J. Phys. Chem. 1987, 91, 5765-5770
5765
Mechanism of Formation of Tri- and Tetrasiiane in the Reaction of Atomic Hydrogen with Monosiiane and the Thermochemistry of the Si,H, Isomers Rosa Becerra and Robin Walsh* Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD, U.K. (Received: March 1 1 , 1987; In Final Form: June 1 1 , 1987)
Product-time evolution curves obtained in earlier studies of the Hg(3Pl) photolysis of H2/SiH4mixtures have been modelled with a complex mechanism in which both silylsilylene, SiH,SiH, and disilene, H2Si=!3H2, play a role. Both Si2H4isomers are necessary to explain the formation of primary Si3H8and Si4Hl,. The source of SiH3SiH is the decomposition of chemically activated disilane formed via silyl radical recombination. H2Si=SiH2 is proposed as arising via isomerization of SiH3SiH in contrast to an earlier mechanism. Both primary and secondary product yields are fitted by the mechanism using reasonable rate constant estimates. RRKM calculations have been carried out which yield the following activation energies: Si2H6 SiH3SiH + HZ,E, = 56.4 i 2.0 kcal mol-'; SiH3SiH H2Si=SiH2;E, = 5.3 f 2.0 kcal mol-'. Together with information on reverse processes these values lead to AHfo(SiH3SiH)= 74.6 & 2.0 kcal mo1-l and AHfo(H2Si=SiH2) 5 62.3 kcal mol-I. The mechanism and these values are in reasonable agreement with recent theoretical studies. The x-bond energy in disilene is discussed.
-
-+
Introduction In a previous publication,' it was shown via product-time evolution studies that, in the reaction of atomic hydrogen (Hg(63Pl) H2 system) with monosilane, Si3HBand Si4H10were formed as primary products (although with additional secondary pathways). Kinetic modelling studies showed that the data could be fitted to a mechanism in which the key processes leading to these higher silanes involved decomposition of chemically activated disilane, viz
+
---
2SiH3 Si2H6* Si2H6* SiH2 SiH4
+
+ H2 H2Si=SiH2 + H2
(3) (4a)
SiH3SiH
-+ + + -
SiH3SiH
SiH4
H2Si=SiH2
Si3H8
SiH3
Si3H7 SiH,
Si3H7
Si4HI0
(4c) (7)
(9) ( 1 1)
The Si2H6*formed in step 3 contains enough excess energy to decompose via step 4a and we proposedl in addition steps 4b and 4c in which the two isomers of Si2H4,viz., SiH3SiH(silylsilylene) and H2Si=SiH2 (disilene), were also formed in parallel although minor yields. SiH3SiH can then lead to Si3H8formation by the insertion step 7 while H2Si=SiH2 can lead to Si4Hlo via the intermediacy of the Si3H7radical formed through the SiH3addition process 9 followed by recombination reaction 1 1 . The modelling exercise was carried out with a slightly more extensive mechanism and from the data fitting process, rate constants for steps 4b and 4c were obtained. Activation energies were then obtained (by means of data fitting to RRKM calculations) and these, in turn, enabled us to set limits on the enthalpies of formation of the two Si2H4isomers and additionally on the r-bond strength in disilene. The present reinvestigation was prompted by recent theoretical s t u d i e ~ which ~ . ~ have suggested that while reaction 4b has an energy barrier only slightly higher than that of (4a), the barrier for (4c) is prohibitively high. On the other hand, the unimolecular, H-atom shift, isomerization process SiH3SiH
-
H2Si=SiH2
(8)
(1) Olbrich, G.; Potzinger, P.; Reimann, B.;Walsh, R. Organometallics 1984, 3, 1267-1 272. (2) Gordon, M. S.; Truong, T. N.; Bonderson, E. K. J . A m . Chem. SOC. 1986, 208, 1421-1427. ( 3 ) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J. Pfiys. Cfiem. 1986, 90, 3399-3406.
0022-3654/87/2091-5765$01.50/0 , ,
I
TABLE I: Reaction Mechanism (in Presence of 200 Torr of H,) no. reaction k/cm' molecule-' s-I a*b 1. Hg* + H2 4.2 x 10-7c Hg + 2H 4.0 x 10-13 2. H + SiH4 H2 + SiH3 3. ZSiH, % si2H6* } 8.0 X IO-" (f= 0.41) L SiH2 + SiH, 4a. Si2H6* SiH, + SiH, 2.2 x 1010d SiH3SiH + H2 4b. 3.9 x 109d 4c. H2Si=SiH2 + H2 0 Si2H6+ M 7.7 X 1O-Il (M = H2) 5. Si2H6*+ M 6. SiH2 + SiH4 Si2H6 1.1 x 10-10 7. SiH3SiH + SiH4 Si3Hs 1.0 x 10-'0 H2Si=SiH2 1.0 x 107d 8. SiH,SiH 9. SiH, + H2Si=SiH2 Si3H7 2.0 x 10-12 Si2HS 5.0 X 10. H + H2Si=SiH2 11. SiH, + Si3H7 Si4HIo 3.2 X IO-" 12. SiH3 + Si2H5 Si& 5.5 x 10-11 13. SiH3 + Si2H6 SiH4 + Si2HJ 1.7 X 14. SiH, + Si3H, SiH, + Si!H, 1.7 X 1.7 X 15. SiH, + Si4Hlo SiH, + S1,Hg 16. H + Si2H6 H2 + Si2H5 2.0 x 1 0 4 3 SiH, + SiH, 6.0 x 10-14 17. 18. H + Si3Hs H, + Si3H7 6.0 X 8.0 X 19. H + Si4Hlo H2 + Si4H, 20. 2Si2H5 Si4Hlo 1.5 X lo-" 1.5 X lo-" 21. ZSi'H, Si,H,, 22. Si2H5+ S13H7 SiSHl2 2.0 x lo-" 23. SiH, + Si4H9 Si5H12 2.0 x lo-" 24. Si2H5+ H2Si=SiH2 Si4H9 2.0 x 10-12 3.0 X 1O-Io 25. SiH2 + Si2H6 5.0 X 26. SiH2 + Si3H8 Si,Hlo 5.0 X 21. SiH2 + Si4Hlo Si5HI2 28. SiH2 + H2 + M SiH, + M 2.5 X (M E H,) 2.0 x 10-'0 29. SiH3SiH + Si2H6 Si4Hlo 30. H2Si=SiH2 SiH,SiH IO.ld
--
---- -- ---- -- ---
'For selection of rate constants, see text. *All rate constants second order unless otherwise stated. CNotionalvalue adjusted to ensure correct overall conversion. d First-order rate constant; units, s - I . apparently has a rather low barrier (similar to the analogous process for methylcarbene but unlike that for methylsilylene). Thus we were intrigued to see whether a mechanism with step 8 instead of (4c) could be fitted to the original experimental data and, if so, what were the kinetic and thermodynamic implications. It should be added that the possible occurrence of reaction 8 has been suggested previously in silane photochemical system^.^^^ (4) Pollock, T. L.; Sandhu, H. S.; Jodhan, A.; Strausz, 0. P. J . Am. Chem. SOC.1973, 95, 1017-1024. ( 5 ) Perkins, G . G. A.; Austin, E. R.; Lampe, F. W. J. Am. Chem. SOC.
1979, l O J , 1109-1115.
0 1987 American Chemical Society
5166
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Kinetic Modelling A kinetic modelling exercise was carried out in which an appropriate set of coupled differential rate equations was integrated numerically under conditions corresponding to the original experiments.' This was achieved by using the FACSIMILE program6 based on a Gear-type algorithm with variable integration step length. Product yields were calculated as a function of time within the range of experimental photolysis times. The overall photolysis rate in the program was controlled by the quenching rate for excited Hg(63Pl) by H2 (step 1). This was adjusted to match overall yields. Individual product yields depended on the mechanistic model and the rate constants chosen for individual steps. The final model, after rate constant optimization, is shown in Table I. The rate constants were adjusted to produce reasonable fits to product-time evolution curves (vide infra). The choice of mechanism and rate constants deserves some comment. The key reactions leading to primary formation of S&H8and Si4Hlo(steps 3, 4a, 4b, 7, 8, 9, and 11 have been mentioned in the Introduction. These were supplemented by further reactions to include known or likely abstraction, addition, insertion, and recombination processes. This potentially leads to a very large set of reactions and so, based on experience with the modelling, the reaction set was eventually restricted to the 30 reactions shown. These include (i) all abstractions by H atom and SiH3 radicals, (ii) recombinations of SiH, with all polysilyl radicals, and some polysilyl mutual and cross combinations, (iii) all insertion reactions of SiH2 and some of SiH3SiH, and (iv) addition processes of the major radicals with H2Si=SiH2. Simplifications include (i) the nondistinction between different isomers of higher silanes (e.g., n-Si,Hlo or i-Si4HI0)and silyl radicals (e.g., n-Si3H, or i-Si3H7)and (ii) the neglect of most disproportionation reactions. In assigning rate constants, we have been guided as far as possible by the literature but most of the reactions have not been individually studied and so, of necessity, some rate constants have had to be estimated. To a first approximation primary product formation is controlled by steps 1-12 and secondary product generation by reactions 13-30. Since the main object of the modelling was to fit the primary yields, some uncertainty in a number of the rate constants was not important. On the other hand, the reactivity of higher silanes toward the intermediates in the scheme is such that secondary reactions become important at very low conversions. Thus the attempt was made to optimize the fit to all data. Rate Constant Selection Rate constants were selected as follows: ( i ) Absfraction by H Atoms. Rate constants for H SiH47,8 were taken from literature measurements. In and H Si2H67s9 the case of the latter there is some disagreement but the most recent value was taken. In the H Si2H6reaction allowance was made for the minor displacement process.' For H Si3H8and H Si4Hlothe rate constants are not known and therefore were varied to improve the fit, particularly to secondary consumption of Si3H8and Si4HI0.For simplification no displacement processes were considered. ( i i ) Abstraction by SiH, Radicals. To our knowledge, no rate constants exist for reactions of SM3with the higher silanes. Values were therefore based on a published study of abstraction in the Me,Si + Si2H, reactionlo which, with the same exothermicity, should be a fair analogy. The original rate constant of 5.4 X cm3 molecule-' s-l was increased to take account of an increase in rate constant of the reference Me3Si Me$i recombination reaction obtained by direct measurement." SiH3 abstractions
+
+
+
+
+
+
Becerra and Walsh with Si3H8 and S i 4 H I were ~ assigned the same rate constant. (iii) Radical + Radical Reactions. The only example of a direct gas-phase study of a recombination reaction between siliconcentered radicals" is that between 2Me3Si for which k = 2.5 X lo-" cm3 molecule-1 s-' has been obtained. However, earlier solution workI2 had suggested the mutual reaction of 2SiH3 radicals was ca 3-4 times faster than that of 2Me3Si at low temperatures. We therefore chose a value in accord with this. Also, in accord with an earlier study by Reimann et al.I3 we assigned a disproportionation as well as recombination channel to this termination. In fact we investigated the effect of variation of the disproportionation to recombination ratio between 0 and 0.7 and were able to fit the data for any value in this range, although the rate constant for step 4b had to be suitably readjusted for any particular ratio. Other recombinations between polysilyl radicals were assigned appropriate, although slightly lower, rate constants in the range lo-" cm3 molecule-' s-' by analogy with alkyl radical recombinations where rate constants decline slightly with increasing radical size. (iu) Radical Addition Reactions to H2Si=SiH2. Once again there are no data. Our values were based on the fact that silyl radicals appear to add faster than alkyl radicals to C=C double bonds4 and that radical addition to Si=C double bonds is faster ~ t i 1 l . l ~However, uncertainty here does not have a major effect on the conclusions since as long as step 9 is reasonably fast more than 90% of disilene molecules are effectively converted to Si3H7 via step 9 in the early stages of reaction. It is interesting to note, however, that the concentration of H2Si=SiH2 barely exceeds those of the free radicals in this system with these high rate constants. (u) Insertion Reactions of SiH2 and SiH3SiH. Recent direct ' ~ provided rate constants measurements by Inoue and S u z ~ k ihave for the reactions of SiH2with H2, SiH,, and Si2H6.Rate constants are very high (much higher than was once thoughtI6). We have taken the values, with allowance for the pressure dependence of SiH2 H2, and a small reduction in the magnitude of k for SiH2 Si2H6(from an almost unreasonably high value). Rate constants for SiH2with Si3H8and Si4HI0were assigned similar high values and, in the absence of any data, it was assumed that SiH3SiH had the same reactivity as SiH2. An argument in support of this comes from the knowledge that whereas electronegative substituents deactivate silylenes, SiH3is an electropositive substituent.
+
+
Comparison of the Model Prediction with Experiment and Optimization of the Fit With the rate constants shown in Table I, product-time curves were generated. These are compared with experiment1 in Figure l a 4 Because of the rapid onset of secondary reactions a further comparison is made in terms of average product formation rate (=product yield/irradiation time). This is shown in Figure 2a-c. The visual fit to these latter figures was the more sensitive indicator. Clearly, within the experimental scatter, the fit is reasonable. In optimizing the fit to the data, the initial yields of Si3H8and Si4Hlowere found to be mainly sensitive to the unimolecular reaction rates, viz, steps 4a, 4b, and 8. Total (primary) Si3H8 and Si4Hloyields are controlled by the extent of SiH3SiH formation in step 4b, while the correct proportion of Si4HI0to Si3H8 is controlled by the extent of the isomerization reaction 8 , viz
SiH3SiH
-
H2Si=SiH2
Good fits were obtained with k4b = 3.9
X
lo9 s-I and k8 = 1.0
(1 1) Shimo, N.; Nakashima, N.; Yoshihara, K. Chem. Phys. Left. 1986, (6) Chance, E. M.; Curtis, A. R.; Jones, I. P.; Kirby, C. R. FACSIMILE: A Computer Program for Flow and Chemical Simulation and General Initial Value Problems; H.M.S.O.: London, 1977; No. C13. (7) Arthur, N. L.; Bell, T. N. Reu. Chem. Intermed. 1978, 2 , 37-74. (8) Ellul, R. Doctoral Dissertation, University of Essen, B.R.D., 1980. (9) Fabry, L.; Potzinger, P.; Reimann, B.; Ritter, A,; Steenbergen, H. P. Organometallics 1986, 5, 1231-1235. (10) Gammie, L.; Safarik, I.; Strausz, 0. P.; Roberge, R.; Sandorfy, C. J . Am. Chem. Sor. 1980, 102, 378-380.
125, 303-306.
(12) Gaspar, P. P.; Haizlip, A. D.; Choo, K. Y . J . Am. Chem. SOC.1972, 94, 9032-9037.
(13) Reimann, B.; Matten, A.; Laupert, R.; Potzinger, P. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 500-504. (14) Bastian, E.; Potzinger, P.; Ritter, A,; Schuchmann, H.-P.; von Sonntag, C.; Weddle, G. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 56-62. (15) Inoue, G.; Suzuki, M. Chem. Phys. Lett. 1985, 122, 361-364. (16) Frey, H. M.; Walsh, R.; Watts, I. M. J. Chem. Sor., Chem. Commun. 1986, 1189-1191.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5767
Thermochemistry of Si2H4Isomers
31
'4
a
a
,800
E M
T I M E Is
./ o-+-?----. 0
,
,
,
,
,
,
,
,
,
,
,
,
,
,
, I-
EO0
,
,
,
,
,
,
,
,
,
,
0-I ,
2400
,
,
0
,
,
,
,
,
,
,
,
,
,
TIME / s
24M
L6M
e00
T I M E Is 34 .
2.w
B M
T I M E /s
E
-
.--------
0-
0
,
BOO
!BOD
,
. .
,
__7_
z1m
T I M E Is
Figure 2. Average product formation rates. Lines represent the model predictions.
0
BOD
I800
2400
TIME I s
Figure 1. Product-time curves. Lines represent the model predictions. X lo7 s-l. These absolute values are necessarily subject to some uncertainty. What is important in step 4 is the ratio k4b/k4a. The value k,, = 2.2 X 1Olos-l was obtained from an RRKM calculation (vide infra) assuming single step (strong) collisional deactivation of vibrationally excited Si2H6*. However, the bath gas, hydrogen, is likely to be a weak collision partner and, therefore, multistep collisional deactivation is more probable. The incorporation of such a detailed collision process into the kinetic
scheme would make it impsibly cumbersome and so this problem was investigated by increasing k4aand k4bby equal factors whilst maintaining a constant ratio and constant collisional rate constant ( k J . This had almost no effect on the fit for an increase of a factor of 10 (in which situation where is no stabilization of Si2H,*). Thus the actual extent of stabilization of Si2H6*is unimportant. This is almost certainly because the SiH2 formed in the major decomposition pathway (4a) anyway mostly ends up as Si2H6by insertion in SiH, (step 6 ) . Uncertainties in the experimental data mean that reasonable fits to the results could be obtained with k4b/k4a = 0.18 f 0.05. The required ratio k4b/k4awould, however, be different if the disproportionation factor f in step 3 were different. We have adopted the published value but iff were zero k4b/k4awould be 0.10 & 0.03. Thus the quantitative measure of klb depends on the extent of disproportionation of silyl radicals. For the isomerization step 8 the important comparison is with step 7. To obtain a reasonable fit ks/k7must be within the range (1.0 0.2) X 1017molecule ~ m - leading ~, under experimental conditions, to approximately 60% of SiH,SiH species reacting by isomerization (and the other 40% by insertion). Thus the uncertainty in k, depends to a large extent on our estimate of k7
*
5768 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 TABLE II: RRKM Parameters for Disilane Decomposition disilane" wavenumbers/cm-'
2150 (6), 940 (2), 929 (2), 909, 844, 625 (2), 434, 380 (2), 200
transition states path degeneracy 6 (both channels) wavenumbers/cm-' (both channels) 2150 (5), 940 (2), 929 (2), 909, 844, 625, 380, 254, 200, 100, 80
assigned at T = 574 K 15.91 (both channels) 52.5 (SiH2 formation) 56.4 (SiH3SiH formation) 50.3 (SiH2 formation) 54.2 (SiH3SiH formation)
log (A/s-1) activation energies/kcal mol-'
critical energies/kcal mol-'
Vibrational assignment as previously.'
(vide supra). ks clearly has the correct order of magnitude but may be uncertain by up to a factor of 3. Another question we explored was the consequence of inclusion of the reverse isomerization of disilene to silysilylene, step 30, viz HzSi=SiHz
-
SiH3SiH
Interestingly the fit to the data was completely spoilt with any value for k3,, 2 lo-' s-I. This means that, without step 30, H2Si=SiH2 has an average lifetime, prior to radical trapping, of several seconds. Clearly, if reverse isomerization takes place, then ultimate formation of Si4HIo(via Si3H7)is prevented. This implies that, if this mechanism is correct, disilene is significantly more stable than silylsilylene. This is discussed further in the next section. The fit is also in accord with measured [HD]/[D2] ratios in the H atom/SiD4 system' provided reasonable isotopic effects are invoked for the hydrogen-producing reactions. Finally it should be added that while the mechanism employed here represents a plausible and effective fit to the experimental results and is in accord with theoretical calculation^,^*^ we cannot on this evidence alone rule out our earlier mechanism' which equally well fitted the results. Kinetic Implications The Decomposition of Vibrationally Excited Si2H6. This was analyzed in the previous study' but is reanalyzed here because of new kinetic information concerning the thermal decomposition of disilane. We have shownI6 that in order to reconcile SiHz insertion rate data, theoretical calculations, and a number of other measurements of the heat of formation of SiHz, the published Arrhenius parameters"J8 for SizH6decomposition had to be too low. Recent r e i n v e ~ t i g a t i o n ' has ~ , ~ dramatically ~ confirmed this prediction. We have, therefore, carried out RRKM calculationsz1 on the decomposition channels 4a and 4b, viz
-
SiH2 4- SiH4
Details of the vibrational assignments and transition-state properties are given in Table 11. The A factor for step 4b is not known but as a working hypothesis was assumed to be the same as that for (4a) on the grounds that both Si2H619aand SiH?2 have similar decomposition A factors and SiH4 decomposition involves Hz elimination as in (4b). (17) Bowrey, M.; Purnell, J. H. €'roc. R . Soe. London 1971,321,341-359. (18) Dzarnoski, J.; Rickborn, S.F.; ONeal, H. E.; Ring, M. A. Organometallics 1982, I, 1217-1220. (19) Martin, J. G.; Ring, M. A,; ONeal, H. E. In?. J . Chem. Kinet. 1987, 19, 715-724. (20) Roenigk, K.F.; Jensen, K. F.; Cam, R. W. J . Phys. Chem., submitted for publication (personal communication). (21) Robinson, P. J.; Holbrook, K . A. Unimolecular Reactions; Wiley: London, 1972. (22) Erwin, J. W.; Ring, M. A.; O'Neal, H. E. Int. J . Chem. Kinet. 1985, 17, 1067-1083.
Becerra and Walsh TABLE III: RRKM Parameters for Silylsilylene Isomerization SiH,SiH" wavenumbers/cm-' 2103, 2084 (2), 1958, 932, 914, 874, 711, 426, 384, 377, 121
transition state path degeneracy wavenumbers/cm-'
3 2103, 2084 (2), 1958, 950, 932, 914, 874, 711, 384, 377
assigned at T = 298 K log (Als-1) 12.87 activation energy/kcal mol-] 6-8 0 (with H2)/.fi 3.8 Vibrational assignment for SiH3SiH obtained by reduction of calculated wavenumbers2by 10%. Disilane produced via silyl radical recombination contains a vibrational energy of 73.7 kcal For reaction 4a the calculations give k4, = 2.18 X 10'O s-I a t an excess (nonfixed) energy of 23.4 kcal mol-'. For reaction 4a the activation energy was adjusted to give k4b = 3.9 X lo9 s-' corresponding to k4b/k4a = 0.18. The value obtained was Ea(4b) = 56.4 kcal mol-'. This value will contain uncertainties from two sources. First the experimental uncertainty in kdb/k4,. This source gives rise to f0.7 s-'. A maxkcal mol-'. Second the assumption of Adb= imum uncertainty of loM6here (estimated to cover the reasonable range) implies f l . 6 kcal mol-' uncertainty in activation energy. The vector sum gives f1.7 kcal mol-'. If, as indicated earlier, step 3 had no disproportionation contribution this would raise Ea(4b) by +1.4 kcal mol-'. We conclude Ea(4b) = 56.4 f 2.0 kcal mol-' covers all reasonable sources of uncertainty. It was also verified in a separate calculation that the ratio k4b/k4awas unaffected by weak collisional stabilization using a stepladder model with a step size (average energy removed in a down collision) of 0.57 kcal mol-'. The Isomerization of SiH3SiH. The value for ks of 1.O X lo7 is so high that a very low activation energy process is implied. In this situation two potential complications need to be considered. The first concerns the question of whether SiH3SiH contains vibrational excess energy. A statistical distribution of energy released in step 4b would give SiH3SiH ca 13 kcal mol-' immediately on formation. However, at pressures of 200 Torr the collision frequency is ca lo9 s-' and therefore approximately 300 collisions take place per isomerization event. This should be sufficient for effective deactivation of SiH3SiH. The second complication is effectively the inverse one of reactivation since step 8 will be a unimolecular reaction in its "fall-off" region. We have, therefore, carried out another RRKM calculation to estimate the degree of fall-off of step 8 under experimental conditions. Details of vibrational assignments and transition-state properties are given in Table 111. The A factor for step 8 is not known but as a working hypothesis we assumed that the effective loss of an internal Si-Si rotation in the transition state leading to A = s-I. Calculations with low barriers (in the range 6-8 kcal mol-') assuming weak collisions (step sizes 0.3-0.6 kcal mol-') produced degrees of fall-off ( k S / k 8 " )of between 0.006 and 0.030 at a pressure of 200 Torr. We thus estimate that ksmis lo9*' s-' where the factor of lO*I covers uncertainties both from the experimental estimate of k8 and from the fall-off calculation. In combination with the A factor estimate this leads to E,(8) = 5.3 f 2.0 kcal mol-'. If the mechanism is correct this represents the first experimentally based estimate of this activation energy. Thermodynamic Implications The Heat of Formation of Silylsilylene. This may be derived from Ea(4b) if it is combined with an estimate of the reverse activation energy. This is not known experimentally but again assuming the reactivity of SiH,SiH matches that of SiH,, we estimate16 from published rate dataI5gz4for the reaction of SiH, with Hz(D2)a small barrier of ca 1.5 kcal mol-I. Unit conversion (23) Walsh, R. Acc. Chem. Res. 1981, 14 246-252. (24) Jasinski, J. J . Phys. Chem. 1986, 90,555-557.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5769
Thermochemistry of Si2H4Isomers and temperature correction lead to m02g8(4b)= 55.4 f 2.0 kcal mol-'. In combination with the known Mfo(Si2H6)= 19.1 kcal mol-' this gives AHfo(SiH3SiH)= 74.6 f 2.0 kcal mol-'. The Heat of Formation and ?r-Bond Energy of Disilene. Limiting values for these quantities may be obtained by consideration of reaction 8. Under the experimental conditions ks = 1.0 X lo7 s-' and k30 Ilo-' s-'. Step 30 is the reverse of (8) and therefore K8,+ = kg/k30 1 lo8. This will be true even though step 8 is in the fall-off region since step 30 will be similarly so. This implies that (-AG"(8)) 1 10.9 kcal mol-'. An entropy estimate of ASO(8) = -2.1 cal K-'mol-' may be made from theoretically calculated vibrational frequencies of the two isomers of Si2H42which leads to ( - W ( 8 ) ) 1 11.5 kcal mol-'. This then, in combination with AHfo(SiH3SiH) gives AHfo(H2Si=SiH2) I63.1 (f2) kcal mol-'. Consideration of the bond energy changes during the notional process 4c
-
Si2H6
H2Si=SiH2
+ H2
gives AHo = 2D(Si-H) - D(H-H) - D,(Si=Si). The published values for bond dissociation energies are D(Si-H) = 86.3 kcal mol-' 23 and D(H-H) = 104.2 kcal mol-' 25 and from the AHf' values in this paper A P ( 4 c ) I44.0 kcal mol-'. This then yields D,(Si=Si) 1 24 ( 1 3 ) kcal mol-'.
Discussion Implications of the Kinetic Model. While the earlier mechanism for this complex reaction has not been ruled out, the present revised mechanism clearly provides an excellent fit to the data. It is necessary to the fit that the isomerization of SiH3SiH to H2Si=SiH, has an activation energy of 5.3 f 2.0 kcal mol-'. The theoretical studies which prompted this work give values of 9 kcal mol-' (Gordon et a12) and 3 kcal mol-' (Ho et a13) which are in reasonable agreement. Apart from this we prefer the present model on the general ground that it appears to us, irrespective of theory, unlikely that H2 elimination from disilane should occur simultaneously by both 1,l- and l,Zpathways, as we (lacking the imagination to envisage alternatives at the time) earlier proposed.' The fact that theory," finds the barrier to 1,2-H2elimination ca kcal mol-' higher than that for l , l - H 2 elimination confirms this view. This must also put in doubt the minor 1,2-H2elimination pathways proposed by Ring and O'Nea126 in alkylsilane decompositions. The kinetic data deduced for silylsilylene forming reaction 4b, viz
-
Si2H6
SiH3SiH + H2
are consistent with thermal studies. The parameters used and deduced in the modelling predict that in pyrolyses carried out under static bulb conditions ( T = 600 K) only ca 3% H2compared with SiH, should be produced initially. H2is not reported as being formed but this would be close to the limit of detection. Pyrolysis under shock tube conditions ( T = 1000 K) is predicted to produce ca 13% H2if unimolecular fall-off effects and secondary processes are ignored. Dzarnoski et all8 reported ca 20% H2 compared with SiH4 and made the first estimate of the rate constant for reaction 4b, viz log (klb/s-') = 15.3-55.3 kcal mol-'/RT In 10 The agreement with our work is not too bad; especially, as seems to us likely, (from the calculations of H o et a13) the following secondary reactions may occur at high temperatures SiH3SiH H2Si=SiH2
-
+ - - +
H2Si=SiH2 H,Si=Si
H2
H,Si=Si
Si(H)2Si
Si,
H2
(25) Benson, S . W. Thermochemical Kinetics; Wiley: New York, 1976. (26) Rickborn, S. F.; Ring, M. A.; ONeal, H. E. In?. J. Chem. Kine?. 1984, 16, 1371-1383 and references cited therein.
TABLE I V Heats of Formation of Si2H4Isomers (kcal mol-') expt (e) or AHHfD(SiH$iH) AHf0(H2Si=SiH2) theory (t) ref 64.5 65.2 68.9 65.1 72.5 80.4 72.4 74.6
f 3.5 f 2.6
f3 f 2.0
e
