced in the reaction cell: about 1 h r later it wa8 condensed out of the reaction cell and analyzed. It was noted that within the limit of experimental uncertainty, acetylene was quantitatively recovered.
Properties of Trapped H and D Atoms Produced by the Photolysis of HI in 3MP-d,, Glass1 by Mervyn A. Long and John E. Willard Department of Chemistry,University of Wisconsin,Madison, Wisconsin 63706 (Received September 18, 1969)
Photolysis of H I dissolved in perdeuterated 3-methylpentane ( 3 M P - d ~ )glass a t any temperature from 4 to 50°K produces trapped H atoms and trapped D atoms observable by esr, as well as C6D13 radicals formed by abstraction of D from the 3MP-dl4 by hot H. CeHl3 radicals, but no trapped H atoms, are produced by photolysis of HI in 3MP-h14 under identical conditions, indicating a major effect of isotopic substitution on the trapping capability of the matrix. The initial quantum yield of trapped H in 3MP-dl4 (ca. 0.03) is independent of temperature over the range of a t least 20-40’K but decreases with time of photolysis until a steady-state concentration is reached, while the concentrations of C6Dl3 and D continue to grow linearly. The fractional rate of decay of trapped H atoms following short illuminations decreases rapidly with time, but the decay curves for samples with different initial concentrations are superimposable after normalization for dose. The properties of trapped H produced in 3MP-d14in Kel-F tubes are the same as when produced in quartz tubes. Trapped H atoms may be produced by photolysis of empty quartz tubes after certain conditions of aging and y radiolysis. Trapped D (or H )atoms are not produced by the radiolysis of 3MP-dl4 (or 3MP-hl4) with or without dissolved H I present, although photolysis of H I in radiolyzed 3MP& produces them. The implications of the data with respect to the mechanism of H atom trapping and decay are discussed.
Introduction Recent investigations have shown that (a) hydrogen and deuterium atoms produced by photolysis of H I in perdeuterated 3-methylpentane (3MP-du) a t 20-50°K can be trapped in the matrix and observed by their ear spectra;2 (b) photolysis of H I in perprotiated 3-methylpentane (3MP-hl4) under identical conditions does not produce trapped H atoms;2 (c) radiolysis of 3 M p - d ~under conditions demonstrated to trap D atoms produced by photolysis of H I does not produce trapped D atoms.* The present paper reports further investigations of
the photolysis of HI in 3MP-dl4 and 3MP-h14designed to answer the following questions. (1) What are the kinetics of growth of trapped H and D atoms and of free radicals during photolysis of HI in 3MP-d14? ( 2 ) What are the kinetics of decay of trapped H and D atoms? (3) What do the kinetics indicate about the mechanisms of trapping and decay? (4) Does D (1) T F s work has been supported in part by U. S. Atomic Energy Commission Contract AT(11-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) D. Timm and J. E. Willard, J. Amer. Chem. SOC., 91,3406 (1969). (3) D. Timm and J. E. Willard, J.Phys. Chem., 73,2403 (1969).
Volume 74, Number 6 March 19, 1970
