Structural and conformational analysis of 1-monofluorosilacyclobutane and 1-monochlorosilacyclobutane. A gas-phase electron diffraction and ab initio investigation

Article abstract of DOI:10.1016/S0022-2860(00)00725-0

The structures of numerous 1,1-disubstituted silacyclobutanes have been investigated thoroughly, but none of the monosubstituted representatives have been studied as yet. In the present work the geometric structures and conformational equilibria of 1-monofluorosilacyclobutane (MFSCB) and 1-monochlorosilacyclobutane (MCSCB) have been investigated by means of gas-phase electron diffraction and ab initio calculations. The conformational analysis reveals that both molecules are present with both equatorial and axial conformations. Consistent with one another, the electron diffraction data and the ab initio results show that the equatorial conformers of MFSCB and MCSCB are more stable than the higher energy axial forms. The experiments demonstrate that the equatorial conformers of MFSCB and MCSCB are lower in energy than the axial conformers by 4.30 (0.21) kJ/mol (corresponding to eq: ax = 85: 15 (5)) and 3.92 (0.23) kJ/mol, (eq: ax = 83: 17 (6)), respectively. For comparison, ab initio calculations at the MP/6-31G(d,p) level predict energy differences of 6.04 and 3.43 kJ/mol, respectively, in favor of the equatorial forms of MFSCB and MCSCB. During the structural refinement it was assumed that all of the structural parameters except the puckering angle Θ for both the equatorial and axial conformers are equal. This assumption was supported by the ab initio calculations. The major (r_a) bond distances and bond angles which were obtained from the final refinement of the experimental data are (with uncertainties of 3σ) for MFSC: r(Si-C) = 1.855(1) A?, r(Si-F) = 1.592(2) A?, r(C-H) = 1.089(3) A?, ∠(H-Si-F) = 106.8(6)°, ∠(C-Si-C) = 80.8(6)°, ∠(C-C-C) = 98.6(19)° and the puckering angles Θ_eq = 37.4(20)° and Θ_ax = 23.5(70)°. For MCSCB the following structural parameters were obtained: r(Si-C) = 1.864(2) A?, r(Si-Cl) = 2.059(3) A?, r(Si-H) = 1.470(12) A?, r(C-C) = 1.591(5) A?, r(C-H) = 1.112(4) A?, ∠(H-Si-Cl = 106.0(6)°, ∠(C-Si-C) = 80.7(14)°, ∠(C-C-C) = 98.7(22)°. The puckering angles were found to be Θ_eq = 34.2(25)° and Θ_ax = 21.5(50)°. The observed simultaneous reduction of the Si-C and the Si-F(Cl) bonds can be examined by electrostatic arguments and other concepts such as bond polarity, negative hyperconjugation and carbon(2pπ)-silicon(3pπ) orbital overlap. By applying various ab initio methods such as HF/6-31G(d,p), MP2/6-31G(d,p) and DFT/b3pw91/6-31G(p) the structures and conformations of mono- and dihalogenated silacyclobutanes of the type (CH₂)₃SiY (Y = HF, HCl, HBr, HI, H₂, F₂, Cl₂, Br₂ and I₂) have been investigated. Our results show that there is a regular increase of the preferability of the equatorial conformer with the increase of the electronegativity of the halogen atom. This finding is consistent with the correlation which was postulated earlier by Jonvik and Boggs [4-7] for monosubstituted cyclobutanes. In order to gain more insight regarding the influence of the electronegativity of the substituent on the degree of strain in silacyclobutanes and for the purpose of comparison of the structures of different mono- and dihalogenated acyclic silanes and silacyclic compounds of larger size, as silacyclopentane and silacyclohexane have been also computed.

Full text of DOI:10.1016/S0022-2860(00)00725-0

Journal of Molecular Structure 559 (2001) 7–24
www.elsevier.nl/locate/molstruc
Structural and conformational analysis of
1-monofluorosilacyclobutane and 1-monochlorosilacyclobutane.
A gas-phase electron diffraction and ab initio investigation
*
M. Dakkouri , M. Grosser
¨
¨
Abteilung fur Elektrochemie, Universitat Ulm, Albert-Einstein-Alle 11, D-89069 Ulm, Germany
Received 22 May 2000; accepted 26 May 2000
Abstract
The structures of numerous 1,1-disubstituted silacyclobutanes have been investigated thoroughly, but none of the mono-
substituted representatives have been studied as yet. In the present work the geometric structures and conformational equilibria
of 1-monofluorosilacyclobutane (MFSCB) and 1-monochlorosilacyclobutane (MCSCB) have been investigated by means of
gas-phase electron diffraction and ab initio calculations. The conformational analysis reveals that both molecules are present
with both equatorial and axial conformations. Consistent with one another, the electron diffraction data and the ab initio results
show that the equatorial conformers of MFSCB and MCSCB are more stable than the higher energy axial forms. The
experiments demonstrate that the equatorial conformers of MFSCB and MCSCB are lower in energy than the axial conformers
by 4.30 (0.21) kJ/mol (corresponding to eq : ax ˆ 85 : 15 (5)) and 3.92 (0.23) kJ/mol, ꢀeq : ax ˆ 83 : 17 (6)), respectively. For
comparison, ab initio calculations at the MP/6-31G(d,p) level predict energy differences of 6.04 and 3.43 kJ/mol, respectively,
in favor of the equatorial forms of MFSCB and MCSCB. During the structural refinement it was assumed that all of the
structural parameters except the puckering angle u for both the equatorial and axial conformers are equal. This assumption was
supported by the ab initio calculations. The major (ra) bond distances and bond angles which were obtained from the final
ꢀ
ꢀ
refinement of the experimental data are (with uncertainties of 3s) for MFSC: rꢀSi–C† ˆ 1:855ꢀ1† A; rꢀSi–F† ˆ 1:592ꢀ2† A;
ꢀ
rꢀC–H† ˆ 1:089ꢀ3† A; ЄꢀH–Si–F† ˆ 106:8ꢀ6†Њ; ЄꢀC–Si–C† ˆ 80:8ꢀ6†Њ; ЄꢀC–C–C† ˆ 98:6ꢀ19†Њ and the puckering angles
ꢀ
ueq ˆ 37:4ꢀ20†Њ and uax ˆ 23:5ꢀ70†Њ: For MCSCB the following structural parameters were obtained: rꢀSi–C† ˆ 1:864ꢀ2† A;
ꢀ
ꢀ
ꢀ
ꢀ
rꢀSi–Cl† ˆ 2:059ꢀ3† A; rꢀSi–H† ˆ 1:470ꢀ12† A; rꢀC–C† ˆ 1:591ꢀ5† A; rꢀC–H† ˆ 1:112ꢀ4† A; ЄꢀH–Si–Cl† ˆ 106:0ꢀ6†Њ;
ЄꢀC–Si–C† ˆ 80:7ꢀ14†Њ; ЄꢀC–C–C† ˆ 98:7ꢀ22†Њ: The puckering angles were found to be ueq ˆ 34:2ꢀ25†Њ and uax
ˆ
21:5ꢀ50†Њ: The observed simultaneous reduction of the Si–C and the Si–F(Cl) bonds can be examined by electrostatic argu-
ments and other concepts such as bond polarity, negative hyperconjugation and carbon(2pp)–silicon(3pp) orbital overlap.
By applying various ab initio methods such as HF/6-31G(d,p), MP2/6-31G(d,p) and DFT/b3pw91/6-31G(p) the structures
and conformations of mono- and dihalogenated silacyclobutanes of the type (CH2)3SiY ꢀY ˆ HF; HCl, HBr, HI, H2, F2, Cl2, Br2
and I2) have been investigated. Our results show that there is a regular increase of the preferability of the equatorial conformer
with the increase of the electronegativity of the halogen atom. This finding is consistent with the correlation which was
postulated earlier by Jonvik and Boggs [4–7] for monosubstituted cyclobutanes. In order to gain more insight regarding the
influence of the electronegativity of the substituent on the degree of strain in silacyclobutanes and for the purpose of comparison
* Corresponding author. Tel.: ϩ49-731-502-2873; fax: ϩ49-731-502-2872.
E-mail address: marwan.dakkouri@chemie.uni-ulm.de (M. Dakkouri).
0022-2860/01/$ - see front matter ᭧ 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00725-0

8
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
of the structures of different mono- and dihalogenated acyclic silanes and silacyclic compounds of larger size, as silacyclo-
pentane and silacyclohexane have been also computed. ᭧ 2001 Elsevier Science B.V. All rights reserved.
Keywords: 1-Monofluorosilacyclobutane; 1-Monochlorosilacyclobutane; Structural and conformational analysis; Correlation between electro-
negativity of the susbtituent and conformational stability
1. Introduction
paper, we present the results of the structural and
conformational analysis for 1-monofluorosilacyclo-
butane (MFSCB) and 1-monchlorosilacyclobutane
(MCSCB) obtained from electron diffraction and ab
initio studies. A comparison of the results of the struc-
tural analysis of MFSCB and MCSCB should provide
valuable information with respect to the role of elec-
tronegativity and size of the substituent by the deter-
mining of the conformational stability in these
molecules.
As a continuation of our work to clarify the bonding
behavior of silicon, we recently prepared some sila-
cyclobutane derivatives of the type: (CH2)3SiHX
(where X ˆ F, Cl, Br) and also (CH2)3Si(CxCH)2 in
order to study their structures and dynamical behavior
[1–3]. One of the main objectives for studying these
novel organosilicon compounds is to investigate the
particular sensitivity of the Si–C bond to the effects of
substituents and whether the variation of this bond as
a result of substitution represents a systematic beha-
vior which can be utilized to predict some chemical
and physical properties of molecules containing
silicon. Another reason for investigating these
compounds is to examine whether the predicted corre-
lations [4–7] between the electronegativity of the
substituent in various monosubstituted pure cyclo-
butanes and the conformational preferability of these
molecules are applicable to heterocyclobutanes.
According to the ab initio studies by Jonvik and
Boggs [4–7], the equatorial conformer becomes
increasingly favored with an increase in the electro-
negativity of the substituent. It is interesting to note
that the conformational distribution as well as the
geometrical parameters in most of the cyclobutane
compounds we have studied [8–14] are in excellent
agreement with the rules postulated by Jonvik and
Boggs (from now on J ϩ B) [4–7].
The fluorine atom with its pronounced electro-
negativity and relatively small size (covalent radius
˚
of 0.64 A) is one of the most interesting substituents
which gives rise to dramatic changes in structures and
energetics of molecules. On the other hand, the
chlorine atom is considered to be a moderate p-elec-
tron donor and distinct s-electron acceptor but is
˚
larger in size (covalent radius of 0.99 A). Attaching
silicon to fluorine or chlorine leads in most cases to an
increase of bond ionicity, i.e. the formation of ionic
structures such as Y3SiϩXϪ and subsequently to a
shortening of both the Si–X and Si–Y bonds. In this
context, it is of particular interest to investigate the
structural consequences of the direct attachment of
fluorine or chlorine to silicon when the latter is incor-
porated in a strained frame such as a four-membered
ring. There are reasons to anticipate that the strain
energy will be the limiting factor for probable drastic
geometrical changes which might be induced by the
halogen atom.
It has been long established that the silacyclobutane
system is one of the most important precursors for the
formation of doubly bonded silicon as in silenes and
the silicon analogues of carbenes, silylenes [27].
Moreover, the stability of the halogen–silicon bond
has been the focal point of numerous papers. One of
the main reasons for the importance of these investi-
gations is to understand the mechanisms of chemical
vapor deposition [28,29] and plasma etching [30,31]
using systems containing Si–F or Si–Cl [32]
fragments.
A large number of substituted silacyclobutanes are
known and some have been the subject of numerous
investigations using
a variety of experimental
methods including thermochemistry [15–18], spec-
troscopy [3,19–20] and electron diffraction [21–26].
While the structures and dynamics of 1,1-halogenated
silacyclobutanes have been studied by applying
different experimental techniques, no monosubtituted
derivatives have been investigated so far. Therefore,
we synthesized the monohalogenated derivatives in
order to investigate their structural peculiarities in
comparison to the dihalogenated compounds. In this

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
9
Fig. 1. Atomic numbering for the equatorial (left) and axial (right) conformers of MFSCB and MCSCB.
2. Ab initio calculations
3. Experimental
The optimization of the structural parameters of the
equatorial and axial conformers of MFSCB and
MCSCB (for atomic numbering see Fig. 1) was carried
out using several quantum mechanical computational
methods, a restricted Hartree–Fock applying 6-31Gءء
basis set, perturbation theory MP2/6-31Gءء [33–35],
and density functional (DFT) hybrid b3pw91/6-31Gء
[36–38]. All calculations were performed with the
gaussian-98 [39] and spartan 5.0 [40] programs.
In order to calculate the vibrational mean ampli-
tudes we performed a normal coordinate analysis for
both conformers of MFSCB and MCSCB. The calcu-
lations were based on the force fields provided by the
HF/6-31Gءء method. The quadratic force fields in
Cartesian displacement coordinates were transferred
to the force fields in internal coordinates by applying
the method of Zhao and Krimm [41]. The scaling
factors applied to the ab initio force field in order to
reproduce the experimental frequencies were 0.90 for
stretching, 0.80 for bending, and 1.00 for the torsional
coordinates.
Many attempts have been undertaken to synthesize
MFSCB by the selective fluorination of only one Si–
H linkage using a large variety of mild fluorination
agents, but these could not afford the desired product.
Finally, the following three-step route has led to
MFSCB. Silacyclobutane was prepared by the reduc-
tion of 1,1-dichloro-1-silacyclobutane with lithium
aluminum hydride according to the method of Laane
[42]. A sample of extremely diluted silacyclobutane
was then treated with anhydrous SnBr4 in diethylether
at about 0ЊC provide 1-bromo-1-silacyclobutane.
Finally, addition of 1-bromo-1-silacyclobutane to
anhydrous ZnF2 in dibutylether at 5–10ЊC afforded
MFSCB (30%): bp 35.0–35.5ЊC.
We synthesized MCSCB using a two-steps route,
which differs from the method reported earlier by
Harthcock et al. [43]. Silacyclobutane was first
prepared according to the method of Laane [42].
The resulting silacyclobutane was then diluted with
anhydrous n-butyl ether and treated with freshly
distilled SnCl4 in n-butyl ether at room temperature
to provide 1-chloro-1-silacyclobutane (23%): bp
83.5–85ЊC/711 Torr, nD20 ˆ 1:4575: The purity of
the product was checked by means of IR-spectroscopy
and mass spectrometry.
For the purpose of comparison and for a more
reasonable discussion of the reasons leading to the
variation of the bond lengths and valence angles
associate with the silicon atom within the title
compounds we calculated the geometries of a variety
of related ring and open chain molecules (see below)
using the same methods and basis sets we employed in
the case of MFSCB and MCSCB.
The gas-phase electron diffraction photographs were
recorded on Kodak electron image plates using a
Balzers KD-G2 diffractometer. Two sets of data for
each compound were obtained at approximate camera

10
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
Fig. 2. Experimental ( × ) and calculated (—) reduced molecular intensity curves sءM(s) for MFSCB.
Fig. 3. Experimental ( × ) and calculated (—) reduced molecular intensity curves sءM(s) for MCSCB.

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
11
Table 1
Experimental geometrical parameters (ra-structure) and vibrational amplitudes (lij) of 1-fluorosilacyclobutane (MFSCB)
˚
lij (A)
Equatorial
Axial
a
(eq)exp
(eq)calc.
˚
Bond distances (A)
Si–C
Si–F
Si–H
C–C
C–H
1.855(1)
0.063(2)
0.051(2)
0.080(14)
0.053b
0.052
0.040
0.087
0.057
0.077
1.592(2)
1.472b
1.586b
1.089(3)
0.090(5)
Bond angles (Њ)
H–Si–F
C–Si–C
C–C–Si
C–C–C
H–C–H
uc
106.8(6)
Si···C3
Si···H7,8
C2,4···H5
0.082(6)
0.115(17)
0.160b
0.067
0.115
0.134
0.099
0.111
0.093
0.177
0.148
0.202
0.131
80.8(6)
85.3b
98.6(19)
110.0b
37.4(20)
5.2b
C
2,4···F
0.102(5)
0.158(33)
0.076(8)
0.150b
C3,4···H7,8
C3···F
F···H9
F···H10
F···H7,11
F···H8,12
23.5(70)
14.0b
15.0b
7.0b
d
g1
g2
14.6b
0.120b
e
g3
%(eq)
6.6b
0.202b
f
85(5)
0.135b
a
Ab initio values using HF/6-31Gءء.
Not refined.
Puckering angle.
Rocking angle of the H–Si–F group.
Rocking angle of the H–C2–H group.
b
c
d
e
f
Rocking angle of the H–C3–H group. Uncertainties in parentheses are 3s.
distances of 25 and 50 cm and with an accelerating
voltage of 60 kV, yielding a total intensity range
our usual procedures which have been described else-
where [45,46]. Figs. 2 and 3 show the molecular inten-
sities for MFSCB and MCSCB in the s range of
ꢀϪ1
from s ˆ 2 to s ˆ 18 A for the short camera distance
ꢀϪ1
ꢀϪ1
and s ˆ 8 to s ˆ 33 A for the long camera distance
s ˆ 2–33 A : The atomic scattering amplitudes and
ꢀϪ1
in intervals of Ds ˆ 0:2 A : The samples were kept at
phases of Haase [47] were used.
Ϫ60ЊC (MFSCB) and Ϫ25ЊC (MCSCB). The inlet
system and nozzle were at room temperature. ZnO
diffraction patterns were used for the calibration of the
electron wavelength providing l250 ˆ 0:048883
4. Structural analysis and discussion
ꢀ
ꢀ
^0:000040 A; l500 ˆ 0:048745 ^ 0:000022 A in the
The experimental results for the structural analysis
of MFSCB and MCSCB are displayed in Tables 1 and
2 and the final radial distribution curves resulting from
the experiment and the fitted models are shown in
Figs. 4 and 5. The present study shows that both
MFSCB and MCSCB exist in two stable conformers
in the gas-phase with the equatorial conformer being
predominant. This finding is consistent with the
conformational preferability predicted by all quantum
mechanical methods we applied in this study. In the
case of MFSCB only eight geometrical parameters
ꢀ
case of MFSCB and l250 ˆ 0:048950 ^ 0:000153 A;
ꢀ
l500 ˆ 0:048777 ^ 0:000071 A in the case of
MCSCB. Three plates from each camera distance
(MFSCB) and two plates at 25 cm camera
distance and three plates from the 50 cm camera
distance (MCSCB) were traced on our computer-
controlled and modified ELSCAN, E-2500 (Optronics
International, Chelmsford, MA, USA) [44] with data
being recorded at intervals of 0.1 mm. Data reduction
and least-squares refinements were carried out using

12
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
Table 2
Experimental geometrical parameters (ra-structure) and vibrational amplitudes (1ij) of 1-chlorosilacyclobutane (MCSCB)
˚
lij (A)
Equatorial
Axial
a
(eq)exp
(eq)calc.
˚
Bond distances (A)
Si–C
Si–Cl
Si–H
C–C
C–H
1.864(2)
0.064(1)
0.059(2)
0.087b
0.053
0.048
0.087
0.056
0.077
2.059(3)
1.470(12)
1.591(5)
1.112(4)
0.056b
0.086(5)
Bond angles (Њ)
H–Si–Cl
C–Si–C
C–C–Si
C–C–C
H–C–H
uc
106.0(6)
Si···C3
0.088(5)
0.155b
0.067
0.117
0.135
0.107
0.197
0.117
0.276
0.146
0.223
0.143
80.7(14)
Si···H7,8
C2,4···H5
C2,4···Cl
C2,4···H11,7
C3···Cl
85.0b
0.160b
98.7(22)
110.0b
34.2(25)
0.0b
0.122(6)
0.197b
21.5(50)
9.6b
0.103(11)
0.375b
d
g1
g2
Cl···H9
e
10.0b
10.0b
7.9b
Cl···H10
Cl···H7,11
Cl···H8,12
0.246b
g3
% (eq)
7.9b
0.223b
f
83(6)
0.143b
a
Ab initio values using HF/6-31Gءء.
Not refined.
Puckering angle.
Rocking angle of the H–Si–F group.
Rocking angle of the H–C2–H group.
b
c
d
e
f
Rocking angle of the H–C3–H group. Uncertainties in parentheses are 3s.
Fig. 4. Experimental ( × ) and calculated (—) radial distribution curves for MFSCB.

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
13
Fig. 5. Experimental ( × ) and calculated (—) radial distribution curves for MCSCB.
and nine vibrational mean-amplitudes could be
refined whereas for MCSCB ten geometrical para-
meters and six vibrational mean-amplitudes were
fitted. For the purpose of more systematic and reason-
able discussion of the structural results we will discuss
first the endocyclic bond distances in MFSCB and
MCSCB and then the bond angles and exocyclic
bond lengths.
ment of the silicon atom to strong electronegative
atoms such as fluorine and chlorine and for reasons
pointed out in the introduction, we will focus first on
the consideration of the alternation of the Si–C bond
in mono and geminally halogenated silacyclobutanes
and some related molecules. In accordance with the
expectation the Si–C bond in MFSCB and MCSCB
(Tables 1 and 2) shortens in comparison to the parent
molecule silacyclobutane, SCB, (Table 5). This bond
˚
length reduction of 0.040 and 0.031 A, respectively, is
substantial. The experimentally determined value for
the Si–C bond in methylsilane is 1.864 A [48] and it
5. Endocyclic parameters
˚
shortens on going to methylmonofluorosilane and
Comparison between the ring parameters in
MFSCB and MCSCB (Tables 1 and 2) reveals that
both the Si–C and C–C bonds in the former molecule
˚
˚
dimethyl-difluorosilane by 0.017 A [49] and 0.028 A
[50], respectively. Although the ab initio calculations
at all levels of theory which we employed in this work
show the same tendency for the shortening of the Si–
C bond on monohalogenation, this bond contraction is
significantly smaller (Tables 3 and 4) than provided
by the experiment. Another interesting feature which
emerges from Tables 3 and 4 is that the Si–C bond
distance predicted by the ab initio calculations is
always overestimated. In several previous papers,
we alluded to this peculiarity and discussed it in detail
˚
are shorter by about 0.009 and 0.005 A, respectively.
This slight bond shortening, which is also reproduced
by the ab initio calculations (Tables 3 and 4), is less
than might be anticipated on the basis of the difference
in the electronegativities of fluorine and chlorine.
Probably the ring strain and the larger puckering
angle of the ring in the case of MFSCB are responsible
for this feature. Because of the presence of a Si–C
bond in a strained four-membered ring and the attach-

Table 3
Comparison between bond distances (A) and bond angles (Њ) of various mono- and disubstituted silacyclobutanes (CH2)3SiXY using MP2/6-31G
ءء
˚
X ˆ Y ˆ H
XY ˆ HCl
XY ˆ HF
X ˆ Y ˆ CL
X ˆ Y ˆ F
X ˆ Y ˆ CH3
Equatorial
Axial
1.8830
Equatorial
Axial
1.8787
Si–C
1.8957
1.8806
2.0648
1.4752
1.8750
1.6224
1.4769
1.8714
1.8612
1.9000
Si–F(Cl)
Si–H
2.0683
1.4762
1.6254
1.4783
Ͻ 2.0559 Ͼ
Ͻ 1.6122 Ͼ
Ͻ 1.4800 Ͼ a
Si–Cmeth
C–C
C–H
Ͻ 1.8840 Ͼ
1.5557
1.5557
Ͻ 1.0900 Ͼ
108.4
78.0
86.3
100.2
108.7
107.5
25.1
1.5591
Ͻ 1.0894 Ͼ
106.9
79.1
1.5584
Ͻ 1.0898 Ͼ
106.4
79.1
1.5607
Ͻ 1.0890 Ͼ
106.8
79.5
1.5599
Ͻ 1.0899 Ͼ
106.0
79.5
1.5616
Ͻ 1.0891 Ͼ
108.4
80.4
85.5
101.3
109.2
108.1
22.4
1.5654
Ͻ 1.0880 Ͼ
106.0
81.5
85.3
101.8
108.9
108.1
20.5
X–Si–X
(C–Si–C)ring
C–C–Si
C–C–C
H–C2–H
H–C3–H
Si–C–C–C
C–Si–C–C
H,Cl,F–Si(CSiC)plane
109.8
77.9
86.6
100.3
108.4
107.2
24.3
85.3
86.4
85.0
86.5
100.4
109.1
107.6
25.8
21.2
131.0
100.6
108.8
107.8
22.4
18.4
120.0
100.4
109.0
107.5
26.2
21.5
131.0
100.8
108.5
107.7
21.2
17.5
121.2
20.5
18.5
17.0
19.8
(131.6)eq
(120.0)ax
33.0
(131.0)eq
(120.7)ax
30.0
(131.0)eq
(123.0)ax
27.5
(129.8)eq
(120.3)ax
32.0
ub
34.3
80.0
2.2
29.6
2.3
35.0
92.0
1.7
28.0
1.8
%eq
mt (Debye)
0.3
2.7
2.4
0.56
a
Ͻ ···Ͼ : Average values.
Puckering angle.
b

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
15
Table 4
˚
Comparison between bond distances (A) and bond angles (Њ) of various mono- and disubstituted silacyclobutanes (CH2)3SiXY from DFT
calculations using b3pw91/6-31Gء
X ˆ Y ˆ H
XY ˆ HCl
XY ˆ HCl
X ˆ Y ˆ CL
X ˆ Y ˆ F
Equatorial
Axial
1.8880
Equatorial
Axial
1.8841
Si–C
1.9000
1.8854
2.0812
1.4885
1.8810
1.6189
1.4922
1.8763
1.8668
Si–F(Cl)
Si–H
C–C
2.0834
1.4902
1.6215
1.4944
Ͻ 2.0714 Ͼ
Ͻ 1.6088 Ͼ
Ͻ 1.4943 Ͼ a
1.5600
1.5630
1.5625
1.5643
1.5643
1.5663
1.5702
C–H
Ͻ 1.0950 Ͼ
Ͻ 1.0930 Ͼ
Ͻ 1.0950 Ͼ
Ͻ 1.0950 Ͼ
Ͻ 1.0960 Ͼ
Ͻ 1.0930 Ͼ
Ͻ 1.0930 Ͼ
H–Si–H(F,Cl)
C–Si–C
107.9
78.6
106.1
79.7
105.6
79.8
106.5
80.0
105.6
80.2
107.9
81.2
105.6
82.1
C–C–Si
86.7
85.7
87.1
85.5
87.1
86.1
85.9
C–C–C
H–C2–H
H–C3–H
101.0
108.4
107.2
101.2
108.8
107.3
101.7
108.4
107.5
101.0
108.7
107.3
101.7
108.1
107.3
102.4
108.8
107.8
102.7
108.5
107.8
Si–C–C–C
C–Si–C–C
21.9
17.9
22.7
18.6
17.3
14.2
23.4
19.3
16.5
13.5
16.9
13.9
15.2
12.5
H,Cl,F–Si(CSiC)plane
(131.0)eq
(121.0)ax
130.2
121.8
130.3
122.7
(129.4)eq
(122.7)ax
(129.8)eq
(124.6)ax
ub
28.8
0.6
30.2
79.9
2.4
23.0
2.4
31.2
92.0
1.7
21.7
1.7
22.5
2.9
20.3
2.4
%eq
mt (Debye)c
a
Ͻ ···Ͼ : Average values.
Puckering angle.
Total dipole moment.
b
c
[51–54]. It is noteworthy that the hybrid DFT method
b3pw91/6-31Gء seems to overestimate the Si–C bond
rather more than the perturbation method MP2/6-
31Gءء.
mono to the geminally chlorinated compound. This
striking bonding behavior will be discussed more
later.
Perhaps the most interesting finding of the present
work is the observed correlation between the steadily
shortening of the Si–C bond with the increase of the
electronegativity of the substituent. Scheme 1 visua-
lizes this systematic behavior of the Si–C bond in
correlation with the electronegativity, x, of the sub-
stituents. In Scheme 1, the following abbreviations
are used: silacyclobutane (SCB), 1,1-dimethylsila-
cyclobutane (DMSCB), 1,1-diethynylsilacyclobutane
(DESCB), 1,1-dichlorosilacyclobutane (DCSCB) and
1,1-difluorosilacyclobutane (DFSCB). The electrone-
gativities of H, CH3, –CxC, Cl and F are 2.08 [56],
2.56 [57], 2.66 [57], 3.07 [56], and 4.00 [56], respec-
tively.
As may be anticipated, the replacement of both
hydrogen atoms on silicon by fluorine or chlorine
atoms leads to an increased contraction of the Si–C
bond. Such bonding behavior is apparent from the
comparison between the mono and geminally haloge-
nated silacyclobutanes in Tables 3–5. The experimen-
tally determined reduction of the Si–C bond length
upon going from MFSCB (the equatorial form has
been chosen for comparison since it is prevailing in
the conformational equilibrium) to the difluoro deri-
˚
vative is about 0.02 A. This bond length shortening is
predicted by MP2/6-31G(d,p) and DFT/b3pw91/6-
˚
31G(d) methods to be about 0.014 A. Although the
Si–C bond in the chlorinated counterpart shows the
same tendency, it decreases only slightly (by about
0.004 A as provided by the experiment and 0.009 A
by the ab initio calculations) upon moving from the
When we presented Scheme 1 for the first time at
the VIIth European Electron Diffraction Meeting in
Prague in 1997 we included the literature value of
˚
˚
˚
1.886(4) A for the Si–C bond in DCSCB reported

16
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
Table 5
˚
Comparison between some experimental bond distances (A) and bond angles (Њ) in various mono- and disubstituted silacyclobutanes
(CH2)3SiXY
(X ˆ Y ˆ H)a
(XY ˆ HF)b
(XY ˆ HCl)b
(X ˆ Y ˆ F)c
(X ˆ Y ˆ CL)d
X ˆ Y ˆ CH3
e
Si–Cring
Si–F
1.895(2)
1.855(1)
1.592(2)
1.864(2)
1.836(3)
1.574(3)
1.860(3)
1.885(2)
Si–Cl
Si–H
2.059(3)
1.470f
2.040(2)
1.496(18)
1.472f
Si–Cmeth
C–C
C–H
X–Si–X
(C–Si–C)ring
C–C–Si
C–C–C
uh
1.872(2)
1.563(4)
1.115(3)
109.9(47)
79.2(11)
86.8g
1.607(6)
1.143(9)
115.6(90)
80.8(5)
85.1(5)
98.6(5)
1.586f
1.089(3)
106.8(6)
80.8(6)
85.3g
1.591(5)
1.112(4)
106.0(6)
80.7(14)
85.0g
1.574(8)
1.099(6)
106.9(5)
82.7(6)
86.8(8)
100.6(8)
25.0(20)
1.557(4)
1.091(8)
105.2(8)
81.1(10)
85.7(12)
102.0(15)
25.9(26)
98.6(19)
37.4(20)
98.7(22)
34.2(22)
99.9g
33.6(2.1)
29.7(45)
a
Ref. [21].
b
This work (equatorial form).
Ref. [24].
Ref. [25].
c
d
e
f
Ref. [26].
Not refined value.
Calculated value.
Puckering angle.
g
h
by Cyvin et al., in 1986 [22]. We showed that this
value did not fit within this systematic behavior and
thus it had to be incorrect. Professor Lev Vilkov, who
was present in the audience, mentioned that the struc-
ture of DCSCB had been reinvestigated, and he
(where R ˆ –CH3, CH3–CH2– and X ˆ H, Cl, F).
These calculations, which have been carried out at
the same level of theory as MFSCB and MCSCB,
were necessary to gain additional information about
the consequences of the incorporation of silicon into
four-membered ring systems and the effect of the ring
strain on the Si–C bond lengths within the title
compounds when compared to unstrained alkyl deri-
vatives and rings of larger size.
Tables 6 and 7 for the monohalogenated silanes
demonstrate the following interesting features (i)
The HF/6-31Gءء and DFT/b3pw91/6-31Gء methods
overestimate the Si–C bond more than the MP2/6-
31Gءء method. This is also true for the Si–Cl bond.
˚
provided the new value of 1.860(3) A for the Si–C
bond length. This fits perfectly into Scheme 1. This
demonstrates once more the advantage of systematic
studies for predicting trends in molecular properties.
In order to obtain as complete a comparison as
possible, we optimized the structures of some related
unstrained compounds of the types R2SiHX and
R2SiX2 as well as other types of strained silacyclic
compounds of the type: (CH2)4SiHX and (CH2)5SiX2
Scheme 1
SCB Ͼ
DMSCB Ͼ
DESCB Ͼ
MCSCB Ͼ
DCSCB Ͼ
MFSCB Ͼ
DFSCB
1.895(2)a
1.878(2)b
1.874(2)c
1.864(2)d
1.860(3)e
1.855(2)d
1.836(3)f
˚
Si–C(A)
a
Ref. [21].
b
c
d
e
f
Ref. [26].
Ref. [55].
Ref. (this work).
Ref. [25].
Ref. [24].

Table 6
Structural parameters of some dialkylsilanes of the type R2SiHX (R ˆ CH3, CH3–CH2 and X ˆ Cl, F) as obtained from ab initio calculations
R ˆ CH3
R ˆ CH3–CH2
X ˆ F
IIb
X ˆ Cl
IIb
X ˆ F
IIb
X ˆ Cl
IIb
Ia
IIIc
1.8670
1.6282
1.4777
Ia
IIIc
1.8701
2.0744
1.4756
Ia
IIIc
1.8714
1.6321
1.4809
Ia
IIIc
1.8760
2.0778
1.4792
Si–C
Si–X
Si–H
1.8722
1.6043
1.4766
1.8702
1.6238
1.4917
1.8750
2.0861
1.4731
1.8734
2.0892
1.4883
1.8773
1.6074
1.4788
1.8769
1.6274
1.4942
1.8816
2.0901
1.4753
1.8814
2.0950
1.4900
X–Si–H
C–Si–C
106.9
112.3
106.9
112.1
107.0
111.6
106.0
112.5
105.9
112.6
106.5
112.0
106.4
112.4
106.4
112.4
106.5
112.6
105.6
111.9
105.5
112.4
106.3
112.7
a
HF/6-31Gءء.
b
c
DFT(b3pw91/6-31Gء).
MP2/6-31Gءء.

Table 7
Structural parameters of 1-fluorosilacyclopentane (MFSCP), 1-chlorosilacyclopentane (MCSCP), 1-fluorosilacyclohexane (MFSCH) and 1-chlorosilacyclohexane (MCSCH)
obtained from ab initio calculations
MFSCPa
IId
MCSCPa
IId
MFSCHb
IId
MCSCHb
IId
Ic
IIIe
1.8704
1.6260
1.4783
Ic
IIIe
1.8735
2.0682
1.4765
Ic
IIIe
1.8693
1.6283
1.4799
Ic
IIIe
1.8727
2.0720
1.4779
Si–C
Si–X
Si–H
1.8737
1.6017
1.4762
1.8759
1.6224
1.4924
1.8762
2.0790
1.4730
1.8787
2.0845
1.4886
1.8744
1.6037
1.4777
1.8746
1.6242
1.4937
1.8775
2.0831
1.4743
1.8779
2.0884
1.4898
X–Si–H
C–Si–C
106.3
94.7
106.2
94.6
106.6
93.8
105.9
94.6
105.8
94.6
106.8
93.6
106.7
105.6
106.7
105.6
107.0
105.1
106.0
105.6
106.0
105.7
106.8
105.1
a
Cs-symmetry (fluorine atom in equatorial position).
Chair form (fluorine atom in equatorial position).
HF/6-31Gءء.
b
c
d
e
DFT(b3pw91/6-31Gء).
MP2/6-31Gءء.

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
19
Table 8
Selected structural parameters of various cyclic silanes (SCP ˆ silacyclopentane, MFSCP ˆ 1-monofluorosilacyclopentane, DFSCP ˆ 1,1,-
difluorosilacyclopentane, MCSCP ˆ 1-monochlorosilacyclopentane, DCSCP ˆ 1,1,-dichlorosilacyclopentane, SCH ˆ silacyclohexane,
MFSCH ˆ 1-monofluorosilacyclohexane, DFSCH ˆ 1,1-difluorosilacyclohexane, MCSCH ˆ 1-monochlorosilacyclohexane, DCSCH ˆ 1,1-
dichlorosilacyclohexane) obtained from ab initio calculations using MP2/6-31Gءء
SCP
MFSCPa
DFSCP
1.8678
MCSPa
DCSCP
1.8771
SCH
MFSCHb
DFSCH
1.8554
MCSCHb
DCSCH
1.8633
Si–C
Si–H
Si–X
1.8860
1.4791
1.8704
1.4783
1.6260
1.8735
1.4765
2.0682
1.8861
1.4809
1.8693
1.4799
1.6283
1.8727
1.4779
2.0720
1.6156
2.0605
1.6187
2.0653
H–Si–H
X–Si–H
X–Si–X
C–Si–C
108.6
108.2
106.6
93.8
106.8
93.6
107.0
105.1
106.8
105.1
105.7
98.3
108.0
97.6
106.1
107.5
108.3
106.6
92.5
104.2
a
Cs-Symmetry (halogen atom in equatorial position).
Chair form (halogen atom in equatorial position).
b
(ii) Despite being overestimated, the values for the
Si–F bond lengths produced by the DFT/b3pw91/6-
31Gء and MP2/6-31Gءء calculations are consistent
with each other but differ from the values predicted
at the HF/6-31Gءء level. (iii) While the HF/6-31 Gءء
and MP2/6-31Gءء methods provide consistently
similar Si–H bond lengths for all compounds listed
in Tables 6 and 7, the DFT/b3pw91/6-31Gء method,
however, produces unreasonably long Si–H bond
distances. (iv) There is almost no difference between
the values predicted by the various methods for the
bond angles around the silicon atom.
upon monofluorination and by 2.9Њ upon difluorina-
tion. This correlation between the X–Si–X angle and
C–Si–C angles at silicon may be interpreted in terms
of a large charge withdrawal from the central silicon
atom which leads to higher bond polarity and orbital
rehybridization. This charge redistribution is also
responsible for the shortening of the Si–C and Si–X
bonds with the increase of the degree of halogenation.
The values presented in Tables 9 and 10 show the
effect of successive chlorination and fluorination on
the most prominent structural parameters in silacyclo-
butane and the unstrained dimethyl- and diethylsilane
From Table 7 it can be seen that all the trends
observed for the dialkyl compounds are also present
for the monohalogenated cyclic compounds silacyclo-
pentane (SCP) and silacyclohexane (SCH). Table 8
compares the influence of successive halogenation
on the geometrical parameters around silicon in the
less strained cyclic systems SCP and SCH. The
following conclusions can be drawn: (1) The general
trend of Si–C and Si–F(Cl) bond reduction upon
progressive halogen substitution is present. (2) Except
for the SCB series, the C–Si–C valence angle is sensi-
tive to mono- and dihalogenation at silicon. For
instance, this angle widens on monofluorination by
1.3Њ and on difluorination by 5.8Њ. In SCH this
widening amounts to 1.1Њ and 3.3Њ on mono and
difluorination, respectively. (3) The X–Si–X bond
angle reduces simultaneously with the increase of
the C–Si–C angle upon progressive halogenation.
For example, this angle decreases in SCP by 2Њ
Table 9
Comparison between some calculated (MP2/6-31Gءء: SCB ˆ sila-
cyclobutane; MCSCB ˆ 1-monochlorosilacyclo-butane; DCSCB ˆ 1,1-
˚
dichlorosilacyclobutane) bond distances (A) and bond angles (Њ) in
SCB, MCSCB, DCSCB and various alkylsilanes and their chlori-
nated derivatives
Si–C
Si–Cl
X–Si–X C–Si–C
SCB
MCSCB
DCSCB
1.8957
(1.8806)eq
1.8714
108.4
106.9
78.6
79.0
80.4
2.0648
Ͻ 2.0559 Ͼ a 108.4
(CH3)2SiH2
(CH3)2SiHCl
(CH3)2SiCl2
1.8829
1.8701
1.8655
107.6
106.5
107.7
111.0
112.0
114.4
2.0744
2.0693
(C2H5)2SiH2
(C2H5)2SiHCl 1.8760
(C2H5)2SiCl2
1.8884
107.4
106.3
107.4
112.4
112.7
112.9
2.0778
2.0733
1.8736
a
Average value.

20
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
Table 10
support that the contraction of the Si–C bond results
from overlap between 2pp orbitals on carbon and 3pp
orbitals on silicon [66]. This overlap is enhanced by
the increase of the positive charge on silicon upon
fluorine substitution and the consequent contraction
of the 3pp(Si) orbital, thus providing a better match
to the size of a 2pp(C) orbital. Obviously this kind of
rationalization explains why the Si–C bond contrac-
tion is more pronounced upon fluorination on silicon
than upon chlorination.
Comparison between some calculated (MP2/6-31Gءء: SCB ˆ sila-
cyclobutane; MFSCB ˆ 1-monofluorosilacyclobutane; DFSCB ˆ 1,1-
˚
difluorosilacyclobutane) bond distances (A) and bond angles (Њ) in
SCB, MFSCB, DFSCB and various alkylsilanes and their fluorinated
derivatives
Si–C
Si–F
X–Si–X C–Si–C
SCB
MFSCB
DFSCB
1.8957
(1.8750)eq
1.8612
108.4
106.8
78.6
79.5
81.5
1.6224
Ͻ 1.6122 Ͼ a 106.0
Table 5 shows that in contrast to the calculated
values presented in Tables 3 and 4, the experimentally
determined C–C bond lengths in silacyclobutanes are
reduced in going from the mono- to the dihalogenated
compounds and do not possess any systematic beha-
vior. In contrast, the theoretically predicted values for
the C–C bond lengths increase successively with
increasing electronegativity of the substituents and
thus with increasing positive charge at silicon. The
consequence of such charge deficiency at silicon is a
charge transfer from the C–C bond region towards
silicon and, therefore, the elongation of this bond.
Electron releasing substituents such as the methyl
groups in dimethylsilacyclobutane (Table 5) seem to
have no effect on the ring C–C bond.
(CH3)2SiH2
(CH3)2SiHF
(CH3)2SiF2
1.8829
1.8670
1.8584
107.6
107.0
105.7
111.0
111.6
115.2
1.6280
1.5916
(C2H5)2SiH2
(C2H5)2SiHF
(C2H5)2SiF2
1.8884
1.8710
1.8636
107.4
106.5
105.3
112.4
112.6
114.9
1.6320
1.5947
a
Average value.
species. From Table 9 it can also be seen that mono-
and dichlorination lead to a systematic reduction of
the Si–C bond length, as expected. However, while
the bond shortening in going from mono- to dichlor-
osilacyclobutane is about 0.0092 A, this shortening
amounts only to 0.0046 and 0.0024 A in going from
˚
˚
the mono- to dichloro derivatives of dimethylsilane
(DMS) and diethylsilane (DES), respectively. The
contraction of the Si–C bond upon monochlorination
6. Bond angles and exocyclic bond distances
˚
of SCB, DMS and DES of 0.015, 0.013 and 0.012 A is
comparable in all these compounds. It is also note-
worthy that the Si–C bond in SCB is longer than in
DMS and DES by about 0.013 and 0.007 A, respec-
Because of the well known correlations between
the endocyclic and exocyclic angles in silacyclobu-
tanes [67], we will not discuss these angles separately.
It is accepted that while the alternation of bond
lengths as a result of successively increasing atom
˚
tively. This bond elongation might be accounted for as
being the result of the ring strain. In all compounds
shown in Table 9 the Si–Cl bond shortens only
slightly in going from the mono- to dichloro deriva-
tives.
As is apparent from Table 10, the shortening of the
Si–C bond is more pronounced in the case of the
fluorinated derivatives than in their chlorinated coun-
terparts. This bond contraction, which is substantiated
by the experiment and parallels the trend found in the
chlorinated compounds, might be rationalized by
invoking various concepts. Among those which are
frequently used in this regard are s–p-hyperconjuga-
tion [58–60], negative hyperconjugation (anomeric
effect) [61–64] and electronegativity criteria and
bond ionicity [65]. Most recent work lends strong
or group electronegativity generally shows
a
systematic behavior and variations of bond angles,
however, demonstrate a less regular tendency upon
substitution (at least within the group IV) and often
defy expectations. The main reasons for this behavior
are most likely the rehybridization of the orbitals on
the central atom and the subsequent deviation from
orthogonality and the increased bond ionicity as a
result of the electronegative substituents.
Tables 3–5 demonstrate that endocyclic Si–C–C
and C–C–C valence angles are little affected by halo-
genation at silicon. More intriguing is the alternation
of the C–Si–C and X–Si–X (X ˆ H, Cl) bond angles.
As found experimentally (Table 5), these angles show

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
21
almost no variation in going from the mono- to the
dihalogenated silacyclobutanes except in the case of
the fluoro derivatives. The difluorination widens the
C–Si–C angle by about 1.9Њ. Of particular interest is
the behavior of the X–Si–X and C–Si–C bond angles
within the series of the compounds shown in Tables 9
and 10. The calculations at MP2/6-31Gءء level indi-
cate that the X–Si–X angle decreases on monochlor-
ination but increases upon dichlorination to produce
almost the H–Si–H angle in the unchlorinated silane.
On the other hand, the C–Si–C angle widens on
successive chlorination of SCB and DMS but it
remains almost unchanged in the case of DES. As
can be seen in Table 9, this angle increases by 1.8Њ
upon going from SCB to DCSCB but this widening
amounts to 3.4Њ on dichlorination of DMS. Therefore,
the smaller enlargement of the C–Si–C angle in the
case of DCSCB might be accounted for by the
increase of the strain energy within the ring as a result
of the shortening of the Si–C bond.
diffraction and ab initio values for the Si–F bond
length in MFSCB (Tables 3–5) reveals that the
MP2/6-31Gءء method overestimates this bond by
ء
˚
about 0.030 A and the DFT/b3pw91/6-31G by
˚
about 0.027 A. As is evident from the experimental
results (Table 5), this bond shortens by about 0.018 A
˚
upon difluorination. The ab initio methods (Tables 3
˚
and 4) predict a value of about 0.01 A for this bond
reduction. The Si–F bond shortening due to difluor-
˚
ination is appreciably higher, about 0.037 A, in the
acyclic compounds shown in Table 10. Unlike the
situation with the Si–F bond, the calculated (MP2/6-
31Gءء) and the experimental values for the Si–Cl
bond in MCSCB are in good agreement. The DFT/
b3pw91/6-31Gء method (Table 4), however, over-
˚
estimates this bond by more than 0.02 A. The dichlor-
˚
ination leads to an Si–Cl bond contraction of 0.019 A,
which parallels the value for the Si–F bond shortening
on difluorination. Although the ab initio methods
which have been applied in this work reflect this
tendency of bond shortening but they provide a
It should be noted that the experimentally deter-
mined [21] H–Si–H angle and C–C bond length in
SCB have not been considered by the discussion of the
experimental results for the following reasons: The
differences between the experimentally determined
˚
smaller value of about 0.01 A.
As was the case for the Si–C bond, several expla-
nations may be used to elucidate the Si–F and Si–Cl
bond reduction upon dihalogenation. All of these are
based on electrostatic considerations, and these
simply imply that the increased shrinkage of the
silicon atomic radius as a consequence of charge with-
drawal by means of the electronegative substituents (F
and Cl) result in shortening of all bonds at silicon.
Invoking negative hyperconjugation may explain
this Si–F(Cl) bond shortening as well.
˚
value for the C–C bond of 1.607 (6) A [21] and the
˚
theoretically predicted one of 1.556 A is clearly larger
than the experimental uncertainty. Another striking
feature between the experimental and calculated
results is that the values for the Si–C bond within
the molecules listed in Tables 3 and 4 are overesti-
˚
mated by the theory on average by 0.017 to 0.020 A
except in the case of SCB where the overestimation
˚
amounts only to 0.001 A. This indicates that the
˚
experimental value of 1.895(2) A is too high. If it
7. Conformational stability of MFSCB and
MCSCB
can be accepted that the average off-set value which
we obtained for MP2/6-31Gءء method for the Si–C
˚
bond of 0.017 A is also valid for SCB, this would lead
to an effective value of 1.878 A which seems to be
Both the electron diffraction experiments as well as
the ab initio calculations carried out in this work have
shown that MFSCB and MCSCB occur in both the
equatorial and axial forms with the equatorial
conformer being more stable. From the experiment
we obtained an eq:ax ratio of 85:15(5) for MFSCB
and 83:17(6) for MCSCB. These values agree fairly
well with those of 84:16(14) [2] and 71:29(21) [1]
which we obtained from the microwave spectra
some years ago. Both the MP2/6-31Gءء and
b3pw91/6-31Gء methods predicted 92% and 80%
˚
reasonable. Further discrepancies between theory and
experiment are evident from the values for the H–Si–
H and C–Si–C angles. All in all, an experimental
reinvestigation of the structure of SCB is warranted
and this would benefit from the knowledge of the
theoretical values we obtained in this work.
We now consider the behavior of the Si-halogen
bond and its variation as a consequence of the geminal
halogenation of SCB. Comparison of the electron

22
M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
Table 11
˚
Comparison between bond distances (A) and bond angles (Њ) in monobromo-, monoiodo-, dibromo- and diiodo silacyclobutanes (CH2)3SiXY
XY ˆ HBra
XY ˆ HIb
X ˆ Y ˆ Bra
X ˆ Y ˆ Ib
Equatorial
Axial
Equatorial
Axial
Si–C
Si–X
Si–H
C–C
1.8866
2.2248
1.4891
1.5623
1.8890
1.8897
2.4593
1.4897
1.5838
1.8905
1.8785
Ͻ 2.2172 Ͼ
1.8845
Ͻ 2.4546 Ͼ
2.2269
1.4911
1.5617
2.4600
1.4915
1.5813
1.5637
1.5828
H–Si–X
X–Si–X
C–Si–C
C–C–Si
C–C–C
H–C2–H
H–C3–H
106.0
105.8
106.8
106.7
110.5
80.5
85.3
101.8
109.2
108.0
111.4
80.7
85.9
101.1
109.8
109.2
79.4
85.3
101.0
109.0
107.4
79.6
86.9
101.4
108.4
107.5
80.4
86.4
100.8
109.5
108.6
79.9
86.6
100.3
109.5
109.1
Si–Xax (CSiC)plane
122.5
24.7
119.6
27.7
118.3
131.2
29.2
118.7
130.0
27.2
Si-Xeq(CSiC)plane
132.3
32.8
78.0
129.6
26.7
56.0
uc
%eq
a
DFT (b3pw91/6-31Gء).
b
DFT(b3pw91/3-21Gء).
c
Puckering angle.
(Tables 3 and 4) for the equatorial form for MFSCB
and MCSCB, respectively. The increased stability of
the equatorial conformer with the increase of the elec-
tronegativity of the substituent is in full agreement
with the J ϩ B rules [4,5] which predict a linear rela-
tionship between the preferability of the equatorial
conformer and the electronegativity of the substituent
in monosubstituted cyclobutanes.
The behavior of the puckering angle u is particularly
interesting. As is apparent from Tables 1 and 2, the
observed puckering angle for the axial conformer uax
is smaller than that for the equatorial conformer ueq by
about 12.7Њ and 13.9Њ for MCSCB and MFSCB, respec-
tively. Also ab initio calculations (Tables 3 and 4)
predict that uax is smaller than ueq for both molecules
(5Њ and 7Њ for the chloro and fluoro compounds, respec-
tively). Considering the fairly large uncertainties asso-
ciated with the experimental values, the agreement is
satisfactory. Particularly interesting is the increase of
the puckering angle in going from SCB to (MCSCB)eq
to (MFSCB)eq and its subsequent narrowing in going to
the dihalogenated derivatives, DCSCB and DFSCB
(Table 5). Similarly, the calculated value for the puck-
ering angle reaches its maximum for (MFSCB)eq and
then drops down for DCSCB and DFSCB (Tables 3
and 4). While the increase of ueq and the decrease of
uax in going from SCB to MCSCB and MFSCB may be
roughly rationalized by invoking the J ϩ B rules, the
further decrease of the puckering angle on dihalogena-
tion is surprising. As we saw earlier, the ring Si–C bond
shortens uniformly in this series. Thus, the irregular
behavior of u is not easy to elucidate. A possible expla-
nation is that the disubstitution at silicon with strong
electronegative atoms (chlorine and fluorine) leads to
a substantial increase of the positive charge at silicon
affecting the Si–C bond polarity. As a result, a charge
flow from the carbon atom opposite to silicon towards
the highlypositive centeroccurs, andthe consequence is
an accentuated 1,3-repulsion. To reduce the enhanced
Dunitz–Schomaker strain [68–70] the ring flattens and
the puckering angle becomes smaller.
For the purpose of more general and complete
comparisonswealsooptimizedthegeometriesofmono-
bromosilacyclobutane (MBSCB), dibromosilacyclobu-
tane (DBSCB), monoiodosilacyclobutane(MISCB) and
diiodosilacyclobutane (DISCB) using the DFT/
b3pw91/6-31Gء basis set for the bromo derivatives
and the same method with the 3-21Gء basis set for the

M. Dakkouri, M. Grosser / Journal of Molecular Structure 559 (2001) 7–24
23
iodo compounds. Table 11 summarizes the most impor-
tant structural results obtained from these calculations.
These results, in addition to those which are shown in
Table 4 (where the same DFT method was applied),
clearly point to the general systematical Si–C bond
reduction with increasing electronegativity of the
substituent. As can be seen from Table 11, the Si–C
higher stability of the equatorial conformer with the
increase of the electronegativity of the substituent paral-
lels the trend which was reported several years ago by
J ϩ B for a variety of monosubstituted cyclobutanes
[4,5].
In conclusion, the present electron diffraction study
has shown the following. First, the Si–C bond length in
MFSCB and MCSCB shortens appreciably (by 0.040
˚
bond shortens by 0.0332, 0.0237, 0.0215 and 0.0155 A
˚
in going from SCB to DFSCB, DCSCB, DBSCB and
DISCB, respectively. Simultaneously, the Si–Br and
Si–I bonds shorten upon dihalogenation. These findings
signify once more the role of the electrostatic effects
which imply the withdrawal of electrons by means of
the electronegative substituents and the subsequent
decrease in the silicon atomic radius leading to a short-
ening of all bonds associated with the central atom.
From Tables 4 and 11 it can also be seen that the
exocyclic X–Si–X bond angle continues to widen and
the endocyclic C–Si–C bond angle continues to
decrease in DBSCB and DISCB. Another feature
emerges from comparing the puckering angle u in all
silacyclobutanes displayed in Tables 4 and 11. Namely,
in monohalogenated silacyclobutanes the puckering
angle for the equatorial conformers is larger than the
puckering angle in the parent molecule SCB but it is
appreciably smaller for the axial forms. Additionally,
while ueq does not show any systematic behavior in
correlation with the increased electronegativity of the
substituent, the puckering angle uax, however, increases
uniformly with decreasing electronegativity of the
halogen atom. It is also noteworthy that the puckering
angle for the dihalogenated compounds drops down
drastically in comparison with the corresponding mono-
halogenated ones. The ring puckering angle, however,
increases steadily from DFSCB to DBSCB. This
increase of the ring puckering angle is probably due to
the decrease of the electronegativity of the substituent
(from F to Br) and thus the weakening of the 1,3 repul-
sion within the ring. It should be pointed out that the
values for the iodo compounds which are displayed in
Table 11 are not appropriate for the comparisons with
the remaining data in Tables 4 and 11 since they have
been obtained by using the basis set 3-21Gء. This basis
set was used since no higher basis set is available for
iodine. Finally, from Tables 4 and 11 it is seen that there
is evidently a systematical correlation between the elec-
tronegativity of the halogen and the preferability of the
equatorial conformer for the silacyclobutanes. The
and 0.031 A, respectively) when compared with the
parent molecule SCB. Second, the Si–F(Cl) bond
contracts upon dihalogenation. Both of these may be
explained using bonding concepts like negative hyper-
conjugation and 3pp(Si)–2pp(C) orbital overlap.
However, these features are most reasonably explicable
by use of electrostatic arguments. Third, the experi-
˚
mental C–C bond length of 1.607(6) A [21] in SCB is
appreciably longer than the theoretically calculated
˚
value of 1.556 A. Furthermore, the Si–C bond of
˚
1.895(2) A is evidently long. This discrepancy signifies
that a reinvestigation of the structure of SCB is recom-
mended. Fourth, the present study has shown that the
higher the electronegativity of the substituent the more
stable the equatorial conformer is which is in accord
with the J ϩ B rules. Fifth and finally, the silacyclobu-
tane ring flattens upon proceeding from the mono- to
dihalogenated derivatives and the puckering angle u
achieves its minimum for DFSCB. This remarkable
behavior is explained by the enhanced Dunitz–Scho-
maker strain (1,3 repulsion) as a result of the increase
of the positive charge on silicon upon difluorination and
the subsequent electron flow from the carbon atom
opposite to silicon.
Acknowledgements
M. Dakkouri gratefully acknowledges the financial
support from the Fonds der Chemischen Industrie.
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