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The molecular structure of gaseous octachlorotrisilane, Si3Cl8, as determined by electron diffraction
Journal of Molecular Structure, 77 (1981) 315-318 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Short communication
THE MOLECULAR STRUCTURE OF GASEOUS OCTACHLOROTRISILANE, Si,Cl& AS DETERMINED BY ELECTRON DIFFRACTION
ARNE ALMENNINGEN Department TORGNY Department (Received
of Chemistry,
University
of Oslo, Blindern,
University
of Trondheim,
Oslo 3 (Norway)
FJELDBERG of Chemistry,
NLHT,
N-7055
DragvoIl
(Norway)
2 July 1981)
A large amount of structural information is now available on the subject of disilanes [l--8]. Simple trisilanes, however, have been much less extensively studied; in fact the title compound is the first trisilane of the type S&X,, all X equal, having been studied by gas-phase electron diffraction techniques. The main purpose of this investigation was to determine the change in the Si-Si bond length on going from a disilane to a trisilane. We also compare the structural parameters of Si,Cls with those found for its carbon analogue [9]. We were aware of the possibility that the molecule might possess largeamplitude motion about an equilibrium conformation of CsV symmetry (i.e. the SiCla groups are both staggered relative to the central SiC12 group), but we chose to refine the twist angle between the central SiCl, group and the terminal SiCl, groups to get an optimum value. A commercial sample of Si&ls (Petrarch Systems Inc.) was used. Data were recorded with the Oslo apparatus [lo] and those in the range s = 2.0032.25 a-’ were used throughout the structural refinement. The electron wavelength was 0.064 69 A, the nozzle-plate distances were 477.67 and 197.63 mm and the temperature of the nozzle was ca. 90°C. The intensity data were corrected according to the standard procedure used by The Norwegian Electron Diffraction Group [ll] to get a final radial distribution curve obtained by Fourier transformation of the intensity cunre. Scattering amplitudes and phase shifts were calculated analytically [12] and the potentials for both Si and Cl were of the Hartree-Fock-Slater type [ 13 ] _ Final structural parameters are given in Table 1 (for numbering of the atoms see Fig. 1) together with standard deviations which have been corrected for systematic errors. As can be seen from the Table, a torsion angle not significantly different from zero gave the best least-squares fit. Initial values for the amplitudes were obtained from a force-field calculation based upon spectroscopic work by Hoefler 1141 assuming a staggered model of Cp, symmetry. 0022-2860/81/0000-0000/$02.75
0 1981 Elsevier Scientific Publishing Company
316 TABLE
1
Structural
parameters
of Si,CI,
Distance
5Pe Si-Si Si-CI, SiCI, Si-Cl, Sic1 2 Si2.-.CllO Sil.n.C13 Si-.-Si Cl1o---clll C13--*Cl4 Cl3.-~cllo c14---Cl10 Cl4---Cl8 c14-.-Cl11 Cl4---Cl9 C13...C18 c13---Cl7
Amplitude (A)
Type
(A)
(ra)
2.329(7) 2.026(7) 2.034(22) 3.531(20) 3.548(14) 3.992(38) 3.186(22) 3.306(3) 4.039(18) 4.149(14) 4.15(5) 5.259(9) 5.27(4) 6.32(3) 7.01(3)
0.068(8) O-046(5) 0.051(9) O-153(23) O-145(23) 0_178(fixed) 0.064(U) 0.083(11) O-382(25) 0.371(fixed) 0_545(fixed) 0.120(19) 0.399(19) O-281(24) O-178(25)
SiSiSi SiSiCI, SiCI, SiSiCl, Sic1 Z Twist
Angle (“I 118.7(1.6) 109.3(0.6) 107.8(1.7) 1.5(9.8)”
=This angle is defmed as zero when Si2-Cl3 is anti to Sil-Si6 (and Si6%17 is anti to Sil-Si2). Both SiCI, groups were assumed to have the same twist angle, and the direction of twisting was such that the two-fold axis through Sil, bisecting both angles SiSiSi and CllOSilClll was not removed.
The experimental radial distribution curve together with the difference curve are shown in Fig. 2. Some of the irregularities are probably due to large-amplitude motion of the SiCl, groups about a staggered equilibrium conformation with twist angle equal to zero. Since thepeak cokesponding to the longest Cl - - -Cl distance is resolved, the rotational motion must be hindered.
n
C'lO
Fig. 1. Numbering
of atoms in Si,CI,.
317
0~
I
2
4
3
5
6
7 ---
D=
Exp.-Th.
Fig. 2. Experimental radial distribution curve for Si,Cl, together with the difference curve (experimental minus theoretical RD curve). The artificial damping constant was 0.0015 Ama,and the intensity curves were modified by s/ I fcl I - I fcl I before being Fourier transformed_
The Si-Si bond length is found to be 2.329(7) 8, compared to 2.324(3) for Si2C16 [8], 2.331(l) for Si2H6 [I], 2.340(9) for Si2(CH& [2], 2.324(6) for S&F6 [3] and 2.338(13) A for (CH&ClSi-SiCl(CH& [4]. The value for Si&le could therefore be said to lx typical of those for disilanes and. in particular, no significant change takes place in the Si-Si bond length on going from Si*Cl, to Si,Cl,. A similar trend is found for the C-C length on going from C2H, [15] to CJHB [16]. The Six1 bond lengths [2.026(7) and 2.034(22) a] are quite normal. The wide SiSiSi angle is hardly due to Cl...Cl repulsion; it could, however, suggest a rehybridization, giving the Si-Si bond a greater s character due to d orbitals on the silicon atoms being involved in bonding. In the carbon analogue, C3Cle [9], however, strong Cl.- -Cl repulsion is thought to be responsible for the wide CCC angle [119.0(4.0)“] and the long C-C bond distance [1.657(30) A]. REFERENCES 1 B. Beagley, A. R. Conrad, J. M. Freeman, J. J. Monaghan, B. G. Norton and G. C. Holywell, J. Mol. Struct., 11 (1972) 371. 2 B. Beagley, J. J. Monaghan and T. G. Hewitt, J. Mol. Struct., 8 (1971) 401. 3 D. W. H. Rankin and A. Robertson, J. Mol. Struct., 27 (1975) 438. 4 K. Kveseth, Acta Chem. Stand., Ser. A, 33 (1979) 453. 5 Y. Morino and E. Hirota, J. Chem. Phys., 28(2) (1958) 185. 6 K. Yamasaki, A. Kotera, K. Tatematsu and M. Iwasaki, J. Chem. Sot. Jpn., 69 (1948) 7 D. A. Swick and I. L. Karle, J. Chem. Phys., 23 (1955) 1499. 8 J. Haase, Z. Naturforsch., Teil A, 28 (1973) 542. 9 L. Femholt and R. St$levik, Acta Chem. !&and., Ser. A, 28 (1974) 963. 10 0. Bastiansen, 0. Hassel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. 11 B. Andersen, H. M. Seip, T. G. Strand and R. StQlevik, Acta Chem. Stand., 23 (1969) 3224.
104
318 12 13 14 15 16
A. T. F. L. T.
C. Yates, Comput. Phys. Commun., 2 (1971) 175. G. Strand and R. A. Bonham, J. Chem. Phys., 40 (1964) 1686. Hoefler, Monatsh. Chem., 104a (1973) 694. S. Eartell and H. K. Higgingbotham, J. Chem. Phys., 42 (1965) 851. Iijima, Bull. Chem. Sot. Jpn., 45 (1972) 1291.
Short communication
THE MOLECULAR STRUCTURE OF GASEOUS OCTACHLOROTRISILANE, Si,Cl& AS DETERMINED BY ELECTRON DIFFRACTION
ARNE ALMENNINGEN Department TORGNY Department (Received
of Chemistry,
University
of Oslo, Blindern,
University
of Trondheim,
Oslo 3 (Norway)
FJELDBERG of Chemistry,
NLHT,
N-7055
DragvoIl
(Norway)
2 July 1981)
A large amount of structural information is now available on the subject of disilanes [l--8]. Simple trisilanes, however, have been much less extensively studied; in fact the title compound is the first trisilane of the type S&X,, all X equal, having been studied by gas-phase electron diffraction techniques. The main purpose of this investigation was to determine the change in the Si-Si bond length on going from a disilane to a trisilane. We also compare the structural parameters of Si,Cls with those found for its carbon analogue [9]. We were aware of the possibility that the molecule might possess largeamplitude motion about an equilibrium conformation of CsV symmetry (i.e. the SiCla groups are both staggered relative to the central SiC12 group), but we chose to refine the twist angle between the central SiCl, group and the terminal SiCl, groups to get an optimum value. A commercial sample of Si&ls (Petrarch Systems Inc.) was used. Data were recorded with the Oslo apparatus [lo] and those in the range s = 2.0032.25 a-’ were used throughout the structural refinement. The electron wavelength was 0.064 69 A, the nozzle-plate distances were 477.67 and 197.63 mm and the temperature of the nozzle was ca. 90°C. The intensity data were corrected according to the standard procedure used by The Norwegian Electron Diffraction Group [ll] to get a final radial distribution curve obtained by Fourier transformation of the intensity cunre. Scattering amplitudes and phase shifts were calculated analytically [12] and the potentials for both Si and Cl were of the Hartree-Fock-Slater type [ 13 ] _ Final structural parameters are given in Table 1 (for numbering of the atoms see Fig. 1) together with standard deviations which have been corrected for systematic errors. As can be seen from the Table, a torsion angle not significantly different from zero gave the best least-squares fit. Initial values for the amplitudes were obtained from a force-field calculation based upon spectroscopic work by Hoefler 1141 assuming a staggered model of Cp, symmetry. 0022-2860/81/0000-0000/$02.75
0 1981 Elsevier Scientific Publishing Company
316 TABLE
1
Structural
parameters
of Si,CI,
Distance
5Pe Si-Si Si-CI, SiCI, Si-Cl, Sic1 2 Si2.-.CllO Sil.n.C13 Si-.-Si Cl1o---clll C13--*Cl4 Cl3.-~cllo c14---Cl10 Cl4---Cl8 c14-.-Cl11 Cl4---Cl9 C13...C18 c13---Cl7
Amplitude (A)
Type
(A)
(ra)
2.329(7) 2.026(7) 2.034(22) 3.531(20) 3.548(14) 3.992(38) 3.186(22) 3.306(3) 4.039(18) 4.149(14) 4.15(5) 5.259(9) 5.27(4) 6.32(3) 7.01(3)
0.068(8) O-046(5) 0.051(9) O-153(23) O-145(23) 0_178(fixed) 0.064(U) 0.083(11) O-382(25) 0.371(fixed) 0_545(fixed) 0.120(19) 0.399(19) O-281(24) O-178(25)
SiSiSi SiSiCI, SiCI, SiSiCl, Sic1 Z Twist
Angle (“I 118.7(1.6) 109.3(0.6) 107.8(1.7) 1.5(9.8)”
=This angle is defmed as zero when Si2-Cl3 is anti to Sil-Si6 (and Si6%17 is anti to Sil-Si2). Both SiCI, groups were assumed to have the same twist angle, and the direction of twisting was such that the two-fold axis through Sil, bisecting both angles SiSiSi and CllOSilClll was not removed.
The experimental radial distribution curve together with the difference curve are shown in Fig. 2. Some of the irregularities are probably due to large-amplitude motion of the SiCl, groups about a staggered equilibrium conformation with twist angle equal to zero. Since thepeak cokesponding to the longest Cl - - -Cl distance is resolved, the rotational motion must be hindered.
n
C'lO
Fig. 1. Numbering
of atoms in Si,CI,.
317
0~
I
2
4
3
5
6
7 ---
D=
Exp.-Th.
Fig. 2. Experimental radial distribution curve for Si,Cl, together with the difference curve (experimental minus theoretical RD curve). The artificial damping constant was 0.0015 Ama,and the intensity curves were modified by s/ I fcl I - I fcl I before being Fourier transformed_
The Si-Si bond length is found to be 2.329(7) 8, compared to 2.324(3) for Si2C16 [8], 2.331(l) for Si2H6 [I], 2.340(9) for Si2(CH& [2], 2.324(6) for S&F6 [3] and 2.338(13) A for (CH&ClSi-SiCl(CH& [4]. The value for Si&le could therefore be said to lx typical of those for disilanes and. in particular, no significant change takes place in the Si-Si bond length on going from Si*Cl, to Si,Cl,. A similar trend is found for the C-C length on going from C2H, [15] to CJHB [16]. The Six1 bond lengths [2.026(7) and 2.034(22) a] are quite normal. The wide SiSiSi angle is hardly due to Cl...Cl repulsion; it could, however, suggest a rehybridization, giving the Si-Si bond a greater s character due to d orbitals on the silicon atoms being involved in bonding. In the carbon analogue, C3Cle [9], however, strong Cl.- -Cl repulsion is thought to be responsible for the wide CCC angle [119.0(4.0)“] and the long C-C bond distance [1.657(30) A]. REFERENCES 1 B. Beagley, A. R. Conrad, J. M. Freeman, J. J. Monaghan, B. G. Norton and G. C. Holywell, J. Mol. Struct., 11 (1972) 371. 2 B. Beagley, J. J. Monaghan and T. G. Hewitt, J. Mol. Struct., 8 (1971) 401. 3 D. W. H. Rankin and A. Robertson, J. Mol. Struct., 27 (1975) 438. 4 K. Kveseth, Acta Chem. Stand., Ser. A, 33 (1979) 453. 5 Y. Morino and E. Hirota, J. Chem. Phys., 28(2) (1958) 185. 6 K. Yamasaki, A. Kotera, K. Tatematsu and M. Iwasaki, J. Chem. Sot. Jpn., 69 (1948) 7 D. A. Swick and I. L. Karle, J. Chem. Phys., 23 (1955) 1499. 8 J. Haase, Z. Naturforsch., Teil A, 28 (1973) 542. 9 L. Femholt and R. St$levik, Acta Chem. !&and., Ser. A, 28 (1974) 963. 10 0. Bastiansen, 0. Hassel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. 11 B. Andersen, H. M. Seip, T. G. Strand and R. StQlevik, Acta Chem. Stand., 23 (1969) 3224.
104
318 12 13 14 15 16
A. T. F. L. T.
C. Yates, Comput. Phys. Commun., 2 (1971) 175. G. Strand and R. A. Bonham, J. Chem. Phys., 40 (1964) 1686. Hoefler, Monatsh. Chem., 104a (1973) 694. S. Eartell and H. K. Higgingbotham, J. Chem. Phys., 42 (1965) 851. Iijima, Bull. Chem. Sot. Jpn., 45 (1972) 1291.