3158
YVESROUSSEAU AND GILBERTJ. MAINS
The (3PJ Mercury-Photosensitized Decomposition of Monogermane
by Yves Rousseau and Gilbert J. Mains1 Department of Chemistry, Carnegk Institute of Technology, Pittsburgh, Pennsylvania (Received April 26, 1966)
16216
The Hg6(3P1)-photosensitizeddecomposition of GeH4 appears to proceed by rupture of a Ge-H bond. Hydrogen, digermane, t,rigermane, tetragermane, and polymer were observed products. The H atoms formed in the primary process were not scavenged by 5% added propylene. The effect of added helium on the (HD)/(H2) ratio obtained from the photolysis of a GeHrCzD6 system suggests the participation of "hot" hydrogen atoms in the reaction mechanism.
Introduction Studies of the mercury-photosensitized decompositiln of paraffin-type hydrocarbons have revealed quantum efficiencies near unity at pressures corresponding to almost complete q u e n ~ h i n g . ~ -A~ notable exception is methane for which the quantum yields of product formation are quite lows5-' Recent investigations of the mercury-photosensitized decomposition of monosilane in this laboratorys and elsewhereQhave shown that this silicon analog behaves very differently from methane, the quantum yields of product formation exceeding unity. I n order to extend this sequence of investigatlions6~8 to additional group IV hydrides, the mercury-photosensitized decomposition of monogermane was studied. The most recent study of the mercury-photosensitized decomposition of monogermane, aside from a preparative investigation,l0 was reported by Romeyn and Noyes" in 1932. Romeyn and Noyes proposed the following over-all equation to describe the photodecomposition of monogermane in the pressure range 0.01-5 torr GeH4
HgPPi)
Ge
+ 2H2
(1)
The rate of the proposed primary process, viz. Hg6(3P1) -4-GeH4---t GeH3
+ H + Hg6('So)
(2)
suggested a quenching cross section of GeH4 for Hg6 (3P1) atoms of 140 A2. The present study was undertaken to extend the investigation of Romeyn and Noyes to a wider range of experimental conditions and, using GeD4 as a tracer, to determine whether or not the primary process involved the production of hydrogen atoms as proposed. The Journal of Physical Chemistry
Experimental Section The cylindrical quartz reaction vessels, 7 cm in diameter and 10 cm long, were filled with GeH4 to the desired pressure after addition of a drop of clean mercury and evacuation using a high-vacuum apparatus. The filled vessels were irradiated with a low-pressure mercury lamp, Hanovia SC 2537, described previously.* The temperature of the reaction vessel was maintained at 25" by flowing air from the air-conditioned laboratory over the cell-lamp system. Unless stated otherwise, all experiments were performed in duplicate. A neutral density filter and a Vycor no. 7910 filter were interposed between the reaction vessel and the lamp to reduce the light flux and to prevent appreciable 1849-A radiation from entering the reaction vessel. Using a nitrous oxidebutane actinometer, the 2537-A radiation absorbed in the vessel was found to be 0.16 peinstein/min. A (1) Department of Chemistry, University of Detroit, Detroit, Mich. (2) R. A. Back, Trans. Faraday SOC.,54, 512 (1958). (3) R. A. Back, Can. J . Chem., 37, 1834 (1959). (4) R. J. Cvetanovic, W. E. Falconer, and K. R. Jennings, J . Chem. Phys., 35, 1225 (1961). (5) K. Morikawa, W. S. Benedict, and H. S. Taylor, ibid., 5, 212 (1937). (6) G. J. Mains and A. S. Newton, J . Phys. Chem., 6 5 , 212 (1961). ( 7 ) R. A. Back and D. van der Auwera, Can. J . Chem., 40, 2339 (1962). (8) H. Niki and G. J. Mains, J . Phys. Chem., 6 8 , 304 (1964). (9) M. A. Nay, G. N. C. Woodall, 0. P. Strausz, and H. E. Gunning, J . Am. Chem. SOC.,87, 179 (1965). (10) G. Gibbon, -IT. Rousseau, C. H. Van Dyke, and G. J. Mains, Inorg. Chem., 5, 114 (1966). (11) H. Romeyn and W. A. Noyes, Jr., J . Am. Chem. SOC.,54, 4143 (1932).
THE("1)
MERCURY-PHOTOSENSITIZED DECOMPOSITION OF MONOGERMANE
-
smaller quartz reaction vessel, 40 mm in diameter and 15 cm long, was used for the experiments in which helium was added. All analyses were performed using a Consolidated Electrodynamics mass spectrometer, Model 21-103C. The mass spectral sensitivities and patterns of the anticipated products were determined from pure samples immediately before or after determination of the mass spectrum of the photolysis products. I n some of the experiments, heavier products were separated from unreacted germane by condensation at - 131" (n-pentane slush) prior to analysis. Germane was prepared by the reduction of Ge02 by NaBH4 as previously described. lo No impurities were detected by mass spectrometry. GeD4 was similarly prepared using NaBD4 and was found to contain approximately 5 atom % light hydrogen, presumably as GeD3H. Commercial helium was used without further purification. The polymer, which deposited in the reaction vessel during irradiation, was removed after each experiment by thorough cleansing in a hot mixture of nitric and sulfuric acid.
0 c
a
.24
I
I
I
I
,
i
:, AFTER STANDING 3 HOURS AT 25OC SUBSEQUENT TO PHOTOLYSIS
E
.00
-1
COOLED IN LIQUID N, IMMEDIATELY AFTER PHOTOLYSIS
TIME (minutes)
Figure 1. Postirradiative hydrogen formation at 25' and P G ~ = H40 ~ mm.
rg 2s
Results The products of the mercury-photosensitized decomposition of germane are H2, Ge2H6,and a polymer which deposited initially on the window of the reaction vessel as a black dusty powder. At high conversions, small yields of Ge3Hs and Ge4H10were also detected. The series of peaks in the mass spectrum of the products between m/e 140 and 158 corresponded to ions of the formula Ge2Hn+,where 0 I n 5 6, were attributed to Ge2He. The variation in intensity of these peaks was as predicted by random combination of five isotopes of germanium and was in agreement with the mass spectrum of an independently prepared sample of Ge2H6. The appearance of peaks in the m/e ranges 210-234 and 280-314 were similarly attributed to GeaHs and Ge4H10. No attempt was made to determine the composition of the polymer. The rate of hydrogen formation as a function of irradiation time is reported in Figure 1 for two different postirradiative procedures. The upper curve was obtained when the reaction vessel was allowed to stand for about 3 hr prior to separation of the hydrogen. The lower sequence of observations was obtained when the reaction vessel was cooled in liquid nitrogen and the hydrogen product separated immediately after irradiation. It is clear that postirradiative effects are significant in the germane system and all subsequent data were obtained by immediate postirradiative separation of photolysis products. Taylor, et ~ 1 . ~re-~
I
3159
.'L 50 100 150 : GERMANE PRESSURE IN mm Hg
Figure 2. Effect of GeHa pressure on the rate of HPproduction a t 25'.
ported that the polymer formed in the pyrolysis of GeH4 catalyzes germane decomposition at 300" but did not observe significant catalysis at 25". I n view of their observations, it seems more reasonable to attribute the postirradiative hydrogen formation to dehydrogenation of the polymer rather than to catalytic effects. However, it should be noted that we have not studied this postirradiative hydrogen production further. The observed decrease in the rate of hydrogen production as a function of irradiation time is attributable to a decrease in the light flux entering the vessel by the polymer. Redetermination of the initial rate of hydrogen formation using a fresh sample of germane in an uncleaned reaction vessel supported this explanation. Figure 2 depicts the observed pressure independence of the rate of hydrogen production in the pressure range 15-175 torr. The quantum yield of hydrogen in this region is 0.92. While no attempt was made to measure the quenching cross section of germane, the data reported in Figure 2 suggest that complete quenching is obtained at 15 torr as would be expected if the 2
(12) K. Tamaru, M. Boudart, and H. Taylor, J. Phys. Chem., 59, 801 (1955).
Volume 70, Number I0 October 1966
YVESROUSSEAU AND GILBERT J. MAINS
3160
cross section is as high as the 140 A2 suggested by Romeyn and Xoyes. The rate of formation of Ge2H6as a function of irradiation time is plotted in Figure 3 along with the rate of formation of hydrogen for comparison. These data suggest that the initial rates of hydrogen and digermane production are equal. The decrease in the rate of Ge2H6production may be attributed to the consumption of this compound in the formation of trigermane, tetragermane, and polymer. Table I shows the effect of propylene on the rate of hydrogen production. It should be noted that the addition of 5.3% propylene has little effect on the rate of hydrogen formation. Table I1 reports the isotopic distribution of hydrogens obtained from the irradiation of GD, and a mixture of GeH4 and GeD4. Figure 4 depicts the effect of helium on the ratio of H2 to H D obtained from the mercury-photosensitized decomposition of a mixture of GeH4 and C2D6. The significance of these observations will be treated in the subsequent discussion.
1
I
I
POaH4= 40 mm Hg
0.20
.-Ee \
I
=r 0.12
w
t0 I
0
I
4 12 20 IRRADIATION TIME (minutes)
Figure 3. Effect of irradiation time on the rates of Hz and GeH4 production at 25".
Table I: Effect of Propylene on Hz in (3P1) Hg-Sensitized Photolysis of GeH4 a t 25" PGeHI,
yo decom-
PCsH,,
torr
torr
43 44
0
5.3
Pa*,
position
pmole/min
0.07 0.07
0.152 0.147
20 40 60 ADDED HELIUM (mm Hg)
Figure 4. Effect of He pressure on the ratio (Hz)/(HD) a t 25'.
Since there is no evidence for (or against) the participation of 6(3Po)Hg atoms in this system, these metastable atoms are not included in the reaction sequence. The observed quantum yield of hydrogen, 0.92, rules % PGeDI, I'GeHI, decomout reactions 4 and 5 as the exclusive primary reactions torr torr %H2 %HD %Dz position since these would result in quantum yields in excess of 17 ... 2.3 4.0 93.7 0.2 unity. Furthermore, the significant yield of H D ob15.9 14.8 30.7 46.2 23.1 0.1 served from the irradiation of a 1:1 mixture of GeH4 and GD, (Table 11) also rules out hydrogen production by either reaction 3 or 5 alone and suggests that Discussion the primary quenching act is best represented by reThe primary process, Le., the quenching of 6(3P~)Hg actions 2 and 4. Since the distribution of H2, HD, and atoms by GeH4, could conceivably be represented by Dz observed from irradiation of the GeHrGeD4 any of the following reactions as they are all energetimixture is nearly random (see Table 11),it would appear cally feasible. that reaction 2, as postulated by Romeyn and Noyes, best represents the primary chemical quenching process. 6(3P1)Hg GeH4+GeH3 H 6('So)Hg (2) However, contributions of a few per cent from reactions +GeH2 H2 6('So)Hg (3) 3,4, and 5 cannot be ruled out. Acceptance of reaction 2 as the primary process +GeH Hz H 6('S0)Hg requires the participation of hydrogen atoms in the (4) photolysis mechanism and suggests that the following abstraction reaction, proposed by Romeyn and Noyes +Ge 2H2 W S O > H ~ (5)
Table 11: Isotopic Hydrogen Yields in the (3P1)Hg-Sensitized Photolysis of a GeH4-GeD4 Mixture a t 25"
+
+ +
+ + + + +
+
The Journal of Physical Chemistry
+
THE(3P1)~IERCURY-PHOTOSENSITIZED DECOMPOSITION OF MONOGERMANE
H
+ GeH4+Hz + GeH3
H*
+ He +H + H e
(7)
because the absence of a temperature coefficient is not definitive. The effect of helium in the mercury-photosensitized decomposition of GeH4 in the presence of C2D6 provides such a test. Since the quenching cross sections of C2D6and He are very small,16 reaction 2 represents the primary process in this mixture. I n addition to reaction 7, the following reactions may be expected to occur
+ GeH4 +H + GeH4 II* + GeH4+Hz + GeH3 H + GeH4+Hz + GeH3 H* + C2D6 H + CzDs ]-I*
---f
+ C2De +H D + CzD5 H + CzDs +H D + CzD5
H*
(6)
must occur with high collision efficiency to produce the observed 0.92 quantum yield for Hz production in the presence of 5.3% propylene (Table I). Niki and Mainss found that olefins were also ineffective in scavenging hydrogen atoms in their SiH4 study and suggested that the silane analog of reaction 6 was rapid because of a very low activation energy. Gunning, et al.,9 on the other hand, supported an activation energy for the silane analog which was only a few kilocalories per mole lower than reported for alkanes. Since the question of the activation energy for the silane analog of reaction 6 is controversial, the interpretation of the high collision efficiency for the germane system merits further consideration here. A high collision efficiency for reaction 6 may be due to a low activation energy, as suggested by Niki and Mains, if thermal H atoms are involved or the high efficiency may not depend upon the activation energy at all if energetic., Le., [‘hot” hydrogen atoms are postulated. The latter explanation, the participation of “hot” hydrogen atoms in these systems, is not entirely unreasonable since the rupture of a Ge-H bond requires only 70 kcal/mole and rupture of the Si-H bond requires approximately 75 kcal/mole. 13-15 Thus, about 42 kcal/mole must appear as translational energy of H, GeH3, and Hg, and as internal energy of GeH3 as a result of the chemical quenching of 6(3P1)Hg by GeH4 as postulated in reaction 2. Under these circumstances, the production of hydrogen atoms with energies in excess of 10 kcal/mole is quite conceivable. I n order to distinguish between the above possible alternative explanations for the high collision efficiency of reaction 2, it is necessary to resort to a test which is unique to “hot” atom reactions, such as collisional deactivation
(9)
(6) (10)
(11)
(12)
Reactions 7, 8, and 10 are deactivation processes and probably occur through a sequence of collisions of the “hot” hydrogen atom, H*. If thermal hydrogen atoms are solely responsible for the hydrogen production, only reactions 6 and 12 need be considered and, since reaction 12 has an activation energy of 13 kcal/mole,” the ratio of Hz to H D would be expected to be quite large and independent of added He. If “hot” hydrogen atoms participate in the formation of hydrogen, the ratio of H2 to H D should be smaller and increase with the addition of helium. The results are shown in Figure 4. It should be noted that D2 was not detected among the hydrogen products. The (H,)/(HD) ratio does increase as the pressure of He is increased as predicted if [‘hot” hydrogen atoms are involved in the system. The mechanism cited above predicts a linear increase if it is assumed that reaction (ks 12 is negligible, i.e., (H2)/(HD) = [k7(He) k9)(GeH4) klO(C2D6) ]/kll(C2D6). Thus, it seems reasonable to conclude that “hot” hydrogen atoms are present in the mercury photosensitization of GeH4 and may be cited to explain the high collision efficiency of reaction 2. We have attempted to obtain further evidence for (‘hot’’ hydrogen atoms by adding large amounts of helium to a mixture of GeH4 and propylene (20:l). The results of the latter experiments were inconclusive because of the analytical problems involved. If, as the above results suggest, “hot” hydrogen atoms are involved in the mercury-photosensitized decomposition of GeH4, a similar mechanism may be invoked in the controversial SiH4 experiments and this system should be reexamined. The formation of digermane, trigermane, and tetragermane probably involves a sequence of radical combination and hydrogen abstraction steps as postulated for the SiH4and CH4analogs, viz.
+
+
2GeH3 +Ge2H6
H
+ GezH6 +Ge2H5 + Hz
GeH3
+ Ge2H5+GeaHs
2GezH5+Ge4H10
+
(13) (14) (15) (16)
etc. t o polymer deposition. ~
(8)
3161
~
~~~
~
~
~
(13) F. E.Saalfeld and H. J. Svec, Inorg. Chem., 2 , 4 6 (1963). (14) S. R. Gunn and L. G. Green, J . Phys. Chem., 65, 729 (1961). (15) M. L. Huggins, J . A m . Chem. Soc., 7 8 , 546 (1956). (16) Y.Rousseau and H. Gunning, Can. J . Chem., 41, 465 (1963). (17) V. V.Voevodsky and V. N. Kondratiev, Progr. Reaction Kinetics, 1, 54 (1961).
Volume 70, Number 10 October 1966
EDWINF. MEYERAND ROBERT E. WAGNER
3162
While the above sequence qualitatively describes the system, it should be emphasized that further study is required before the reaction sequence is accepted in detail. For example, the isotopic distributions of deuterium in digermane and trigermane products from the irradiation of the GeH4-GeD4 mixture strongly suggest formation via reactions 13 and 15. However, the abstraction reaction which produces Ge2H6 could involve GeH3 as well as the H atoms as suggested in reaction 14. Examination of the mass spectrum of GeH4-GeDl after irradiation should, in principle, permit a decision as to whether GeH3 radicals were involved in abstraction reactions. Because of the low conversions used
in these experiments and the large number of isotopes of germanium, we could not find evidence for GeH3D and GeD3H formation. Future experiments, utilizing a high-resolution mass spectrometer, may permit a conclusion regarding the participation of GeH4 in abstraction reactions. Acknowledgments. The authors wish to thank Mr. Eric Daby for assistance with some experiments and Mr. Wrbican for determining the mass spectra. The financial support of the U. S. Atomic Energy Commission (Contract No. AT(30-1-3007)) and the U. S. Air Force Office of Scientific Research is gratefully acknowledged.
Cohesive Energies in Polar Organic Liquids
by Edwin F. Meyer' and Robert E. Wagner U.S. A r m y Coating & Chemical Laboratory, Aberdeen Proving Ground, M a r y h n d
(Received M a y 8, 1966)
A method which allows quantitative estimation of the dipole-dipole (orientation), dipoleinduced dipole (induction), and dispersion energies in polar organic liquids is presented and illustrated with the methyl n-alkyl ketones. Use is made of the temperature variation of density and vapor pressure for homologous series of organic compounds. Data were obtained for the odd-numbered 2-ketones from Csto C13. As an example of the results, it is estimated that the cohesion in liquid 2-butanone a t 40" is comprised of 8% orientation, 14% induction, and 78% dispersion energies. The relatively high value for induction is surprising in view of the general opinion in the literature, but reconsideration of the usual expressions for these energies as applied to the liquids in question makes it not unreasonable. The contribution of induction to cohesion is larger than is generally appreciated.
A knowledge of the relative amounts of the different types of cohesive energies in liquids will lead to a better understanding of liquid Properties, Particularly sohbiljties, as well as provide necessary information for the development of a satisfactory theory of the liquid state. It can be shown that such knowledge is obtainable from the temperature dependence of vapor pressures and densities for homologous series of organic compounds.
Theoretical Section We assume herein that the cohesive energy (the energy required to separate the component molecules to infinity without changing the average internal energy of
Y
The Journal of Physical Chemistrg
(1) Correspondence should be directed to Capt. E. F. Meyer, NAS-NRC Postdoctoral Research Associate, at the U. S. Naval Research Laboratory, Washington, D. C.
