Infrared and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-fluoro-1-methylsilacyclobutane

Article abstract of DOI:10.1016/S1386-1425(99)00092-X

The infrared (3500-40 cm^-1) spectra of gaseous and solid 1-fluoro-1-methylsilacyclobutane, c-C₃H₆SiF(CH₃), have been recorded. Additionally, the Raman spectrum (3500-30 cm^-1) of the liquid has been recorded and quantitative depolarization values have been obtained. Both the axial and equatorial (with respect to the methyl group) conformers have been identified in the fluid phases. Variable temperature (-55--100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 267±10 cm^-1 (3.19±0.12 kJ mol^-1), with the axial conformer being the more stable form and the only conformer remaining in the polycrystalline solid. A complete vibrational assignment is proposed for the axial conformer and many of the fundamentals for the equatorial conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311++G** basis sets at the levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules.

Full text of DOI:10.1016/S1386-1425(99)00092-X

Spectrochimica Acta Part A 55 (1999) 2753–2769
www.elsevier.nl/locate/saa
Infrared and Raman spectra, conformational stability,
normal coordinate analysis, ab initio calculations, and
vibrational assignment of 1-fluoro-1-methylsilacyclobutane
1
2
Gamil A. Guirgis , Todor K. Gounev, Pengqian Zhen , James R. Durig *
Department of Chemistry, Uni6ersity of Missouri-Kansas City, Kansas City, MO 64110-2499, USA
Received 17 November 1998; received in revised form 6 April 1999; accepted 6 April 1999
Abstract
The infrared (3500–40 cm ) spectra of gaseous and solid 1-fluoro-1-methylsilacyclobutane, c-C H SiF(CH ), have
−
1
3
6
3
−
1
been recorded. Additionally, the Raman spectrum (3500–30 cm ) of the liquid has been recorded and quantitative
depolarization values have been obtained. Both the axial and equatorial (with respect to the methyl group)
conformers have been identified in the fluid phases. Variable temperature (−55– −100°C) studies of the infrared
spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has
−
1
−1
been determined to be 267910 cm
(3.1990.12 kJ mol ), with the axial conformer being the more stable form
and the only conformer remaining in the polycrystalline solid. A complete vibrational assignment is proposed for the
axial conformer and many of the fundamentals for the equatorial conformer have also been identified. The vibrational
assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium
geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311+ +
G** basis sets at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The
results are discussed and compared to those obtained for some similar molecules. © 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Infrared; Raman; Conformational stability; Ab initio calculations; 1-Fluoro-1-methylsilacyclobutane
1
. Introduction
*
Corresponding author. Tel.: +1-816-2351136; fax: +1-
For the initial vibrational [1–6] and rotational
7,8] investigations of the halocyclobutanes, it was
8
16-2355191.
[
E-mail address: durigj@umkc.edu (J.R. Durig)
inferred that large values of the enthalpy differ-
ences, DH, between the two conformers should
result in little or none of the high energy axial
form being present at ambient temperature. How-
ever, more recent vibrational and ab initio studies
1
Present address: Analytical R/D Department, Performance
Products Division, Bayer, Bushy Park Plant, Charleston, SC
2
9411, USA.
2
Taken in part from the dissertation of P. Zhen which will
be submitted to the Department of Chemistry in partial fulfill-
ment of the Ph.D degree.
1
386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00092-X

2
754
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
[
9–12] showed that several modes of the high
plicable. The results of these spectroscopic and
theoretical studies are reported herein.
energy axial conformers of these molecules were
readily observable in both the liquid and vapor
states at ambient temperature. Similar studies of
methylcyclobutane [13,14] showed the enthalpy
difference between the low energy equatorial form
and the high energy axial form as less than 400
2
. Experimental
−
1
A sample of 1-chloro-1-methylsilacyclobutane
cm
for both the vapor and liquid so that some
was prepared by the reaction of dichloro (3-
choropropyl) methylsilane with magnesium in dry
diethyl ether according to the method of Dam-
raner [16] .This sample was then used to prepare
of the fundamentals of the axial form could be
observed. These studies have been followed by an
investigation [15] of the conformational stability
of the corresponding silicon compound, 1-methyl-
silacyclobutane. The enthalpy difference for c-
1
-fluoro-1-methylsilacyclobutane by fluorination
of the chloride with freshly prepared SbF . The
−
1
3
C H SiH(CH ) was 122926 cm
(1.4690.32
3
6
3
sample was purified with a low-temperature, low-
pressure fractionation column.
−
1
kJ mol ) with the equatorial conformer more
stable which made it possible to observe most of
the fundamentals for the axial conformer. The
equatorial-to-axial puckering barrier of 686 cm
(
calculation compared well with the experimentally
determined similar inversion barrier of 641920
cm
The mid-infrared spectra of the gas and solid
−
1
(
Fig. 1) were obtained from 3200 to 300 cm
on
−
1
a Perkin–Elmer model 2000 Fourier transform
spectrometer equipped with a Ge–CsI beamsplit-
ter and a DTGS detector. The gas was contained
in a 10 cm cell fitted with CsI windows. This
−
1
8.21 kJ mol ) obtained from the MP2/6-31G*
−
1
−1
(7.6790.24 kJ mol
) obtained for
methylcyclobutane [13] which was obtained from
the observed ring puckering transitions in the
low-frequency Raman spectrum.
As a continuation of the study of 1-methylsila-
cyclobutane, we have investigated the conforma-
tional stability of 1-fluoro-1-methylsilacyclobu-
tane. It is expected that the electronegativity of
the fluorine atom would favor this atom in the
equatorial position as found for the halocyclobu-
tane molecules, [9–12] whereas the steric effect,
because of the size of the methyl group, would
favor the methyl group to be in the equatorial
position. Therefore, we initiated an infrared and
Raman spectroscopic study of methylsilacyclobu-
tane to determine the presence or absence of
conformers at ambient temperature and if they
are present to determine their relative stabilities.
As an aid in interpreting the vibrational spectra
we have carried out ab initio calculations at the
RHF/3-21G*, RHF/6-31G* and MP2/6-31G*
levels. The optimized geometries, conformational
stabilities, harmonic force fields, infrared intensi-
ties, Raman activities, depolarization ratios, and
vibrational frequencies have been obtained to
compare with the experimental results where ap-
Fig. 1. Mid-infrared spectra of 1-flouro-1-methylsilacyclobu-
tane in the: (A) gas; and (B) annealed solid.

G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
2755
Fig. 2. Far infrared spectra of 1-flouro-1-methylsilacyclobutane in the: (A) gas; insert with high vapor pressure, the sharp lines
marked with an asterisk are due to the rotational transitions of HCl; (B) amorphous solid; and (C) annealed solid.
−
1
spectrum was obtained at 0.5 cm
resolution
chamber fitted with KBr windows. The tempera-
ture was maintained with boiling liquid nitrogen
and monitored with two Pt thermoresistors. The
complete cell was connected to a pressure mani-
fold, allowing the filling and evacuation of the
system. After cooling to the desired temperature,
a small amount of the compound was condensed
into the cell. Next, the system was pressurized
with the noble gas, which immediately started to
condense in the cell, allowing the compound to
dissolve.
The far infrared spectrum of the gas (Fig. 2A)
was recorded with a Bomen model DA3.002
Fourier transform spectrometer equipped with a
vacuum bench, a 6.25 mm Mylar beamsplitter,
and a liquid helium cooled Ge bolometer with a
wedged sapphire filter and polyethylene window.
The spectra were obtained from the sample con-
tained in a 1 m folded path cell equipped with
mirrors coated with gold, and fitted with
polyethylene windows. To remove traces of water,
and transformed with boxcar truncation function.
The spectrum of the solid was obtained by con-
densing the sample onto a liquid nitrogen cooled
CsI plate contained in an evacuated cell equipped
with CsI windows, and 256 scans were collected
for both the reference and sample interferograms
−
1
at 1 cm resolution and then transformed with a
boxcar truncation function.
The mid-infrared spectra of the sample dis-
solved in liquefied xenon as a function of temper-
ature were recorded on a Bruker model IFS 66
Fourier transform spectrometer equipped with a
globar source, a Ge–KBr beamsplitter and a
DTGS detector. In all cases, 100 interferograms
−
1
were collected at 1.0 cm
resolution, averaged
and transformed with a boxcar truncation func-
tion. For these studies, a specially designed
cryostat cell was used. It consisted of a copper cell
with a pathlength of 4 cm with wedged silicon
windows sealed to the cell with indium gaskets.
The copper cell was enclosed in an evacuated
an activated 4 A molecular sieve was used to dry
,

2
756
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
the sample. Interferograms were recorded 512
In order to obtain a more complete description
of the molecular motions involved in the normal
modes of 1-fluoro-1-methylsilacyclobutane, we
have carried out a normal coordinate analysis.
The force fields in Cartesian coordinates were
calculated by the Gaussian-94 program [18] with
the MP2/6-31G* basis set. Internal coordinates
(Table 3) were used to calculate the G and B
matrices using the structural parameters given in
Table 2. Using the B matrix, [20] the force field in
Cartesian coordinates was then converted to a
force field in internal coordinates, and the pure ab
initio vibrational frequencies were reproduced.
The force constants for both the axial and equato-
rial conformers can be obtained from the authors.
Subsequently, scaling factors of 0.9 for stretching
and bending and 1.0 for the torsional coordinates,
and the geometric average of scaling factors for
interaction force constants were used to obtain
the fixed scaled force field and resultant
wavenumbers. A set of symmetry coordinates was
used (Table 4) to determine the corresponding
−
1
times at a resolution of 0.10 cm
and trans-
formed with a boxcar truncation function. The
spectra of the solid (Fig. 2B and 2C) was obtained
with the Perkin–Elmer model 2000 spectrometer
equipped with a metal grid beamsplitter and a
DTGS detector.
The Raman spectra were recorded on a SPEX
model 1403 spectrophotometer equipped with a
Spectra-Physics model 164 argon ion laser operat-
ing on the 514.5 nm line. The laser power used
−
1
was 0.5 W with a spectral bandpass of 3 cm
.
The spectra of the liquid were recorded with the
sample sealed in a Pyrex glass capillary held in a
Miller–Harney apparatus [17] .Depolarization
measurements were obtained for the liquid sample
using a standard Ednalite 35 mm camera polarizer
with 38 mm of free aperture affixed to the SPEX
instrument. Depolarization ratio measurements
were checked by measuring the state of polariza-
tion of the Raman bands of CCl immediately
4
before depolarization measurements were made
on the liquid sample. The measurements of Ra-
man frequencies are expected to be accurate to
−
1
9
2 cm and typical spectra are shown in Fig. 3.
All of the observed bands in both the infrared and
Raman spectra, along with the proposed assign-
ments, are listed in Table 1.
3. Ab initio calculations
The LCAO-MO-SCF restricted Hartree–Fock
calculations were performed with the Gaussian-94
program [18] using Gaussian-type basis functions.
The energy minima with respect to nuclear coor-
dinates were obtained by the simultaneous relax-
ation of all of the geometric parameters consistent
with the symmetry restrictions using the gradient
method of Pulay [19].The structural optimization
for both the equatorial and the axial (with respect
to the methyl group) conformers were carried out
with initial parameters taken from those of
methylsilacyclobutane [15].The 6-31G* and/or 6-
311+ +G** basis sets were employed at the level
of restricted Hartree−Fock (RHF) and Moller–
Plesset (MP2) to second order. The determined
structural parameters are listed in Table 2.
Fig. 3. Raman spectra of 1-flouro-1-methylsilacyclobutane in
the: (A) liquid; and (B) annealed solid.

Table 1
Observed infrared and Raman frequencies (cm 1) and assignments for 1-fluoro-1-methylsilacyclobutanea
−
Infrared
Gas
Raman
Liquid
Assignment
b
Rel.
int.
Xenon
solution
Rel.
int.
Solid
Rel.
int.
Rel. int. Solid
& dp.
Rel.
int.
6i
Approximate description
2
2
995 R
989 Q
vs
vs
2980
2978
2964
sh,s
s
2980
2970
sh,m,dp
m,p
2984
2980
2967
m
624
(CH ) Antisymmetric stretch
2 2
2
2
985 Q
974 P
2979
s
s
sh,m
w
625
CH3 Antisymmetric stretch
2969
m
61,62
(CH ) Antisymmetric stretch, CH3
2 2
antisymmetric stretch
2
2
2
970 R
960 Q,A
950 P
/
vs
s
2951
939
s
s
2947
2931
w
w
2944
2929
sh,s,p
s,p
2951
2933
w
s
63
CH2 Antisymmetric stretch
2
64,626
(CH ) Symmetric stretch
2 2
2
2
945 R
935 Q
2926
s
2918
2902
2883
1462
1421
s
2910
s,p
2919
2905
2891
1456
1421
s
65,66
CH2 Symmetric stretch, CH3 sym-
metric stretch
2
2
1
925 P
887 Q
462 Q
vw
w
m
w
w
w
m
2872
1456
m
2881
1454
sh,m,p
w,dp
(
1
9
9
9
)
vw
w
vw
67,6%7
68,6%8
CH2 Deformation
1423
sh,w
w
(CH ) Deformation
2 2
2
7
5
3
1
414
w
1
1
415 Q
405 Q
m
m
1409
1401
m
sh,w
1405
1398
w
sh,w
w
1410
w,dp
1408
1398
w
sh,w
69,6%9
627,6%27
CH3 Antisymmetric deformation
CH3 Antisymmetric deformation
2
7
6
9
1
395
1
391 Q
m
1387
m
1383
377
sh,w
m
1388
vw
628,6%28
(CH ) Deformation
2 2
1
1
1
1
1
1
1
269 R
265 Q
263 min B
261 Q
256 P
s
s
1258
1245
s
1259
1254
s
1261
1250
w,p
1261
1249
w
w
610,6%10
629,6%29
CH3 Symmetric deformation
CH2 Wag
sh,w
255 Q
sh,w
1247
m
sh,w,dp

Table 1 (Continued)
Infrared
Raman
Liquid
Assignment
b
Gas
Rel.
int.
Xenon
solution
Rel.
int.
Solid
Rel.
int.
Rel. int. Solid
& dp.
Rel.
int.
6i
Approximate description
1
219 Q
vw
vw
w
6%
CH2 Twist
CH2 Twist
3
0
1
223
vw
w
1224
1220
w
sh,w
1
1
1
1
1
1
1
1
214 Q
197 R
189 Q
185 P
138 R
132 Q
125 Q
075 max
1211
1184
w
w
1215
1214
1186
w,dp
w,p
630
1188
w
1186
w
611,6%11
(CH ) Twist
2 2
s
s
vw
1127
1068
s
1124
1113
m
sh,w
1128
1059
w,p
1132
1054
w
612,6%12
(CH ) Wag
2 2
vw
6%
(CH ) Wag
3
1
2 2
/
1
1
053
046
vw
w
vw,dp
vw
631
(CH ) Wag
2 2
9
9
9
40 Q
sh,s
945
928
916
sh,w
vs
6%
C–C Ring stretch
3
3
28 max
16 Q
bd,vs
sh,s
927
917
s
923
m,p
924
s
613,6%16
Si–F stretch, (CH2)2 rock
s
vs
sh,s
632,6%32
633,6%14
614
(CH ) Twist
2 2
9
13
8
8
8
8
8
8
8
8
8
99 Q
86 R
79 Q
77 min, B
75 Q
68 P
61 Q
15 R
09 Q
s
s
895
875
sh,w
s
898
875
m
s
901
877
sh,m,p
m,p
C–C Ring stretch, C–C ring stretch
C–C Ring stretch
(
1
9
9
9
)
873
m
2
7
5
3
2
7
6
9
s
s
s
s
857
805
797
770
m
sh,s
s
845
802
797
s
849
800
793
775
w
615
CH2 Rock
sh,vw
vs
803
774
m,p
w,p
sh,vw
m
634,6%15
616,6% ,6%
CH3 Rock, CH2 rock
802 Q
780 R
774 Q
768 P
730 R
723 Q
714 P
790
vs
(CH ) Rock, Si–F stretch, CH3 rock
2 2
13
34
s
779
775
vs
sh,s
m
617,6%17
CH3 Rock
m
723
w
731
716
w
sh,vw
733
722
m
w
618
6%
C–Si Ring stretch
C–Si Stretch
715
sh,w
717
w,p
1
9

Table 1 (Continued)
Infrared
Raman
Liquid
Assignment
b
Gas
Rel.
int.
Xenon
solution
Rel.
int.
Solid
675
Rel.
int.
Rel. int. Solid
& dp.
Rel.
int.
6i
Approximate description
C–Si Ring stretch
6
6
6
81 R
74 Q, C
66 P
s
673
m
s
678
626
m,dp
s,p
681
w
635,6%35
6
6
23
12
vw
vw
sh,w
m
624
612
465
448
w
w
w
w
624
611
s
629
609
vs
636,6%18
619,6%36
6%
(CH ) Rock, C–Si ring stretch
2 2
w
vw
C–Si Stretch, (CH2)2 rock
Ring deformation
/
467 Q
457 R
448 Q
439 P
468
452
sh,m,p
s,p
2
0
450
277
m
s
453
vs
620
Ring deformation
2
79 Q
s
283
269
w,dp
287
272
w
637,6%37
FSiC Wag
2
2
67 Q
43 Q
s
sh,w,p
m
621
6%
FSiC Rock
Ring puckering
sh,m
2
3
(
1
9
9
9
)
2
22
vw
vw
230
191
133
vw
w
229
192
w,p
234
198
140
w
s
622,6%22
638,6%38
FSiC Deformation
FSiC Twist
1
78 Q, C
w,dp
2
7
5
3
1
1
10 Q
01 Q
vw
vw
w
w
623
6%
Ring puckering
FSiC Rock
2
7
6
9
2
1
1
02
w
w
w
w
93
75
sh,w
m
Lattice modes
Lattice modes
Lattice modes
Lattice modes
7
5
4
3
9
7
a
Abbreviations used: s, strong; m, moderate; w, weak; v, very; bd, broad; sh, shoulder; p, polarized; dp, depolarized; A, B, and C refer to infrared band envelopes;
P, Q, and R refer to the rotational–vibrational branches.
b
6
i and 6i% refer to the axial and equatorial conformers, respectively.

2
760
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
Table 2
Structural parameters, rotational constants, dipole moments, and energy for 1-fluoro-1-methylsilacyclobutanea
Parameter
RHF/6-31G*
MP2/6-31G*
MP2/6-311++G**
Eq. Ax.
1.875
Eq.
Ax.
1.882
Eq.
Ax.
1.880
Si–C (C )
1.883
1.873
1.606
1.566
1.083
1.087
1.083
1.087
1.084
1.085
1.087
1.087
80.06
101.4
87.28
106.9
107.6
108.0
108.0
107.9
107.6
113.9
110.0
115.0
111.4
121.1
112.7
111.3
111.0
121.7
112.4
21.9
1.882
1.869
1.633
1.562
1.092
1.096
1.092
1.097
1.095
1.095
1.094
1.094
79.6
100.9
86.42
107.1
107.6
108.4
108.4
108.1
107.9
114.4
109.7
115.7
111.0
122.1
111.7
110.7
110.9
122.4
111.7
28.1
1.872
1.859
1.631
1.565
1.095
1.091
1.095
1.091
1.093
1.095
1.094
1.094
79.75
100.2
85.39
107.6
107.9
109.1
109.1
108.1
108.1
110.1
114.2
109.9
116.3
111.2
122.6
110.4
111.0
116.1
117.8
33.3
2
3
Si–C11
Si–F
1.874
1.604
1.565
1.086
1.083
1.086
1.083
1.084
1.085
1.087
1.087
1.869
1.630
1.562
1.095
1.092
1.095
1.092
1.094
1.096
1.094
1.094
79.53
100.6
85.51
107.8
107.4
108.7
108.7
108.0
108.0
110.1
114.3
110.3
116.1
111.9
122.4
110.5
111.1
115.9
117.9
33.5
1.858
1.635
1.564
1.091
1.096
1.091
1.096
1.094
1.094
1.094
1.094
79.74
100.5
86.20
106.9
108.1
108.9
108.9
108.2
108.1
114.4
109.6
116.0
110.6
121.9
111.5
110.7
110.8
123.7
110.3
29.7
C –C (C )
4
2
3
C–H7
C–H8
C–H9
C–H10
C–H5
C–H6
C–H13
C–H14(H15
)
ÚC SiC3
79.8
2
ÚC C C
101.0
86.74
107.3
107.4
108.2
108.2
107.8
107.7
110.2
114.0
110.8
115.4
112.2
121.8
111.1
111.1
117.6
116.5
26.0
2
4 3
ÚCCSi
ÚFSiC11
ÚH C H
5
4
6
8
ÚH C H
7
2
ÚH C H
9
3
10
ÚH13CH14(H15
ÚH14CH15
ÚCCH6
)
ÚCCH5
ÚCCH (H )
7
9
ÚCCH (H10)
8
ÚSiCH (H )
7
9
ÚSiCH (H10)
8
ÚSiCH13
ÚSiCH14(H15
ÚCSiC11
)
ÚFSiC (C )
2
3
Pucking angle
~
(FSiC11H13
)
180.0
4.154
2.277
2.079
0.268
1.725
0.000
1.7457
5.13195
182
180.0
4.015
2.325
2.077
1.062
1.267
0.000
1.6532
5.13278
180.0
4.130
2.293
2.078
0.219
1.874
0.000
1.8868
5.90684
364
180.0
3.974
2.348
2.077
1.100
1.401
0.000
1.7812
5.90850
180.0
4.178
2.304
2.095
0.222
2.164
0.000
2.175
180.0
3.997
2.359
2.089
1.323
1.587
0.000
2.066
A
B
C
ꢀ
ꢀ
ꢀ
ꢀ
v ꢀ
a
v ꢀ
b
v ꢀ
c
v ꢀ
~
−
(E+540)H
6.34635
376
6.34806
−
1
E (cm
)
a
Bond distances in A, bond angles in degrees, rotational constants in GHz, dipole moments in Debye, and energies in Hartrees.
,
potential energy distributions (P.E.D.). A com-
parison between the observed and calculated fre-
man activities, depolarization ratios and P.E.D.
are given in Table 5.
quencies
of
1-fluoro-1-methylsilacyclobutane
Raman and infrared spectra for 1-fluoro-1-
methylsilacyclobutane were calculated using the
along with the calculated infrared intensities, Ra-

G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
2761
calculated frequencies, scattering activities and
intensities determined from the ab initio calcula-
tions. The Gaussian-94 program [18] with the
option of calculating the polarizability deriva-
tives (RHF/6-31G*) was used. The Raman scat-
tering cross sections, #|j/#d, which are
proportional to the Raman intensities, can be
calculated from the scattering activities and the
predicted frequencies for each normal mode us-
ing the relationship [21]:
and k are universal constants, and S
j
is the cor-
responding Raman scattering activity. To obtain
the polarized Raman scattering cross section, the
polarizabilities are incorporated into Sj by
S [(1−z )/(1+z )] where z is the depolarization
j
j
j
j
ratio of the jth normal mode. The Raman scat-
tering cross sections and calculated fixed scaled
frequencies are used together with a Lorentzian
line shape function to obtain the calculated spec-
trum. The predicted Raman spectra of the pure
equatorial and axial conformers are shown in
Fig. 4D and C, respectively. The predicted Ra-
man spectrum of the mixture of the two con-
formers with an assumed DH of 267 cm
DS=0 is shown in Fig. 4B which should be
compared to the experimental spectrum of the
liquid (Fig. 4A). The utilized DH value is taken
from the temperature dependent study of the
xenon solution spectra.
4
4
4
(
(
|j
ꢁ
2 y
ꢂ
:
(w −w )
;
ꢁ
h
ꢂ
0
j
=
Sj
2
V
45
ꢃ
−hcwj
n
8y cw
j
−1
1
−exp
and
kT
where w is the exciting frequency, w is the vibra-
tional frequency of the jth normal mode, h, c
0
j
Infrared intensities were also calculated based
on the dipole moment derivatives with respect to
the Cartesian coordinates. The derivatives were
taken from the ab initio calculations at the MP2/
6-31G* level and transformed to normal coordi-
nates by:
Table 3
Internal coordinate definitions for 1-fluoro-1-methylsilacy-
clobutane
Coordinate
involved
Definition Coordinate
involved
Definition
ꢁ
(vvꢂ ꢁ(vv
ꢂ
=
%
Lji
(
Qi
(Xj
j
Si C Stretch
R1
R2
R3
R4
r1
C C H Bend
k2
k3
k4
k5
k6
h5
h6
1
2
2
4
5
Si C Stretch
C C H Bend
1
3
2 4 6
C C Stretch
C C H Bend
3 4 5
2
4
where the Q is the ith normal coordinate, X is
i
j
C C Stretch
C C H Bend
3
4
3 4 6
the jth Cartesian displacement coordinate, and
Lji is the transformation matrix between the
Cartesian displacement coordinates and normal
coordinates. The infrared intensities were then
calculated by
C H Stretch
H C H Bend
5 4 6
2
7
C H Stretch
r2
C C H Bend
2
8
4 2 7
C H Stretch
r3
C C H Bend
4 2 8
3
9
C H Stretch
r4
Si C H Bend
i2
3
10
1
3
9
C C Stretch
r5
Si C H Bend
i3
4
5
1
3
10
C H Stretch
r6
C C H Bend
i5
i6
i4
€1
l1
€2
l2
4
6
4
3
9
Si C Stretch
R5
r7
C C H Bend
1
11
4 3 10
Si F Stretch
H C H Bend
2
2
2
1
12
9
3
10
Ny ꢃ ꢁ(vxꢂ ꢁ(vyꢂ ꢁ(vz ꢂ n
C11H13 Stretch r8
C11H14 Stretch r9
C11H15 Stretch r10
C Si C Bend
Ii=
+
+
2
1
11
2
3
c
(Qi
(Qi
(Qi
C Si F Bend
2
1 12
C Si H Bend
3
1
11
Si C C Bend
h1
i1
k1
C Si F Bend
C11Si F Bend q2
1 12
1
2
4
3 1 12
In Fig. 5C and D, the predicted infrared spec-
tra of the axial and equatorial conformers, re-
spectively, are shown. The combination of the
Si C C Bend
1
3
4
C C C Bend
Si C11H13 Bend u1
2
4
3
1
C Si C Bend
q1
Si C11H14 Bend u2
1
2
1
3
Si C H Bend
h2
Si C11H Bend u3
1
2
7
1
15
spectra of the two conformers with a DH of 267
Si C H Bend
h3
H14C11H15 Bend |1
H13C11H14 Bend |3
H13C11H15 Bend |2
−1
1
2
8
cm
is shown in Fig. 5B and the experimental
H C H Bend
h4
~
7
2
8
spectrum of the sample dissolved in liquid xenon
is shown in Fig. 5A for comparison.
Si–CH3 Torsion

2
762
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
Table 4
Symmetry coordinates for 1-fluoro-1-methylsilacyclobutanea
Species Description
A% (CH2)2 Antisymmet- S =r −r +r −r
4
Symmetry coordinate
Species Description
C–Si Stretch
Symmetry coordinate
1
1
2
3
S19=R5
ric stretch
CH3 Antisymmetric S =2r −r −r
Ring deformation
FSiC Rock
S20=h +i −k −q
S21=€ +€ −l −l
1 2 1 2
1
1
1
1
2
8
9
10
stretch
CH2 Antisymmetric S =r −r
FSiC Deformation
Ring puckering
S22=4q −€ −€ −l −l
2 1 2 1 2
3
5
6
S23=h +i +k +q
1
1
1
1
stretch
CH2)2 Symmetric
A%%
(CH2)2 Antisymmet- S24=r −r −r +r
ric stretch
1
2
3
4
(
S4=r +r +r +r
1
2
3
4
stretch
CH3 Antisymmetric S25=r −r
9
10
CH2 Symmetric
stretch
CH3 Symmetric
stretch
S =r +r
6
stretch
(CH2)2 Symmetric
stretch
5
5
S26=r +r −r −r
1 2 3
4
S =r +r +r
6 8 9 10
CH3 Antisymmetric S27=| −|
2
3
CH2 Deformation
S =4k −k −k −k −k
5
deformation
7
6
2
3
4
(
CH2)2 Deformation S =4h −h −h −h −h
(CH ) Deformation S28=4h −h −h −h −h
2 2 4 2 3 5 6
8
4
2
2
5
6
+
4i −i −i −i −i
−4i +i +i +i +i
4 2 3 5 6
4
2
3
5
6
CH3 Antisymmetric S =2| −| −|
CH2 Wag
CH2 Twist
(CH ) Wag
2 2
S29=k +k −k −k
S30=k −k −k +k
2 3 4 5
9
1
2
3
2 3 4 5
stretch
CH3 Symmetric de- S10=| +| +| −u −u −u
1
2
3
1
2
3
S31=h +h −h −h −i −i
2
3
5
6
2
3
formation
+
i5+i6
(CH2)2 Twist
S11=h −h −h +h +i −i
2
3
5
6
2
3
(
CH2)2 Twist
S32=h −h −h +h −i +i
2
3
5
6
2
3
−
i5+i6
+
i5−i6
(CH2)2 Wag
S12=h +h −h −h +i +i
2
3
5
6
2
3
C–C Stretch in ring S33=R −R
3
4
−
i5−i6
CH3 Rock
S34=u −u
2 3
Si–F Stretch
S13=r7
C–Si Stretch in ring S35=R −R
1
2
C–C Stretch in ring S14=R +R
(CH ) Rock
2 2
S36=h −h +h −h −i +i
3
4
2
3
5
6
2
3
CH2 Rock
CH2)2 Rock
S15=k −k +k −k
2 3 4 5
−
i5+i6
(
S16=h −h +h −h +i −i
2
3
5
6
2
3
FSiC Wag
FSiC Twist
S37=€ −€ +l −l
1 2 1
2
2
+
i5−i6
S38=€ −€ −l +l
1 2 1
CH3 Rock
S17=2u −u −u
Si–CH3 Torsion
S39=~
1
2
3
C–Si Stretch in ring S18=R +R
1
2
a
Not normalized.
4
. Conformational stability
From the mid-infrared spectra shown in Fig. 1,
it is clear that the feature centered at about 450
−
1
A careful examination of the observed wavenum-
cm
in the spectrum of the gas is actually a
−
1
bers for 1-fluoro-1-methylsilacyclobutane listed in
Table 1 shows that for each fundamental of the
axial conformer there is a fundamental of the
equatorial form with nearly coincident frequency.
In some cases the approximate descriptions of such
fundamentals may be different because of differ-
ences between the P.E.D.s (Table 5) for the two
rotamers. However, regardless of these descrip-
tions, it is impossible to completely separate the
spectral bands of the two conformers which makes
the conformational analysis much more difficult.
doublet with Q-branches at 467 and 448 cm . On
the basis of the ab initio predicted frequency order,
these two bands are assigned to the ring deforma-
tion modes of the equatorial and axial conformers,
respectively. The above mentioned modes are also
present in the Raman spectrum of the liquid (Table
1) and partially resolved in the infrared spectra of
the xenon solution (Fig. 6). However, in the spec-
−
1
trum of the solid only a single band at 450 cm
is observed in this spectral region. A comparison
of the far infrared spectra of the amorphous and

G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
2763
annealed solids (Fig. 2B and C) reveals that the
high frequency member of the conformer doublet
is the one which disappears upon crystallization.
This and some other, much less obvious spectral
changes show that the axial conformer is the only
form present in the crystalline solid.
In order to determine the enthalpy difference,
DH, between the conformers of c-C H SiF(CH ),
3
6
3
the relative band intensities of the conformer dou-
−
1
blet, 465–448 cm , in the infrared spectrum of
the sample dissolved in liquid xenon were mea-
sured as a function of temperature. Ten sets of
spectral data were obtained for these bands in the
Fig. 5. Mid infrared spectra of 1-flouro-1-methylsilacyclobu-
tane: (A) xenon solution at −100°C; (B) calculated spectrum
of the mixture of both conformers; (C) calculated spectrum of
the equatorial conformer; (D) calculated spectrum of the axial
conformer. Bands marked with asterisks are due to impurities.
temperature range −55 to −100°C (Table 6). The
temperature dependent spectra are shown in Fig. 6
−
1
where the intensity of the band at 448 cm clearly
increases as the temperature is decreased compared
−
1
to the band at 465 cm . This is indicative for the
higher stability of the axial conformer of 1-fluoro-
1
-methylsilacyclobutane (Fig. 7).
The intensity data for the conformer pair were
fit to the van’t Hoff equation −ln K=(DH/
Fig. 4. Raman spectra of 1-flouro-1-methylsilacyclobutane: (A)
experimental spectrum of the liquid; (B) calculated spectrum
of the mixture of both conformers; (C) calculated spectrum of
the equatorial conformer; (D) calculated spectrum of the axial
conformer.
RT)−(DS/R), where K is substituted with the
intensity ratio (I /I ), and it is assumed that DH
eq ax
is not a function of temperature. Using a least-
squares fit and the slope of the van’t Hoff plot, a

Table 5
Observed and calculated frequencies (cm−1) for 1-fluoro-1-methylsilacyclobutane
Fundamental
Axial
Equatorial
Ab
initioa
Fixed
scaledb
IR
int.
Raman
act.
dp
ratiod
Obs.e
P.E.D.f
Ab
initioa
Fixed
scaledb
IR
int.c
Raman
act.
dp
ratiod
Obs.e
P.E.D.f
c
d
d
A%
1
2
3
4
5
6
(CH
stretch
CH Antisymmetric
stretch
CH Antisymmetric
stretch
(CH
stretch
CH Symmetric
stretch
CH Symmetric
stretch
CH Deformation
(CH Deformation
CH Antisymmetric
deformation
CH Symmetric
deformation
2
)
2
Antisymmetric 3202
3038
3036
3007
2968
2953
2947
17.3
5.4
77.6
101.5
78.9
0.68
0.63
0.15
0.14
0.60
0.01
(2969)
(2969)
2960
83S
94S
90S
91S
98S
1
2
3
4
5
3200
3202
3172
3120
3114
3104
3036
3037
3009
2960
2954
2945
4.6
75.8
89.0
0.74
0.45
0.29
0.08
0.37
0.01
(2969)
(2969)
2960
46S
56S
89S
79S
92S
1
2
3
4
5
,44S
2
1
3
3200
3170
3128
3113
3106
19.2
13.7
5.0
,37S
2
18.1
2.9
89.9
2
)
2
Symmetric
222.5
54.8
(2939)
2935
163.8
107.8
105.2
(2939)
2935
,12S
1
2
28.5
0.6
39.1
0.4
3
107.7
2935
100S
6
2935
100S
6
7
8
9
2
1565
1525
1522
1485
1447
1444
2.2
5.5
7.3
6.6
7.3
28.8
0.70
0.73
0.75
1462
(1423)
1415
93S
49S
47S
7
8
9
1561
1523
1517
1481
1445
1439
0.5
12.4
2.6
6.6
26.8
11.3
0.71
0.75
0.74
1462
(1423)
1415
92S
78S
75S
7
8
9
/
2
)
2
,47S
,47S
9
,17S
,18S
9
3
8
8
1
0
3
1370
1300
43.6
1.0
0.64
1263
97S10
1369
1299
42.4
0.9
0.61
1263
97S10
1
1
1
1
1
1
2
3
4
5
(CH
(CH
Si–F Stretch
2
)
)
2
Twist
Wag
1256
1211
986
932
905
1191
1149
935
885
859
5.0
49.1
121.1
45.0
4.5
3.0
7.9
14.0
2.7
0.67
0.75
0.11
0.14
0.57
1189
1132
928
877
861
47S11,24S15,17S16,12S12
68S12,10S11
26S13,19S14,10S16,10S17
48S14,17S11,10S18
1258
1213
848
951
871
1193
1151
804
902
826
2.8
52.0
72.6
24.6
13.5
5.6
1.9
0.4
19.6
1.1
0.74
0.75
0.42
0.13
0.33
1189
1132
802
899
809
47S11,24S15,15S16,12S12
69S12,10S11
33S13,21S11,15S15,14S14
71S14,11S18
2
2
C–C Stretch in ring
CH
(CH
CH
2
Rock
Rock
Rock
35.9
24S15,24S13,16S12,14S14
,
24S15,22S18,18S20,14S16
,
1
2S18
12S12
16
2
)
2
857
813
64.4
3.3
0.13
(790)
27S16,30S20,12S13
981
931
122.0
1.9
0.15
928
11S16,27S13,15S11,14S17
,
12S15
17
18
19
20
21
22
23
3
820
756
640
464
283
227
131
777
717
607
440
268
216
124
3041
34.2
31.2
2.9
5.6
12.7
1.6
3.0
2.6
12.2
6.7
0.7
0.9
0.46
0.44
0.13
0.24
0.68
0.32
0.73
0.75
774
723
612
448
267
222
110
2989
49S17,28S13,12S19
15S18,31S19,20S15,14S17
41S19,19S16,16S18
53S20,25S18
27S21,33S23,23S22,11S16
66S22,18S21
817
662
746
480
115
231
256
3203
775
628
707
456
109
219
243
3038
45.3
6.5
35.4
17.9
0.9
3.3
3.0
4.4
3.3
6.2
7.3
11.7
0.3
0.3
0.36
0.06
0.24
0.25
0.31
0.48
0.65
0.75
774
623
714
467
101
222
243
2989
51S17,24S13,11S19
23S18,26S16,17S15,14S20
49S19,18S17,11S20,10S16
44S20,23S18,20S19
53S21,44S23
71S22,11S21
35S23,24S21,16S22,13S16
100S24
C–Si Stretch in ring
C–Si Stretch
Ring deformation
FSiC Rock
FSiC Deformation
Ring puckering
(
1
9
9
9
)
2
7
5
3
1.5
5.3
0.2
93.7
52S23,44S21
100S24
0.7
107.1
2
7
6
9
A%%
24
(CH
stretch
CH Antisymmetric
stretch
(CH
stretch
CH Antisymmetric
deformation
(CH Deformation
2
)
2
Antisymmetric 3206
25
26
27
3
3201
3128
1519
3037
2967
1441
10.6
11.1
1.0
97.0
15.8
16.4
0.75
0.75
0.75
2985
(2939)
1405
94S25
94S26
92S27
3200
3118
1519
3036
2958
1441
8.5
9.6
2.6
98.8
17.3
12.5
0.75
0.75
0.75
2985
(2939)
1405
88S25,12S26
88S26,12S25
95S27
2
)
2
Symmetric
3
28
29
30
2
)
2
1506
1328
1278
1428
1260
1212
12.1
1.1
1.0
3.3
0.7
9.8
0.75
0.75
0.75
1391
1255
1214
95S28
74S29,12S30
54S30,21S31,11S29
1503
1334
1283
1426
1265
1217
10.7
0.8
2.6
5.9
1.3
8.4
0.75
0.75
0.75
1391
1255
1219
99S28
77S29,10S30
57S30,19S31
CH
CH
2
Wag
Twist
2

Table 5 (Continued)
Fundamental
Axial
Equatorial
Ab
initioa
Fixed
scaledb
IR
int.
Raman
act.
dp
ratiod
Obs.e
P.E.D.f
Ab
initioa
Fixed
scaledb
IR
int.c
Raman
act.
dp
ratiod
Obs.e
P.E.D.f
c
d
d
31
32
33
34
35
36
37
38
39
(CH
(CH
2
)
)
2
Wag
Twist
1118
988
982
867
699
670
282
179
153
1060
938
932
822
663
636
267
170
151
0.6
7.4
6.4
67.7
25.0
4.5
10.9
0.1
2.3
0.3
10.0
1.4
5.4
0.2
0.6
0.7
0.0
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
(1053)
916
899
809
674
623
279
178
71S31
1128
985
999
847
724
638
281
173
150
1071
934
948
803
687
606
267
165
149
0.4
4.4
7.8
74.2
30.6
1.3
9.7
0.0
0.0
2.4
2.8
7.8
0.6
5.0
1.1
0.7
0.6
0.0
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
1075
73S31
/
2
2
42S32,30S33,20S30
61S33,23S32
64S34
71S35,17S34
70S36,10S32
81S37
916
940
802
674
612
279
178
39S32,38S33,18S30
51S33,28S32
70S34,12S35,10S37
38S35,35S36,11S34
44S36,39S35
81S37
C–C Stretch in ring
CH Rock
C–Si Stretch in ring
(CH Rock
3
2 2
)
FSiC Wag
FSiC Twist
Si–CH
84S38
99S39
78S38
91S39
3
Torsion
0.0
a
b
c
d
e
f
Calculated with the MP2/6-31G* basis set.
Scaling factors of 0.9 for stretching and bending coordinates and 1.0 for torsional coordinates.
−1
Calculated infrared intensities in km mol
.
4
−1
Calculated Raman activities in A
,
amu , using RHF/6-31G* basis set.
Frequencies are taken from the infrared spectrum of the gas, except the ones in parentheses, which are taken from the infrared spectrum of the solid.
For a description of the symmetric coordinates see Table 4.
(
1
9
9
9
)
2
7
5
3
2
7
6
9

2
766
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
−
1
−1
Table 6
DH value of 267910 cm (3.1990.1 kJ mol
)
Temperature and intensity ratio for the conformational study
of 1-fluoro-1-methylsilacyclobutane
was obtained with the axial conformer being more
stable. This value should be representative for DH
in the gas phase [22–26] since the molecular sizes
of both conformers are almost the same.
a
T (°C)
1000/T (K)
K=I465(eq.)/I448(ax.) −ln K
−
−
−
−
−
55
60
65
70
75
4.58
4.65
4.80
4.92
5.08
5.18
5.31
5.46
5.61
5.78
0.3547
0.3236
0.3144
0.3061
0.2921
0.2723
0.2581
0.2461
0.2306
0.2220
1.0365
1.1283
1.1571
1.1838
1.2307
1.3009
1.3544
1.4020
1.4671
1.5051
5
. Vibrational assignment
Both the axial and equatorial conformers of
-fluoro-1-methylsilacyclobutane have a plane of
−80
−
−
−
85
90
95
1
symmetry and their 39 normal modes are
classified by the symmetry species of the Cs sym-
metry group, A% and A%%. There are 23 A% vibra-
tions which are expected to produce polarized
Raman bands and A–B-type hybrid infrared
band contours. The remaining 16 A%% modes
should give rise to depolarized lines in the Raman
spectrum and yield C-type infrared band en-
−
100
a
−1
DH=267910 cm (3.2090.11 kj mol−1) with the axial
form the more stable conformer.
velopes. Thus, to identify the A% and A%% vibra-
tions, we utilized the measured depolarization
ratios and observed infrared band contours,
whereas to distinguish between the equivalent
modes of the two conformers we relied on the
predicted frequency order, infrared intensities and
Raman activities from the ab initio calculations.
The assignments (Table 1) of the carbon–hy-
drogen stretching and deformational modes of
1
-fluoro-1-methylsilacyclobutane are essentially
the same as for 1-methylsilacyclobutane with only
minor frequency shifts and they have been dis-
cussed elsewhere [15].The CH waging and twist-
2
ing fundamentals are observed as weak,
−
1
depolarized bands at 1250 and 1214 cm , respec-
tively, in the Raman spectrum of the liquid. The
Fig. 6. Temperature dependent infrared spectrum (420–500
−
1
cm ) of 1-flouro-1-methylsilacyclobutane dissolved in liquid
xenon.
Fig. 7. Axial conformation of 1-flouro-1-methylsilacyclobu-
tane.

G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
2767
latter mode shows a pair of Q-branches for the
two conformers in the infrared spectrum of the
gas. The corresponding A% (CH ) wag and twist
The wavenumber separation of the fundamen-
tals of the two conformers becomes much more
apparent in the low frequency portion of the
mid-infrared region, especially for the C–Si
stretching and ring deformation modes. The C–Si
2
2
−
1
are located at 1127 and 1184 cm , respectively,
in the infrared spectrum of the sample dissolved
in liquid xenon. In the Raman spectrum of the
ring stretches of the axial conformer are associ-
−
1
−1
liquid, a depolarized band at 1059 cm
is as-
ated with the Q-branch at 723 cm
and the
−
1
C-type band at 674 cm in the infrared spectrum
of the gas. For the equatorial form, the methyl
C−Si stretch is attributed to the weak shoulder
signed to the A%% (CH ) wag, whereas the respec-
2
2
tive twist gives rise to a strong band in the
−
1
infrared spectrum at 916 cm
.
−
1
at 715 cm
in the spectrum of the xenon solu-
Since there are eight expected fundamentals of
each conformer in the interval from 750 to 950
tion, whereas for the axial form this mode pro-
−
1
−
1
duces the weak Raman band at 609 cm
in the
cm , the vibrational spectra are quite complex in
this spectral range. The vibrations of the axial
form can be identified from the spectra of the
spectrum of the solid. The strong and polarized
−
1
band at 626 cm
in the Raman spectrum of the
liquid is assigned to the A% C–Si ring stretch of
the equatorial rotamer, and is coincident with the
A%% (CH2)2 rock of the axial form. From the
normal coordinate analysis, the A%% C–Si ring
stretch of the equatorial conformer is expected to
crystalline solid. For example, the two CH rocks
3
−
1
are located at 797 and 779 cm
in the infrared
spectrum of the solid, whereas the CH and the
2
A%% (CH ) rocks are observed at 845 and 790
2
2
−
1
cm in the same spectrum. The respective modes
of the equatorial conformer are coincident with
modes of the axial form (Table 1). A prominent
−
1
be ꢀ20 cm higher in frequency than the corre-
sponding mode of the axial form which appears at
−
1
ꢀ
674 cm . However, no bands are observed
−
1
B-type band centered at 877 cm
in the infrared
−
1
around 694 cm
thermore, the infrared band at 673 cm
in the vibrational spectra. Fur-
spectrum of the gas is assigned to the A% C–C ring
stretch of the axial conformer. The band exhibits
two relatively broad Q-branches at 879 and 875
−
1
in the
spectrum of the xenon solution appears to be
about twice as intense as those at 723 and 715
−
1
cm
with equal intensities for which an alterna-
−1
cm , whereas the predicted intensities (Table 5)
tive assignment can be given as being the A% C–C
ring stretches of the equatorial and axial conform-
ers, respectively. However, we have assigned the
above mentioned fundamental of the equatorial
of these three fundamentals are about the same.
−1
This implies that the band at 673 cm
should be
assigned to the A%% C–Si ring stretches of both
conformers.
−
1
conformer to the much weaker band at 895 cm
in the spectrum of the xenon solution, since it is
The ring deformations of the equatorial and
axial conformers are observed at 465 and 448
−
1
expected to appear ꢀ20 cm
higher in fre-
−1
cm , respectively, in the infrared spectrum of the
quency and have weaker infrared intensity. Fur-
thermore, a B-type band with very similar band
contour having two Q-branches was observed for
xenon solution. The former band is not found in
the spectrum of the solid. Since for both rotamers
the ring puckering and FSiC rocking modes are
another A% fundamental, the CH symmetric de-
3
extensively mixed (Table 5), their assignment to
−
1
formation at 1263 cm . The A%% C–C ring
−1
the pairs of Q-branches at 267 and 243 cm
and
−
1
stretches are then assigned to the medium intense
1
10 and 101 cm
in the far infrared spectrum of
−
1
solid phase band at 898 cm
for the axial ro-
the gas is rather optional. However, it is definite
that the first member of each pair of bands be-
longs to the axial conformer. Finally, the depolar-
−
1
tamer and the Q-branch at 940 cm
in the
infrared spectrum of the gas for the equatorial
form. Quite large frequency shifts are encountered
between the Si–F stretches of the axial and equa-
torial conformers, which appear as very intense
−
1
ized Raman bands at 283 and 192 cm
are
assigned to the FSiC wag and twist, respectively,
whereas the corresponding deformation is associ-
−
1
−1
bands at 928 and 797 cm
in the infrared spec-
ated with the weak absorption at 230 cm
in the
trum of the sample dissolved in liquid xenon.
far infrared spectrum of the annealed solid.

2
768
G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
−
1
6. Discussion
ab initio predicted value of 364 cm
using the
MP2/6-31G* calculation. When the basis set size
is increased to 6-311+ +G**, the predicted en-
The assignment of the carbon–hydrogen
stretching modes must be considered somewhat
−
1
ergy difference of 374 cm
is still larger than the
tentative since selective deuteration of the b- and
experimental value. However, it should be noted
g-CH groups of the ring was not carried out. In
that the MP2/6-31G* calculation gave an en-
2
−
1
general, the assignment follows those given earlier
for the corresponding modes of the 1-methylsila-
cyclobutane molecule where the assignments were
made mainly on the basis of the ab initio pre-
dicted frequencies. In many cases the same, or
nearly the same, frequencies are assigned for the
corresponding bands for the two conformers. In
fact, there is only a limited number of bands
which could readily be assigned confidently to the
high energy equatorial conformer.
thalpy difference of 137 cm
for methylsilacy-
clobutane with the equatorial conformer more
stable which agreed with the value of 122926
−
1
cm
experimentally obtained from the tempera-
ture study of the xenon solution [15]. Thus, it
appears that MP2/6-31G* calculations can give
reasonable estimates of the conformational stabil-
ities of these silicon substituted silacyclobutane
molecules.
The structural parameters for the two conform-
ers are remarkably similar. For the most part, the
heavy atom distances differ by 0.003 A or less and
,
the carbon-hydrogen distances are the same ex-
The potential energy distribution for the A%%
modes of the axial conformer is relatively pure
except for the (CH ) twist (6 ) which is com-
2
2
32
posed of 30% C–C ring stretch and 20% CH2
twist in addition to the 42% contribution from the
cept for those of the g-CH groups. Similarly, the
2
angles differ by less than 1° for the corresponding
parameters of the two conformers. The puckering
angle is the only major parameter that differs
between the two conformers by about 4.4° with
the axial conformer having the larger value as
expected from the relative conformer stabilities.
Since no experimental structural parameters
have been reported for 1-fluoro-1-methylsilacy-
clobutane it is not possible to assess how well the
parameters are predicted from the ab initio calcu-
lations at the MP2 level. Nevertheless, based on
some of our earlier studies [9–12] of four-mem-
bered rings, we believe the carbon-hydrogen and
carbon-carbon bond distances should be well pre-
(
CH ) twist. However, for the A% modes there is
2 2
extensive mixing from the (CH ) twist (6 ) at
2
2
11
−
1
1
cm
189 cm
through the FSiC rock (6 ) at 267
21
−
1
with the exception of the ring deformation
−
20 2 2 12
1
(
6 ) at 448 cm and the (CH ) wag (6 ) at 1132
−
1
cm . For example, the CH rock (6 ) at 861
2
15
−
1
cm has five different contributions with none of
them greater than 24%. Therefore, the descrip-
tions for many of these modes are rather arbitrary
and they are used for bookkeeping rather than to
give accurate descriptions of the molecular mo-
tions. Similar mixing is also found for the equato-
rial conformer.
The more stable axial conformer for 1-fluoro-1-
methylsilacyclobutane is the exact opposite of the
one found for 1-methylsilacyclobutane where the
equatorial conformer was found to be more stable
dicted, i.e. the C–H distances within 0.005 A
C–C distances within 0.008 A. The angles are
,
and
,
usually predicted within about one degree from
the MP2/6-31G* calculations so we expect the
listed parameters given in Table 2 are about as
accurate as one could obtain from an electron
diffraction or microwave investigation of this
molecule.
The similarity of the geometric parameters of
the two conformers is reflected in the similar
values for the diagonal elements of the two force
constant matrices. The only significant difference
is found in the values of the CCSi and CCC ring
bending force constants, which are about 5%
−
1
−1
by 122926 cm
(1.4690.32 kJ mol ). There-
fore, the electronegative fluorine atom takes the
equatorial position which is consistent with the
relative stabilities of the conformers of fluorocy-
clobutane [12]. The steric effect of the relatively
large methyl group is clearly not as important as
the electronegativity of the fluorine atom in deter-
mining the relative conformer stability.
−
1
The enthalpy value of 267910 cm
obtained
from the xenon solution is slightly lower than the

G.A. Guirgis et al. / Spectrochimica Acta Part A 55 (1999) 2753–2769
2769
[
[
10] J.R. Durig, M.J. Lee, T.S. Little, J. Raman Spectrosc. 21
1990) 529.
11] J.R. Durig, M.J. Lee, T.S. Little, Struct. Chem. 2 (1991)
95.
larger for the equatorial form. This fact results in
a lower frequency of the ring deformation for the
(
−
1
axial conformer (448 cm ) compared to that for
1
−
1
the equatorial rotamer (467 cm ). The only
other doublets with significantly different frequen-
cies are those for the A%% C–C stretch of the ring
[12] J.R. Durig, M.J. Lee, W. Zhao, T.S. Little, Struct. Chem.
3 (1992) 329.
[
13] J.R. Durig, T.J. Geyer, T.S. Little, V.F. Kalasinsky, J.
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581.
−
1
at 899 and 940 cm
and for the FSiC rock at
of the axial and equatorial
[
−
1
267 and 243 cm
conformers, respectively.
[
15] J.R. Durig, P. Zhen, Y. Jin, T.K. Gounev, G.A. Guirgis,
J. Mol. Struct. 477 (1999) 31.
[
[
16] E. Damraner, J. Organomet. Chem. 43 (1971) 121.
17] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970)
Acknowledgements
2
91.
[
18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill,
B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith,
G.A. Petersson, J.A. Montgomery, K. Raghavachari,
M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Fores-
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JRD acknowledges the University of Kansas
City Trustees for a Faculty Fellowship award for
partial financial support of this research.
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