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Symmetrically substituted silanes: (XH2C)2SiH2, (XH2C)2SiX2, (X2HC)2SiH2 and (X2HC)2SiX2 with X F, Cl or Br. Conformational energies, structures and torsional force constants obtained by molecular-mechanics calculations
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 372 (1995) 275-284
Symmetrically substituted silanes: (XH2C)2SiH2, (XH2C)2SiX2, (XzHC)zSiH2 and (XzHC)zSiX2 with X - F, C1 or Br. Conformational energies, structures and torsional force constants obtained by molecular-mechanics calculations Tore H. Johansen,
Reidar Stolevik*
Department of Chemistry, A VH, University of Trondheim, N-7055 Dragvoll, Norway
Received 17 March 1995; accepted in final form 7 June 1995
Abstract
A series of twelve symmetrically halogen-substituted silanes (XH2C-SiY2-CH2X and X2HC-SiY2-CHX2, with Y = H or X, and X = F, C1 or Br) have been investigated using molecular-mechanics calculations, with bonding parameters and interaction potentials derived from earlier gas-phase studies on halogenated alkanes. For the molecules FH2C-SiH2-CH2F and FH2C-SiF2-CH2F the low-energy conformers are GG and GG", while for the molecules F2HC-SiH2-CHF 2 and F2HC-SiFz-CHF2 only AA is a low-energy form. The low-energy form of the molecules (X = C1 or Br) XHzC-SiH2-CHzXand XH2C-SiX2-CH2X is GG. For C12HC-SiH2-CHC12 low-energy forms are AG and GG, while for C12HC-SiCIz-CHC12 low-energy forms are AA, AG and GG. Only GG is a low-energy conformer in BrzHC-SiBr2-CHBr2. The conformers AA, AG, GG and GG" have staggered terminal groups relative to the central group. However, in Br2HC-SiH2-CHBr2 one of the low-energy forms found is AS, possessing one eclipsed terminal group, while the other low-energy form is GG.
1. Introduction
Using gas-phase electron-diffraction data and molecular-mechanics calculations a large number of halogen substituted propanes have been studied [1-5]. Thus, the conformational structures and compositions have been established. Also, nonbonding a t o m . . , atom interaction potentials [1-4] have been derived based on these observations (Table 1). Recently we have started an investigation of the * Corresponding author.
conformational structures and energies in the Si-analogues to the halopropanes having skeletons C - S i - C , S i - C - S i and Si-Si-Si. Given time and money ab initio calculations will also be carried out, and hopefully synthetic work will provide us with samples for gas-phase electron-diffraction investigations. Recently a study of the reliability of ab initio calculations on the chloroethanes and chloropropanes regarding geometries and energies has been carried out [6], and a similar work on the silanes and trisilanes has already been started. One family of molecules of special interest [3] concerning conformational equilibria is represented
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276
Table 1 Values of the nonbonding a t o m . . , atom interactions V(R) in the Morse-potential formulation a Interaction
Ref.
R0 (A,)
Rm (,~)
c (kcal mo1-1)
H-.. H H... C H • -- F H . . . CI H . . . Br F... F F . . •C C1... C1 C1. • - C B r . . . Br Br... C
2 2 4 2 2 4 4 2 2 2 2
2.76 2.90 2.37 2.69 2.78 1.97 2.74 3.20 2.98 3.45 3.13
3.15 3.30 2.90 3.24 3.35 2.64 3.26 3.80 3.60 4.18 3.80
0.023 0.043 0.29 0.55 0.55 3.60 0.61 3.00 1.22 3.00 1.22
a The parameters R0, R,~ and e are related as follows: V(R0) = 0 and V(Rm) = - e corresponding to the minimum of
V(R). by the formulas X H 2 C - C Y 2 - C H 2 X and X 2 H C CY2-CHX2, where X is one of the halogens and Y may be H or X. This work is a study of the conformational structures and energies in the Si-analogues XH2C-SiY2-CH2X and X2HC SiY2-CHX 2. Conformational possibilities for these types of molecules have been indicated in Table 2. Thus, the conformers AA, AG, G G and G G " have staggered terminal groups relative to the central SiY2 group, while in AS one terminal group eclipses the central group as indicated. Previous work [5] showed that G G is the lowenergy form of the family X H 2 C - C H 2 - C H 2 X , while GG", having parallel C1-X. • • X - C 3 bonds on the same side of the C - C - C skeleton, is a high energy form not present in detectable amounts in the gas at about room temperature. However, for F H z C - C H 2 - C H 2 F a small percent (~ 10%) of G G " was probably present [4] at 20°C. A G and AA have energies about 0.51.5 kcal mo1-1 in excess of G G [5]. The largest separation of the halogen atoms exists in the AA conformer, while the shortest X . . . X distance is obtained in GG". However, A G has a lower energy than AA, and G G has the lowest energy in every case [5]. The energetically unfavourable X . . . X distance in the G G " form is about 3 ]k [3], while the longest X . - . X distance in AA is about 5-6 ,~ for X = C1
and Br. The energetically favourable X - . . X distances are found in the G G form, having antiparallel C1-X- - - X - C 3 bonds on opposite sides of the C - C - C skeleton. These distances are found [3] in the regions about 3.6-4.0 A (X = C1) and 4.0-4.4 A (X = Br). For F . . . F, the energetically favourable distances are found in the region 2.4-2.8 ,~ as indicated by the Rm values shown in Table 1. The non-bonding distances in a molecule with the skeleton C - S i - C are about 17% longer than the corresponding distances in a molecule with a C - C - C skeleton (the factor 1.17 is found by an analysis of average experimental values in propane and silane molecules containing these C - C - C and C - S i - C skeletons, respectively). Thereby a distance of 3 ,~ ~ C - C - C ) is increased to 3.5 (C-Si-C), and 4 A ( C - C - C ) is increased to 4.7 ,~ (C-Si-C). Thus, the C - S i - C molecules ought to have different conformational energies from the C - C - C molecules. It is also possible that the changes in internuclear distances combined with the reduction in rotational barrier heights may result in different types (AS) of conformers, as well as different energies among the AA, AG, G G and G G " forms (Table 2). Also torsional force constants and rotational barrier heights ought to be significantly smaller in the C - S i - C molecules compared to the corresponding halopropanes. It is expected that the chloro- and the bromo-substituted molecules resemble each other in conformational properties, while the fluoro-substituted molecules have different conformational distributions from the heavyhalogen substituted molecules.
2. Computational method Molecular-mechanics (MM) calculations on molecules with a C - S i - C skeleton where silicon is in the central position require only those nonbonding a t o m . . . a t o m interaction potentials recorded in Table 1. Thus, we do not need the interaction terms Si.--C, S i . . . H and S i . . . halogen in our calculations. Parameter values for the interaction potentials in the Morse formulation [1] have been established from studies on the
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277
Table 2 Conformational possibilities for molecules of the types XH2C-SiY2-CH2X and X2HC SiY2-CHX 2 with Y = H or X, and X = F, CI, Br XH2C-SiY 2 CH2X
Symbol
Symmetry
Ma
thl2(X-CI-Si-C3) b ~bl2( H - C I - S i - C 3 ) b
q~23(X-C3-Si-CI) b q523( H - C 3 - S i - C I ) b
HIH X-C-Si-C-X HIH
AA
CEv
1
180
180
AG
Cl
4
180
-60
H-C-Si-C-X X[X
GG
C2
2
+60
+60
X-C-Si C-X
XIX H-C-Si-C-H XIX
UlX X-C-Si-C-H H[H
XIH
HrX H-C-Si-C-H
X2HC SiY2-CHX2
XIH
XIH
HIX
XlX H-C-Si C-H HIH
GG"
Cs
2
+60
-60
HIH X-C-Si-XC n l n
AS
C~
2
180
0
HIH X-C-Si-C-X XIX XlX H-C-Si-HC XIX
Classical multiplicities [15]. b q~12and t~23 a r e torsional angles as indicated for exactly staggered conformations (AA, AG, GG, GG"), while for AS one terminal group (CH2X or CHX2) eclipses the central Si-group. a
chloro- and bromo-alkanes [3] and from the fluoro-alkanes [1,4]. The energy of a given conformer was calculated according to the expression
E = Er + Eo + E~ + Em + Ec
where the terms have the followingmeaning: 1 - ] F r ( r - r0)2 Strain in bonds" Er = ~
where Fr is the force constant of the bond r, and r ° is the reference value for Er = 0. Values for Fr and r ° are recorded in Table 3. Strain in bond angles:
E0 = ~1 S Fo(O- 00)2
where Fo is the force constant of the bond angle 0 and 0° is the reference value for Eo = 0. Values for Fo and 0° are recorded in Table 3. Torsional s t r a i n ' Ee = 1 V30 Z [ 1 + cos(34~)] where ~ represents a torsional angle and V 3o is the intrinsic threefold rotational barrier height, with
the value 1.2 kcal mo1-1 for rotation around each of the C - S i bonds. Coulomb interactions" Ec = Z
D/R
with D = 332QQ', where Q and Q' are the excess charges on two atoms and R is the distance between two atoms not bonded to the same atom. The excess charges were calculated according to the method described in Ref. [7]. Values of the parameters D are recorded in Table 4. The actual charges used in calculating the D values were reduced according to our experience based on earlier applications to haloalkanes. Thus, for the F-substituted molecules the calculated charge value is divided by 2.0, while for Cl-substituted molecules this factor is 1.6 and for Br-substituted molecules the factor 1.3 was applied. Nonbonding interactions in the Morse formulation •Em
= Z V(R)
The Morse-function formulation of nonbonding a t o m . . , atom interactions has been described in
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Table 3 Force constants (Fr and Fo) and reference values (r° and 0°) Bond-stretch H Si F-Si ( - S i F 2 ) C1-Si (-SiCI2-) Br Si ( SiBr2- ) H-C F C ( CH2F ) C1 C ( CH2C1) Br-C (-CH2Br) F - C (-CHF2, -CF3) CI-C ( CHCI2, -CC13) Br-C (-CHBr2, -CBr3) Si-C
r°/A 1.485 1.580 2.050 2.190 1.100 1.390 1.780 1.960 1.350 1.770 1.960 1.865
Table 4 Coulomb parameters D = 332 QQ' in kcal A mol 1 where Q and Q~ are atomic excess charges a
F~ mdyn ,~ 1 2.90 5.20 2.80 2.10 4.75 6.05 3.30 2.55 6.05 3.30 2.55 2.80
Angle-bend
O°/deg
Fo mdyn A(rad) -2
C Si C H Si H F-Si-F C1-Si-CI Br-Si-Br H C Si/H Si C F-C-Si/F-Si-C CI-C-Si/C1-Si-C Br C Si/Br Si C H C H F C F C1-C-C1 Br-C-Br H C F H-C-C1 H C Br
111.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5
0.70 0.30 0.40 0.50 0.50 0.40 0.90 0.80 0.70 0.50 0.70 1.10 1.10 0.85 0.75 0.70
R e f . [1]. H e r e R is the d i s t a n c e b e t w e e n t w o a t o m s n o t b o n d e d to the s a m e a t o m . T h e V ( R ) t e r m s are M o r s e - f u n c t i o n s d e f i n e d by the p a r a m e t e r s Ro, R m a n d c, w i t h v a l u e s r e c o r d e d in T a b l e 1. T h e s e p a r a m e t e r s are r e l a t e d as f o l l o w s [1]: V ( R o ) = 0 a n d V(Rm) = - c c o r r e s p o n d i n g to the m i n i m u m o f
v(R). P a r a m e t e r v a l u e s for t h e M o r s e p o t e n t i a l s , the f o r c e c o n s t a n t s , the r e f e r e n c e v a l u e s as well as the charge parameters have been established empirically f r o m g a s - p h a s e c o n f o r m a t i o n a l d a t a o n haloethanes and haloalkanes of the following kinds: X H z C - C H 2 X , XzHC-CH2X , X2HCC H X 2, X H z C - C H z - C H 3, X z H C - C H z - C H 3 , XH2-XCH-CH3, XHzC-CHz-CH2X, XHzCXCH-CH2X, X2HC-CX2-CH3, X2HC-CH 2-
X = halogen
F
CI
Br
XH2C SiH2-CH2X X.. - C H--. C H...X X- • - X H-- • H
-0.45 0.09 -2.35 12.11 0.45
0.44 -0.08 -1.61 8.65 0.30
1.03 -0.18 -1.28 7.27 0.22
X2HC-SiH2-CHX 2 H...X X. -. X X.-.C C. - • H H-..H
-4.25 8.02 -2.59 1.37 2.25
-2.87 5.61 -1.18 0.60 1.47
-2.30 4.68 -0.43 0.21 0.13
XH2C-SiX2-CH2X and X2HC-SiX2-CHX 2 H . - . X (1 .-.3) H . - . X (1 --.2) X. -. X (1. -. 2) X . . . X (1 ...3) C. - •X C. -. H H..-H
-5.12 -5.34 4.66 4.47 -3.81 4.37 5.87
-3.62 -4.16 3.42 2.82 -2.20 2.68 4.16
-2.70 -3.86 2.96 2.07 - 1.44 1.88 3.52
a The electrostatic energy terms are D/R where R is the interatomic distance in ,~ units. C H X 2, X 2 H C - X C H - C H X 2, X 2 H C - C X 2 - C H X 2 , ( X H 2 C ) 2 C ( C H 3 ) 2 a n d ( X H 2 C 4 ) C . All o f these molecules have X = chlorine, many have X = bromine and a smaller number have X = fluorine. S t r u c t u r a l p a r a m e t e r v a l u e s for 25 m o l e c u l e s o b t a i n e d by o u r M M c a l c u l a t i o n s h a v e b e e n systema t i c a l l y c o m p a r e d w i t h the c o r r e s p o n d i n g experim e n t a l v a l u e s [8] w h e n X = c h l o r i n e . G e n e r a l l y the a g r e e m e n t b e t w e e n o b s e r v a t i o n s a n d t h e calculated v a l u e s lies w i t h i n the e x p e r i m e n t a l e r r o r limits. E n e r g y m i n i m a w e r e f o u n d by a d j u s t i n g all structural parameters simultaneously, but one or b o t h t o r s i o n a l angles w e r e h e l d c o n s t a n t in finding the e n e r g i e s o f the t r a n s i t i o n f o r m s . T h e S i - C H 2 X a n d S i - C H X 2 g r o u p s possess Cs s y m m e t r y , while the c e n t r a l C - S l Y 2 C g r o u p possess C2v s y m m e t r y in o u r m o d e l o f the m o l e c u l e s . T h e utilized m o l e c u l a r - m e c h a n i c s c o m p u t e r p r o g r a m , w h i c h uses a c o m b i n a t i o n o f s t e e p e s t - d e s c e n t and Newton-Raphson m e t h o d s , w a s w r i t t e n by S. R u s t a d [9].
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279
Table 5 MM results for FH2C-SiH2 CH2F and F H z C - S i F 2 - C H z F molecules Parameter
FH2C-SiH2-CH2F AA
Torsional a angles (deg): q~12: ~23: Energies b: (kcal/mol): Torsional force constantsC: Fo12: F~23: Conformational composition (%)d:
AG
FH2C-SiF2-CH2F GG
AG
GG
GG"
180.0 180.0
180.0 -60.0
44.8 44.8
50.2 -50.2
180.4 -55.3
1.7
0.9
0
0.001
4.7
2.7
0
0.2
0.028 0.028
0.049 0.039
0.041 0.041
0.037 0.037
0.040 0.040
0.038 0,048
0.047 0.047
0,049 0 049
0
1
18
41
57.8 -57.8
AA
180.0 180.0
0
53.5 53.5
GG"
41
57
42
a See Table 2. h Relative to G G (E = 0). c Diagonal values in mdynA(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
3. Results for individual molecules Table 6 Torsional barrier heights in XH2C-SiY2-CH2X and X2HC SiY2-CHX~ molecules with X = F, C1, Br and Y = H, X Molecule
Conformational change
F
CI
Br
XH2C-SiH2-CH2X
AA ~ AG AG ~ GG" GG" ---* AA AG ~ G G G G ~ AA
1.1 1.3 4.1 1.1 4.0
1.0 0.4 3.0 1.1 5.5
1.0 0.5 1.8 1.2 5.2
XH2C SiX2-CH2X
AA ---* AG AG ~ GG" GG" ~ AA AG ~ G G G G ~ AA
1.0 0.9 6.7 0.9 6.8
3.6 3.6 9.9 4.1 12.9
3.8 3.9 9.6 4.5 13.9
X2HC-SiHz-CHX 2
AA ~ AG AG ~ GG" GG" ~ AA AG ~ GG G G ~ AA
2.2 2.0 2.6 2.0 2.8
2.6 4.2 10.6 4.9 14.4
2.1 (_)a (_)a 5.8 17.7
X2HC-SiX2 CHX2
AA ~ AG AG ---* GG" GG" ~ AA AG ---, G G G G ~ AA
2.4 2.5 2.0 2.5 2.1
5.6 6.9 15.4 7.4 19.0
6.1 7.5 15.9 8.4 21.7
a GG" is not a conformational minimum for Br2HC-SiH z CHBr 2.
Several halogen-substituted propanes, being analogues to the silanes studied in this work, have already been studied by gas-phase electron diffraction. Results for the corresponding propanes will be compared with the results for molecules treated in this work in the summary at the end of this paper.
3.1. FHeC-SiH2-CH2F and FH2C-SiF2-CH2F Results for these molecules are recorded in Table 5. Clearly the low-energy forms in both cases are G G and GG". The G G " conformer possesses one F 1 - . . F 3 parallel interaction in both molecules (Table 2). At room temperatures the gas-phase compositions of the molecules ought to be about 80-100% of G G and G G " conformers. These conformers both have approximately staggered terminal groups relative to the central group. However, in F H z C - S i F 2 - C H 2 F the deviations from exactly staggered form are 15° in the torsional angles. The values of the torsional force constants are not very different in the two molecules. Values of the rotational barrier heights (Table 6) are obtained in the ranges (kcal mo1-1) 1.1-4.1 and
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Table 7 M M results for CIH2C-SiH2-CH2C1 and CIH2C SiCI 2 CH2CI molecules Parameter
CIH2C Sill 2 CH2CI AA
Torsional a angles (deg): q~12: q~23: Energiesb: (kcal/mol): Torsional force constantsC: F~,12: F~,23: Conformational composition (%)d:
180.0 180.0 3.2
AG
CIH2C-SiCI2-CH2CI GG
181.1 -57.7 1.8
0.030 0.030
0.046 0.063
0
9
55.5 55.5 0 0.116 0.116 89
GG"
63.7 -63.7 2.2
AA
180.0 180.0 6.1
AG
GG
GG"
177.1 -49.4 3.4
49.7 49.7 0
64.0 -64.0 2.9
-
0.160 0.160
0.120 0.160
2
0
1
0.190 0.190 98
0.130 0.130 1
a See Table 2. b Relative to G G (E = 0). c Diagonal values in mdynA(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
0.9-6.8 for F H 2 C - S i H 2 - C H 2 F and F H 2 C - S i F 2 CHzF respectively.
3.2. CIH2C-SiH2-CH2CI and CIH2C-SiCI2CH2CI Results are recorded in Table 7. The low-energy conformer is G G in both molecules. In this conformer parallel X1 ... X3 interactions are not present. Deviations from exactly staggered form are 5-10 ° in the torsional angles of GG. In the gas phase at about room temperature the contributions of G G conformers alone ought to be about 90-99%. For CIHzC-SiH2-CHzC1 the G G " form, possessing one parallel CI1..-C13 interaction, is not a stable conformer, while for C1HzC SiCI2-CHzCI the G G " conformer corresponds to a minimum on the potential energy surface. The values of the torsional force constants for the tetrachlorosilane are significantly larger than for the dichlorosilane. The ranges of rotational barrier heights (kcal mol-l), involving G G in the transitions, are 1 5 and 4-13 for the dichloro- and tetrachlorosilanes, respectively (Table 6).
3.3. BrH2C-SiH2 CH2Br and BrH2C-SiBr2CH2Br Results are recorded in Table 8. In both cases the only low-energy conformer is GG. For
dibromosilane G G " is not a stable form. Deviations from exactly staggered conformations are about 9 ° in tetrabromosilane, while for dibromosilane the deviations are less than 1° in the torsional angles. At about room temperature the gases ought to consist of 80-99% G G conformers. The values of the torsional force constants are significantly larger in the tetrabromosilane than in the dibromosilane. Rotational barrier heights (kcal mol-l), involving the G G conformer, were obtained in the regions 1-5 and 4 14 for the dibromosilane and the tetrabromosilane, respectively. Thus, the values of these barrier heights are approximately equal in these compounds and the corresponding chlorocompounds.
3.4. F2HC-SiH2-CHF 2 and F2HC-SiF2CHF2 Results are recorded in Table 9. For these molecules AA, possessing two parallel F1 .. • F3 interactions, is the low-energy conformer. The AA conformer have exactly staggered terminal groups relative to the central group, corresponding to C2v symmetry. At room temperature the gases ought to consist of about 70-90% of the AA conformers alone. Torsional force constants in the tetrafluorosilane have larger values than in the hexafluorosilane, when the AA conformer is considered. Rotational barrier heights involving the AA conformer, were
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Table 8 MM results for BrH2C-SiH2-CH2Br and BrHzC-SiBr2 CH2Br molecules Parameter
BrH2C-SiH2-CH2Br AA
Torsional a angles (deg): ~b12:
~23: Energiesb: (kcal/mol): Torsional force constantsC: F~,j2:
180.0 180.0 2.9
F4,23:
0.047 0.047
Conformational composition (%)d:
0
AG
BrH2C-SiBr2-CH2 Br GG
179.6 --60.3 1.5 0.043 0.063 14
GG"
59.2 59.2 0
66.0 --66.0 2.9
0.110 0.110 85
AA
180.0 180,0 6.7
AG
GG
GG"
179.0 -48.0 3.4
51.0 51.0 0
61.0 -61.0 4.0
-
0.120 0.120
0.119 0.093
1
0
1
0.220 0.220 99
0.150 0.150 0
a See Table 2. b Relative to GG (E = 0). c Diagonal values in mdyn.A(rad-2). d Boltzmann distribution including the multiplicities of Table 2. o b t a i n e d in the r e g i o n 2 - 3 k c a l m o l - l . T h u s , these b a r r i e r s h a v e a p p r o x i m a t e l y e q u a l h e i g h t s in the two molecules.
3.5. CI2HC-SiH2-CHCI2 and CI2HC-SiCI2-
cI-ICb R e s u l t s are r e c o r d e d in T a b l e 10. T h e l o w - e n e r g y f o r m o f t e t r a c h l o r o s i l a n e is A G , w h i l e for h e x a c h l o r o s i l a n e A A , A G a n d G G are l o w - e n e r g y
f o r m s . I n b o t h m o l e c u l e s G G " is a h i g h - e n e r g y c o n f o r m e r c o n t r i b u t i n g less t h a n 1% to the gasp h a s e c o m p o s i t i o n s . E s s e n t i a l l y t h e gas o f the t e t r a c h l o r o s i l a n e o u g h t to be a t w o - c o n f o r m e r m i x ture o f A G a n d G G , w h i l e f o r t h e h e x a c h l o r o s i l a n e the gas is a t h r e e - c o n f o r m e r m i x t u r e o f A A , A G and GG. T o r s i o n a l f o r c e c o n s t a n t s in t e t r a c h l o r o s i l a n e h a v e significantly s m a l l e r v a l u e s t h a n t h o s e o f hexachlorosilane. The rotational barrier heights
Table 9 MM results for F2HC-SiHz-CHF 2 and FzHC SiF2-CHF 2 molecules Parameter
F2HC-SiH 2 CHF 2 AA
AG
F2HC-SiF2-CHF 2 GG
ASe
AA
AG
GG
GG"
180.0 180.0 0
173.8 -69.5 2.0
73.0 73.0 4.0
70.0 -70.0 4.1
Torsional" angles (deg):
(~12: ~23: Energiesb: (kcal/mol): Torsional force constantsC: F~2: F~23: Conformational composition (%)d:
180.0 180.0 0 0.065 0.065 72
175.4 -67.0 1.5 0.061 0.044 25
70.2 70.2 2.7
64.1 -64.1 2.8
0.042 0.042
0.046 0.046
2
1
a See Table 2. b Relative to AA (E = 0). c Diagonal values in mdyn~,(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
0.012 0.012 88
0.071 0.048 12
0.047 0.047
0.047 0.047
0
0
282
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
Table 10 M M results for C12HC SiH2-CHC12 and CI2HC-SiC12 CHC12 molecules Parameters
CI2HC-SiH2CHC12 AA
AG
C12HC-SiC12 CHC12 GG
GG"
AA
AG
GG
GG"
164.0 -59,9 0.1
65.9 65.9 0.2
54.4 -54.4 3.8
Torsional a angles (deg):
q~12: ~z3: Energiesb: (kcal/mol): Torsional force constantsC: F¢I2: Fe~23: Conformational composition (%)d:
180.0 180.0 0.9
158.6 - 51.4 0
0.065 0.065 4
0.153 0.090 79
64.2 64.2 0.5
43.0 -43.0 2.3
0.097 0.097
0.049 0.049
16
1
180.0 180.0 0 0.232 0.232 17
0.305 0.265 59
0.265 0.265 24
0.259 0.259 0
a See Table 2. b Relative to A G (E = 0). c Diagonal values in mdyn,~(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
of these molecules (Table 6) have values in the regions 3-14 and 6-19 kcal mol 1 for tetrachlorosilane and hexachlorosilane, respectively. 3.6. Br2HC-SiH2-CHBr2 and Br2HC-SiBr2CHBr 2 Results are recorded in Table 11. For tetrabromosilane AS and G G are low-energy conformers, while G G " is not a stable form of this molecule. In hexabromosilane G G is the low-energy form. The
gas of tetrabromosilane, at room temperature, is a rather complex mixture of AG, G G and AS. The gas-phase of hexabromosilane is a mixture of AA, A G and GG. In both molecules the deviations from exactly staggered conformation might be as large as 24 ° . However, AA has exactly staggered conformation in both molecules. Values of the torsional force constants are much larger in hexabromosilane than in tetrabromosilane. Rotational barrier heights (kcal tool -1) were obtained in the region
Table 11 M M results for Br2HC-SiH2 CHBr2 and B r 2 H C - S i B r 2 - C H B r 2 molecules Parameters
Br2HC-SiH2-CHBr 2 AA
Torsional a angles (deg): 0~2: 4~23: Energiesb: (kcal/mol): Torsional force constantsC: F,12: F~23: Conformational composition ( % f :
180.0 180.0 3.1 0.044 0.044 0
AG
Br2HC SiBr2-CHBr 2 GG
156.0 -44.5 0.6 0.133 0.080 30
60.0 60.0 0.2 0.094 0.094 29
a See Table 2. b Relative to AS (E = 0) and G G (E = 0). c Diagonal values in mdynA.(rad-2). d Boltzmann distribution including the multiplicities of Table 2. e GG" is not a conformational m i n i m u m for BrzHC S i H 2 - C H B r 2.
GG"
180.0 0.0 0 0.138 0.015 41
AA
180.0 180.0 0.7 0.182 0.182 8
AG
GG
GG"
163.0 -58.5 0.7
64.7 64.7 0
57.0 -57.0 5.7
0.239 0.186 36
0.320 0.320 56
0.379 0.379 0
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284 Table 12 S u m m a r y of low-energy conformers a in X H 2 C - S i Y 2 CH2X and X 2 H C - S i Y 2 - C H X 2 molecules, Y = H or X Molecule
X = F
X = CI
X = Br
XH2C-SiH2-CH2 X XH2C SiX2-CH2X
GG, G G " GG, G G "
GG GG
GG GG
X2HC S i H 2 - C H X 2 X2HC-SiX2-CHX 2
AA AA
AG, G G AA, A G , G G
AS, G G GG
a See Table 2.
2-18 and 6-22 for tetrabromosilane and hexabromosilane, respectively.
283
Table 14 Torsional frequencies (wl and ~2) and torsional force constants (Fe) in the G G conformer of XH2C-ZY2-CH2X and X 2 H C Z Y 2 - C H X 2 molecules, X = chlorine, Y = H or X and Z = Si or Ca Molecules and values: F~ co 1 ( - r - 0 - ) b c d 2 ( + - q - ) c (mdynA,(rad) -2 (cm 1) (cm-X) F,(C-C)/F,(Si-C) XHzC-ZH2-CH2 X XH2C Z X 2 - C H 2 X X2HC ZH 2 C H X 2 X2HC Z X 2 - C H X 2
0.17/0.12 0.32/0.19 0.24/0.10 0.46/0.27
60/46 72/49 57/21 75/51
145/111 121/82 87/32 77/53
a Values for Z = C are taken from ref. [14]. b C2 symmetry.
c C~ symmetry.
4. Summary and discussion Conformational possibilities for the symmetrically substituted silanes XHzC-SiYz-CH2X and XzHC-SiYz-CHX 2 have been summarised in Table 12. For these molecules the staggered conformers GG, AG, AA and GG" as well as AS (with one eclipsed terminal group) have to be considered. For all molecules, except FzHC-SiYz-CHF 2, GG is a low-energy form and AA is a high energy form. However, for FzHC-SiYz-CHF 2 molecules AA is a low-energy conformer (see Table 9). Only for FH2C-SiYz-CH2F molecules is GG" a low-energy form (Table 5), and only for Br2HC-SiH2-CHBr 2 is AS a low-energy form (Table 11). The most complex conformational mixture should exist for C12HC-SiC12-CHC12, including AA, AG and GG conformers in the gas phase (Table 10).
Gas-phase electron diffraction results exist for 1,3-dihalo(X)propanes: F [4], Cl [10] and Br [11]. The conformational compositions at about room temperature, for these molecules and the corresponding silanes treated in this work, are recorded in Table 13. Clearly GG is the abundant conformer in all six molecules. AA is present only in small amounts, while AG is present in detectable amounts. Only in FH2C-SiH2-CH2F is GG" a low-energy form, while in FH2C-CH2-CH2F gas-phase experiment shows that the form GG" is present with about 10% contribution [4]. Thus, the following simplified rule may be remembered: GG is a low-energy conformer and AA is a high-energy form in these molecules. Diffraction data for CI2HC-CH2-CHC12 showed [12] that the gas-phase at 62°C existed essentially of only GG conformers, while for the CI2HC-SiH2-CHC12 molecule AG is the low-
Table 13 Gas-phase conformational compositions a o f X H 2 C - S i H 2 CH2X and X H 2 C - C H 2 - C H 2 X molecules C1H2C-ZH2-CH2CI
BrH2C-ZH2-CH2Br
Molecule
FH2C-ZH2-CH2F
Conformer b
Z = C [4]
Z = Sic
z = c [1o]
Z = Sic
Z=C[ll]
Z = Sic
GG AG AA GG"
63 27 0 10
41 l8 0 41
73 24 3 0
89 9 0 2
67 30 3 0
85 14 0 1
a At about room temperatures. b See Table 2. c This work.
284
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
energy form according to our calculations (Table 10). The electron diffraction data [13] for C12HCCCI2-CHCI2 showed that at 112°C the gas-phase existed essentially of G G conformers. For the corresponding silane, ClzHC-SiClz-CHC12, the calculations predicted that the gas is a mixture of AG, G G and AA conformers. Barrier heights corresponding to rotation of one of the X H z C - or X2HC-groups around the Si-C bond have values in the region 0.9-2.5 kcal mo1-1 for fluoro-substituted molecules, while for the heavy-halogen substituted molecules this region is 0.4-8.4 kcal mol 1. Generally the rotational barriers in C - S i - C molecules are lower than the barriers in the corresponding C C - C molecules. Thus, the barrier heights in X H 2 C - S i H 2 - C H 2 X (X = C1) are close to 1.0 kcal mo1-1 while in X H 2 C C H 2 - C H 2 X ( X = C1) the values are close to 4.0 kcal mo1-1 [5]. However, the barrier heights found in X 2 H C - S i H 2 - C H X 2 and X 2 H C - C H 2CHX 2 (X = C1) overlap with values in the ranges 2 - 4 ( C - C - C ) [3] and 3-5 ( C - S i - C ) kcal tool -1 (see low-barrier height values in Table 6). Torsional force constants in the molecules treated in this work have their values in the regions: 0.03-0.07 (fluorides), 0.03-0.31 o(chlorides) and 0.05-0.38 (bromides) in mdynA(rad) -2 units. Using the approximation formulas established earlier [14] the wavenumbers of the fundamental low-frequency oscillations for the G G conformer of the chlorosilanes have been estimated and compared with the corresponding chloropropanes in Table 14. Values of the torsional force constants and wavenumbers corresponding to the chloropropanes are taken from Ref. [14]. The ranges of wavenumbers (cm -1) are 57-75 (c~l) and 77-145 (co2) for the
chloropropanes, while for the chlorosilanes these ranges are 21-51 (col) and 32-111 (a~2). Thus, in order to establish experimentally the torsional frequencies of the molecule C12HC-SiH2-CHC12 one has to detect wavenumbers as low as 20-30 cm a according to our estimates (Table 14). In conclusion, we have found that in the silanes XH2C-SiY2-CH2X and X 2 H C - S i Y 2 - C H X 2 as well as in the corresponding propanes X H 2 C CY 2 CH2X and X 2 H C - C Y 2 - C H X 2 (Y = H or X) the staggered GG, AG, G G " and AA conformers have to be expected in the gas phase. Only for B r z H C - S i H 2 CHBr2 is an eclipsed form (AS) expected to be a low-energy conformer.
References [1] R.J. Abraham and R. Stolevik, Chem. Phys. Lett., 77 (1981) 181. [2] R. St~levik, J. Mol. Struck (Theochem), 109 (1984) 397. [3] T. Rydland and R. Stolevik, J. Mol. Struct., 105 (1983) 157. [4] P. Kl~eboe, D.L. Powell, R. Stolevik and gl. Vorren, Acta Chem. Scand., Ser. A, 36 (1982) 471. [5] L. Postmyr, J. Mol. Struct., 319 (1994) 211. [6] R. Stolevik and K. Hagen, Ab initio calculations of conformational structures and energies in chlorosubstituted ethanes and propanes, J. Mol. Struck, 352/353 (1995) 23. [7] R.T. Sanderson, J. Inorg. Chem., 28 (1966) 1553. [8] R. Stolevik, 3. Mol. Struct., 291 (1993) 301. [9] S. Rustad, Thesis in Chemistry, University of Oslo, Oslo, 1973. [10] S. Grindheim and R. Stolevik, Acta Chem. Scand., Ser. A, 30 (1976) 625. [11] P.E. Farup and R. Stolevik, Acta Chem. Scand., Ser. A, 28 (1974) 680. [12] M. Braathen, D.H. Christensen, P. Klzeboe, R. Seip and R. Stolevik, Acta Chem. Scand., A33 (1979) 437. [13] L. Fernholt and R. Stolevik, Acta Chem. Scand., A29 (1975) 651. [14] R. Stolevik and P. Bakken, J. Mol. Struck, 244 (1991) 27. [15] R. Stolevik, Acta Chem. Scand., A28 (1974) 455.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 372 (1995) 275-284
Symmetrically substituted silanes: (XH2C)2SiH2, (XH2C)2SiX2, (XzHC)zSiH2 and (XzHC)zSiX2 with X - F, C1 or Br. Conformational energies, structures and torsional force constants obtained by molecular-mechanics calculations Tore H. Johansen,
Reidar Stolevik*
Department of Chemistry, A VH, University of Trondheim, N-7055 Dragvoll, Norway
Received 17 March 1995; accepted in final form 7 June 1995
Abstract
A series of twelve symmetrically halogen-substituted silanes (XH2C-SiY2-CH2X and X2HC-SiY2-CHX2, with Y = H or X, and X = F, C1 or Br) have been investigated using molecular-mechanics calculations, with bonding parameters and interaction potentials derived from earlier gas-phase studies on halogenated alkanes. For the molecules FH2C-SiH2-CH2F and FH2C-SiF2-CH2F the low-energy conformers are GG and GG", while for the molecules F2HC-SiH2-CHF 2 and F2HC-SiFz-CHF2 only AA is a low-energy form. The low-energy form of the molecules (X = C1 or Br) XHzC-SiH2-CHzXand XH2C-SiX2-CH2X is GG. For C12HC-SiH2-CHC12 low-energy forms are AG and GG, while for C12HC-SiCIz-CHC12 low-energy forms are AA, AG and GG. Only GG is a low-energy conformer in BrzHC-SiBr2-CHBr2. The conformers AA, AG, GG and GG" have staggered terminal groups relative to the central group. However, in Br2HC-SiH2-CHBr2 one of the low-energy forms found is AS, possessing one eclipsed terminal group, while the other low-energy form is GG.
1. Introduction
Using gas-phase electron-diffraction data and molecular-mechanics calculations a large number of halogen substituted propanes have been studied [1-5]. Thus, the conformational structures and compositions have been established. Also, nonbonding a t o m . . , atom interaction potentials [1-4] have been derived based on these observations (Table 1). Recently we have started an investigation of the * Corresponding author.
conformational structures and energies in the Si-analogues to the halopropanes having skeletons C - S i - C , S i - C - S i and Si-Si-Si. Given time and money ab initio calculations will also be carried out, and hopefully synthetic work will provide us with samples for gas-phase electron-diffraction investigations. Recently a study of the reliability of ab initio calculations on the chloroethanes and chloropropanes regarding geometries and energies has been carried out [6], and a similar work on the silanes and trisilanes has already been started. One family of molecules of special interest [3] concerning conformational equilibria is represented
0022-2860/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)08978-0
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
276
Table 1 Values of the nonbonding a t o m . . , atom interactions V(R) in the Morse-potential formulation a Interaction
Ref.
R0 (A,)
Rm (,~)
c (kcal mo1-1)
H-.. H H... C H • -- F H . . . CI H . . . Br F... F F . . •C C1... C1 C1. • - C B r . . . Br Br... C
2 2 4 2 2 4 4 2 2 2 2
2.76 2.90 2.37 2.69 2.78 1.97 2.74 3.20 2.98 3.45 3.13
3.15 3.30 2.90 3.24 3.35 2.64 3.26 3.80 3.60 4.18 3.80
0.023 0.043 0.29 0.55 0.55 3.60 0.61 3.00 1.22 3.00 1.22
a The parameters R0, R,~ and e are related as follows: V(R0) = 0 and V(Rm) = - e corresponding to the minimum of
V(R). by the formulas X H 2 C - C Y 2 - C H 2 X and X 2 H C CY2-CHX2, where X is one of the halogens and Y may be H or X. This work is a study of the conformational structures and energies in the Si-analogues XH2C-SiY2-CH2X and X2HC SiY2-CHX 2. Conformational possibilities for these types of molecules have been indicated in Table 2. Thus, the conformers AA, AG, G G and G G " have staggered terminal groups relative to the central SiY2 group, while in AS one terminal group eclipses the central group as indicated. Previous work [5] showed that G G is the lowenergy form of the family X H 2 C - C H 2 - C H 2 X , while GG", having parallel C1-X. • • X - C 3 bonds on the same side of the C - C - C skeleton, is a high energy form not present in detectable amounts in the gas at about room temperature. However, for F H z C - C H 2 - C H 2 F a small percent (~ 10%) of G G " was probably present [4] at 20°C. A G and AA have energies about 0.51.5 kcal mo1-1 in excess of G G [5]. The largest separation of the halogen atoms exists in the AA conformer, while the shortest X . . . X distance is obtained in GG". However, A G has a lower energy than AA, and G G has the lowest energy in every case [5]. The energetically unfavourable X . . . X distance in the G G " form is about 3 ]k [3], while the longest X . - . X distance in AA is about 5-6 ,~ for X = C1
and Br. The energetically favourable X - . . X distances are found in the G G form, having antiparallel C1-X- - - X - C 3 bonds on opposite sides of the C - C - C skeleton. These distances are found [3] in the regions about 3.6-4.0 A (X = C1) and 4.0-4.4 A (X = Br). For F . . . F, the energetically favourable distances are found in the region 2.4-2.8 ,~ as indicated by the Rm values shown in Table 1. The non-bonding distances in a molecule with the skeleton C - S i - C are about 17% longer than the corresponding distances in a molecule with a C - C - C skeleton (the factor 1.17 is found by an analysis of average experimental values in propane and silane molecules containing these C - C - C and C - S i - C skeletons, respectively). Thereby a distance of 3 ,~ ~ C - C - C ) is increased to 3.5 (C-Si-C), and 4 A ( C - C - C ) is increased to 4.7 ,~ (C-Si-C). Thus, the C - S i - C molecules ought to have different conformational energies from the C - C - C molecules. It is also possible that the changes in internuclear distances combined with the reduction in rotational barrier heights may result in different types (AS) of conformers, as well as different energies among the AA, AG, G G and G G " forms (Table 2). Also torsional force constants and rotational barrier heights ought to be significantly smaller in the C - S i - C molecules compared to the corresponding halopropanes. It is expected that the chloro- and the bromo-substituted molecules resemble each other in conformational properties, while the fluoro-substituted molecules have different conformational distributions from the heavyhalogen substituted molecules.
2. Computational method Molecular-mechanics (MM) calculations on molecules with a C - S i - C skeleton where silicon is in the central position require only those nonbonding a t o m . . . a t o m interaction potentials recorded in Table 1. Thus, we do not need the interaction terms Si.--C, S i . . . H and S i . . . halogen in our calculations. Parameter values for the interaction potentials in the Morse formulation [1] have been established from studies on the
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
277
Table 2 Conformational possibilities for molecules of the types XH2C-SiY2-CH2X and X2HC SiY2-CHX 2 with Y = H or X, and X = F, CI, Br XH2C-SiY 2 CH2X
Symbol
Symmetry
Ma
thl2(X-CI-Si-C3) b ~bl2( H - C I - S i - C 3 ) b
q~23(X-C3-Si-CI) b q523( H - C 3 - S i - C I ) b
HIH X-C-Si-C-X HIH
AA
CEv
1
180
180
AG
Cl
4
180
-60
H-C-Si-C-X X[X
GG
C2
2
+60
+60
X-C-Si C-X
XIX H-C-Si-C-H XIX
UlX X-C-Si-C-H H[H
XIH
HrX H-C-Si-C-H
X2HC SiY2-CHX2
XIH
XIH
HIX
XlX H-C-Si C-H HIH
GG"
Cs
2
+60
-60
HIH X-C-Si-XC n l n
AS
C~
2
180
0
HIH X-C-Si-C-X XIX XlX H-C-Si-HC XIX
Classical multiplicities [15]. b q~12and t~23 a r e torsional angles as indicated for exactly staggered conformations (AA, AG, GG, GG"), while for AS one terminal group (CH2X or CHX2) eclipses the central Si-group. a
chloro- and bromo-alkanes [3] and from the fluoro-alkanes [1,4]. The energy of a given conformer was calculated according to the expression
E = Er + Eo + E~ + Em + Ec
where the terms have the followingmeaning: 1 - ] F r ( r - r0)2 Strain in bonds" Er = ~
where Fr is the force constant of the bond r, and r ° is the reference value for Er = 0. Values for Fr and r ° are recorded in Table 3. Strain in bond angles:
E0 = ~1 S Fo(O- 00)2
where Fo is the force constant of the bond angle 0 and 0° is the reference value for Eo = 0. Values for Fo and 0° are recorded in Table 3. Torsional s t r a i n ' Ee = 1 V30 Z [ 1 + cos(34~)] where ~ represents a torsional angle and V 3o is the intrinsic threefold rotational barrier height, with
the value 1.2 kcal mo1-1 for rotation around each of the C - S i bonds. Coulomb interactions" Ec = Z
D/R
with D = 332QQ', where Q and Q' are the excess charges on two atoms and R is the distance between two atoms not bonded to the same atom. The excess charges were calculated according to the method described in Ref. [7]. Values of the parameters D are recorded in Table 4. The actual charges used in calculating the D values were reduced according to our experience based on earlier applications to haloalkanes. Thus, for the F-substituted molecules the calculated charge value is divided by 2.0, while for Cl-substituted molecules this factor is 1.6 and for Br-substituted molecules the factor 1.3 was applied. Nonbonding interactions in the Morse formulation •Em
= Z V(R)
The Morse-function formulation of nonbonding a t o m . . , atom interactions has been described in
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T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
Table 3 Force constants (Fr and Fo) and reference values (r° and 0°) Bond-stretch H Si F-Si ( - S i F 2 ) C1-Si (-SiCI2-) Br Si ( SiBr2- ) H-C F C ( CH2F ) C1 C ( CH2C1) Br-C (-CH2Br) F - C (-CHF2, -CF3) CI-C ( CHCI2, -CC13) Br-C (-CHBr2, -CBr3) Si-C
r°/A 1.485 1.580 2.050 2.190 1.100 1.390 1.780 1.960 1.350 1.770 1.960 1.865
Table 4 Coulomb parameters D = 332 QQ' in kcal A mol 1 where Q and Q~ are atomic excess charges a
F~ mdyn ,~ 1 2.90 5.20 2.80 2.10 4.75 6.05 3.30 2.55 6.05 3.30 2.55 2.80
Angle-bend
O°/deg
Fo mdyn A(rad) -2
C Si C H Si H F-Si-F C1-Si-CI Br-Si-Br H C Si/H Si C F-C-Si/F-Si-C CI-C-Si/C1-Si-C Br C Si/Br Si C H C H F C F C1-C-C1 Br-C-Br H C F H-C-C1 H C Br
111.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5
0.70 0.30 0.40 0.50 0.50 0.40 0.90 0.80 0.70 0.50 0.70 1.10 1.10 0.85 0.75 0.70
R e f . [1]. H e r e R is the d i s t a n c e b e t w e e n t w o a t o m s n o t b o n d e d to the s a m e a t o m . T h e V ( R ) t e r m s are M o r s e - f u n c t i o n s d e f i n e d by the p a r a m e t e r s Ro, R m a n d c, w i t h v a l u e s r e c o r d e d in T a b l e 1. T h e s e p a r a m e t e r s are r e l a t e d as f o l l o w s [1]: V ( R o ) = 0 a n d V(Rm) = - c c o r r e s p o n d i n g to the m i n i m u m o f
v(R). P a r a m e t e r v a l u e s for t h e M o r s e p o t e n t i a l s , the f o r c e c o n s t a n t s , the r e f e r e n c e v a l u e s as well as the charge parameters have been established empirically f r o m g a s - p h a s e c o n f o r m a t i o n a l d a t a o n haloethanes and haloalkanes of the following kinds: X H z C - C H 2 X , XzHC-CH2X , X2HCC H X 2, X H z C - C H z - C H 3, X z H C - C H z - C H 3 , XH2-XCH-CH3, XHzC-CHz-CH2X, XHzCXCH-CH2X, X2HC-CX2-CH3, X2HC-CH 2-
X = halogen
F
CI
Br
XH2C SiH2-CH2X X.. - C H--. C H...X X- • - X H-- • H
-0.45 0.09 -2.35 12.11 0.45
0.44 -0.08 -1.61 8.65 0.30
1.03 -0.18 -1.28 7.27 0.22
X2HC-SiH2-CHX 2 H...X X. -. X X.-.C C. - • H H-..H
-4.25 8.02 -2.59 1.37 2.25
-2.87 5.61 -1.18 0.60 1.47
-2.30 4.68 -0.43 0.21 0.13
XH2C-SiX2-CH2X and X2HC-SiX2-CHX 2 H . - . X (1 .-.3) H . - . X (1 --.2) X. -. X (1. -. 2) X . . . X (1 ...3) C. - •X C. -. H H..-H
-5.12 -5.34 4.66 4.47 -3.81 4.37 5.87
-3.62 -4.16 3.42 2.82 -2.20 2.68 4.16
-2.70 -3.86 2.96 2.07 - 1.44 1.88 3.52
a The electrostatic energy terms are D/R where R is the interatomic distance in ,~ units. C H X 2, X 2 H C - X C H - C H X 2, X 2 H C - C X 2 - C H X 2 , ( X H 2 C ) 2 C ( C H 3 ) 2 a n d ( X H 2 C 4 ) C . All o f these molecules have X = chlorine, many have X = bromine and a smaller number have X = fluorine. S t r u c t u r a l p a r a m e t e r v a l u e s for 25 m o l e c u l e s o b t a i n e d by o u r M M c a l c u l a t i o n s h a v e b e e n systema t i c a l l y c o m p a r e d w i t h the c o r r e s p o n d i n g experim e n t a l v a l u e s [8] w h e n X = c h l o r i n e . G e n e r a l l y the a g r e e m e n t b e t w e e n o b s e r v a t i o n s a n d t h e calculated v a l u e s lies w i t h i n the e x p e r i m e n t a l e r r o r limits. E n e r g y m i n i m a w e r e f o u n d by a d j u s t i n g all structural parameters simultaneously, but one or b o t h t o r s i o n a l angles w e r e h e l d c o n s t a n t in finding the e n e r g i e s o f the t r a n s i t i o n f o r m s . T h e S i - C H 2 X a n d S i - C H X 2 g r o u p s possess Cs s y m m e t r y , while the c e n t r a l C - S l Y 2 C g r o u p possess C2v s y m m e t r y in o u r m o d e l o f the m o l e c u l e s . T h e utilized m o l e c u l a r - m e c h a n i c s c o m p u t e r p r o g r a m , w h i c h uses a c o m b i n a t i o n o f s t e e p e s t - d e s c e n t and Newton-Raphson m e t h o d s , w a s w r i t t e n by S. R u s t a d [9].
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
279
Table 5 MM results for FH2C-SiH2 CH2F and F H z C - S i F 2 - C H z F molecules Parameter
FH2C-SiH2-CH2F AA
Torsional a angles (deg): q~12: ~23: Energies b: (kcal/mol): Torsional force constantsC: Fo12: F~23: Conformational composition (%)d:
AG
FH2C-SiF2-CH2F GG
AG
GG
GG"
180.0 180.0
180.0 -60.0
44.8 44.8
50.2 -50.2
180.4 -55.3
1.7
0.9
0
0.001
4.7
2.7
0
0.2
0.028 0.028
0.049 0.039
0.041 0.041
0.037 0.037
0.040 0.040
0.038 0,048
0.047 0.047
0,049 0 049
0
1
18
41
57.8 -57.8
AA
180.0 180.0
0
53.5 53.5
GG"
41
57
42
a See Table 2. h Relative to G G (E = 0). c Diagonal values in mdynA(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
3. Results for individual molecules Table 6 Torsional barrier heights in XH2C-SiY2-CH2X and X2HC SiY2-CHX~ molecules with X = F, C1, Br and Y = H, X Molecule
Conformational change
F
CI
Br
XH2C-SiH2-CH2X
AA ~ AG AG ~ GG" GG" ---* AA AG ~ G G G G ~ AA
1.1 1.3 4.1 1.1 4.0
1.0 0.4 3.0 1.1 5.5
1.0 0.5 1.8 1.2 5.2
XH2C SiX2-CH2X
AA ---* AG AG ~ GG" GG" ~ AA AG ~ G G G G ~ AA
1.0 0.9 6.7 0.9 6.8
3.6 3.6 9.9 4.1 12.9
3.8 3.9 9.6 4.5 13.9
X2HC-SiHz-CHX 2
AA ~ AG AG ~ GG" GG" ~ AA AG ~ GG G G ~ AA
2.2 2.0 2.6 2.0 2.8
2.6 4.2 10.6 4.9 14.4
2.1 (_)a (_)a 5.8 17.7
X2HC-SiX2 CHX2
AA ~ AG AG ---* GG" GG" ~ AA AG ---, G G G G ~ AA
2.4 2.5 2.0 2.5 2.1
5.6 6.9 15.4 7.4 19.0
6.1 7.5 15.9 8.4 21.7
a GG" is not a conformational minimum for Br2HC-SiH z CHBr 2.
Several halogen-substituted propanes, being analogues to the silanes studied in this work, have already been studied by gas-phase electron diffraction. Results for the corresponding propanes will be compared with the results for molecules treated in this work in the summary at the end of this paper.
3.1. FHeC-SiH2-CH2F and FH2C-SiF2-CH2F Results for these molecules are recorded in Table 5. Clearly the low-energy forms in both cases are G G and GG". The G G " conformer possesses one F 1 - . . F 3 parallel interaction in both molecules (Table 2). At room temperatures the gas-phase compositions of the molecules ought to be about 80-100% of G G and G G " conformers. These conformers both have approximately staggered terminal groups relative to the central group. However, in F H z C - S i F 2 - C H 2 F the deviations from exactly staggered form are 15° in the torsional angles. The values of the torsional force constants are not very different in the two molecules. Values of the rotational barrier heights (Table 6) are obtained in the ranges (kcal mo1-1) 1.1-4.1 and
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Table 7 M M results for CIH2C-SiH2-CH2C1 and CIH2C SiCI 2 CH2CI molecules Parameter
CIH2C Sill 2 CH2CI AA
Torsional a angles (deg): q~12: q~23: Energiesb: (kcal/mol): Torsional force constantsC: F~,12: F~,23: Conformational composition (%)d:
180.0 180.0 3.2
AG
CIH2C-SiCI2-CH2CI GG
181.1 -57.7 1.8
0.030 0.030
0.046 0.063
0
9
55.5 55.5 0 0.116 0.116 89
GG"
63.7 -63.7 2.2
AA
180.0 180.0 6.1
AG
GG
GG"
177.1 -49.4 3.4
49.7 49.7 0
64.0 -64.0 2.9
-
0.160 0.160
0.120 0.160
2
0
1
0.190 0.190 98
0.130 0.130 1
a See Table 2. b Relative to G G (E = 0). c Diagonal values in mdynA(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
0.9-6.8 for F H 2 C - S i H 2 - C H 2 F and F H 2 C - S i F 2 CHzF respectively.
3.2. CIH2C-SiH2-CH2CI and CIH2C-SiCI2CH2CI Results are recorded in Table 7. The low-energy conformer is G G in both molecules. In this conformer parallel X1 ... X3 interactions are not present. Deviations from exactly staggered form are 5-10 ° in the torsional angles of GG. In the gas phase at about room temperature the contributions of G G conformers alone ought to be about 90-99%. For CIHzC-SiH2-CHzC1 the G G " form, possessing one parallel CI1..-C13 interaction, is not a stable conformer, while for C1HzC SiCI2-CHzCI the G G " conformer corresponds to a minimum on the potential energy surface. The values of the torsional force constants for the tetrachlorosilane are significantly larger than for the dichlorosilane. The ranges of rotational barrier heights (kcal mol-l), involving G G in the transitions, are 1 5 and 4-13 for the dichloro- and tetrachlorosilanes, respectively (Table 6).
3.3. BrH2C-SiH2 CH2Br and BrH2C-SiBr2CH2Br Results are recorded in Table 8. In both cases the only low-energy conformer is GG. For
dibromosilane G G " is not a stable form. Deviations from exactly staggered conformations are about 9 ° in tetrabromosilane, while for dibromosilane the deviations are less than 1° in the torsional angles. At about room temperature the gases ought to consist of 80-99% G G conformers. The values of the torsional force constants are significantly larger in the tetrabromosilane than in the dibromosilane. Rotational barrier heights (kcal mol-l), involving the G G conformer, were obtained in the regions 1-5 and 4 14 for the dibromosilane and the tetrabromosilane, respectively. Thus, the values of these barrier heights are approximately equal in these compounds and the corresponding chlorocompounds.
3.4. F2HC-SiH2-CHF 2 and F2HC-SiF2CHF2 Results are recorded in Table 9. For these molecules AA, possessing two parallel F1 .. • F3 interactions, is the low-energy conformer. The AA conformer have exactly staggered terminal groups relative to the central group, corresponding to C2v symmetry. At room temperature the gases ought to consist of about 70-90% of the AA conformers alone. Torsional force constants in the tetrafluorosilane have larger values than in the hexafluorosilane, when the AA conformer is considered. Rotational barrier heights involving the AA conformer, were
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T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
Table 8 MM results for BrH2C-SiH2-CH2Br and BrHzC-SiBr2 CH2Br molecules Parameter
BrH2C-SiH2-CH2Br AA
Torsional a angles (deg): ~b12:
~23: Energiesb: (kcal/mol): Torsional force constantsC: F~,j2:
180.0 180.0 2.9
F4,23:
0.047 0.047
Conformational composition (%)d:
0
AG
BrH2C-SiBr2-CH2 Br GG
179.6 --60.3 1.5 0.043 0.063 14
GG"
59.2 59.2 0
66.0 --66.0 2.9
0.110 0.110 85
AA
180.0 180,0 6.7
AG
GG
GG"
179.0 -48.0 3.4
51.0 51.0 0
61.0 -61.0 4.0
-
0.120 0.120
0.119 0.093
1
0
1
0.220 0.220 99
0.150 0.150 0
a See Table 2. b Relative to GG (E = 0). c Diagonal values in mdyn.A(rad-2). d Boltzmann distribution including the multiplicities of Table 2. o b t a i n e d in the r e g i o n 2 - 3 k c a l m o l - l . T h u s , these b a r r i e r s h a v e a p p r o x i m a t e l y e q u a l h e i g h t s in the two molecules.
3.5. CI2HC-SiH2-CHCI2 and CI2HC-SiCI2-
cI-ICb R e s u l t s are r e c o r d e d in T a b l e 10. T h e l o w - e n e r g y f o r m o f t e t r a c h l o r o s i l a n e is A G , w h i l e for h e x a c h l o r o s i l a n e A A , A G a n d G G are l o w - e n e r g y
f o r m s . I n b o t h m o l e c u l e s G G " is a h i g h - e n e r g y c o n f o r m e r c o n t r i b u t i n g less t h a n 1% to the gasp h a s e c o m p o s i t i o n s . E s s e n t i a l l y t h e gas o f the t e t r a c h l o r o s i l a n e o u g h t to be a t w o - c o n f o r m e r m i x ture o f A G a n d G G , w h i l e f o r t h e h e x a c h l o r o s i l a n e the gas is a t h r e e - c o n f o r m e r m i x t u r e o f A A , A G and GG. T o r s i o n a l f o r c e c o n s t a n t s in t e t r a c h l o r o s i l a n e h a v e significantly s m a l l e r v a l u e s t h a n t h o s e o f hexachlorosilane. The rotational barrier heights
Table 9 MM results for F2HC-SiHz-CHF 2 and FzHC SiF2-CHF 2 molecules Parameter
F2HC-SiH 2 CHF 2 AA
AG
F2HC-SiF2-CHF 2 GG
ASe
AA
AG
GG
GG"
180.0 180.0 0
173.8 -69.5 2.0
73.0 73.0 4.0
70.0 -70.0 4.1
Torsional" angles (deg):
(~12: ~23: Energiesb: (kcal/mol): Torsional force constantsC: F~2: F~23: Conformational composition (%)d:
180.0 180.0 0 0.065 0.065 72
175.4 -67.0 1.5 0.061 0.044 25
70.2 70.2 2.7
64.1 -64.1 2.8
0.042 0.042
0.046 0.046
2
1
a See Table 2. b Relative to AA (E = 0). c Diagonal values in mdyn~,(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
0.012 0.012 88
0.071 0.048 12
0.047 0.047
0.047 0.047
0
0
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Table 10 M M results for C12HC SiH2-CHC12 and CI2HC-SiC12 CHC12 molecules Parameters
CI2HC-SiH2CHC12 AA
AG
C12HC-SiC12 CHC12 GG
GG"
AA
AG
GG
GG"
164.0 -59,9 0.1
65.9 65.9 0.2
54.4 -54.4 3.8
Torsional a angles (deg):
q~12: ~z3: Energiesb: (kcal/mol): Torsional force constantsC: F¢I2: Fe~23: Conformational composition (%)d:
180.0 180.0 0.9
158.6 - 51.4 0
0.065 0.065 4
0.153 0.090 79
64.2 64.2 0.5
43.0 -43.0 2.3
0.097 0.097
0.049 0.049
16
1
180.0 180.0 0 0.232 0.232 17
0.305 0.265 59
0.265 0.265 24
0.259 0.259 0
a See Table 2. b Relative to A G (E = 0). c Diagonal values in mdyn,~(rad-2). d Boltzmann distribution including the multiplicities of Table 2.
of these molecules (Table 6) have values in the regions 3-14 and 6-19 kcal mol 1 for tetrachlorosilane and hexachlorosilane, respectively. 3.6. Br2HC-SiH2-CHBr2 and Br2HC-SiBr2CHBr 2 Results are recorded in Table 11. For tetrabromosilane AS and G G are low-energy conformers, while G G " is not a stable form of this molecule. In hexabromosilane G G is the low-energy form. The
gas of tetrabromosilane, at room temperature, is a rather complex mixture of AG, G G and AS. The gas-phase of hexabromosilane is a mixture of AA, A G and GG. In both molecules the deviations from exactly staggered conformation might be as large as 24 ° . However, AA has exactly staggered conformation in both molecules. Values of the torsional force constants are much larger in hexabromosilane than in tetrabromosilane. Rotational barrier heights (kcal tool -1) were obtained in the region
Table 11 M M results for Br2HC-SiH2 CHBr2 and B r 2 H C - S i B r 2 - C H B r 2 molecules Parameters
Br2HC-SiH2-CHBr 2 AA
Torsional a angles (deg): 0~2: 4~23: Energiesb: (kcal/mol): Torsional force constantsC: F,12: F~23: Conformational composition ( % f :
180.0 180.0 3.1 0.044 0.044 0
AG
Br2HC SiBr2-CHBr 2 GG
156.0 -44.5 0.6 0.133 0.080 30
60.0 60.0 0.2 0.094 0.094 29
a See Table 2. b Relative to AS (E = 0) and G G (E = 0). c Diagonal values in mdynA.(rad-2). d Boltzmann distribution including the multiplicities of Table 2. e GG" is not a conformational m i n i m u m for BrzHC S i H 2 - C H B r 2.
GG"
180.0 0.0 0 0.138 0.015 41
AA
180.0 180.0 0.7 0.182 0.182 8
AG
GG
GG"
163.0 -58.5 0.7
64.7 64.7 0
57.0 -57.0 5.7
0.239 0.186 36
0.320 0.320 56
0.379 0.379 0
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284 Table 12 S u m m a r y of low-energy conformers a in X H 2 C - S i Y 2 CH2X and X 2 H C - S i Y 2 - C H X 2 molecules, Y = H or X Molecule
X = F
X = CI
X = Br
XH2C-SiH2-CH2 X XH2C SiX2-CH2X
GG, G G " GG, G G "
GG GG
GG GG
X2HC S i H 2 - C H X 2 X2HC-SiX2-CHX 2
AA AA
AG, G G AA, A G , G G
AS, G G GG
a See Table 2.
2-18 and 6-22 for tetrabromosilane and hexabromosilane, respectively.
283
Table 14 Torsional frequencies (wl and ~2) and torsional force constants (Fe) in the G G conformer of XH2C-ZY2-CH2X and X 2 H C Z Y 2 - C H X 2 molecules, X = chlorine, Y = H or X and Z = Si or Ca Molecules and values: F~ co 1 ( - r - 0 - ) b c d 2 ( + - q - ) c (mdynA,(rad) -2 (cm 1) (cm-X) F,(C-C)/F,(Si-C) XHzC-ZH2-CH2 X XH2C Z X 2 - C H 2 X X2HC ZH 2 C H X 2 X2HC Z X 2 - C H X 2
0.17/0.12 0.32/0.19 0.24/0.10 0.46/0.27
60/46 72/49 57/21 75/51
145/111 121/82 87/32 77/53
a Values for Z = C are taken from ref. [14]. b C2 symmetry.
c C~ symmetry.
4. Summary and discussion Conformational possibilities for the symmetrically substituted silanes XHzC-SiYz-CH2X and XzHC-SiYz-CHX 2 have been summarised in Table 12. For these molecules the staggered conformers GG, AG, AA and GG" as well as AS (with one eclipsed terminal group) have to be considered. For all molecules, except FzHC-SiYz-CHF 2, GG is a low-energy form and AA is a high energy form. However, for FzHC-SiYz-CHF 2 molecules AA is a low-energy conformer (see Table 9). Only for FH2C-SiYz-CH2F molecules is GG" a low-energy form (Table 5), and only for Br2HC-SiH2-CHBr 2 is AS a low-energy form (Table 11). The most complex conformational mixture should exist for C12HC-SiC12-CHC12, including AA, AG and GG conformers in the gas phase (Table 10).
Gas-phase electron diffraction results exist for 1,3-dihalo(X)propanes: F [4], Cl [10] and Br [11]. The conformational compositions at about room temperature, for these molecules and the corresponding silanes treated in this work, are recorded in Table 13. Clearly GG is the abundant conformer in all six molecules. AA is present only in small amounts, while AG is present in detectable amounts. Only in FH2C-SiH2-CH2F is GG" a low-energy form, while in FH2C-CH2-CH2F gas-phase experiment shows that the form GG" is present with about 10% contribution [4]. Thus, the following simplified rule may be remembered: GG is a low-energy conformer and AA is a high-energy form in these molecules. Diffraction data for CI2HC-CH2-CHC12 showed [12] that the gas-phase at 62°C existed essentially of only GG conformers, while for the CI2HC-SiH2-CHC12 molecule AG is the low-
Table 13 Gas-phase conformational compositions a o f X H 2 C - S i H 2 CH2X and X H 2 C - C H 2 - C H 2 X molecules C1H2C-ZH2-CH2CI
BrH2C-ZH2-CH2Br
Molecule
FH2C-ZH2-CH2F
Conformer b
Z = C [4]
Z = Sic
z = c [1o]
Z = Sic
Z=C[ll]
Z = Sic
GG AG AA GG"
63 27 0 10
41 l8 0 41
73 24 3 0
89 9 0 2
67 30 3 0
85 14 0 1
a At about room temperatures. b See Table 2. c This work.
284
T.H. Johansen, R. Stolevik/Journal of Molecular Structure 372 (1995) 275-284
energy form according to our calculations (Table 10). The electron diffraction data [13] for C12HCCCI2-CHCI2 showed that at 112°C the gas-phase existed essentially of G G conformers. For the corresponding silane, ClzHC-SiClz-CHC12, the calculations predicted that the gas is a mixture of AG, G G and AA conformers. Barrier heights corresponding to rotation of one of the X H z C - or X2HC-groups around the Si-C bond have values in the region 0.9-2.5 kcal mo1-1 for fluoro-substituted molecules, while for the heavy-halogen substituted molecules this region is 0.4-8.4 kcal mol 1. Generally the rotational barriers in C - S i - C molecules are lower than the barriers in the corresponding C C - C molecules. Thus, the barrier heights in X H 2 C - S i H 2 - C H 2 X (X = C1) are close to 1.0 kcal mo1-1 while in X H 2 C C H 2 - C H 2 X ( X = C1) the values are close to 4.0 kcal mo1-1 [5]. However, the barrier heights found in X 2 H C - S i H 2 - C H X 2 and X 2 H C - C H 2CHX 2 (X = C1) overlap with values in the ranges 2 - 4 ( C - C - C ) [3] and 3-5 ( C - S i - C ) kcal tool -1 (see low-barrier height values in Table 6). Torsional force constants in the molecules treated in this work have their values in the regions: 0.03-0.07 (fluorides), 0.03-0.31 o(chlorides) and 0.05-0.38 (bromides) in mdynA(rad) -2 units. Using the approximation formulas established earlier [14] the wavenumbers of the fundamental low-frequency oscillations for the G G conformer of the chlorosilanes have been estimated and compared with the corresponding chloropropanes in Table 14. Values of the torsional force constants and wavenumbers corresponding to the chloropropanes are taken from Ref. [14]. The ranges of wavenumbers (cm -1) are 57-75 (c~l) and 77-145 (co2) for the
chloropropanes, while for the chlorosilanes these ranges are 21-51 (col) and 32-111 (a~2). Thus, in order to establish experimentally the torsional frequencies of the molecule C12HC-SiH2-CHC12 one has to detect wavenumbers as low as 20-30 cm a according to our estimates (Table 14). In conclusion, we have found that in the silanes XH2C-SiY2-CH2X and X 2 H C - S i Y 2 - C H X 2 as well as in the corresponding propanes X H 2 C CY 2 CH2X and X 2 H C - C Y 2 - C H X 2 (Y = H or X) the staggered GG, AG, G G " and AA conformers have to be expected in the gas phase. Only for B r z H C - S i H 2 CHBr2 is an eclipsed form (AS) expected to be a low-energy conformer.
References [1] R.J. Abraham and R. Stolevik, Chem. Phys. Lett., 77 (1981) 181. [2] R. St~levik, J. Mol. Struck (Theochem), 109 (1984) 397. [3] T. Rydland and R. Stolevik, J. Mol. Struct., 105 (1983) 157. [4] P. Kl~eboe, D.L. Powell, R. Stolevik and gl. Vorren, Acta Chem. Scand., Ser. A, 36 (1982) 471. [5] L. Postmyr, J. Mol. Struct., 319 (1994) 211. [6] R. Stolevik and K. Hagen, Ab initio calculations of conformational structures and energies in chlorosubstituted ethanes and propanes, J. Mol. Struck, 352/353 (1995) 23. [7] R.T. Sanderson, J. Inorg. Chem., 28 (1966) 1553. [8] R. Stolevik, 3. Mol. Struct., 291 (1993) 301. [9] S. Rustad, Thesis in Chemistry, University of Oslo, Oslo, 1973. [10] S. Grindheim and R. Stolevik, Acta Chem. Scand., Ser. A, 30 (1976) 625. [11] P.E. Farup and R. Stolevik, Acta Chem. Scand., Ser. A, 28 (1974) 680. [12] M. Braathen, D.H. Christensen, P. Klzeboe, R. Seip and R. Stolevik, Acta Chem. Scand., A33 (1979) 437. [13] L. Fernholt and R. Stolevik, Acta Chem. Scand., A29 (1975) 651. [14] R. Stolevik and P. Bakken, J. Mol. Struck, 244 (1991) 27. [15] R. Stolevik, Acta Chem. Scand., A28 (1974) 455.