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Ab Initio study of isomerization reaction from c-OSiH2O to t-OSiHOH
THEOCH 6013
Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Ab Initio study of isomerization reaction from c-OSiH2O to t-OSiHOH S. Kondo a,*, K. Tokuhashi a, H. Nagai a, A. Takahashi a, M. Kaise a, M. Sugie a, M. Aoyagi b, S. Minamino b a
National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305-0046, Japan b Institute for Molecular Science, Okazaki, Aichi, 444-8585, Japan Received 16 June 1998; received in revised form 12 October 1998; accepted 12 October 1998
Abstract The isomerization reaction from c-OSiH2O to t-OSiHOH, a vital reaction to understand the spontaneous ignition of silane, has been reinvestigated with Gaussian-2 theory and the CASSCF(6,6) method. It has been found that the reaction proceeds through two consecutive steps; i.e., c-OSiH2O undergoes isomerization to yield w-OSiH2O, and then the latter is converted to tOSiHOH. The G-2 energy of the transition state of the latter process is 4.3 kcal/mol higher than that of the former. However, the G-2 energy of this higher transition state plus H atom is still 4.8 kcal/mol lower than that of the original reactants of SiH3 1 O2. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Silane; Isomerization; Gaussian-2; CASSCF
1. Introduction Silane is known for its spontaneous ignition in air at room temperature [1–4]. As this phenomenon occurs at room temperature, the elementary reactions active at low temperatures are important for elucidation of the phenomenon. In particular, it has been well recognized that SiH3 1 O2 is the key reaction for the occurrence of the spontaneous ignition of silane [5–9]. Hartman et al. have proposed that the reaction of SiH3 1 O2 proceeds through three different pathways, among which there is a reaction route to yield SiH3O 1 O [5]. If it is true, since O atom can readily react with SiH4 to yield two active radicals of SiH3 and OH at room temperature, it may make a chain branching process. Then, Koshi et al. have studied
on the branching ratio of this reaction [6,8]. They have determined the branching ratio between the OH producing and H producing reactions but have not determined the O producing one. However, according to the calculation made by Darling and Schlegel [9], the total energy of SiH3O 1 O is 10.8 kcal/mol higher than that of the original reactants of SiH3 1 O2. If it is the case, the reaction to yield an oxygen atom can not proceed at a favorable rate at room temperature. Then, we have suggested in a previous paper that the reaction route from SiH3 1 O2 to OSiOH 1 2H is a key to understand the spontaneous ignition at room temperature [10]. The whole route of this reaction was as follows; SiH3 1 O2 ! SiH3 OO* ! c 2 OS : H2 O 1 H
* Corresponding author. Fax: 181-298-54-4487. E-mail address: [email protected] (S. Kondo) 0166-1280/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0166-128 0(98)00515-6
! OSiHOH 1 H ! OSiOH 1 2H;
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Table 1 Optimized structures by the MP2 FULL/6-31G* method a Molecule
SiO1
SiO2
SiH1
SiH2
aO1SiO2
aO1SiH1
aO1SiH2
bH1SiO1O2
bH2SiO1O2
c-OSiH2O TS1 w-OSiH2O b TS2 t-OSiHOHc
1.6719 1.6206 — 1.6413 1.6399
1.6713 1.6207 — 1.6413 1.5356
1.4735 1.5048 — 1.4844 1.4700
1.4737 1.5049 — 1.5281 2.2409
58.26 100.40 — 133.80 128.20
118.83 134.31 — 111.06 105.29
118.91 95.19 — 85.80 23.04
107.92 107.99 — 154.53 180.00
2 107.86 2 136.86 — 2 80.07 0.00
a
Bond lengths are in angstroms and bond angles are in degrees. The optimization calculation has not been converged with the MP2 FULL/6-31G* method for w-OSiH2O. c For t-OSiHOH, O1H2 0.9733 and aSiO1H2 115.72. b
where SiH3OO* denotes excited species of SiH3OO and c-OSiH2O is a cyclic molecule. This suggestion has been made based on the study on the isomerization reaction from c-OSiH2O to OSiHOH which was done by Murakami et al. [11]. They have claimed that the transition state from c-OSiH2O to OSiHOH was identified by the CASSCF(6,6) method, although they have not shown any details of the calculation. They have just stated that the barrier height to this reaction is 6.4 kcal/mol, and the energy of this transition state plus that of H atom is about the same as that of the transition state (TS0) of the reaction from SiH3OO to c-OSiH2O 1 H. However, in an attempt to follow the result reported by Murakami et al. [11], we could not find the transition state that connects c-OSiH2O directly to OSiHOH. Instead, we have found the transition state (TS1) which connects c-OSiH2O to w-OSiH2O, another isomer of c-OSiH2O. w-OSiH2O has been found to undergo further isomerization through a transition state (TS2) to yield t-OSiHOH which is H-trans form of OSiHOH. In the following, the result of the present calculation is reported.
2. Results and discussion 2.1. (1) Calculation by using the HF/6-31G* and MP2 FULL/6-31G* methods At first, the investigation to find out the reaction route from c-OSiH2O to OSiHOH was made by using the HF/6-31G* and MP2 FULL/6-31G* methods. Actually, the transition state which seems to connect c-OSiH2O directly to OSiHOH (H-cis
form) was found by the HF/6-31G* method. Then, a similar effort was paid to find the corresponding one by using the MP2 FULL/6-31G* method. However, the latter effort was not successful. In this procedure, we have at first tried to see what happens if one of the two H atoms connected to the Si atom in c-OSiH2O is forced to move from the original position toward one of the two end O atoms. As, if one of the H atoms connected to Si atom in c-OSiH2O is shifted to an adjacent O atom, we may obtain OSiHOH molecule. Actually, when the geometry optimization is carried out fixing the H atom at a certain distance shifted toward one of the O atoms from the original position, we have found that both the SiO bonds of the optimized structure become longer than the original values. The SiO bond along which the H atom is shifted is more conspicuously prolonged than the other. Eventually, a transition state which connects c-OSiH2O to HSiOOH has been found both by the HF/6-31G* and MP2 FULL/6-31G* methods in stead of the one that connects c-OSiH2O to OSiHOH. Actually, this transition state has already been reported in our previous paper [10]. In the second place, we have attempted to see what happens if the OSiO angle of c-OSiH2O molecule is increased. If there is a reaction route which enables cOSiH2O to change directly to OSiHOH, we can expect that one of the two H atoms tends to move from the Si atom toward either of the two O atoms when the OSiO angle is increased. However, there has been found no clear indication of such tendency. Even if the OSiO angle is increased quite a bit, e.g. ten degrees or so, the two H atoms stick to their original positions near the Si atom. By increasing the OSiO
S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Fig. 1. The optimized structure of w-OSiH2O by the CASSCF(6,6) method.
angle step by step, we finally arrived at a saddle point of the energy surface (TS1). As is shown in Table 1, the geometry of this transition state is very similar to that of c-OSiH2O molecule except that the OSiO angle is much larger than that in c-OSiH2O. For comparison, the optimized structures of c-OSiH2O and OSiHOH are also listed in Table 1. Here, the numbering of the atoms in the molecules are to be referred to Fig. 1. The optimized structures of c-OSiH2O and OSiHOH are essentially the same as in the literature [12]. Considering the change in geometry from that of c-OSiH2O, TS1 may hardly be the transition state that connects cOSiH2O directly to OSiHOH. Rather, this transition state seemed to be owing to the one connecting cOSiH2O to another isomer that may result from c-OSiH2O by opening the OSiO angle (w-OSiH2O). Unfortunately, however, we could not find out any stable structure of this isomer by the MP2 FULL/ 6-31G* method. In the next step, we have tried to follow the reaction coordinate in the reverse way. We have made calculation starting both from H-cis and H-trans forms of OSiHOH. The reaction coordinate in the potential surface was investigated by shifting the H atom connected to O atom toward the Si atom in these molecules. In this way, we have arrived at a saddle point (TS2) when we started from H-trans form of OSiHOH. The optimized structure of the transition state, which is listed in Table 1, strongly suggests that it connects OSiHOH with some isomer of c-OSiH2O that has a large OSiO angle. However, again the optimization calculation to find this isomer was not successful.
27
Thus, the optimum structures of the saddle points TS1 and TS2 in the potential surface were found by starting from c-OSiH2O and t-OSiHOH, respectively. Both these transition states seem to probe from two different directions the same and a single molecule which may result from c-OSiH2O by opening the OSiO angle. However, the optimization calculation for this molecule did not converge at all either by the HF/6-31G* method nor by the MP2 FULL/631G* method. This result suggests that the molecule has the bi-radical character around the geometry of the transition states, TS1 and TS2. If it is the case, the treatment with such a method as CASSCF may be necessary to obtain the optimized structure. 2.2. (2) Calculation by using the CASSCF(6,6) method At this stage, we have switched the calculation method from HF/6-31G* and MP2 FULL/6-31G* to CASSCF(6,6)/6-31G*. A part of the calculations has also been carried out by using the CASSCF(10,8) method. Very similar results have been obtained for the two method. Therefore, only the results obtained with the CASSCF(6,6) method will be shown here. In an attempt to identify the optimum structure of w-OSiH2O, we have arrived at a few somewhat different optimum structures, which are different in their energies as well. The one with the minimum energy was considered to be the ground electronic state. However, it is remarkable that the lowest levels of the singlet and triplet states of this molecule have been found to have almost the same energy. The reason is because there are two unpaired electrons which are distributed almost equally to the two end O atoms, and because there is very little interaction between them. As we have explored the dominant route from c-OSiH2O to OSiHOH, bifurcation path through the interconversion at the w-OSiH2O intermediate is not considered here. We have followed the reaction coordinate starting from w-OSiH2O in the singlet state. By shifting one of the two H atoms in w-OSiH2O along one of the two SiO bonds step by step, we have reached a transition state. This transition state is very similar in its geometry to TS2 obtained with the MP2 FULL/6-31G* method in the above, and can be definitely identified as the same one. Pursuing further the reaction
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Table 2 Optimized structures by the CASSCF(6,6) method a Molecule
SiO1
SiO2
SiH1
SiH2
aO1SiO2
aO1SiH1
aO1SiH2
bH1SiO1O2
bH2SiO1O2
c-OSiH2O TS1 w-OSiH2O TS2 t-OSiHOHb
1.6425 1.6689 1.6710 1.5858 1.6174
1.6606 1.6936 1.6956 1.6084 1.5257
1.4636 1.4668 1.4643 1.4600 1.4583
1.4636 1.4672 1.4642 1.5366 2.2303
62.02 97.36 116.37 128.03 127.49
117.90 116.11 105.83 114.72 105.29
117.97 107.41 105.76 72.37 22.01
110.63 114.47 117.21 167.55 180.00
2110.55 2121.30 2117.53 251.51 0.00
a b
Bond distances are in angstroms and bond angles in degrees. For t-OSiHOH, O1H2 0.9494 and aSiO1H2 118.32.
coordinate to the other side of this transition state finally leads to the optimum structure of t-OSiHOH. However, by decreasing the OSiO angle of wOSiH2O, the reaction coordinate lead to c-OSiH2O molecule via a transition state of TS1. Thus, a reaction route from c-OSiH2O via wOSiH2O to t-OSiHOH was completely identified by the CASSCF(6,6) method. The optimum structures of the three molecules, c-OSiH2O, w-OSiH2O, and tOSiHOH, and the two transition states, TS1 and TS2, were all obtained by this method. The optimized structures of these molecules and transition states are summarized in Table 2. As for c-OSiH2O and tOSiHOH, there are not much difference between the optimized structures by the MP2 FULL/6-31G* method and the corresponding ones by the CASSCF method. In contrast, as for TS1 and TS2, there are some differences between them. Table 3 shows the CASSCF(6,6) energies for these species. In this calculation, the energy of w-OSiH2O is 3.1 kcal/mol lower than that of TS1, and 28.3 kcal/mol lower than that of TS2. Now, our concern is the energy of w-OSiH2O 1 H relative to the original reactant of SiH3 1 O2. For that purpose, we have tried to
calculate the energies of these molecules and transition states on the Gaussian-2 theoretical basis. 2.3. (3) Energetics relevant to the Reaction Route from c-OSiH2O to t-OSiHOH The energy values of both TS1 and TS2 have successfully been obtained by the Gaussian-2 theory. The result is summarized in Table 3 together with those of relevant molecules. The energy of TS2 is 4.3 kcal/mol higher than TS1. Also, the energies of TS1 and TS2 plus that of H atom are, respectively, 13.6 and 17.9 kcal/mol higher than that of TS0. It should be noted that the higher barrier of the two (TS2) is still 4.8 kcal/mol lower than the energy of the original reactants of SiH3 1 O2. However, the energy of w-OSiH2O 1 H may be estimated from these values combined with the energy differences between w-OSiH2O and these transition states obtained by the CASSCF(6,6) method. In this case, although it is possible that the energy of wOSiH2O is slightly underestimated owing to the biradical character, the resulting argument (see below) on the reaction route from SiH3 1 O2 all the way to
Table 3 Energy values (in a.u.) of c-OSiH2O, TS1, w-OSiH2O, TS2 and t-OSiHOH Molecule
CASSCF(6,6)
MP2 FULL/6-31G*
G-1 a
G-2 a
c-OSiH2O TS1 w-OSiH2O TS2 t-OSiHOH
2439.829164 2439.813965 2439.818922 2439.773751 2439.949255
2440.173403 2440.102688 — 2440.154578 2440.305857
2440.463320 2440.437070 — 2440.430535 2440.593683
2440.461921 2440.436153 — 2440.429117 2440.593597
a The zero-point energies contained in G-1 and G-2 energies are for the HF/6-31G* method with a correction factor of 0.893 for c-OSiH2O and t-OSiHOH and for the MP2 FULL/6-31G* method with a correction factor of 0.925 for TS1 and TS2.
S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
29
Fig. 2. Energy diagram (in kcal/mol) for the reaction route from SiH3 1 O2 to OSiOH 1 2H.
OSiOH 1 2H may not be affected much. Now, since the energy of w-OSiH2O is 3.1 kcal/mol lower than that of TS1 (by CASSCF), and the energy of TS1 1 H is 9.1 kcal/mol lower than the original reactants of SiH3 1 O2 (by G-2), w-OSiH2O 1 H is expected to be 12.2 kcal/mol lower than the original reactants. However, the same w-OSiH2O is 28.3 kcal/mol lower than TS2 (by CASSCF), and the energy of TS2 1 H is 4.8 kcal/mol lower than the reactants (by G-2), w-OSiH2O 1 H is in this case expected to be 33.1 kcal/mol lower than the reactants. At any rate, the true energy of w-OSiH2O 1 H may be somewhere between 12.2 and 33.1 kcal/mol lower than the reactants. Considering the present result, the energy diagram given in the literature [9–11] should be revised as in Fig. 2. Actually, Fig. 2 shows the energy diagram of the reaction route from SiH3 1 O2 to OSiOH 1 2H, where the species of w-OSiH2O and the transition states TS1 and TS2 have been added in the diagram, while most of the remaining part has just been lifted from in Ref. [10], Fig. 2. Here, a tentative value of
223 kcal/mol is employed for the energy of w-OSiH2O 1 H. The present findings are of interest in interpreting the spontaneous ignition of silane particularly when it is compared with the previous one [10,11]. According to the report by Murakami et al., the energy of the transition state from c-OSiH2O to OSiHOH plus that of H atom was much the same as that of TS0. If it is true, once produced c-OSiH2O molecules may readily undergo the subsequent isomerization to yield OSiHOH. Then, since OSiHOH is much more stable than c-OSiH2O, it is expected to have an enormous excess energy which is enough for OSiHOH to proceed further to the subsequent decomposition reactions. Some of them may decompose to yield OSiOH 1 H, and others to yield SiO 1 H2O and so forth [10,11]. However, according to the present calculation, cOSiH2O 1 H resulting from the original reactants encounters two more potential barriers to get to OSiHOH 1 H, which are definitely higher than TS0. In this case, therefore, c-OSiH2O may not necessarily
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
undergo straightforward isomerization to yield OSiHOH. Rather, the reaction may have a sort of sojourn before it proceeds to the subsequent reaction. The significance of the present result in interpreting the spontaneous ignition at room temperature will be discussed in more detail in a subsequent paper [13]. 3. Conclusion The reaction route from c-OSiH2O to OSiHOH has been reinvestigated with Gaussian-2 theory and the CASSCF(6,6) method. No reaction route was found in our calculation that connects c-OSiH2O directly to OSiHOH. Instead, it has been found that the reaction proceeds via another isomer of c-OSiH2O to reach OSiHOH; at first c-OSiH2O undergoes isomerization to yield w-OSiH2O, and then the latter is further converted to t-OSiHOH. The energy of the transition state of the latter process (TS2) is 4.3 kcal/mol higher than that of the former (TS1). However, the energy of TS2 1 H is still 4.8 kcal/mol lower than that of the original reactants of SiH3 1 O2. Acknowledgements A part of this work was supported by a Grant-in-aid
for Priority Area and for Scientific Research by the Ministry of Education, Science and Culture, Japan (No. 08230229).
References [1] H.J. Emerius, K.J. Stewart, J. Chem. Soc. (London) (1935) Part II, 1182. [2] A.N. Baratov, L.P. Vogman, L.D. Petrova, Combust. Explos. Shock Waves 5 (1969) 407. [3] D. Conrad, Arbeitsschulz 9 (1976) 299. [4] Y. Urano, S. Horiguchi, K. Tokuhashi, H. Ohtani, M. Iwasaka, S. Kondo, Y. Hashiguchi, Kagaku Gijutsu Kenkyusho Hokoku 84 (1989) 585. [5] J.R. Hartman, J. Famil-Ghiriha, M.A. Ring, H.E. O’Neal, Combust. Flame 68 (1987) 43–56. [6] M. Koshi, A. Miyoshi, H. Matsui, J. Phys. Chem. 95 (1991) 9869. [7] S. Koda, Prog. Energy Combust. Sci. 18 (1992) 513. [8] M. Koshi, N. Nishida, Y. Murakami, H. Matsui, J. Phys. Chem. 97 (1993) 4473. [9] C.L. Darling, H.B. Schlegel, J. Phys. Chem. 98 (1994) 8910. [10] S. Kondo, K. Tokuhashi, H. Nagai, A. Takahashi, M. Kaise, M. Sugie, M. Aoyagi, K. Mogi, S. Minamino, J. Phys. Chem. A101 (1997) 6015. [11] Y. Murakami, M. Koshi, H. Matsui, K. Kamiya, H. Umeyama, J. Phys. Chem. 100 (1996) 17501. [12] C.L. Darling, H.B. Schlegel, J. Phys. Chem. 97 (1993) 8207. [13] S. Kondo, K. Tokuhashi, A. Takahashi, M. Kaise, Combustion Science and Technology (1999) submitted.
Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Ab Initio study of isomerization reaction from c-OSiH2O to t-OSiHOH S. Kondo a,*, K. Tokuhashi a, H. Nagai a, A. Takahashi a, M. Kaise a, M. Sugie a, M. Aoyagi b, S. Minamino b a
National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305-0046, Japan b Institute for Molecular Science, Okazaki, Aichi, 444-8585, Japan Received 16 June 1998; received in revised form 12 October 1998; accepted 12 October 1998
Abstract The isomerization reaction from c-OSiH2O to t-OSiHOH, a vital reaction to understand the spontaneous ignition of silane, has been reinvestigated with Gaussian-2 theory and the CASSCF(6,6) method. It has been found that the reaction proceeds through two consecutive steps; i.e., c-OSiH2O undergoes isomerization to yield w-OSiH2O, and then the latter is converted to tOSiHOH. The G-2 energy of the transition state of the latter process is 4.3 kcal/mol higher than that of the former. However, the G-2 energy of this higher transition state plus H atom is still 4.8 kcal/mol lower than that of the original reactants of SiH3 1 O2. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Silane; Isomerization; Gaussian-2; CASSCF
1. Introduction Silane is known for its spontaneous ignition in air at room temperature [1–4]. As this phenomenon occurs at room temperature, the elementary reactions active at low temperatures are important for elucidation of the phenomenon. In particular, it has been well recognized that SiH3 1 O2 is the key reaction for the occurrence of the spontaneous ignition of silane [5–9]. Hartman et al. have proposed that the reaction of SiH3 1 O2 proceeds through three different pathways, among which there is a reaction route to yield SiH3O 1 O [5]. If it is true, since O atom can readily react with SiH4 to yield two active radicals of SiH3 and OH at room temperature, it may make a chain branching process. Then, Koshi et al. have studied
on the branching ratio of this reaction [6,8]. They have determined the branching ratio between the OH producing and H producing reactions but have not determined the O producing one. However, according to the calculation made by Darling and Schlegel [9], the total energy of SiH3O 1 O is 10.8 kcal/mol higher than that of the original reactants of SiH3 1 O2. If it is the case, the reaction to yield an oxygen atom can not proceed at a favorable rate at room temperature. Then, we have suggested in a previous paper that the reaction route from SiH3 1 O2 to OSiOH 1 2H is a key to understand the spontaneous ignition at room temperature [10]. The whole route of this reaction was as follows; SiH3 1 O2 ! SiH3 OO* ! c 2 OS : H2 O 1 H
* Corresponding author. Fax: 181-298-54-4487. E-mail address: [email protected] (S. Kondo) 0166-1280/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0166-128 0(98)00515-6
! OSiHOH 1 H ! OSiOH 1 2H;
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Table 1 Optimized structures by the MP2 FULL/6-31G* method a Molecule
SiO1
SiO2
SiH1
SiH2
aO1SiO2
aO1SiH1
aO1SiH2
bH1SiO1O2
bH2SiO1O2
c-OSiH2O TS1 w-OSiH2O b TS2 t-OSiHOHc
1.6719 1.6206 — 1.6413 1.6399
1.6713 1.6207 — 1.6413 1.5356
1.4735 1.5048 — 1.4844 1.4700
1.4737 1.5049 — 1.5281 2.2409
58.26 100.40 — 133.80 128.20
118.83 134.31 — 111.06 105.29
118.91 95.19 — 85.80 23.04
107.92 107.99 — 154.53 180.00
2 107.86 2 136.86 — 2 80.07 0.00
a
Bond lengths are in angstroms and bond angles are in degrees. The optimization calculation has not been converged with the MP2 FULL/6-31G* method for w-OSiH2O. c For t-OSiHOH, O1H2 0.9733 and aSiO1H2 115.72. b
where SiH3OO* denotes excited species of SiH3OO and c-OSiH2O is a cyclic molecule. This suggestion has been made based on the study on the isomerization reaction from c-OSiH2O to OSiHOH which was done by Murakami et al. [11]. They have claimed that the transition state from c-OSiH2O to OSiHOH was identified by the CASSCF(6,6) method, although they have not shown any details of the calculation. They have just stated that the barrier height to this reaction is 6.4 kcal/mol, and the energy of this transition state plus that of H atom is about the same as that of the transition state (TS0) of the reaction from SiH3OO to c-OSiH2O 1 H. However, in an attempt to follow the result reported by Murakami et al. [11], we could not find the transition state that connects c-OSiH2O directly to OSiHOH. Instead, we have found the transition state (TS1) which connects c-OSiH2O to w-OSiH2O, another isomer of c-OSiH2O. w-OSiH2O has been found to undergo further isomerization through a transition state (TS2) to yield t-OSiHOH which is H-trans form of OSiHOH. In the following, the result of the present calculation is reported.
2. Results and discussion 2.1. (1) Calculation by using the HF/6-31G* and MP2 FULL/6-31G* methods At first, the investigation to find out the reaction route from c-OSiH2O to OSiHOH was made by using the HF/6-31G* and MP2 FULL/6-31G* methods. Actually, the transition state which seems to connect c-OSiH2O directly to OSiHOH (H-cis
form) was found by the HF/6-31G* method. Then, a similar effort was paid to find the corresponding one by using the MP2 FULL/6-31G* method. However, the latter effort was not successful. In this procedure, we have at first tried to see what happens if one of the two H atoms connected to the Si atom in c-OSiH2O is forced to move from the original position toward one of the two end O atoms. As, if one of the H atoms connected to Si atom in c-OSiH2O is shifted to an adjacent O atom, we may obtain OSiHOH molecule. Actually, when the geometry optimization is carried out fixing the H atom at a certain distance shifted toward one of the O atoms from the original position, we have found that both the SiO bonds of the optimized structure become longer than the original values. The SiO bond along which the H atom is shifted is more conspicuously prolonged than the other. Eventually, a transition state which connects c-OSiH2O to HSiOOH has been found both by the HF/6-31G* and MP2 FULL/6-31G* methods in stead of the one that connects c-OSiH2O to OSiHOH. Actually, this transition state has already been reported in our previous paper [10]. In the second place, we have attempted to see what happens if the OSiO angle of c-OSiH2O molecule is increased. If there is a reaction route which enables cOSiH2O to change directly to OSiHOH, we can expect that one of the two H atoms tends to move from the Si atom toward either of the two O atoms when the OSiO angle is increased. However, there has been found no clear indication of such tendency. Even if the OSiO angle is increased quite a bit, e.g. ten degrees or so, the two H atoms stick to their original positions near the Si atom. By increasing the OSiO
S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Fig. 1. The optimized structure of w-OSiH2O by the CASSCF(6,6) method.
angle step by step, we finally arrived at a saddle point of the energy surface (TS1). As is shown in Table 1, the geometry of this transition state is very similar to that of c-OSiH2O molecule except that the OSiO angle is much larger than that in c-OSiH2O. For comparison, the optimized structures of c-OSiH2O and OSiHOH are also listed in Table 1. Here, the numbering of the atoms in the molecules are to be referred to Fig. 1. The optimized structures of c-OSiH2O and OSiHOH are essentially the same as in the literature [12]. Considering the change in geometry from that of c-OSiH2O, TS1 may hardly be the transition state that connects cOSiH2O directly to OSiHOH. Rather, this transition state seemed to be owing to the one connecting cOSiH2O to another isomer that may result from c-OSiH2O by opening the OSiO angle (w-OSiH2O). Unfortunately, however, we could not find out any stable structure of this isomer by the MP2 FULL/ 6-31G* method. In the next step, we have tried to follow the reaction coordinate in the reverse way. We have made calculation starting both from H-cis and H-trans forms of OSiHOH. The reaction coordinate in the potential surface was investigated by shifting the H atom connected to O atom toward the Si atom in these molecules. In this way, we have arrived at a saddle point (TS2) when we started from H-trans form of OSiHOH. The optimized structure of the transition state, which is listed in Table 1, strongly suggests that it connects OSiHOH with some isomer of c-OSiH2O that has a large OSiO angle. However, again the optimization calculation to find this isomer was not successful.
27
Thus, the optimum structures of the saddle points TS1 and TS2 in the potential surface were found by starting from c-OSiH2O and t-OSiHOH, respectively. Both these transition states seem to probe from two different directions the same and a single molecule which may result from c-OSiH2O by opening the OSiO angle. However, the optimization calculation for this molecule did not converge at all either by the HF/6-31G* method nor by the MP2 FULL/631G* method. This result suggests that the molecule has the bi-radical character around the geometry of the transition states, TS1 and TS2. If it is the case, the treatment with such a method as CASSCF may be necessary to obtain the optimized structure. 2.2. (2) Calculation by using the CASSCF(6,6) method At this stage, we have switched the calculation method from HF/6-31G* and MP2 FULL/6-31G* to CASSCF(6,6)/6-31G*. A part of the calculations has also been carried out by using the CASSCF(10,8) method. Very similar results have been obtained for the two method. Therefore, only the results obtained with the CASSCF(6,6) method will be shown here. In an attempt to identify the optimum structure of w-OSiH2O, we have arrived at a few somewhat different optimum structures, which are different in their energies as well. The one with the minimum energy was considered to be the ground electronic state. However, it is remarkable that the lowest levels of the singlet and triplet states of this molecule have been found to have almost the same energy. The reason is because there are two unpaired electrons which are distributed almost equally to the two end O atoms, and because there is very little interaction between them. As we have explored the dominant route from c-OSiH2O to OSiHOH, bifurcation path through the interconversion at the w-OSiH2O intermediate is not considered here. We have followed the reaction coordinate starting from w-OSiH2O in the singlet state. By shifting one of the two H atoms in w-OSiH2O along one of the two SiO bonds step by step, we have reached a transition state. This transition state is very similar in its geometry to TS2 obtained with the MP2 FULL/6-31G* method in the above, and can be definitely identified as the same one. Pursuing further the reaction
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
Table 2 Optimized structures by the CASSCF(6,6) method a Molecule
SiO1
SiO2
SiH1
SiH2
aO1SiO2
aO1SiH1
aO1SiH2
bH1SiO1O2
bH2SiO1O2
c-OSiH2O TS1 w-OSiH2O TS2 t-OSiHOHb
1.6425 1.6689 1.6710 1.5858 1.6174
1.6606 1.6936 1.6956 1.6084 1.5257
1.4636 1.4668 1.4643 1.4600 1.4583
1.4636 1.4672 1.4642 1.5366 2.2303
62.02 97.36 116.37 128.03 127.49
117.90 116.11 105.83 114.72 105.29
117.97 107.41 105.76 72.37 22.01
110.63 114.47 117.21 167.55 180.00
2110.55 2121.30 2117.53 251.51 0.00
a b
Bond distances are in angstroms and bond angles in degrees. For t-OSiHOH, O1H2 0.9494 and aSiO1H2 118.32.
coordinate to the other side of this transition state finally leads to the optimum structure of t-OSiHOH. However, by decreasing the OSiO angle of wOSiH2O, the reaction coordinate lead to c-OSiH2O molecule via a transition state of TS1. Thus, a reaction route from c-OSiH2O via wOSiH2O to t-OSiHOH was completely identified by the CASSCF(6,6) method. The optimum structures of the three molecules, c-OSiH2O, w-OSiH2O, and tOSiHOH, and the two transition states, TS1 and TS2, were all obtained by this method. The optimized structures of these molecules and transition states are summarized in Table 2. As for c-OSiH2O and tOSiHOH, there are not much difference between the optimized structures by the MP2 FULL/6-31G* method and the corresponding ones by the CASSCF method. In contrast, as for TS1 and TS2, there are some differences between them. Table 3 shows the CASSCF(6,6) energies for these species. In this calculation, the energy of w-OSiH2O is 3.1 kcal/mol lower than that of TS1, and 28.3 kcal/mol lower than that of TS2. Now, our concern is the energy of w-OSiH2O 1 H relative to the original reactant of SiH3 1 O2. For that purpose, we have tried to
calculate the energies of these molecules and transition states on the Gaussian-2 theoretical basis. 2.3. (3) Energetics relevant to the Reaction Route from c-OSiH2O to t-OSiHOH The energy values of both TS1 and TS2 have successfully been obtained by the Gaussian-2 theory. The result is summarized in Table 3 together with those of relevant molecules. The energy of TS2 is 4.3 kcal/mol higher than TS1. Also, the energies of TS1 and TS2 plus that of H atom are, respectively, 13.6 and 17.9 kcal/mol higher than that of TS0. It should be noted that the higher barrier of the two (TS2) is still 4.8 kcal/mol lower than the energy of the original reactants of SiH3 1 O2. However, the energy of w-OSiH2O 1 H may be estimated from these values combined with the energy differences between w-OSiH2O and these transition states obtained by the CASSCF(6,6) method. In this case, although it is possible that the energy of wOSiH2O is slightly underestimated owing to the biradical character, the resulting argument (see below) on the reaction route from SiH3 1 O2 all the way to
Table 3 Energy values (in a.u.) of c-OSiH2O, TS1, w-OSiH2O, TS2 and t-OSiHOH Molecule
CASSCF(6,6)
MP2 FULL/6-31G*
G-1 a
G-2 a
c-OSiH2O TS1 w-OSiH2O TS2 t-OSiHOH
2439.829164 2439.813965 2439.818922 2439.773751 2439.949255
2440.173403 2440.102688 — 2440.154578 2440.305857
2440.463320 2440.437070 — 2440.430535 2440.593683
2440.461921 2440.436153 — 2440.429117 2440.593597
a The zero-point energies contained in G-1 and G-2 energies are for the HF/6-31G* method with a correction factor of 0.893 for c-OSiH2O and t-OSiHOH and for the MP2 FULL/6-31G* method with a correction factor of 0.925 for TS1 and TS2.
S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
29
Fig. 2. Energy diagram (in kcal/mol) for the reaction route from SiH3 1 O2 to OSiOH 1 2H.
OSiOH 1 2H may not be affected much. Now, since the energy of w-OSiH2O is 3.1 kcal/mol lower than that of TS1 (by CASSCF), and the energy of TS1 1 H is 9.1 kcal/mol lower than the original reactants of SiH3 1 O2 (by G-2), w-OSiH2O 1 H is expected to be 12.2 kcal/mol lower than the original reactants. However, the same w-OSiH2O is 28.3 kcal/mol lower than TS2 (by CASSCF), and the energy of TS2 1 H is 4.8 kcal/mol lower than the reactants (by G-2), w-OSiH2O 1 H is in this case expected to be 33.1 kcal/mol lower than the reactants. At any rate, the true energy of w-OSiH2O 1 H may be somewhere between 12.2 and 33.1 kcal/mol lower than the reactants. Considering the present result, the energy diagram given in the literature [9–11] should be revised as in Fig. 2. Actually, Fig. 2 shows the energy diagram of the reaction route from SiH3 1 O2 to OSiOH 1 2H, where the species of w-OSiH2O and the transition states TS1 and TS2 have been added in the diagram, while most of the remaining part has just been lifted from in Ref. [10], Fig. 2. Here, a tentative value of
223 kcal/mol is employed for the energy of w-OSiH2O 1 H. The present findings are of interest in interpreting the spontaneous ignition of silane particularly when it is compared with the previous one [10,11]. According to the report by Murakami et al., the energy of the transition state from c-OSiH2O to OSiHOH plus that of H atom was much the same as that of TS0. If it is true, once produced c-OSiH2O molecules may readily undergo the subsequent isomerization to yield OSiHOH. Then, since OSiHOH is much more stable than c-OSiH2O, it is expected to have an enormous excess energy which is enough for OSiHOH to proceed further to the subsequent decomposition reactions. Some of them may decompose to yield OSiOH 1 H, and others to yield SiO 1 H2O and so forth [10,11]. However, according to the present calculation, cOSiH2O 1 H resulting from the original reactants encounters two more potential barriers to get to OSiHOH 1 H, which are definitely higher than TS0. In this case, therefore, c-OSiH2O may not necessarily
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S. Kondo et al. / Journal of Molecular Structure (Theochem) 469 (1999) 25–30
undergo straightforward isomerization to yield OSiHOH. Rather, the reaction may have a sort of sojourn before it proceeds to the subsequent reaction. The significance of the present result in interpreting the spontaneous ignition at room temperature will be discussed in more detail in a subsequent paper [13]. 3. Conclusion The reaction route from c-OSiH2O to OSiHOH has been reinvestigated with Gaussian-2 theory and the CASSCF(6,6) method. No reaction route was found in our calculation that connects c-OSiH2O directly to OSiHOH. Instead, it has been found that the reaction proceeds via another isomer of c-OSiH2O to reach OSiHOH; at first c-OSiH2O undergoes isomerization to yield w-OSiH2O, and then the latter is further converted to t-OSiHOH. The energy of the transition state of the latter process (TS2) is 4.3 kcal/mol higher than that of the former (TS1). However, the energy of TS2 1 H is still 4.8 kcal/mol lower than that of the original reactants of SiH3 1 O2. Acknowledgements A part of this work was supported by a Grant-in-aid
for Priority Area and for Scientific Research by the Ministry of Education, Science and Culture, Japan (No. 08230229).
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