Growth patterns of chlorosiloxanes

THEO CHEM ELSEVIER

Journal of Molecular

Structure (Theochem)

398-399

(1997)X-379

Growth patterns of chlorosiloxanes’ Karl Jug*, Daniel Wichmann

Abstract The growth pattern ofchlorosiloxanes Si,,O,,,Cl,,, was studied with respect to increasing, decreasing and constant ratio of n’ln. The relative stability of isomers was determined with the SINDOI method. The calculations allow us to draw cqnclusions about stahilities of chains, monocyclic and oligocyclic molecules and the preferable ring sizes. Special attention is given to the cage structures of silsesquioxanes with n’ln = I .5 and II = m. It is possible to explain experimental data from mass spectra of mixtures of chlorosiloxane Kqwords:

compounds

Chlorosiloxane;

with

more than 100 atoms. 0 1997 Elsevier Science B.V.

Silsesquioxane;

Structure; Stability

1. Introduction Chlorosiloxanes are formed from silicon tetrachloride and oxygen at high temperatures above 1300 K. This process is known as chemical vapor deposition and is of great technological importance in the production of optical waveguides [ 11. Chlorosiloxanes are intermediates in a well defined deposition of solid silicon dioxide. Although mass spectrometry shows us the great variety of species being formed [2], only a few of them have yet been isolated and their structure determined [3-61. Since purification of the reaction products was found to be difficult, preparation and structure determination has been achieved by an alternative approach for SixO12C18 [S], namely by synthesis of the hydridosiloxanes and subsequent photochlorination 171. From our recent semiempirical calculations with the SINDO 1 method [8], we have learned about the stability of different building units. Now, with the growing amount of * Corresponding author. ’ Presented at WATOC ‘96, Jerusalem, Israel, 7- 12 July 1996.

experimental information, calculations on chlorosiioxane clusters of up to 210 atoms can give more insight into the growth patterns of this interesting class of molecules, which eventually leads to solid SiOZ.

2. Structure growth towards equal oxygen to silicon ratio The product distribution of chlorosiloxanes Si,O,b,Z, ‘C/mmeasured by mass spectrometry [9] is best visualized by plotting the oxygen to silicon ratio n’ln versus the number n of silicon atoms, i.e. the size of the molecule. The results are shown in Fig. l(a) and Fig. l(b) for a typical mixture of reaction products. Fig. l(a) presents the ingredients of an oily, distillable fraction of chlorosiloxanes. While the product distribution strongly varies in the case of smaller molecules, it becomes more compact for chlorosiloxanes with more than twenty silicon atoms. A slowly increasing relative oxygen content of n’ln = 1.2 to 1.3 is found in this region. Smaller chlorosiloxanes may

0 166. I280/97/$17.00 0 I997 Elsevier Science B.V. All rights reserved PII SO 166.1280(96)04927-5

366

K. Jug, il.

Wichmarm/Journal

of Molrcular

Smtcture

(Theochem)

398-399

(1997) 365-379

4 n’

(0) 1.8

n (Si)

1.0

0.8

0.6 0

20

10

40

30 n (Si)

b) n’ (0) n (Si)

1.8

1.6

1.0

0.8 max. ht. 0.6

Ia 1 0

min. ht.

.

01

I

,

I

I

I

10

20

30 n (Si)

1

A

40

Fig. 1. Experimental distribution of chlorosiloxanes with high, medium and low intensity (a) oily fraction. (b) high boiling fraction

K. Jug. D. Wic~hmanrdJoumal

oj’Molecuiar

have an oxygen to silicon ratio below 1.0, which is typical for chains, or as large as 1.5, which corresponds to chlorosilsesquioxane stoichiometry. There is even a high boiling fraction containing chlorosiloxanes with greater oxygen content, 1.4 to 1.6, that are shown in Fig. l(b). Gaps in this presentation at about 22 and 30 silicon atoms are caused by the superposition of different mass spectrometric measurements of the same sample. In the region below 20 silicon atoms, homologous series with a characteristic ratio of n’(O)ln(Si) show up. They can be classified by their oxygen content as follows: Group 1: Si,O,_,Cl *,,+?,chains with increasing oxygen to silicon ratio. Group 2: Si ,,O,Cl z,,, rings with equal ratio of oxygen and silicon atoms. Group 3: Si,0,+,C1~n-2m, oligocycles with decreasing oxygen to silicon ratio. Chlorosiloxanes of group 1 can be be found on curve I in Fig. 1, chlorosiloxanes of group 2 on curve 11. Oligocyclic compounds of group 3 are distinguished as bicyclic (m = 1, curve III), tricyclic (m = 2, curve IV) and tetracyclic (m = 3, curve V) structures. Growth within these homologous series leads to an equal oxygen to silicon ratio in all cases. Formal reaction equations for the growth of chain structures can look like Sic& + 1/202 -

SiOC12 + Cl*

Si,,O,_iCIZ,,+z+SiOCl~ 2SiClJ + l/20,

-

-

Sin+10,,Clz(,+1j+2

SiZOCl, + Cl2

(1) (2) (3)

Srructure

(Throchrm)

4

398-399

(19971

361

365-379

n’(O)/n(Si)

E/n(Si) [Hartree]

1.0

0.80, ,

i 0.9

11; -:_i;.;t0.65

1

2

3

4

5

6

7

0.5

n(Si)

b)

E/n&)

[Haflree]

n’(O)/n(Si)

0.80 \

r 1.0

n(Si)

Fig. 2. Stabilization and oxygen content of chlorosiloxane Si,,O,._ , Cl :,,+? and b) rings Si,,O,,CI ?,!.

(a) chains

so it becomes clear that they do not correspond to a prevalent growth pattern. In the following, we present SINDOl [lo-131 results for structures and binding energies of chlorosiloxanes. Plots of binding energies per silicon atom for the most stable isomers reveal the preferred stabilization in connection with a varying oxygen content. 2.1. Chain-like and monocyclic

molecules

Si,,O,_ ,Clzrl+? + Si20C16 WL

Si,,.

10,C12(,+

1Cbrl+2

-

+

1)+2 + Sic&

(4)

SiCIQ + I /20,

Si,+1O,,Cl2(,~+1)+2+Cl2

(5)

Eqs. (1) and (2) are rather speculative, because the elusive SiOC12 has not yet been observed in the gas phase. The net result of Eqs. (l)-(5) is always the insertion of SiOCll units, no matter whether this reactive species is actually involved or not. However, all these series of chlorosiloxanes end at moderate molecular size in favour of more oxygen-rich species and

In our previous article [S] we have extensively described structural features of chlorosiloxane chains and rings. Both series Sin0,,-IC12n+2 (n = 2-4) and Si, 0,C12, (n = 2-5) were extended to the next homologue with n = 5 and n = 6, respectively. We found for both SisOJCl I: and SihOhCl I? the branched isomer is most stable. It is remarkable that the substituted tenmembered ring is calculated to be 0.9 kcal mol-’ lower in energy than the favourable conformation of the twelve-membered ring, which has boat form. In Fig. 2 the binding energy per silicon atom for both chains and rings is plotted. The stabilization of chains with increasing n(Si) (Fig. 2(a)) is mainly

caused by the increasing oxygen content, because a standard SiO bond is more stable than the Sic1 bond. Branching plays a minor role. Consequently in chlorosiloxane rings (Fig. 2(b)) with n’(O)/rz(Si) = 1, convergence is reached after removal of the ring strain.

(1)

Si405Clfi

si607c110

2.2. Bicyclic molecules Even in bicyclic chlorosiloxanes with the formula Si,,0,,+lC12,z-1, the main stability criterion is the decreasing amount of ring strain with growing molecular size. Structures of these molecules with n = 3-7

(2)

(C2u)

(c2v)

(3b)

S&06&

cD3h)

SisO<a

(4) si708ch2 (cd Fig. 3. Structures

(I-4)

of most stable bicyclic chlorosiloxanes

S&O,,+ 1C12n_?rn = 4-7.

K. Jug. D. Wichmnnn/Journul

of MolecularStructure (Theochent) 398-399

are presented in Fig. 3. Si40sClh (1) necessarily contains two annelated six-membered rings, whereas the most stable isomer of SisOsClx (2) is constructed only of eight-membered rings. Experimental data are available for Si607Cl I,). In our calculations we found isomer (3a) most stable, which contains one eight- and one ten-membered ring. An isomer with

(5)

si507c16

(7)

Si709Cb0

(c2v)

(S-9)

two annelated eight-membered rings and Cz, symmetry (3b) was calculated to be 8.9 kcal mol-’ higher in energy. However, this isomer was separated in a crystalline form from the product mixture and structurally characterized [6]. It is slightly distorted to C, symmetry due to crystal packing. Other isomers are at least partly built of six-membered rings and consequently

(6)

si6oScl8

(‘%I

(C3v)

(8b)si8olOc~12 (&) Fig. 4. Structures

369

(I 997) 365-379

(9) %0~12~~16

of most stable tricyclic chlorosiloxanes

Si,,0,,+zCI,,,4,

(Td) n = 5-8,

IO.

calculated to be much higher in energy. The combination of gas chromatography with mass spectrometry revealed that at least two isomers of this compound exist in the chlorosiloxane mixture. Therefore, it is strongly assumed from the calculated stabilities that these are the two isomers with maximum ring size. For II = 7 only the annelated ten-membered ring structure (4) was optimized, because this structure is expected to be free of ring strain, whereas other isomers of this stoichiometry must contain energetically unfavourable eight-membered rings. 2.3. Tricyclic tnolecules Chlorosiloxanes

of the formula

Si,~0,,+2C12,_j are

(10)

(11)

Fig. 5. Structures

%0013C114

(10-12)

collected in Fig. 4 for n = 5, 6, 7, 8 and 10. The smallest tricyclic molecule with n = 4 will be discussed in the next section in connection with the higher chlorosilsesquioxanes. For tz = 5 we calculated structure (5). It contains two annelated highly strained six-membered rings, which are forced to nonplanarity. Even in an open trigonal prism-like structure (6) with one siloxane bridge missing, one such ring remains. Si709C1 ,,) (7) consists of annelated eight-membered rings arranged as in a part of a cube. The energetically favoured structure with tz = 8 (8a) features only the preferred ten-membered rings. A cube-like siloxane which is open on one side (Sb) would have the same molecular formula as @a), but necessarily contains eight-membered rings and is therefore

~~8~11~~10

(CZ~)

of most stable tetracyclic

(c2)

(12)

%2~15~~18

chlorosiloxanes

(D3h)

Si,,0,,+3C12,,+, n = 8. IO, 12.

K. Jug, D. Wichmunn/Journal

o~Molecular

the 29.9 kcal mol-’ higher in energy. Finally, adamantane-like structure (9) is another unstrained tricyclic chlorosiloxane, useful for modelling solid state growth.

4

n’(O)/n(Si)

E/n(%) [Hartree]

1.5

0.85 -

- 1.4 0.80-

Strumre

(Throchem)

398-399

2.4. Tetracyclic

(1997) 365-379

371

molecules

Tetracyclic structures Si,,0n+3C1?n-6 are presented for IZ= 8, 10 and 12 in Fig. 5. The most stable isomer of SigOIICll,, (10) is from previous work [8]. For n = 10 (11) there is only one eight-membered ring left. The other rings are ten- or twelve-membered. Strainfree twelve-membered rings are reached in Si ,?O t&l t8 (12). A double layer of twelve-membered rings in a chair conformation was optimized in Dih symmetry. This molecule represents a structural unit of the tridymite modification of solid SiO?.

- 1.3

2.5. Stahilizution

in oligocyclic

molecides

- 1.2 0.75-

< * E/n(Si)

p

* n’O)/n(Si) 0.70

345678

9

10

11

12

13

1.0

n(Si) b)

E/n(%) [Hartree] 0.851 \

n’(O)/n(Si) [ 1.5

I 1.4

1.3

1.2

t * E/n(Si)

- 1.1

* n’(O)/n(Si) 0.70

3

4

5

6

7

8

9

10

11

12

13

I.0

n(Si)

1.2

t * E/n(Si)

- 1.1

* n’(O)/n(Si) 0.70

345678

9

10

11

12

13

1.0

n(Si) Fig. 6. Stabilization and (b) tricyclic S&O,,+ 1C12,,-2, Si,,0,,+3C12n_h chlorosiloxanes.

tricyclic and tetracyclic compounds Bicyclic, (Fig. 6) show an increase of binding energy per silicon atom towards saturation, despite a decreasing oxygen content for these oligocyclic series. This is caused by ring strain of the first siloxanes, built of six- and eightmembered rings. For molecules with about ten silicon atoms this stabilization is reached. From Fig. 1 we learn, that neither long siloxane chains nor highly enlarged rings occur. So chlorosiloxane growth must work via branching, as demonstrated in the case of siloxane chains and rings. This growth of side-chains must be followed by ring closure via substitution of terminal chlorine with bridging oxygen atoms, as this process yields more oxygen-rich chlorosiloxanes. While from perchlorodisiloxane SilOC1,, as starting material, instead of SiCI+ a similar product distribution is reached [2], including molecules with an odd number of silicon atoms, obviously cleavage of certain siloxane bonds plays an important role. As the strength of a standard SiO bond (111.2 kcal mol-’ in SiOl) is higher than that of Sic1 bonds (95.6 kcal mol -’ in SiClJ [14], such fragmentation processes will predominantly work in small rings, where siloxane bonds are weakened due to ring strain.

oxygen content of (a) bicyclic Si,,O,+zC1~,,_~ and (c) tetracyclic

3. Silsesquioxanes Silsesquioxanes of the general molecular formula Si,O3,&1,, were chosen as a model series, because their oxygen content of 1.5 per silicon atom shows an upper limit of the experimental product distribution

372

K. Jug, D. WichmanrdJournul

ofMolrcular

for high intensity measurements (curve VI in Fig. 1). Such a high oxygen content of some small silsesquioxanes does not correspond to a growth pattern that leads to the structure of solid SiOZ. Rather it contrasts solid state growth as has already been shown in case of oligocyclic compounds. In the last decade there has been great interest in these spherosilicates with various organic and

Structure (Theochrm)

398-399

(I 997) 365-379

inorganic substituents [ 1.51. Lately work has been focused on the hydrogen-terminated species. Silsesquioxanes of this kind have been synthesized and studied for n = 8, 10, 12, 14, 16 and 18 only recently [16-201, but just for II = 8 in the case of the chlorine substituted compounds [5]. We started with calculating the tricyclic SijOhCll (13) and the tetracyclic Sih0&16 (14), shown in Fig. 7.

-

(13) si406c14 (Td)

(14)

si609c16

(D3h)

V

(15) s~8olZc18 (Oh)

(17a)

Sil20&ll2

Fig. 7. Structures

(13-17)

(16) SWl&11o (h)

(D2d) of chlorosilsesquioxanes

Si,,Ol,,~CI,, n = 4, 6. 8, 10, 12.

K. Jug. D. WichmantdJournal

of Moleculur

These molecules were described in the literature only with hydrocarbon substituents instead of chlorine [15]. SiOSi angles of 114.5” in the former (13) and 126.9 and 136.6” in the latter (14) case show that these molecules are highly strained. It is therefore not clear, whether mass spectrometric peaks in this region originate from stable molecules or result from fragmentation of chlorosiloxanes with greater molecular weight. Structure (15) is the well known perchlorooctasilsesquioxane. It has Oh symmetry in the gas phase. Tornroos found a distortion towards Sh in the crystal 151. Our calculated SiOSi angle of 145.4” is slightly smaller than the experimental SiOSi angle of 148.0 and 148.8”, respectively. The calculated SiO bond length of 1.566 A (exp. 1.595-1.610 A) is somewhat shorter, the Sic1 bond length of 2.038 A (exp. 1.989- 1.992 A) longer, than in the crystal. This deviation of SINDOl bond lengths from experimental values is in the expected range of accuracy and holds

(18a) Sil4021Cl14 (h)

Structure

(Theochem)

398-399

(1997) 365-379

for all structures, where experimental values exist. It was described in the presentation of the smaller chlorosiloxanes [8]. Structure (16) was optimized in Dsh symmetry. Si 12018C11~(17a) was found to have DZd symmetry. This isomer is 22.6 kcal mall’ more stable than a double layer of twelve-membered rings (17b) optimized in Dbh symmetry. In contrast to the structures (15), (16) and (17a) in Fig. 7, this latter species has not been observed as a hydrogen terminated molecule, but is known as a building unit in double-ring silicates [ 191. Two isomers of SilJO?,HII were described in the literature [ 161. A major fraction of the preferred D3,, structure was isolated. Consistent with these experimental findings the corresponding chlorosilsesquioxane (Ma) in Fig. 8 was calculated to be 15.9 kcal mol-’ more stable than the CzV isomer (Mb) that shows just another arrangement of six ten-membered and three eight-membered rings. As expected, the unobserved DTh isomer with a greater

(18b) %&Cl14

(G,)

(18c) Si14021Ch4 (h) Fig. 8. Structures

(18a-c)

313

of chlorosilsesquioxanes

Si,,01,,2CI,,

II = 14.

314

K. Jug, D. WichmannLJournal

of Molecular

number of eight-membered rings (1%) is as much as 68.7 kcal mol-’ higher in energy. So a distribution of equal ring sizes on the spherosiloxane surface is strongly preferred to a combination of large and small rings. Especially unstrained ten- and twelve-membered rings are favourable for these cage-like structures. This was taken into account for the choice of greater perchlorosilsesquioxanes models, where experimental information becomes less. Si 16O&1 r6 must contain at least two eight-membered rings (19) (Fig. 9). This is also the structure that was recently proposed to be the preferred Si rh isomer in a mixture of hydridosilsesquioxanes. This conclusion was drawn based on topological considerations and ?Si chemical shift values [20].

Structure

(19-22)

398-399

(I 997) 365-379

The dodecahedral molecule (20) consists only of the preferred ten-membered rings. Si2j03hC1Z1 (21) represents the sodalite cage, known from zeolite chemistry. For n = 36 a structure of ten- and twelvemembered surface-rings (22) was considered and optimized in Dhh symmetry. For n = 48 in analogy to zeolite chemistry, the so called o-cage (23) in Fig. 10 was chosen. Optimization in the proposed Oh symmetry leads to a significant distortion of this framework by rotation of the SiOICl units towards the large sixteen-membered rings. Siloxane bridges are bent towards the inner cage and the corresponding angles are reduced. Therefore this arrangement of 8-, lo- and 16-membered rings turned out to be unfavourable.

(21) si24036c124 (6) Fig. 9. Structures

(Theochem)

of chlorosiisesquioxanes

(22) si36054c136 (&h) Si,,O&l,,,

II = 16, 20, 24, 36.

Finally, it was possible to calculate a structure (24) as large as Si6a09&lh0 with assumed Ii, symmetry and in analogy to the Buckminster fullerene ChO. Fig. 11 shows the stabilization of silsesquioxanes at a constant ratio n’(O)ln(Si) = 1.5. This plot reveals that even for a structure with 210 atoms, saturation of binding energy per silicon is not reached.

Furthermore, the silsesquioxane models with IZ = 24 and II = 48 do not fit in the global trend of stabilization as they contain 6 (sodalite cage) and 12 (a-cage) significantly strained eight-membered rings. SiOSi angles and sizes of surface rings of the most stable structures in Figs. 7-10 are collected in Table 1. The angles in molecules with the formula

(23) si48072c148 (OIL)

Fig. IO. Structures

(23, 24) of chlorosilsesquioxanes

Si,,01,,,7CI,,, n = 48, 60

ofMolecular Structure (Themhem)

K. Jug, D. Wichmann/Journnl

376

ss; - 1.5

0.90

- 1.4

0.85

- 1.3

0.80 - 1.2

0.75

Si,O

* n’(0 0.65

’ 0

5

10 15 20

25

30

35 40

45

50

5! 5 60

65

1.0

-

n(Si) Fig.

I I. Stabilization of chlorosilsesquioxanes

Si,,O&I,,.

growth towards silsesquioxanes

In Section 2 we presented results for chlorosiloxanes of decreasing oxygen content with growing molecular size. However, this growth pattern is contrary to the global trend towards oxygen-rich compounds and solid Si02 as the final reaction product. General molecular formulas for such species are Group 4: SinOZ,,_,,,Clz,,, Group 5: Sin03ni2_,,,C1n+Zm Table I SiOSi angles (degrees) and ring sizes r of most stable chlorosilsesquioxanes Molecule

SiO,Si

Si400C13 Sib09Clb SixOlzCIK

114.5 136.6 145.4 151.6 174.4 168.1 179.3 175.6 162.8 164.9 165.1 164.3

Si 100 ISC~10 Si ,2OlxCl I2 Si 140Kl 14 Si 16024Cl 16 Si 200 ~Cl20 Si zJO &I 24 Si 36O54Cljb SL~O72Cl4K SiMO&IhO

SiOhSi

3n/2-mCL1+2,,1

+Si20C16+02

Si,l+203(,~+2)/2-,nCln+2+2,,,

+2C12

SiO,Si

(for definition of angles see Figs. 7-10) SiOdSi

r 6

126.9 150.6 156.6 157.9 158.4 156.8 161.4 126.5 147.0

(6)

So the net result is an addition of SiZ03C12. But there is no detailed knowledge of the actual reaction mechanism. The series Si,,O~,,,~-&Zln+~(i.e. m = 2) includes the disiloxane Si20C16 and the monocyclic SilOIClx as well as the bicyclic (3a), the tricyclic (8a) and the tetracyclic structure (ll), which were already presented. These molecules have no common structural features. . The homologous series Si,OiniZ_) Cl n+h is more instructive. Except for the acyclic Si40$Jlo, it is possible to construct molecules only with twelvemembered rings in a very straightforward way. The SihOhCIIZ ring is followed by the above described adamantane-like structure (9) with n = 10 and the double twelve-membered ring structure (12) with II = 12. Annelation of a third twelve-membered ring results in structure (25) (Fig. 12). Now one could add more and more layers of twelve-membered rings in this way. However, we enlarged the previous structure

Si,Oj,&I,, vary in the range from 145” to almost linear SiOSi-bridges for n 2 8, and excluding the distorted o-cage (n = 48). Most favourable molecules in the energy plot (Fig. 11) have ring sizes r = 10 and 12, exclusively.

4. Structure

365-379

1.1

+- E/n(!

0.70

(1997)

Fig. 1 shows the curves for the group 5 series for m = 2 (curve VII) and m = 3 (curve VIII). Increasing n with constant m means siloxane growth towards SiO, in group 4 and towards silsesquioxanes in group 5. This latter group has been studied in more detail for m = 2 and 3, as molecules of this stoichiometry are identified in the experimental product distribution. A formal reaction equation within group 5 chlorosiloxanes can be written as

n’(0 ~)/n(Si)

E/n(Si) [Hartree] 0.95

398-399

151.7 155.7 151.4

149.9

159.6 125.2

155.7

6,8 8 8,lO 8,lO 8,lO 8,10 IO 8,12 IO.12 8,12,16 IO,12

K. Jug, D. Wichmann/Jounml

ofM&c&r

Structure

by additional rings on its side to Si300&136 (26) to simulate horizontal growth, too. This growth is equivalent to the reaction suggested in Eq. (6), carried out six times. Stabilization in the series with m = 2 is shown in Fig. 13(a) and is both due to the decrease of ring strain and the increasing oxygen content. Fig. 13(b) shows the stabilization in the series with m = 3. It goes along with the increasing oxygen content, but is distinctly lower than the silsesquioxane stabilization for molecules of this size, although at n = 30 already a ratio n’(O)/n(Si) = 1.4 is reached. Finally we calculated Si~s07~C11s (27) within D?h symmetry. This molecule may be understood as a

(25)

%8024c124

Fig. 12. Structures

(25-27)

of polycyclic

Si48072C148 chlorosiloxanes

39X-399

(1997) 365-379

311

chlorine terminated part of tridymite, a modification of crystalline SiO?. It allows a direct comparison to the cage presented as structure (23). However, the lower symmetry of this model required restrictions in the optimization process, a common technique in cluster optimizations. To prevent deformations at the edges, only SiO bond lengths in and between the planes and the terminating Sic1 bonds were relaxed. This bulk structure (27) is 347 kcal mol-’ higher in energy than the cage structure (23). So even at a molecular size of 168 atoms, chlorosiloxane particles deposited from a gas phase reaction do not have the shape of modifications of crystalline SiO?.

(26)

(D3d)

(27)

(Throchem)

si30042c136

(D3d)

(D3h) Si,,0q,,/2-3C1,,,.

n = 18, 30 and Si,,0,2C14x

378

of Moleculrr

K. Jug, D. Wichmarm/Journul

4

n’(O)/n(Si)

E/n(Si) [Hartree] 0.95,

r 1.5 Il.4

0.90

1.3

1’

- 1.2

0.85 -

- 1.1 0.80 -

- 1.0 -0.9

0.75 -

(1997) 365-379

small chlorosiloxane rings in favour of ten- and twelve-membered rings is the preferred growth pattern. In case of chlorosilsesquioxanes Si,,03,&1,, ring strain is removed at about n = 20. Although this series does not lead to solid state growth, even for the largest molecules studied, these cage-like molecules are energetically preferred over oxygen-rich bulk structures.

-0.7

* n’(O)/n(Si)

1 2

3

4

5

6

7

8

9

10

11

12

o.6 0.5

n(Si) b)

39X-399

-0.8 * E/n(Si)

0.70 0.65 k

Srrucrure (Throchrm)

E/n(Si) [Hartreej 0.95 7

n’(O)/n(Si) r 1.5

0.90 0.85 0.80 -

Acknowledgements This work was partially supported by Deutsche Forschungsgemeinschaft. The calculations were performed at the Siemens-Nixdorf S400/40 at RRZN Hannover and several IBM workstations. The structures were drawn with the program SCHAKAL. We thank Prof. M. Binnewies and his coworkers for regular discussions on this topic and information on mass spectrometric and crystallographic data.

0.75 0.70 -

* n’(O)/n(Si) 0.65

0

5

10

15

20

25

30

0.6 0.5

[I] M. Binnewies,

n(Si) Fig. 13. Stabilization and oxygen content of polycyclic anes (a) Si,,01,,/2-2Cl,,+j and (h) Si,,Ol,,i~_lCl,,+h.

References

chlorosilox-

[2] [3] [4]

5. Conclusions

[5]

In chlorine-rich species, namely the acyclic and monocyclic chlorosiloxanes primarily formed, growth of side-chains is energetically favourable in comparison to ring enlargement or chain elongation. As a consequence, chlorosiloxane growth will proceed via branching followed by ring closure in a reaction with oxygen and under loss of chlorine. On the other hand, insertion of SiOC12 units will not result in oxygen-rich products and becomes unfavourable compared to other growth patterns. Oxygen-rich chlorosiloxanes with less than 20 silicon atoms must have cage-like structures. For these polycyclic species the decrease of ring strain is the reason for stabilization. Fragmentation of

[6] [7] [8] [9] [IO] [l l] [I21 [13] [I41

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