2706
J. E". SCHMIDTAND F. W. LAMPE
by Hagemann and SchwarzZais exactly the same as that found in this work and, although partly fortuitous, such agreement is particularly gratifying in view of the completely different experimental methods used. The results obtained for the cyclohexyl radical show a larger disparity than one might have expected. In units of 108 M-' sec-l, kt values range from a low of 0.6 found in the present work to 2.0.2c It is not clear at this time
why the range of values is so large for this particular radical. Acknowledgments. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The author is grateful to Professor C. 0. Guss and Mr. Gary Lee, who synthesized the cyclohexanethiol used in this work.
The ?-Ray Radiolysis of Monosilane and Monosilane-Ethylene Mixtures' by J. F. Schmidt and F. W.Lampe Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania (Received January 17, 1069)
16809
The y-ray radiolysis of pure monosilane at 300'K produces hydrogen and disilane with 100-eV yields of 17.0 and 5.3, respectively, and consumes monosilane with an estimated G(-SiH4) of 22. Considerable amounts of a brown polymeric substance are also observed. Ethylene partly inhibits the formation of hydrogen and disilane producing ethylsilane and diethylsilane in chain processes that are at least partially independent. The yields of all products in both the pure monosilane and in the mixtures with ethylene are affected by the surface-to-volume ratio. The most reasonable mechanism in accord with all observations is based on the conclusion that, in pure monosilane, disilane and hydrogen are produced simultaneously in free-radical and ionic processes. In the monosilane-ethylene mixtures, ethylsilane appears to be formed exclusively by a freeradical reaction and diethylsilane by an ionic process.
Introduction In contrast to the case of simple paraffinic hydrocarbons and mixtures of these paraffins with olefins, very little is known of the radiation chemistry of the structurally similar silanes and of mixtures of these compounds with olefins. Ando and Oae2studied the gas-phase w a y radiolysis of SiHe and of mixtures of SiH4 with C02, H20, and CZHZ. I n the case of pure SiH4, these authors reported as products Hg, SiZHe, traces of higher Si,Hz,+z compounds, and a polymeric compound of empirical formula (SiH2)* as products. For a given initial pressure of SiH4 they found that G(H2) decreased with increasing conversion from a value of 42.8 to about 23, remaining essentially constant thereafter. For a given total dose per gram, Ando and Oae2 found that G(H2) decreased with initial pressure of SiH, while the addition of COZ,G H z , and H 2 0 resulted in marked increase of G(H2). Others3 have reported, a t very high pressures and in the temperature range - 195-450", G values for SiH4 consumption of 3000-35,000. Since SiH4 just begins to decompose thermally at a measurable rate at it is possible that in some temperatures about 400°,4-6 of the higher temperature experiments some thermal initiation obtained;z nonetheless, while we must have some T h e Journal of Physical Chemistry
reservation about the exact magnitude of G values reported,a it is clear that very large G values for SiH, consumption by radiation initiation can be obtained. Mains and Tiernan' have studied the high-energy electron radiolysis of SiH4 and have found G(H2) and G(Si2Ho) to behave with pressure and dose as reported by Ando and 0ael2but to be lower, by a factor of over 2. Mains and Tiernan reported, in addition, that the results of radical-scavenging techniques indicate that a considerable amount of the Hz produced is formed from reactions other than those of hydrogen atom abstraction by hydrogen atoms. The magnitudes of G values for hydrogen formation and for silane consumption point to the conclusion that the radiation-induced conversion of SiH4 takes place via a chain reaction. (1) AEC Document NYO-3570-6. (2) W. Ando and S. Oae, Bull. Chem. Soc. Jap., 35, 1540 (1962). (3) K. Held and R. J. Goldman, Atomic Energy Commission Report NYO-10472. (4) J. Ogier, Ann. Chim. Phys., 20, 37 (1880). (6) T. R. Hogness, T. L. Wilson, and W. C. Johnson, J . Amer. Chem. SOC.,58, 108 (1936). (6) G. Fritz, 2. Naturforsch., B , 7, 507 (1952). (7) (a) G. J. Mains and T. Tiernan, U. S. Atomic Energy Commission Report NYO-2007-8. (b) T. Tiernan, Ph.D. Thesis, Carnegie Institute of Technology, Pittsburgh, Pa., 1966.
^/-RAYRADIOLPS~S OF SiH4AXD SiH4-C2H,
2707
Ando and Oae2 suggested a radical chain involving the reaction SiHa
+ SiH4
----f
SizH6
+H
(A)
but in view of Mains and Tiernan's observation' with radical-scavenging systems and the fact that (A) is endothermic by over 12 kcal/mols it is not likely that (A) is significant in this system. The radiation-induced addition of silanes to olefins has been shown by a number of authors to be an efficient chain p r o c e ~ s . ~ - 'Most ~ authorsg-ll have treated this addition as a free-radical chain reaction, although evidence has been presented12 to the effect that there is an importalk ionic chain component to the addition. Experimental Section All irradiations were conducted in the gas phase in cylindrical Pyrex vessels of 26-cm length and of 240cm3 volume. ?-Rays from the CosOirradiation Facility a t the Pennsylvania State University were used in all experiments as the initiating ionizing radiation. Dose rates (to 50 Torr of SiHS ranged from 4.9 X 1015 to 8.3 X 10'5 eV cm+ hr-l as determined by propylene dosimetry (G(H2) = l . l ) 1 3 and standard electron energyloss data.'* Analyses of the irradiated gases were accomplished by a combination of gas chromatography and mass spectrometry. All quantitative analyses were carried out by gas chromatography using two 8-ft columns, one containing high-activity silica gel (Burrell 341-173) and the other 20% hexadecane on 45-60 Chromosorb P. A Burrell Model K-2 Chromotog with thermal conductivity detection was employed in all gas chromatographic measurements. For purposes of product identification in the monosilane-ethylene mixture irradiations, the effluent from the chromatograph was fed directly (after pressure reduction) to a Nuclide 12-90 G-sector mass spectrometer. After product identification was made in this way it was checked, when possible, by retention times of the pure compounds on the gas chromatogra,ph. Monosilane was prepared by the lithium aluminum hydride reduction of silicon tetrachloride in anhydrous diethyl ether solvent.16 The monosilane and the diethyl ether solvent were separated by distillation on the vacuum line, the monosilane being collected a t -195" after passage of the evolved gases successively through traps held at -95 and -131'. Disilane was prepared by a similar reduction of Si2Cla.16 Ethylsilane and n-butylsilane were similarly prepared from ethyltrichlorosilane and n-butyltrichlorosilane, respectively, which were obtained from Peninsular Chemresearch, Inc. Diethylsilane, obtained from the same source, was purified by vacuum line distillation. Nitric oxide and prepurified hydrogen were obtained from the Matheson Co. The nitric oxide was degassed at -195" and distilled at -159" just prior to use. The
ethylene used was Phillips Research grade, which was further purified before use by the same procedure described for nitric oxide. Results and Discussion Pure Monosilane. The products observed in the y-ray radiolysis of monosilane at 50 Torr pressure and room temperature are hydrogen, disilane, and, at higher doses, a light brown deposit on the vessel surfaces. At the gas chromotographic sensitivities used, higher silanes were not detected. The concentrations of hydrogen and disilane produced as a function of total dose are shown in Figure 1. It is apparent from this figure that both G(H2) and G(SizH6) decrease quite rapidly with increasing dose, an effect which is likely due to radical reactions with the disilane product. To evaluate better the initial G(H2) and G(SitHe), we have plotted in Figure 2 the apparent G values vs. dose. Here we see that both G(H2) and G(SizH6) decrease rapidly with dose to constant values of 12.5 and 1.7, respectively. The solid lines drawn through the points of Figure 2 are calculated from a least-squares fit of the data to the relationship G,,,
=
Go
+ aD + bD2
(1)
where a and b are empirical constants, D is the dose, and GOis the yield at zero dose. The initial G values so obtained for pure monosilane are G(H2) = 17.0 rfi 1.1 G(SizH6)= 5.3
f. 0.4
Considering these data in the light of both our observations of insignificant formation of higher silanes (or, indeed, of anything else in the gas phase) and Ando and Oae's observation2 that the solid deposit has the empirical formula (SiHz),, we may estimate G( -SiH4) to be 22. Using this estimate we calculate that a dose of 7.4 X 1Ol8 eV/cm3 corresponds to 1% conversion of monosilane. Within the precision of the data in Figures 1 and 2 there was no effect on the G values of variation of the dose rate by a factor of 1.7. The thereby suggested weak dependence of yields on dose rate is borne out by comparison of our yields with those of Mains and (8) W.C.Steele, L. D. Nichols, and F. G. A. Stone, J. Amer. Chent. SOC.,84,4441 (1962). (9) G. Rabilloud, BuEL. SOC.Chim. Fr., 2152 (1965). (10) D. W. Roper, Thesis, University of Michigan, Ann Arbor, Mich., 1961. (11) A. M. El-Abbady, J. Chem. U.A . R . , 9, 281 (1966). (12) F.W. Lampe, J. 8. Synderman, and W. H. Johnston, J. Phgs. Chem., 70, 3934 (1966). (13) J. Yang and P. Gant, ibid., 65, 1861 (1961). (14) National Bureau of Standards Circular No. 577, U. 8. Govt. Printing Office, Washington, D. C., 1968. (15) A. E.Finholt, A. C. Bond, K. E. Wilzbach, and H. I. Schlesinger, J. Amer. Chem. SOC.,69, 2692 (1947). (16) L. G.L.Ward and A. G. MacDiarmid, ibid., 82, 2151 (1960). V ~ l u m e79, Number 8 August I969
2708
J. F. SCHMIDT AND F. W. LAMPE
DOSE (
cs x
Figure 1. Product formation in pure SiHd radiolysis: 0,Hz;A, SizH6.
DOSE (ev./cmJ~xlo-ls
Figure 2. Apparent G values of product formation function of dose: 0, Hp; d, SizHB.
RS
a
Tiernan.' Our plateau values (Figure 2) for G(H2)and G(SizH6) are 12.5 and 1.7, respectively, while Mains and Tiernan find plateau values of 9.0 and 3.0, respectively; yet these latter authors used high-energy eIectrons as the radiation source, and we estimate that their dose rate was 40 times higher than ours. Examination of their data and comparison with ours suggest that there is a small, but real, dose rate dependence in which G(H2) decreases while G(Si2He) increases with increasing dose rate. This is to be expected if disilane arises, in part, at least, in a reaction second order in transients (such as the combination of silyl radicals) and if hydrogen is not produced to any significant extent by reactions of second order in transients. On the other hand, working a t higher pressures and with dose rates lower than ours by a factor of 2, Ando and Oae2 found values of G(H2) and G(Si,HB) significantly higher than ours or those of Mains and Tiernan.' This is particularly evident when the results of Ando and Oae2 are extrapolated to our pressure of 50 Torr and suggests a very strong dose rate effect, and an unusual one too, in that the effect on G(SizH6)is opposite to that expected and found by comparison of our work with that of Mains and Tiernan.' We have no explanat,ion for this contradiction, but since we see no reason to mistrust our data and since the results of Ando and Oae2 are in The Journal of Physical Chemistry
conflict with the probable direction of any dose rate effect on the ratio G(H2)/G(Si2Ha), we accept the validity of the comparison of our work with that of Mains and Tiernan' and conclude that there exists a slight dose rate dependence in which G(SizHs) increases and G(HJ decreases with increasing dose rate. The effect of added ethylene on the yield of' disilane at a constant total pressure of 60 Torr is shown in Figure 5 . Here we see that the yield of disilane is decreased by the addition of ethylene to a constant value that is 38% of the yield in pure monosilane. Under total scavenging conditions (16.4% CzH4) the yield of hydrogen is reduced to 65% of its value in pure monosilane. Nitric oxide preserit at 0.7% had an identical effect on the yield of disilane. The results of these radical-scavenging experiments are in excellent agreement with the results of Mains and Tiernan.T A few studies of the effect of KO on the hydrogen yield were inconclusive. Ethylene is known to be an effective scavenger for hydrogen atoms17 and, as has been suggested by White and Rochow'* and as also will be shown later, it is an effective scavenger for silyl radicals. Thus we conclude that 38% of the disilane yield and 65% of the hydrogen yield in the radiolysis of monosilane arise from reactions not involving hydrogen atoms or silyl radicals. Increasing the surface-to-volume ratio, to the extent of a factor of roughly 100, by packing the reaction vessel with quartz wool resulted in a threefold increase in the hydrogen yield and a 48% decrease in the disilane yield. When the radiolysis is carried out in the packed reaction vessel under complete radical scavenging conditions (16.4% of ethylene present), the yield of disilane is reduced by a factor of 2.5 as compared with radiolysis of the radical-scavenged system in the unpacked vessel; this observation suggests that the nonradical precursors of disilane are terminated on the surface. The hydrogen yield is also increased with increasing surface-to-volume ratio in the radical-scavenged system and by a factor of about 3.5. However, since ethylene (a hydrogen-containing substance) is used as the radical scavenger, this observation provides us with no information on the mechanism of radiolysis of pure monosilane, It is evident that the radiolysis of monosilane is a very complex process and that it is at present impossible to write down a complete detailed mechanism. n'onetheless, the above observations coupled with the known ion-molecule reactions in m o n ~ s i l a n eprovide ~~ us with a guide toward at least a partial mechanism that we think encompasses the principal elementary reactions. Thus we write, as a mechanistic scheme consistent with the observations, the following reactions. (17) F. W. Lampe, J . Amer. Chem. Soc., 82, 1551 (1960). (18) D. G. White and E. G. Rochow, ibid., 76, 3897 (1954). (19) G. G. Hess and F. W. Lampe, J . Chern. Phys., 44, 2257 (1966).
2709
?-RAY RADIOLYSIS OF SiH4 AND SiH4-CzHl
+H SiH2 + H2 -+SiHa+ + H + e 1SiH2+ + Hz + e
1-
SiH3
--t
SiH4-m-
H:+ SiH4+H2 + SiH3
+ SiH3+Si2H6 SiHz + SiH4-+ SizHs SiHz+ + SiH, -+ SiHs+ + SiH3 SiH2+ + SiH4 +Si2H4+ + H2 Si2H4++ SiH4 -+Bid%+ + H2 SiH3
+ SiH4 Sin+iHzn+z++ H2 Si,Hzn+ + SiH4+SinH2,+2 + SiH2+ Si,Hz,+ + e- (surface) * polymer Si,Hzn+
SiH3+
---t
+ e-
(surface)
* polymer
Additional initiation steps undoubtably occur but, in view of the increasing energetic requirement to remove more hydrogen in neutral initiation steps and the fact that SiHa and SiHz+are the major ions in the mass spectrum of mon~silane,'~ we feel an adequate representation of the mechanism is possible by (la)-(ld). Reactions 2 and 3 are eliminated by the addition of ethylene (above about 7 4 % ) with a consequent reduction but not elimination of the hydrogen and disilane yields. The insertion reaction of SiH2 reaction 4 is suggested as the most plausible reaction of these radicals with SiH4. The hydrogen abstraction reaction to form two SiH3 radicals is probably endothermic, since the bond dissociation energy D(H3Si-H) is 94 kcal/mols and the sum of the bond energies D(HSi-H) and D(H2Si-H) is 121 kcal/mo1.8~20We do not think that SiH2 is formed in very significant abundance in this system because when ethylene is added the most likely product of its reaction with ethylene, namely vinylsilane, is not observed. Reaction 5 is the major ion-molecule reaction observed in r n o n ~ s i l a n e ,although ~~ the ionic product of this reaction, namely %He+, is inactive. Hence, as far as chemical conversion is concerned reaction 5 represents simply another source of SiH3radicals. Reaction 6 is the second major ion-molecule reaction observed in monosilane. l 9 SiH2+ ions react also with ethylene, 21 but the rates of the two processes are such that the presence of 16% ethylene would be expected to inhibit (6) only by about 10%. Thus ethylene is not an efficient scavenger of the ion-molecule reactions but is of the freeradical processes. I n addition to ( 5 ) and (6), other ionic products of the SiH2+-SiH4 reaction are SizHs+, SizHa+, Si2Hz+,and SizH+ with the appropriate stoichiometric amounts of Hz and H. However, these other ionic processes make up a minor amount of the
SiH2+-SiH4 interaction and, since they add nothing to this partial representation of the mechanism, we have, for simplicity, omitted them. Reactions 7 and 8 have not been observed but are proposed as extensions of (6) to explain the radiationinduced polymeri~ation.~.~ Reaction 9, which is an H2- transfer from SiH4, could not be observed in mass spectrometric studies of pure mon~silane'~ since no suitable reactant ions were present in sufficient abundance. However, the H2- transfer from SiH4has been observed to occur when C2H2+ and C&4+ ions attack SiH4. We propose (9) as a mechanistic step, in analogy to the Hz- transfer to C2H2+ and CZH4+)in order to account for the unscavenged yield of disilane and the reNote that (9) is a ported presence of higher chain-transfer step in the polymerization since SiHz+is regenerated. The termination reactions 10 and 11 are proposed to account for the effect of increasing the surface-tovolume ratio on the radical-scavenged yield of disilane. Monosilane-Ethylene Mixtures. We have already pointed out that the addition of ethylene to monosilane has an inhibitory effect on the radiolytic yields of Hz and Si2&. This inhibition reaction is particularly interesting because the products of the inhibition are formed with yields larger than the expected free-radical yields of the radiolysis. Thus, when 5 : 1 mixtures of monosilane-ethylene are irradiated, four major products appear. These are H2 and Siz&, formed in reduced yields, as already mentioned, and CzHsSiHa and (C2H6)2SiH2. The latter two products were positively identified by mass spectrometric analysis of the gas chromatographic peaks and retention time measurements of samples of the pure compounds; it is particularly noteworthy that the 1: 2 telomer is diethylsilane and not n-butylsilane. The yields of these four major products as functions of total dose (to a 5 : 1 mixture of SiH4/CzH4)are shown in Figures 4 and 5 , from which we derive the following 100-eV yields.
G(H2)
= 11.1
G(Si2He) = 1.7
* 0.7 f
G(C2H6SiH3) = 30.9
0.1 f
2.9
G(C~H~S~H~= C Z7.8 H ~f) 0.9 Trace amounts of other products have been observed which we have tentatively identified as Si3Hs, C2H&%Hs, and SiHsCHzSiHa. The yields of Hz and SizHein the monosilane-ethylene mixtures do not decrease with increasing dose as was observed in pure monosilane (Cj. Figures 1, 4,and 5 ) . Thus the presence of ethylene appears to eliminate the auto-scavenging effect observed in pure monosilane (20) A. E. Douglas, Can. J . Phys., 35, 76 (1957). (21) D. P. Beggs and F. W. Lampe, J . Phys. Chem., in press 1969.
Volume 79, Number 8 August 1969
2710
J. F. SCHMIDTAND E’. W. LAMPE
0.6
I
0
5 0.5.
. x
U U
-l-
0.4.
t
E
L
E 0.3. 1 0
W
U
=’
0.2-
2
0.1-
( I
d
1
4
PERCENTAOE
1‘2 ETHYLENE
Ib
Figure 5 . Inhibition of disilane formation by ethylene. Figure 3. Ethylsilane and hydrogen formation in the radiolysis of SiH4-C2H, mixtures: 0, Hz; A, CrHaSiHs.
(Figures 1 and 2 ) ) and since ethylene is a good atom and radical scavenger, this tends to confirm the suggestion made earlier that radical scavenging by SizHa was responsible. It has already been mentioned that an increase of the surface-to-volume ratio by about 100-fold results in an increase in the yield of hydrogen and a decrease in the yield of disilane; the yields of ethylsilane and diethylsilane are also reduced and by the same factor of reduction as for disilane. This is particularly significant in the case of the disilane yield because the radical contribution t;o disilane formation (reaction 3) has been eliminated by the competition reaction of SiHa radicals with ethylene (Figure 3). This shows conclusively that the ionic component of disilane formation is reduced by an increase in the surface-to-volume ratio suggesting that SiH2+ and Si2%Htn+2+ neutralization a t the wall are the most important ionic termination steps. Since the principal reaction of Si&+ ions is the formation of SiH3 radicals via ( 5 ) ) and hence, since this formation mode of SiH3 will be reduced by an increase in surface area, the decrease in yields of ethylsilane and diethylsilane do not permit us to conclude that these telomer products arise from ionic reactions. Nitric oxide is known to be good scavenger of silyl radicals,22,2aand its effect on the formation of the telomer products in the monosilane-ethylene system is particularly interesting. The addition of 0.6% nitric
DOSE (e.v.k m 3 ) x 10-16
Figure 4. Diethylsilane and disilane formation in the radiolysis of SiHd-C2H4 mixtures: 0, (CzH&SiHg; A, SizHe. The Journal of PhysdcaE Chemistry
(22) M. A. Nay, G . N. C. Woodall, 0. P.Strausz, and H. E. Gunning, J. Amer. Chern. SOL,87, 179 (1966). (23) E. Kaniaratos and F. W. Lampe, unpublished results.
2711
*/-RAYRADIOLYSIS OF SiH4AND SiH4-CzH4 oxide reduces the yield of ethylsilane to zero (or at least to undetectable quantities) but a t the same time reduces the yield of diethylsilane only by a factor of about 2. The use of higher concentrations of nitric oxide reduces the yield of diethylsilane further, so that at 1.6% NO its yield is essentially zero. This difference in behavior of the ethylsilane and the diethylsilane toward nitric oxide addition indicates that ethylsilane and diethylsilane arise, a t least in part, from independent mechanisms. The same conclusion is reached when one examines the effect of the [CzH4]/[SiH4]ratio on the yields of ethylsilane and diethylsilane. Over a fourfold range in tJhis ratio, namely from 0.05 to 0.20, no effect on the relative yields of ethylsilane and diethylsilane is observed. From the magnitude of the 100-eV yields, a chain reaction must be occurring. Any free-radical chain requires either a dependence of the relative yields of ethylsilane and diethylsilane on the [C2H4]/[SiH,] ratio or an induction period in diethylsilane formation. Since neither is observed, we conclude that these products cannot both be formed exclusively in freeradical chain processes. We suggest that, to a first approximation, ethylsilane is formed in a free-radical chain process and that the diethylsilane formation is ionic. Indirect support for this point of view comes from results of studies of the mercury-photosensitized addition of silane to ethylene.ls In this reaction system, which of necessity is purely free radical, ethylsilane was observed along with n-butysilane, but no diethylsilane was reported. On the other hand, in our radiolytic experiments only trace amounts, if any, of n-butylsilane are formed. As additional support, we may mention that the rearrangement required in the reaction intermediates to produce diethylsilane are thought to occur much more readily in ions than in free radicals.24 We should also mention that our experiments were conducted at considerably higher ratios of the [SiH4]/ [CzHA] than obtained in the work of White and Rochow.18 Our failure to observe n-butylsilane is explained by the effect of the [SiHd]/ [CzH4]ratio on the relative yields of ethylsilane and butylsilane which, free radically, are formed in the competition processes SiHaCH2CH2
+ SiH,
--f
+ SiHs
SiHaC2HI,
(12)
SiHaCHzCH2 3. C2H4 --j 0
SiHsCHzCH2CH2CH2 (13) SiH&H2CHzC€12CH2
+ SiH4-+
+
n-C4H9SiH3 SiHa (14) I n mass spectrometric studies of the ion-molecule reactions in monosilane-ethylene mixtures21 we have observed that H2- transfer from SiH4 to an attacking ion occurs readily if it is energetically feasible, and we think that it is a quite general reaction. We have also
observed that SiHt+ adds to C2H4 and that C2H4+ adds to &Ha. Since, in our radiolysis system, SiH4 is always present in considerable excess ([SiHd]/ [ C Z H ~2] 3)) we may, as an approximation, consider that radiolytic initiation occurs only in SiH4 via (1a)-(ld), and we need consider for the proposed ionic formation-mode of (C2H&SiH2 only the addition of SiH2+to C2H4. suggest this occurs by the following steps.
/\
H
H
/ \
H
H H
+
C2H5SiH+
/H \H
\C--C H/ \+/ C2H4 + Si
/\
H
H\
CzH6
/H
\
/c--c
\+/Si
-+ (C2H5)2Si+
/ \
H
C2IIs
The ionic addition to ethylene stops with (18) because no further proton shifts to produce more stable siliconium ions are possible. The formation of diethylsilane then occurs by Hz- transfer from SiH4, viz.
+
(CzH~)2Si+ SiH4 + (CtH&SiHz
+ SiHz+
(19)
Mass spectrometric studies of the isotopic distribution in the ionic products of SiDz+-CzH4 collisions2’ show that the H and D are essentially equivalent in the collision complex. This observation supports the cyclic structure drawn in (15). In terms of a sequence of elementary reaction steps, we suggest that the ethylene-inhibited radiolysis of monosilane ( [SiH4]/[CzH4]2 3) occurs as follows: (1) the initiation is represented by (1a)-(ld), described in the section on radiolysis of pure monosilane; (2) all reactions that occur in pure monosilane radiolysis occur to some extent (depending on the ethylene concentration) in the mixture; (3) the additional elementary reactions below also occur. (24) F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press, New York, N. Y . , 1967.
Volume 78, Number 8 August 1969
2712
T. ELLINGSEN AND J. SMID
+ C2H4 +SiH3CH2CH2 SiHaCHzCHz. + SiH4 +SiHaC2H5+ SiH, SiH3
SiH2+ 4- C Z H---f ~ SiCd%+
+
{
~
~
:
SiH4
+~ e-~
--f
(20) (12)
(15), (16)
(CzH5)2SiH2 SiH2+ (19)
(wall) o ~ ---f } polymer
+ H~
(21)
The proposed details of the elementary steps (15)-(19) were discussed previously. While not complete, we believe this mechanism is the most reasonable representation of this complex
Acknowledgment. This work was supported by Contract No. AT (30-1)-3570 with the U. s. Atomic Energy Commission.
Studies of Contact and Solvent-Separated Ion Pairs of Carbanions. VI.
Conductivities and Thermodynamics of Dissociation of
Fluorenyl Alkali Salts in Tetrahydrofuran and Dimethoxyethane by T. Ellingsen and J. Smid Chemistry Department, State University of N e w Y o r k College of Forestry, Syracuse, N e w York (Received January BO, 1969)
13BlO
The conductance behavior of the lithium, sodium,,potassium, and cesium salts of the fluorenyl carbanion in 1,Z-dimethoxyethaneand that of fluorenylpotassium in tetrahydrofuran was studied over a temperature range of 25" to -70". I n DME, none of the salts shows a dissociation behavior consistent with the simple "sphere in continuum" model. Only the cesium ion pair remains a contact ion pair over the whole temperature range. The potassium salt changes from a predominantly contact ion pair structure at 20" t o a solvent-separated ion pair at - 60°,with the enthalpy change being -4.6 kcal/mol as determined from spectroscopic measurements. Its behavior resembles that of fluorenyl sodium in THF. The potassium salt in THF remains a contact ion pair, and its behavior is very similar to that of the cesium salt in DME. The lithium salt in DME is solvent separated over the entire temperature range, the sodium below 0". It was shown that reasonabIy good thermodynamic data can often be obtained from a measurement of the temperature dependence of the conductance at one salt concentration only. Direct physical evidence for the existence of contact and solvent-separated ion pairs has come chiefly from studies of the optical absorption spectra of alkali salts of certain carbanions' and of radical anions'12 and from nmr and esr investigations, both of which have provided detailed information on the structures of ion pairs and their solvates.288 I n addition, conductance measurements on carbanion and radical ion salts (and those of certain inorganic salts) in ethereal solutions have yielded valuable information on the solvation state of both the alkali ion pairs and that of the free alkali ion^.^-^ I n a previous p~blication,~ we reported on the conductivities and thermodynamics of dissociation of fluorenyllithium, sodium, and cesium in tetrahydrofuran over the temperature range of 25 to -75". Correlation of these conductance data with spectral studies T h e Journal of Physical Chemistry
on fluorenyl alkali salts made it possible to rationalize the values of the respective dissociation constants and of (1) T. E. Hogen-Esch and J. Smid, J . Amer. Chem. SOC.,87, 669 (1966); 88, 307 (1966); L. L. Chan and J. Smid, ibid., 90, 4654 (1968). (2) E. de Boer, Ree. Trav. Chim., 84, 609 (1965);R. V. Slates and M. Szwarc, J . Amer. Chem. Soc., 89, 6043 (1967); N. Hirota, ibid., 90, 3603 (1968). (3) J. A. Dixon, P. A. Gwinner, a n d D . C. Lini, ibid., 87,1379 (1965); L. L. Chan and J. Smid, ibid., 89, 4547 (1967); K. H. Wong and J. Smid, ibid., in press. (4) T.E.Hogen-Esch and J. Smid, ibid., 88, 318 (1966). (5) R. V. Slates and M. Szwarc, J . Phys. Chem., 69, 4124 (1965). (6) D. N.Bhattacharyya, C. L. Lee, J. Smid, and M. Szwaro, ibid., 69, 608 (1965). (7) C. Carvajal, K. J. Tolle, J. Smid, and M. Szwarc, J . Amer. Chem. Soc., 87, 5548 (1965). (8) D. Nioholls, C. Sutphen, and M. Szwaro, J . Phys. Chem., 72, 1021 (1968).
