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Molecular mechanics calculations of conformational structures, energies, rotational barrier heights and torsional force constants in halogenated disilanes, hexachloroethane and trichloromethyl-trichlorosilane
Journal of Molecular Structure (!l%eochem), 124 (1985) 133-142 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MOLECULAR MECHANICS CALCULATIONS OF CONFORMATIONAL STRUCTURES, ENERGIES, ROTATIONAL BARRIER HEIGHTS AND TORSIONAL FORCE CONSTANTS IN HALOGENATED DISIIANES, HEXACHLOROETHANE AND TRICHLOROMETHYL-TRICHLOROSILANE
REIDAR STCLEVIK Department
and PIRKKO BAKKEN
of Chemistry,
University of !lYondheim (Norway)
(Received 2 January 1985)
ABSTRACT Results from molecular-mechanics calculations of conformational structures, energies, barrier heights and torsional force constants in halogenated disilanes, Cl,C-CCl, and Cl,Si-CCl, are given, together with force constants and reference values for bond lengths and bond angles in such molecules. INTRODUCTION
Parameter values of several non-bonded atom-atom interactions involving halogen atoms have been established [l] . These parameter values reasonably well reproduced the experimental gas-phase data on a wide variety of haloalkanes, halopropenes, halo-l,&butadienes and halo-biphenyl compounds. References to these compounds and their relevant force constants are found in ref. 1. In order to carry out MM calculations on systems like X$i-SiXJ or XH#i-SiH2X only non-bonded atom-atom interaction potentials of the types X-*-X, X* 0-H and Ho 0H already established [l] are needed. Interactions involving the Si atoms do not enter the calculations if Si is on the central axis. However, force constants and reference values for bond lengths and bond angles involving Si have to be included in the calculations. It is reasonable to use spectroscopically determined values of the force constants when such values are available. Most of the values in Tables 1 and 2 are averages for several molecules, but some are only rough estimates when experimental data were not available. Also the reference values are averages from several molecules, but in some cases rough estimates had to be used. For bond angles all reference values were taken to be 109.5“. For disialanes and methyl-silanes the intrinsic torsional potential have the assumed form l
V(@) = (V,“/2)(1 - cos( 3@)) with V,"= 1.2kcal mol-’ as suggested in ref. 2.
0166-1280/85/$03.30
0 1985 Elsevier Science Publishers B.V.
(1)
134 TABLE 1 Force constantsa for bond angles in mdyn A rad-‘. All reference values were 109.5”
x-
H
F
Cl
Br
X-Si-X X-%-C X-C-& X-Si-Si X-Si-H
0.3 0.4 0.4 0.4 0.3
0.4 0.7 0.9 0.7 0.4
0.5 0.6 0.8 0.6 0.4
0.5 0.5 0.7 0.5 0.4
*In order to convert from mdyu Ir(molecule-’ to kcal mol-’ multiply by 143.8. TABLE 2 Reference values and force constants for bond distances Distance type
Reference value (A)
Force constant? (mdyn A-‘)
Si-Si C-Si H-G
2.330 1.865 1.485
2.0 2.8 2.9
x=
F
X-Si in various groups XH,Si1.595 X,HSi1.580 X,Si1.565
Cl
Br
F
Cl
Br
2.075 2.050 2.025
2.210 2.190 2.175
4.4 5.2 5.8
2.3 2.8 3.2
1.8 2.1 2.5
aIn order to convert from mdyn 16 molecule-’
to kcal mol-’ multiply by 143.8.
For C1&+CC13 V,O= 2.65 kcal mol-’ was used [l],and force constants as well as reference values for this molecule are found in ref. 3. Excess charges on the atoms were calculated as explained in ref. 3. For fluorocompounds the charges were reduced by division by 2.0. CALCULATIONS
AND RESULTS
Energy minima corresponding to stable conformers were found by simultaneously adjusting bond lengths, bond angles and torsion angles. When calculating barrier heights the torsion angle was not varied. The results for hexahalocompormds are shown in Table 3, and the agreement with experimental data is first discussed. Hexachloroe thane, C13C-CC13 The molecular structure and internal rotation of hexachloroethane have been studied by several workers [4-71. The potential barrier height has been found to be 7-15 kcal mol-‘.
135 TABLE 3 Hexahalocompounds Molecule Barrier height (kcal moP) Torsional force constant (FQ) (mdyn A rad”) Torsionala frequency (v) ( cm-’ ) Bond angles (“) X-&-X
-results F,Si-SiF,
from molecular mechanics calculations Cl,Si-SiCl,
Br,Si+GBr,
Cl,Si-CCl,
1.2
3.7
4.1
7.9
0.4
0.13
0.18
0.31
Cl,-CCl,
17.1
0.59
33
34
26
56
111.3
111.2
110.2
108.2
-
-
-
-
109.0
106.4
X-C-X
aCalculatedfrom the formula v = (1/2r1)(F&Z)“~ around the central axis.
84
where Z-isthe reduced moment of inertia
However, the experimental results are not consistent. A bond length of 1.49 f 0.03 A as reported [5] for C-C seems unreasonable. The three other investigations agree that this bond length is about 1.56-1.57 A. The barrier height was computed from the observed mean square amplitude of the gauche chlorine pair [5], but here the disagreement is even more dramatic. Three values werereported: 0.136 [5], 0.101 [6] and 0.171 [7] in A, with 0.136+ 0.015 A corresponding to 10.8 f 3 kcai mol-‘. The error limit k3 kcal mol-’ seems rather optimistic. Using the formula u2 = A + B/V,” derived in ref. 5 with A = 0.01153 A2 and B = 0.07517 W2 kcal mol-’ the value V,” = 17.3 kcai mol-’ corresponds to u = 0.126 A where u is the root mean square amplitude of the gauche Cl- - Cl pair. Thus u = 0.126 A is well within the experimental error limits 0.136 f 0.015 A. A new experimental estimate may be derived from the formula of the torsional force constant of an ethan-iike fragment as explained in ref. 8. This estimate is 0.60 mdyn A radm2for the torsional force constant (FQ) corresponding to a barrier height of 19.2 kcai mol-’ if the potential is of the form in eqn. (1) above. Our MM results are 17.1 kcal mol-’ for the barrier height and 0.59 mdyn A rad” for F* as shown in Table 3. The agreement with the experimental estimates based on ref. 8 is striking. The difference (19.2-17.1) for the barrier height is probably due to deviations from the cos(3Q) form when the eclipsed transition form is approached. It is also clear that such a high value for the barrier height is consistent with the experimental data as explained above. The barrier heights derived from calculations are always upper limits due to the geometrical constraints of the transition state. If our intrinsic torsional potential with Vi = 2.65 kcaI mol-’ is subtracted, the value of the barrier height due to the non-bonded Cl. -Cl interactions is 14.5 kcai mol-‘. l
l
136
Our value 106.4” for the angle X-C-X agrees with the experimental value of ref. 5, however, the values of refs. 4,6, ‘7 are found in the range 109.0”109.5”. The calculated value r(C-C) = 1.571 R agrees with the experimental values [4,6, 71 found between 1.56 and 1.57 A. Neglecting the excess charges on the atoms leads to insignificant changes in the parameter values for this molecule and for all other hexahalodisilanes. Trichlorome thy&trichlorosilane, C1,Si-CC13 The experimental result [5] for this molecule is u(gauche) = 0.190 f 0.020 A corresponding to a barrier height between 2.8 and 7.8 kcal mol-’ with 4.3 as the most probable value. The calculated value is 7.9, and 6.7 if the intrinsic barrier [2] of 1.2 kcal mol-’ is subtracted. Thus the barrier due to non-bonded Cl*** Cl interactions has been dramatically reduced compared with hexachloroethane. The torsional force constant is reduced to about half the value in C1,C-CCIB. The observed values of the structural parameters are [5] r(Si-C) = 1.93 + 0.04 A and LXCX 2 LX&Y = 110” * 1”. However, the error limits seem rather optimistic. Our values indicate smaller bond angles and a shorter Si-C bond length. Hexachlorodisilune,
Cl&‘i-SiC13
A decrease in potential barrier from C1&-CC13 to C1,Si-CC13 and from the latter to C!l$i-SiC13 is expected. The experimental values [5, 41 are about 1 kcal mol-‘. Error limits were not reported, however, their values have to be large [ 51. The calculated value is 3.7 kcal mol-’ and 2.5 if the intrinsic barrier height [2] of 1.2 kcal mol-’ is subtracted. If the experimental error limits are comparable to those of C1&-CC13 then the calculated value is not significantly different from the experimental one. Both the experimental value of the Si-Si bond length equal to 2.29 f 0.05 a and the calculated value of 2.30 A indicate a shortening relative to the typical values of 2.33 A. The calculations indicate an opening of the X-Si-X angles to 111.2”, a little larger than the experimental value [5] of 110” f. 1”. Hexafluorodisilane,
F3Si-SiF3
The molecular structure and internal rotation were investigated by gasphase electron diffraction [9]. The barrier height was found to be in the range 0.4-0.9 kcal mol-‘. The calculated result is 1.2 kcal mol-l, and if the intrinsic potential with V,” = 1.2 kcal mol-’ is subtracted, free rotation is obtained. The agreement with the experimental value is quite satisfactory.
137
Hexabromodisilane,
Br$%-SiBr3
Experimental data were not found for this compound. However, our calculations show that the barrier height, 4.7 kca.l mol-‘, or 3.5 if the intrinsic barrier of 1.2 kcal molsl is subtracted, is between that of C!l$i-SiCls and Cl$i-CC&. From the results of Table 3 it is seen that the ratio between the torsional force constant and the barrier height of each molecule is only approximately constant, indicating small deviations from the cos(3cP) functional form. 1,2-Dihalodisilanes, XH,Si-SiH& Experimental data for the three compounds with X equal to F, Cl or Br were not available. However, these compounds would be very interesting in the study of non-bonded X. m-X and X***H interactions. Results from MM calculations are found in Table 4. Since the results are quite dependent on the value of the excess atomic charges, values obtained without these charges are also given in Table 4. Typical charges in a -SiH&l group are +0.143 e(Si), -0.173 e(C1) and +0.015 e(H) as fractions of one electronic charge. According to our calculations gauche is the most stable conformer in all three cases, however, anti is only 0.4-1.4 kcal mol” higher in energy. The highest barrier is less than 2 kcal mol-‘, while the lowest barrier is greater than 0.8 kcal mol-‘. For ClH&-CH&l and BrH2C-CH2Br the situation is very different due to the much shorter bond lengths and greater intrinsic barrier height [3]. However, gauche is still the most stable conformer for FHzC-CHzF [lo]. TABLE 4 1,2-Dihalodisilanes, XH,Si-SiH,X, results from molecular mechanics calculations (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol-’ at different torsion angles @ (“) @=o l.O(O.8) 1.5( 1.2) @ (gauche) O(0) O(0) @ = 120 1.q 1.9) 1.7(2.3) @ = 180 (anti) 0.4(0.9) 0.7( 1.4) Torsion angle (“) for gauchea conformer @ (X-Si-Si-X) 56(52) 58(54) Torsionalb force constants (FQ)~ in mdyn A rad-* gauche 0.047(0.033) 0.050(0.060) anti 0.032(0.032) 0.034(0.033)
Br
1.9(1.8) O(0) 1.7(2.0) 0.7( 1.1) 62( 60) O.OSO(O.056) 0.032(0.043)
YZee Fig. 1 for drawing of conformations. bFQ = (&T/W). ‘In order to convert from mdyn PI molecule-’ to kcal mol-’ multiply by 143.8.
138
The torsional angle of the gauche conformer is found between 52” and 62” for these molecules. The bond lengths and the bond angles have values close to those of Table 1 and Table 2. The influence of atomic charges on bond lengths and bond angles is small. 1,1,2-Trihabdisilanes,
X2HSi-SiH2X
Results of the MM calculations are found in Table 5. According to our calculations the conformer anti is higher in energy than gauche, but not more than ca. 1 kcal molV1. Figure 1 shows a drawing of the conformations. The gauche conformer has two X. *X gauche distances while the anti conformer has only one gauche and one anti distance of the type X. lX. The highest barrier is less than ca. 3 kcal mol-’ while the lowest barrier is greater than ca. 1 kcal mol-‘. For the analogous ethan-compounds the situation is very different due to shorter bond lengths and a greater intrinsic barrier height [3]. Results with excess atomic charges and without are given. Clearly the value of the charges are important in some cases. However, the main conclusions remain unaltered. The torsion angle Q (H-Si-Si-X) of the anti conformer is found between 56” and 69” for these molecules. The bond lengths and bond angles have values close to those in Table 1 and Table 2. l
l
1,1,2,2-Tetrahabdisilanes,
&HSb!M&
The gauche conformer has one more gauche X. lX interaction than the anti form, and again anti is about 1 kcal mol-’ higher in energy for X = Cl l
TABLE 5 1,1,2Trihslodiiilanes, X,HSi-SiHH,X, results from molecular mechanics (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol-’ at different torsion angles CP(” ) 2.1(3.3) 1.3(2.6) 0.6( 1.3) biti) O.l(O.9) = 120 1.2( 1.4) 2.0(2.3) = 180 (gauche)a O(0) O(0) Torsion angle (“) for anti* conformer @ (H-Si-Si-X) 61( 69) 61(66) Torsionalb force constants (F,J,)~ in mdyn .&rad-’ 0.048(0.053) anti 0.031(0.038) 0.033(0.050) 0.070(0.080) gauche
calculations
Br
1.9(2.7) 0.6(1.1) 2.5(2.7) O(0) 56( 59) 0.053(0.066) 0.089(0.083)
%ee Fig. 1 for drawing of conformations. bFa = (a2E/aoZ). cIn order to convert from mdyn A molecule-’ to kcal mol-’ multiply by 143.8.
139
(a)
X H H
@
X
H
H
H
H
H @
X
antiA
X
H
gauche G 1C, 1
lC2,,l
(b)
H
H H
X
X
H
X
X
X
@
@ H ontiA
Ii
X
goucho G ( C, 1
(C,)
(cl
n X
X
$r $r
X
X
X
X
X
H
X
H
o/M A (C2h1
H
qottche G CC, 1
Fig. 1. Newman projections of the conformers, names and symmetry (a) For XH,Si-SiH,X, (b) for X,HS-SiHH,X, (c) X,HSi-SiHX,.
in parentheses.
and Br. However, for X = F the two forms have equal energy if the charges are included. If the charges are neglected anti is 1 kcal mol-’ higher in energy than gauche. The highest barrier is less than ca. 3 kcal mol-’ and the lowest barrier is greater than ca. 1 kcal mol-‘. For the analogous ethan-compounds the calculated barrier heights are as high as 7-15 kcal mol-’ for the chloroand bromocompounds [ 31. The torsion angle @(H-Si-Si-H) for the gauche conformer is found between 52” and 62” for these molecules. The values of the bond lengths and the bond angles are close to those in Tables 1 and 2. Force constant formula for chloro-disilanes Based on the same kind of arguments as used in ref. 8 the following formula for the chlorodisilanes was derived with the torsional force constant F@ in mdyn A rad*
140
F@ = 0.022Nxx + 0.0055Hxn + 0.0063Nnn
(2)
Here, Nxx is the number of Cl* *Cl gauche interactions, Nxn is the number of Cl. ’ H gauche interactions and Nnn is the number of H* lH gauche interactions in a Si-Si fragment, (Nxx +Nxn +Nnn = 6). This formula reproduces the force constant values of the chlorodisilanes with an average deviation of lo%, the largest deviation being 20%. Using this formula, the torsional force constants of the following molecules were calculated with F, in mdyn W rad-* l
l
l
ClH$i-SiH3
0.036
Cl*HSi-SiH3
0.035
Cl$Si-SiH3
0.033
Cl$i-SiH?X
0.066
Cl$Si-SiHC&
0.099
Assuming that the torsional potential of these molecules is described by eqn. (1) above, the relationship between V,”and FQ is given by V,” = (2/9)F,. With the units above we then get the expression for the barrier height V,” (kcal mol-‘) = 31.95 F@ (mdyn A radw2)
(3)
and the following values for V,” in kcal mol-’ are obtained C1H2Si-SiH3
1.2
C12HSi-SiHs
1.2
Cl$i-SiH3
1.1
Cl$i-SiH2C1
2.1
Cl$i-SiHC12
3.2
The uncertainties of these values are probably about +20%. Force constant formula for bromo-disilanes For bromo-disilanes the following formula was derived with F, A radm2 Fe = 0.030 Nxx + 0.0053 NXH + 0.0063 NHH
in mdyn (4)
Using this expression and eqn. (3) above the following results were obtained Molecule
FQ (mdyn A rade2)
V,” (kcal mol-’ )
BrH2Si-SiH3 Br2HSi-SiHJ Br3Si-SiH3 Br,Si-SiH2Br Br,Si-SiHBrz
0.036 0.034 0.032 0.081 0.131
1.2 1.1 1.0 2.6 4.2
141
The uncertainties of these values are around 20% both for FG and Vf. Fluoro-compounds: F3Si-SiHF2
FH,Si-SiH,,
FzHSi-SiH3,
F3Si-SiH3,
F,Si-SiH2F
and
For these compounds the torsional potential is probably of the form given in eqn. (1) above. The barrier heights ought to have values between 1.0 and 1.4 kcal mol-’ and the torsional force constants between 0.030 and 0.045 mdyn A rad”. CONCLUSIONS
Previous applications of our non-bonded potential parameters [l] to haloalkanes [3, lo] have shown that the MM calculations reproduce a wide variety of conformational properties in all cases where experimental data existed. Moreover, the same potential parameters together with new force constants and reference values where Si atoms are involved, reproduced the experimental data for F$i-SiF3, Cl,Si-SiC13, C1$+CC13 and C13C-CC13 well. One should remember that the barrier heights within these molecules range from ca. 1 kcal mol’ up to ca. 1’7 kcal mol-‘. As shown in ref. 3 a barrier height of ca. 12 kcal mol-’ in C12HC-CC12 - CHB was correctly reproduced. It is also important to keep in mind that the calculated barrier heights are upper limits due to the constraints of the structural model used. Although it is not very likely, the possibility exists that the intrinsic barrier V,” is significantly lower than 1.2 kcal mol-‘. If that is the case the TABLE 6 1,1,2,2-Tetrahalodisilanes, X,HSi-SiHX,, results from molecular mechanics calculations (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol“ at different torsion angles cp (“) QJ=o 2.2(2.1) 1.2(0.7) Q,(gauche) 0.04(O) O(0) @ = 120 2.5( 3.1) 1.20(2.0) @ = 180 (anti) 0.7( 1.4) O(1.0) Torsion angle (“) for gauche* conformer @ (H-Si-Si-H) 60(52) 59(57) Torsionalb force constant (Fm)C in mdyn A rad-z gauche 0.076(0.073) 0.040( 0.050) anti 0.038(0.027) 0.048(0.060)
Br
3.2(3.0) O(0) 2.8( 3.2) 0.6(1.1) 62(61) O.lOO(O.106) 0.093(0.066)
*See Fig. 1 for drawing of conformation. bFq = (PE/a@). ‘In order to convert from mdyn A molecule-’ to kcal mol-’ multiply by 143.8.
142
barrier heights of the compounds XH2Si-SiH2X, X2HSi-SiH2X and X,HSiSiHx, are also reduced. The contributions to the barrier heights from nonbonded interactions are quite small as seen from the Tables 4-6. In particular, the fluoro-compounds would then have virtually free internal rotation around the Si-Si bond. The effects of including electrostatic terms in the non-bonded potentials are clearly demonstrated in Tables 4-6. However, the main conclusions remain unaltered whether the charges are included or not. No attempt has been made here to correct for the difference in vibrational energies between conformers of the same molecule. Such energy differences may well amount to ca. 0.5 kcal mol-’ in molecules like the halodisilanes. At present the compounds in the Tables 4-6 are not available. However, experimental studies by gas-phase electron diffraction will be started as soon as some of these halodisilanes become available to us. ACKNOWLEDGEMENT
Financial support from Norges almenvitenskapelige forskningsrgd (NAVF) is acknowledged. REFERENCES 1 R. Stelevik, J. MoL Struct., Theochem., 109 (1984) 397. 2 J. P. Hunnel, J. Stackhouse and K. Mislow, Tetrahedron, 33 (1977) 1925. 3 T. Rydland and R. Stdlevik, J. Mol. Struct., Theochem, 105 (1983) 157. 4 D. A. Swick, I. L. Karle and J. Karle, J. Chem. Phys., 22 (1954) 1242. 5 Y. Morino and E. Hirota, J. Chem. Phys., 28 (1958) 185. 6 A. Aimenningen, B. Andersen and M: Traetteberg, Acta Chem. Stand., 18 (1964) 7’B. G. Prater, Ph.D. Thesis, University of Texas, Austin, Texas, 1969. 8 R. Stblevik, Acta Chem. Scan., A31 (1977) 359. 9 H. Oberhsmmer, J. MoL Struct., 31(1976) 237. 10 R. J. Abraham and R. Stblevik, Chem. Phys. Lett., 77 (1981) 181.
603.
MOLECULAR MECHANICS CALCULATIONS OF CONFORMATIONAL STRUCTURES, ENERGIES, ROTATIONAL BARRIER HEIGHTS AND TORSIONAL FORCE CONSTANTS IN HALOGENATED DISIIANES, HEXACHLOROETHANE AND TRICHLOROMETHYL-TRICHLOROSILANE
REIDAR STCLEVIK Department
and PIRKKO BAKKEN
of Chemistry,
University of !lYondheim (Norway)
(Received 2 January 1985)
ABSTRACT Results from molecular-mechanics calculations of conformational structures, energies, barrier heights and torsional force constants in halogenated disilanes, Cl,C-CCl, and Cl,Si-CCl, are given, together with force constants and reference values for bond lengths and bond angles in such molecules. INTRODUCTION
Parameter values of several non-bonded atom-atom interactions involving halogen atoms have been established [l] . These parameter values reasonably well reproduced the experimental gas-phase data on a wide variety of haloalkanes, halopropenes, halo-l,&butadienes and halo-biphenyl compounds. References to these compounds and their relevant force constants are found in ref. 1. In order to carry out MM calculations on systems like X$i-SiXJ or XH#i-SiH2X only non-bonded atom-atom interaction potentials of the types X-*-X, X* 0-H and Ho 0H already established [l] are needed. Interactions involving the Si atoms do not enter the calculations if Si is on the central axis. However, force constants and reference values for bond lengths and bond angles involving Si have to be included in the calculations. It is reasonable to use spectroscopically determined values of the force constants when such values are available. Most of the values in Tables 1 and 2 are averages for several molecules, but some are only rough estimates when experimental data were not available. Also the reference values are averages from several molecules, but in some cases rough estimates had to be used. For bond angles all reference values were taken to be 109.5“. For disialanes and methyl-silanes the intrinsic torsional potential have the assumed form l
V(@) = (V,“/2)(1 - cos( 3@)) with V,"= 1.2kcal mol-’ as suggested in ref. 2.
0166-1280/85/$03.30
0 1985 Elsevier Science Publishers B.V.
(1)
134 TABLE 1 Force constantsa for bond angles in mdyn A rad-‘. All reference values were 109.5”
x-
H
F
Cl
Br
X-Si-X X-%-C X-C-& X-Si-Si X-Si-H
0.3 0.4 0.4 0.4 0.3
0.4 0.7 0.9 0.7 0.4
0.5 0.6 0.8 0.6 0.4
0.5 0.5 0.7 0.5 0.4
*In order to convert from mdyu Ir(molecule-’ to kcal mol-’ multiply by 143.8. TABLE 2 Reference values and force constants for bond distances Distance type
Reference value (A)
Force constant? (mdyn A-‘)
Si-Si C-Si H-G
2.330 1.865 1.485
2.0 2.8 2.9
x=
F
X-Si in various groups XH,Si1.595 X,HSi1.580 X,Si1.565
Cl
Br
F
Cl
Br
2.075 2.050 2.025
2.210 2.190 2.175
4.4 5.2 5.8
2.3 2.8 3.2
1.8 2.1 2.5
aIn order to convert from mdyn 16 molecule-’
to kcal mol-’ multiply by 143.8.
For C1&+CC13 V,O= 2.65 kcal mol-’ was used [l],and force constants as well as reference values for this molecule are found in ref. 3. Excess charges on the atoms were calculated as explained in ref. 3. For fluorocompounds the charges were reduced by division by 2.0. CALCULATIONS
AND RESULTS
Energy minima corresponding to stable conformers were found by simultaneously adjusting bond lengths, bond angles and torsion angles. When calculating barrier heights the torsion angle was not varied. The results for hexahalocompormds are shown in Table 3, and the agreement with experimental data is first discussed. Hexachloroe thane, C13C-CC13 The molecular structure and internal rotation of hexachloroethane have been studied by several workers [4-71. The potential barrier height has been found to be 7-15 kcal mol-‘.
135 TABLE 3 Hexahalocompounds Molecule Barrier height (kcal moP) Torsional force constant (FQ) (mdyn A rad”) Torsionala frequency (v) ( cm-’ ) Bond angles (“) X-&-X
-results F,Si-SiF,
from molecular mechanics calculations Cl,Si-SiCl,
Br,Si+GBr,
Cl,Si-CCl,
1.2
3.7
4.1
7.9
0.4
0.13
0.18
0.31
Cl,-CCl,
17.1
0.59
33
34
26
56
111.3
111.2
110.2
108.2
-
-
-
-
109.0
106.4
X-C-X
aCalculatedfrom the formula v = (1/2r1)(F&Z)“~ around the central axis.
84
where Z-isthe reduced moment of inertia
However, the experimental results are not consistent. A bond length of 1.49 f 0.03 A as reported [5] for C-C seems unreasonable. The three other investigations agree that this bond length is about 1.56-1.57 A. The barrier height was computed from the observed mean square amplitude of the gauche chlorine pair [5], but here the disagreement is even more dramatic. Three values werereported: 0.136 [5], 0.101 [6] and 0.171 [7] in A, with 0.136+ 0.015 A corresponding to 10.8 f 3 kcai mol-‘. The error limit k3 kcal mol-’ seems rather optimistic. Using the formula u2 = A + B/V,” derived in ref. 5 with A = 0.01153 A2 and B = 0.07517 W2 kcal mol-’ the value V,” = 17.3 kcai mol-’ corresponds to u = 0.126 A where u is the root mean square amplitude of the gauche Cl- - Cl pair. Thus u = 0.126 A is well within the experimental error limits 0.136 f 0.015 A. A new experimental estimate may be derived from the formula of the torsional force constant of an ethan-iike fragment as explained in ref. 8. This estimate is 0.60 mdyn A radm2for the torsional force constant (FQ) corresponding to a barrier height of 19.2 kcai mol-’ if the potential is of the form in eqn. (1) above. Our MM results are 17.1 kcal mol-’ for the barrier height and 0.59 mdyn A rad” for F* as shown in Table 3. The agreement with the experimental estimates based on ref. 8 is striking. The difference (19.2-17.1) for the barrier height is probably due to deviations from the cos(3Q) form when the eclipsed transition form is approached. It is also clear that such a high value for the barrier height is consistent with the experimental data as explained above. The barrier heights derived from calculations are always upper limits due to the geometrical constraints of the transition state. If our intrinsic torsional potential with Vi = 2.65 kcaI mol-’ is subtracted, the value of the barrier height due to the non-bonded Cl. -Cl interactions is 14.5 kcai mol-‘. l
l
136
Our value 106.4” for the angle X-C-X agrees with the experimental value of ref. 5, however, the values of refs. 4,6, ‘7 are found in the range 109.0”109.5”. The calculated value r(C-C) = 1.571 R agrees with the experimental values [4,6, 71 found between 1.56 and 1.57 A. Neglecting the excess charges on the atoms leads to insignificant changes in the parameter values for this molecule and for all other hexahalodisilanes. Trichlorome thy&trichlorosilane, C1,Si-CC13 The experimental result [5] for this molecule is u(gauche) = 0.190 f 0.020 A corresponding to a barrier height between 2.8 and 7.8 kcal mol-’ with 4.3 as the most probable value. The calculated value is 7.9, and 6.7 if the intrinsic barrier [2] of 1.2 kcal mol-’ is subtracted. Thus the barrier due to non-bonded Cl*** Cl interactions has been dramatically reduced compared with hexachloroethane. The torsional force constant is reduced to about half the value in C1,C-CCIB. The observed values of the structural parameters are [5] r(Si-C) = 1.93 + 0.04 A and LXCX 2 LX&Y = 110” * 1”. However, the error limits seem rather optimistic. Our values indicate smaller bond angles and a shorter Si-C bond length. Hexachlorodisilune,
Cl&‘i-SiC13
A decrease in potential barrier from C1&-CC13 to C1,Si-CC13 and from the latter to C!l$i-SiC13 is expected. The experimental values [5, 41 are about 1 kcal mol-‘. Error limits were not reported, however, their values have to be large [ 51. The calculated value is 3.7 kcal mol-’ and 2.5 if the intrinsic barrier height [2] of 1.2 kcal mol-’ is subtracted. If the experimental error limits are comparable to those of C1&-CC13 then the calculated value is not significantly different from the experimental one. Both the experimental value of the Si-Si bond length equal to 2.29 f 0.05 a and the calculated value of 2.30 A indicate a shortening relative to the typical values of 2.33 A. The calculations indicate an opening of the X-Si-X angles to 111.2”, a little larger than the experimental value [5] of 110” f. 1”. Hexafluorodisilane,
F3Si-SiF3
The molecular structure and internal rotation were investigated by gasphase electron diffraction [9]. The barrier height was found to be in the range 0.4-0.9 kcal mol-‘. The calculated result is 1.2 kcal mol-l, and if the intrinsic potential with V,” = 1.2 kcal mol-’ is subtracted, free rotation is obtained. The agreement with the experimental value is quite satisfactory.
137
Hexabromodisilane,
Br$%-SiBr3
Experimental data were not found for this compound. However, our calculations show that the barrier height, 4.7 kca.l mol-‘, or 3.5 if the intrinsic barrier of 1.2 kcal molsl is subtracted, is between that of C!l$i-SiCls and Cl$i-CC&. From the results of Table 3 it is seen that the ratio between the torsional force constant and the barrier height of each molecule is only approximately constant, indicating small deviations from the cos(3cP) functional form. 1,2-Dihalodisilanes, XH,Si-SiH& Experimental data for the three compounds with X equal to F, Cl or Br were not available. However, these compounds would be very interesting in the study of non-bonded X. m-X and X***H interactions. Results from MM calculations are found in Table 4. Since the results are quite dependent on the value of the excess atomic charges, values obtained without these charges are also given in Table 4. Typical charges in a -SiH&l group are +0.143 e(Si), -0.173 e(C1) and +0.015 e(H) as fractions of one electronic charge. According to our calculations gauche is the most stable conformer in all three cases, however, anti is only 0.4-1.4 kcal mol” higher in energy. The highest barrier is less than 2 kcal mol-‘, while the lowest barrier is greater than 0.8 kcal mol-‘. For ClH&-CH&l and BrH2C-CH2Br the situation is very different due to the much shorter bond lengths and greater intrinsic barrier height [3]. However, gauche is still the most stable conformer for FHzC-CHzF [lo]. TABLE 4 1,2-Dihalodisilanes, XH,Si-SiH,X, results from molecular mechanics calculations (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol-’ at different torsion angles @ (“) @=o l.O(O.8) 1.5( 1.2) @ (gauche) O(0) O(0) @ = 120 1.q 1.9) 1.7(2.3) @ = 180 (anti) 0.4(0.9) 0.7( 1.4) Torsion angle (“) for gauchea conformer @ (X-Si-Si-X) 56(52) 58(54) Torsionalb force constants (FQ)~ in mdyn A rad-* gauche 0.047(0.033) 0.050(0.060) anti 0.032(0.032) 0.034(0.033)
Br
1.9(1.8) O(0) 1.7(2.0) 0.7( 1.1) 62( 60) O.OSO(O.056) 0.032(0.043)
YZee Fig. 1 for drawing of conformations. bFQ = (&T/W). ‘In order to convert from mdyn PI molecule-’ to kcal mol-’ multiply by 143.8.
138
The torsional angle of the gauche conformer is found between 52” and 62” for these molecules. The bond lengths and the bond angles have values close to those of Table 1 and Table 2. The influence of atomic charges on bond lengths and bond angles is small. 1,1,2-Trihabdisilanes,
X2HSi-SiH2X
Results of the MM calculations are found in Table 5. According to our calculations the conformer anti is higher in energy than gauche, but not more than ca. 1 kcal molV1. Figure 1 shows a drawing of the conformations. The gauche conformer has two X. *X gauche distances while the anti conformer has only one gauche and one anti distance of the type X. lX. The highest barrier is less than ca. 3 kcal mol-’ while the lowest barrier is greater than ca. 1 kcal mol-‘. For the analogous ethan-compounds the situation is very different due to shorter bond lengths and a greater intrinsic barrier height [3]. Results with excess atomic charges and without are given. Clearly the value of the charges are important in some cases. However, the main conclusions remain unaltered. The torsion angle Q (H-Si-Si-X) of the anti conformer is found between 56” and 69” for these molecules. The bond lengths and bond angles have values close to those in Table 1 and Table 2. l
l
1,1,2,2-Tetrahabdisilanes,
&HSb!M&
The gauche conformer has one more gauche X. lX interaction than the anti form, and again anti is about 1 kcal mol-’ higher in energy for X = Cl l
TABLE 5 1,1,2Trihslodiiilanes, X,HSi-SiHH,X, results from molecular mechanics (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol-’ at different torsion angles CP(” ) 2.1(3.3) 1.3(2.6) 0.6( 1.3) biti) O.l(O.9) = 120 1.2( 1.4) 2.0(2.3) = 180 (gauche)a O(0) O(0) Torsion angle (“) for anti* conformer @ (H-Si-Si-X) 61( 69) 61(66) Torsionalb force constants (F,J,)~ in mdyn .&rad-’ 0.048(0.053) anti 0.031(0.038) 0.033(0.050) 0.070(0.080) gauche
calculations
Br
1.9(2.7) 0.6(1.1) 2.5(2.7) O(0) 56( 59) 0.053(0.066) 0.089(0.083)
%ee Fig. 1 for drawing of conformations. bFa = (a2E/aoZ). cIn order to convert from mdyn A molecule-’ to kcal mol-’ multiply by 143.8.
139
(a)
X H H
@
X
H
H
H
H
H @
X
antiA
X
H
gauche G 1C, 1
lC2,,l
(b)
H
H H
X
X
H
X
X
X
@
@ H ontiA
Ii
X
goucho G ( C, 1
(C,)
(cl
n X
X
$r $r
X
X
X
X
X
H
X
H
o/M A (C2h1
H
qottche G CC, 1
Fig. 1. Newman projections of the conformers, names and symmetry (a) For XH,Si-SiH,X, (b) for X,HS-SiHH,X, (c) X,HSi-SiHX,.
in parentheses.
and Br. However, for X = F the two forms have equal energy if the charges are included. If the charges are neglected anti is 1 kcal mol-’ higher in energy than gauche. The highest barrier is less than ca. 3 kcal mol-’ and the lowest barrier is greater than ca. 1 kcal mol-‘. For the analogous ethan-compounds the calculated barrier heights are as high as 7-15 kcal mol-’ for the chloroand bromocompounds [ 31. The torsion angle @(H-Si-Si-H) for the gauche conformer is found between 52” and 62” for these molecules. The values of the bond lengths and the bond angles are close to those in Tables 1 and 2. Force constant formula for chloro-disilanes Based on the same kind of arguments as used in ref. 8 the following formula for the chlorodisilanes was derived with the torsional force constant F@ in mdyn A rad*
140
F@ = 0.022Nxx + 0.0055Hxn + 0.0063Nnn
(2)
Here, Nxx is the number of Cl* *Cl gauche interactions, Nxn is the number of Cl. ’ H gauche interactions and Nnn is the number of H* lH gauche interactions in a Si-Si fragment, (Nxx +Nxn +Nnn = 6). This formula reproduces the force constant values of the chlorodisilanes with an average deviation of lo%, the largest deviation being 20%. Using this formula, the torsional force constants of the following molecules were calculated with F, in mdyn W rad-* l
l
l
ClH$i-SiH3
0.036
Cl*HSi-SiH3
0.035
Cl$Si-SiH3
0.033
Cl$i-SiH?X
0.066
Cl$Si-SiHC&
0.099
Assuming that the torsional potential of these molecules is described by eqn. (1) above, the relationship between V,”and FQ is given by V,” = (2/9)F,. With the units above we then get the expression for the barrier height V,” (kcal mol-‘) = 31.95 F@ (mdyn A radw2)
(3)
and the following values for V,” in kcal mol-’ are obtained C1H2Si-SiH3
1.2
C12HSi-SiHs
1.2
Cl$i-SiH3
1.1
Cl$i-SiH2C1
2.1
Cl$i-SiHC12
3.2
The uncertainties of these values are probably about +20%. Force constant formula for bromo-disilanes For bromo-disilanes the following formula was derived with F, A radm2 Fe = 0.030 Nxx + 0.0053 NXH + 0.0063 NHH
in mdyn (4)
Using this expression and eqn. (3) above the following results were obtained Molecule
FQ (mdyn A rade2)
V,” (kcal mol-’ )
BrH2Si-SiH3 Br2HSi-SiHJ Br3Si-SiH3 Br,Si-SiH2Br Br,Si-SiHBrz
0.036 0.034 0.032 0.081 0.131
1.2 1.1 1.0 2.6 4.2
141
The uncertainties of these values are around 20% both for FG and Vf. Fluoro-compounds: F3Si-SiHF2
FH,Si-SiH,,
FzHSi-SiH3,
F3Si-SiH3,
F,Si-SiH2F
and
For these compounds the torsional potential is probably of the form given in eqn. (1) above. The barrier heights ought to have values between 1.0 and 1.4 kcal mol-’ and the torsional force constants between 0.030 and 0.045 mdyn A rad”. CONCLUSIONS
Previous applications of our non-bonded potential parameters [l] to haloalkanes [3, lo] have shown that the MM calculations reproduce a wide variety of conformational properties in all cases where experimental data existed. Moreover, the same potential parameters together with new force constants and reference values where Si atoms are involved, reproduced the experimental data for F$i-SiF3, Cl,Si-SiC13, C1$+CC13 and C13C-CC13 well. One should remember that the barrier heights within these molecules range from ca. 1 kcal mol’ up to ca. 1’7 kcal mol-‘. As shown in ref. 3 a barrier height of ca. 12 kcal mol-’ in C12HC-CC12 - CHB was correctly reproduced. It is also important to keep in mind that the calculated barrier heights are upper limits due to the constraints of the structural model used. Although it is not very likely, the possibility exists that the intrinsic barrier V,” is significantly lower than 1.2 kcal mol-‘. If that is the case the TABLE 6 1,1,2,2-Tetrahalodisilanes, X,HSi-SiHX,, results from molecular mechanics calculations (values in parentheses were obtained without excess charges on the atoms) X = halogen
F
Cl
Energy (E) in kcal mol“ at different torsion angles cp (“) QJ=o 2.2(2.1) 1.2(0.7) Q,(gauche) 0.04(O) O(0) @ = 120 2.5( 3.1) 1.20(2.0) @ = 180 (anti) 0.7( 1.4) O(1.0) Torsion angle (“) for gauche* conformer @ (H-Si-Si-H) 60(52) 59(57) Torsionalb force constant (Fm)C in mdyn A rad-z gauche 0.076(0.073) 0.040( 0.050) anti 0.038(0.027) 0.048(0.060)
Br
3.2(3.0) O(0) 2.8( 3.2) 0.6(1.1) 62(61) O.lOO(O.106) 0.093(0.066)
*See Fig. 1 for drawing of conformation. bFq = (PE/a@). ‘In order to convert from mdyn A molecule-’ to kcal mol-’ multiply by 143.8.
142
barrier heights of the compounds XH2Si-SiH2X, X2HSi-SiH2X and X,HSiSiHx, are also reduced. The contributions to the barrier heights from nonbonded interactions are quite small as seen from the Tables 4-6. In particular, the fluoro-compounds would then have virtually free internal rotation around the Si-Si bond. The effects of including electrostatic terms in the non-bonded potentials are clearly demonstrated in Tables 4-6. However, the main conclusions remain unaltered whether the charges are included or not. No attempt has been made here to correct for the difference in vibrational energies between conformers of the same molecule. Such energy differences may well amount to ca. 0.5 kcal mol-’ in molecules like the halodisilanes. At present the compounds in the Tables 4-6 are not available. However, experimental studies by gas-phase electron diffraction will be started as soon as some of these halodisilanes become available to us. ACKNOWLEDGEMENT
Financial support from Norges almenvitenskapelige forskningsrgd (NAVF) is acknowledged. REFERENCES 1 R. Stelevik, J. MoL Struct., Theochem., 109 (1984) 397. 2 J. P. Hunnel, J. Stackhouse and K. Mislow, Tetrahedron, 33 (1977) 1925. 3 T. Rydland and R. Stdlevik, J. Mol. Struct., Theochem, 105 (1983) 157. 4 D. A. Swick, I. L. Karle and J. Karle, J. Chem. Phys., 22 (1954) 1242. 5 Y. Morino and E. Hirota, J. Chem. Phys., 28 (1958) 185. 6 A. Aimenningen, B. Andersen and M: Traetteberg, Acta Chem. Stand., 18 (1964) 7’B. G. Prater, Ph.D. Thesis, University of Texas, Austin, Texas, 1969. 8 R. Stblevik, Acta Chem. Scan., A31 (1977) 359. 9 H. Oberhsmmer, J. MoL Struct., 31(1976) 237. 10 R. J. Abraham and R. Stblevik, Chem. Phys. Lett., 77 (1981) 181.
603.