Conformational stability, ab initio calculations and vibrational assignment for 1,1-difluoro- and 1,1-dichloro-1-silacyclopentane

Article abstract of DOI:10.1016/j.molstruc.2011.01.065

The infrared spectrum (3200-400 cm^-1) of the gas and the Raman spectrum (3200-50 cm^-1) of liquid and solid 1,1-difluoro-1- silacyclopentane and 1,1-dichloro-1-silacyclopentane have been recorded. In all of these physical states only the twisted (C₂) conformer was detected. The conformational energies have been calculated with the Moller-Plesset perturbation method to the second order; MP2(Full) as well as the density functional theory by the B3LYP method utilizing a variety of basis sets up to 6-311 + G(2df,2pd). All the calculations predict only the twisted form as the stable conformer of 1,1-difluoro-1-silacyclopentane with the average barrier to planarity of 2548 cm^-1 (30.49 kJ/mol) from the MP2 calculations and a significantly lower value of 1422 cm^-1 (17.01 kJ/mol) from the density functional calculations. Neither calculation was significantly affected by the inclusion of diffuse functions. The C_s conformer has a lower energy of 1703 cm^-1 (MP2) and 1334 cm ^-1 (B3LYP) than the planar form. Thus the path between the two identical C₂ conformers is by pseudorotational motion rather than through the planar form. Similar results obtain for 1,1-dichloro-1- silacyclopentane. The optimized geometry calculated with the 6-311 + G(2df,2pd) basis is given together with a complete vibrational assignment for the twisted (C₂) conformer. These assignments are supported by normal coordinate calculations with scaled force constants from MP2(Full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for silacyclopentane.

Full text of DOI:10.1016/j.molstruc.2011.01.065

Journal of Molecular Structure 992 (2011) 1–8
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Conformational stability, ab initio calculations and vibrational assignment
for 1,1-difluoro- and 1,1-dichloro-1-silacyclopentane
Stephen Bell a, Howard D. Stidham b, Arthur J. LaPlante b, Ya Ying Zheng c, Gamil A. Guirgis c,
⇑
a Department of Physics, University of Dundee, Dundee DD1 4HN, UK
b Department of Chemistry, University of Massachusetts Amherst, Amherst, MA 01003, USA
c Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA
a r t i c l e i n f o
a b s t r a c t
The infrared spectrum (3200–400 cmꢀ1) of the gas and the Raman spectrum (3200–50 cmꢀ1) of liquid
and solid 1,1-difluoro-1-silacyclopentane and 1,1-dichloro-1-silacyclopentane have been recorded. In
all of these physical states only the twisted (C2) conformer was detected. The conformational energies
have been calculated with the Moller–Plesset perturbation method to the second order; MP2(Full) as well
Article history:
Received 18 January 2011
Accepted 28 January 2011
Available online 13 February 2011
as the density functional theory by the B3LYP method utilizing
a variety of basis sets up to
Keywords:
6-311 + G(2df,2pd). All the calculations predict only the twisted form as the stable conformer of 1,1-
difluoro-1-silacyclopentane with the average barrier to planarity of 2548 cmꢀ1 (30.49 kJ/mol) from the
MP2 calculations and a significantly lower value of 1422 cmꢀ1 (17.01 kJ/mol) from the density functional
calculations. Neither calculation was significantly affected by the inclusion of diffuse functions. The Cs
conformer has a lower energy of 1703 cmꢀ1 (MP2) and 1334 cmꢀ1 (B3LYP) than the planar form. Thus
the path between the two identical C2 conformers is by pseudorotational motion rather than through
the planar form. Similar results obtain for 1,1-dichloro-1-silacyclopentane. The optimized geometry cal-
culated with the 6-311 + G(2df,2pd) basis is given together with a complete vibrational assignment for
the twisted (C2) conformer. These assignments are supported by normal coordinate calculations with
scaled force constants from MP2(Full)/6-31G(d) calculations. The results of these spectroscopic and the-
oretical studies are discussed and compared to the corresponding results for silacyclopentane.
Ó 2011 Elsevier B.V. All rights reserved.
Silacyclopentane
IR
Raman
Vibrational analysis
1. Introduction
calculations with full electron correlations using Moller–Ploessett
perturbation to the second order (MP2 = full). The results were
Five membered ring molecules have three possible conforma-
tions; planar, twisted and envelope. The conformational stabilities
of these conformers are of interest to structural chemists and phys-
icists. Recently the conformational stability of silacyclopentane
[1,2] and germacyclopentane [3] was investigated, and the results
indicate that these molecules are found to exist in the twist form.
This finding is in agreement with the previously reported vibra-
tional spectra and assigned on the basis of C2v symmetry although
the authors [2] pointed out that the molecule is C2 symmetry.
Therefore, we initiated this study to investigate the conformations
of similar silicon containing molecules, but with geminal halogen
atoms attached to the silicon atom. Our objective in this work
was to explore the influence of geminal chlorine or fluorine atoms
on the pseudorotational structural parameters of these molecules.
The vibrational assignment was supported and initial structural
parameters obtained at the restricted Hartree–Fock level with
the 6-31G(d) basis set. These calculations were followed by
optimized geometries, conformational stabilities, harmonic force
fields, infrared intensities, Raman activities, depolarization ratios,
and vibrational frequencies.
Larger basis sets up to 6-311 + G(2df,2pd) with electron correla-
tion were used to obtain more accurate optimized geometries and
conformational stabilities.
We also carried out density functional theory calculations by
the B3LYP method with the corresponding basis sets used for the
MP2 calculations. The optimized geometries, conformational sta-
bilities, harmonic force fields, infrared intensities, Raman activities
and depolarization ratios were all obtained using the 6-31G(d) ba-
sis set. The results of these theoretical studies are combined with
experimental observations of the infrared and Raman spectra and
are reported herein. A comparison with similar quantities of some
corresponding molecules is given.
2. Experimental
⇑
The 1,1-dichloro-1-silacyclopenane sample was prepared by
adding a double Grignard reagent of 1,4-dibromobutane dissolved
Corresponding author.
E-mail address: guirgisg@cofc.edu (G.A. Guirgis).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.01.065

2
S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
in 60 mL of anhydrous ethyl ether to a solution of silicon tetrachlo-
ride in anhydrous ethyl ether under nitrogen gas similar to re-
ported method of West [4]. The purity of the sample was
checked by infrared and nuclear magnetic resonance. The sample
of 1,1-difluoro-1-silacyclopentane was prepared from the dichloro
derivative by fluorination of 1,1-dichloro-1-silacyclopentane with
freshly sublimed antimony trifluoride without solvent.
The mid-infrared spectra of the gas shown in Figs. 1 and 2 were
obtained from 3200 to 400 cmꢀ1 with a Perkin Elmer model 100
Fourier transform spectrometer equipped with a KBr beam splitter
and a DTGS detector. Atmospheric water was removed from the
spectrometer housing by a drying agent. The theoretical resolution
used to obtain the spectrum of the gas was 1.0 cmꢀ1, and 128 inter-
ferograms were recorded, co-added and Fourier transformed with
a boxcar truncation function.
Fig. 3. Raman spectrum of liquid 1,1-dichloro-1-silacyclopentane at room
temperature.
The Raman spectra of the liquids shown in Figs. 3 and 4 were
obtained using glass capillaries filled on an all-glass vacuum line
and sealed with a gas blowing torch under running vacuum while
chilled to liquid nitrogen temperatures. The Raman instrument
was briefly described in earlier work [5].
3. Conformational stability
From the ab initio calculations using different basis sets, the
twist form with C2 symmetry was identified as a stable conformer.
The major computational effort was a search for another stable
conformer. The spectrum of the Raman liquid and solid displayed
no significant change in the relative intensities of the bands as
the temperature was decreased. This observation clearly indicates
either that there is only one conformer present, or if more than one
the enthalpy difference is near zero as previously reported by
Guirgis et al. [1]. The ab initio restricted Hartree Fock calculations
Fig. 4. Raman spectrum of 1,1-difluoro-1-silacyclopentane at room temperature.
support the presence of only one conformer. The average of the
MP2 calculations with full electron correlation for 1,1-difluoro-1-
silacyclopentane predict the envelope conformer with Cs symme-
try to be higher in energy than that of the twist form of C2
symmetry by 1714 37 cmꢀ1. In addition, the frequency calcula-
tions indicate that the Cs conformer is a transition state. The
same prediction was also made by the density functional B3LYP
calculations with energy difference of 1276 36 cmꢀ1. Similar
calculations were done for 1,1-dichloro-1-silacyclopentane using
MP2(Full) and DFT (B3LYP) methods. The average energy
differences between the C2 and Cs conformers were 1334 75
and 1276 36 cmꢀ1, respectively.
Since the energy barrier is relatively large, there can be only one
stable conformer in the fluid phases. The conformer present in li-
quid and gas is most probably the twisted form of C2 symmetry.
Fig. 1. Infrared spectra of 1,1-dichloro-1-silacycopentane gas at room temperature.
4. Vibrational assignment
The previously reported vibrational assignments [6] were based
on the infrared spectrum of the gas and liquid phases, and on the
depolarization values found in the Raman spectrum. The authors
recognized that the molecule was twisted with C2 symmetry but
utilized C2v for their assignments and normal coordinate calcula-
tions to clarify the description of the vibrations. In the present pa-
per the assignments are made according to C2 symmetry and are
shown in Tables 1 and 2.
Assignments were made using the observed frequencies in the
Raman spectrum of the liquid or the infrared spectrum of the va-
por, and the primary basis for the assignments was proximity to
the calculated frequencies in the scaled B3LYP/6-311 + G(2d,2p)
normal coordinate calculation. These assignments can be readily
Fig. 2. Infrared spectra of 1,1-difluoro-1-silacycopentane gas at room temperature.

S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
3
Table 1
Vibrational frequencies of 1,1-dichloro-1-silacyclopentane derived from ab initio calculations compared with observed spectroscopic measurements.
Approx. descriptiona
HF
6-
31G(d)
MP2
6-
31G(d)
B3LYP
6-
31G(d)
B3LYP
6-
B3LYP 6-
311 +
Observed
wave-
IR
Raman
Act.e
DP
P.E.D.e
Int.e
Ratioe
311 + GPb
GPb scaledc
numbersd
A
m1
m2
m3
m4
m5
m6
m7
m8
m9
CH symm stretch (2,3)
CH symm stretch (4,5)
CH symm stretch (2,3)
CH symm stretch (4,5)
CH2 scissors (4,5)
CH2 scissors (2,3)
CH2 wag (4,5)
3281
3254
3218
3201
1654
1596
1496
1416
1340
1230
1104
1022
909
800
736
564
372
288
181
156
3187
3156
3116
3097
1563
1508
1399
1324
1263
1145
1077
972
892
770
715
543
358
292
172
148
3129
3090
3064
3042
1530
1479
1372
1296
1234
1119
1035
945
856
741
685
523
343
264
167
142
3107
3066
3044
3018
1503
1452
1352
1282
1222
1105
1021
936
848
737
678
519
342
260
163
142
2947
2908
2888
2863
1465
1415
1319
1253
1193
1078
1018
913
845
730
670
519
342
261
171
148
2949
2910
0.4
149.4
258.6
135.2
199.3
1.1
11.1
1.9
2.8
1.8
3.1
3.4
0.5
10.4
8.8
3.0
9.9
12.2
0.4
0.50
0.13
0.34
0.09
0.54
0.75
0.70
0.49
0.06
0.51
0.65
0.14
0.09
0.08
0.19
0.01
0.07
0.54
0.75
0.74
84S1, 16S3
82S2
76S3, 13S1, 11S2
92S4
96S5
94S6
60S7, 30S8
46S8, 31S10
40S9, 32S12, 10S14
51S10, 19S8, 187
84S11
49S12, 42S9
51S13, 38S15
45S14, 23S13, 15S15
26S15, 38S14, 15S13
44S16, 40S17
45S17, 39S16
82S18
31.6
11.6
17.1
1.2
0.4
1.4
2.4
0.2
31.6
12.9
0.2
0.3
5.8
73.5
74.8
0.7
0.0
2.5
0.1
2867
1451
1404
1317
1250
1191
1076
1013
915
844
735
667
516
CH2 twist (4,5)
CH2 twist (2,3)
m10 CH2 wag (2,3)
m11 Ring breathing
m12 CH2 rock (4,5)
m13 Ring breathing
m14 CH2 rock (2,3)
m15 Ring breathing
m16 Ring deformation
m17 SiCl2 symm stretch
m18 Ring pucker
346.5
230
169.5
150
m19 SiCl2 scissors
m20 SiCl2 twist
2.1
1.8
89S19
82S20, 11S18
B
m21 CH antisymm stretch
(2,3)
m22 CH antisymm stretch
(4,5)
m23 CH antisymm stretch
(2,3)
m24 CH antisymm stretch
(4,5)
m25 CH2 scissors (4,5)
m26 CH2 scissors (2,3)
m27 CH2 wag (4,5)
m28 CH2 twist (4,5)
m29 CH2 wag (2,3)
m30 CH2 twist (2,3)
m31 CH2 rock (4,5)
3282
3258
3218
3205
3187
3162
3116
3100
3129
3096
3064
3046
3107
3071
3044
3024
2947
2914
2888
2869
2949
2910
15.1
36.3
4.7
7.9
69.7
7.3
0.75
0.75
0.75
0.75
81S21, 17S23
59S22, 32S24
79S23, 13S21
65S24, 34S22
2867
1459
30.2
36.4
1641
1598
1478
1407
1320
1141
1030
1555
1510
1381
1322
1230
1078
995
1521
1480
1357
1297
1198
1060
964
1496
1452
1344
1286
1183
1051
956
1458
1416
1311
1254
1159
1025
944
6.2
18.0
0.5
0.4
3.5
10.2
0.4
0.5
4.4
1.6
0.2
3.7
0.75
0.75
0.75
0.75
0.75
0.75
0.75
100S25
100S26
91S27
54S28, 21S30, 14S29
61S29, 15S36
61S30, 32S28
20S31, 23S34, 19S29
16S32
1155
1020
942
16.7
0.7
,
m32 Ring breathing
m33 CH2 rock (2,3)
m34 Ring breathing
938
900
754
909
860
732
875
835
702
865
824
698
856
811
694
7.0
64.2
49.3
0.2
0.5
2.1
0.75
0.75
0.75
61S32, 35S31
61S33
51S34, 15S32, 15S31
804
705
,
,
10S33
m35 SiCl2 antisymm stretch
m36 Ring deformation
590
520
586
498
550
485
537
482
536
478
542
484
123.0
11.5
2.4
1.3
0.75
0.75
72S35, 15S36
52S36, 22S35, 14S31
12S33
m37 SiCl2 rock
m38 SiCl2 wag
m39 Ring pucker
258
243
64
250
231
55
238
223
58
237
223
55
244
233
56
1.2
7.5
0.4
1.5
1.8
0.8
0.75
0.75
0.75
60S37, 24S39
91S38
71S39, 29S37
228
a
b
c
Order corresponding to vibrational modes from B3LYP/6-311 + G(2d,p). The numbers in parentheses correspond to the numbering of C atoms used.
Polarization functions P = (2d,p) in usual Gaussian notation.
Scale factors for force constants: 0.9 for C–H stretches only, 0.95 for all CH2 bends, 1.1 for SiCl2 distorsions, 1.0 for other stretches, other bends, ring distortions, and twists.
Experimental from this study.
d
e
The intensities and activities are from Gaussian output: infrared intensities in km/mol, Raman activities in Å4/amu, DP ratios, and PED taken from B3LYP/6-311 + G(2d,p)
unscaled,
correlated with the previous ones [6] since the A1 and A2 symmetry
species in C2v produce the A species in C2 and the B1 and B2 species
produce B. Depolarization factors for the Raman lines were not
used in the current study Differences with the earlier work can
be attributed to the unavailability of ab initio calculations at that
time. In the present work, the advantage of actually calculating
ab initio all of the force constants and then compounding these
in the frequencies is evident.
were obtained by the simultaneous relaxation of all geometric
parameters using the gradient method of Pulay [8]. Different ba-
sis sets with and without diffuse functions were used in geome-
try optimizations. Electron correlation was approximated to the
second order by the Moller–Plesset perturbation method built
into the Gaussian suite of programs by specifying the key word
MP2 = full on the command line of the input file. We also
applied Beck’s three-parameter hybrid exchange functional in
combination with the gradient corrected correlation functional
of Lee, Yang and Parr by using the B3LYP functional called in
the Gaussian 03. The basis sets: 6-31G(d), 6-311G(d,p), 6-
311G(2d,2p), 6-311G(2df,2pd) were used both with and without
diffuse functions. The energies for all conformers are collected
in Table 3.
5. Ab Initio calculations
We utilized the Gaussian 03 package [7] using LCAO-MO-SCF
restricted Hartree–Fock calculations applying Gaussian-type basis
functions. The energy minima with respect to nuclear coordinates

4
S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
Table 2
Vibrational frequencies of 1,1-difluoro-1-silacyclopentane derived from ab initio calculations compared with observed spectroscopic measurements.
Approx.
HF 6-
31G(d)
MP2 6-
31G(d)
B3LYP 6-
31G(d)
B3LYP 6-
311 + GPb
B3LYP 6-
Observed wave-
IR Int.e Raman
Act.e
DP
P.E.D.e
descriptiona
311 + GPb scaledc numbersd
Ratioe
A
m1
CH symm stretch 3269
(2,3)
CH symm stretch 3252
(4,5)
CH symm stretch 3207
(2,3)
CH symm stretch 3196
(4,5)
3188
3158
3116
3097
3120
3088
3055
3036
3099
3066
3037
3015
2940
2909
2881
2860
2947
2904
2870
2863
1.4
34.2
11.5
13.2
172.1
205.7
157.3
214.0
0.46
0.13
0.24
0.12
85S1, 14S3
89S2
m2
m3
m4
82S3, 12S1
94S4
m5
m6
m7
m8
m9
CH2 scissors (4,5) 1654
CH2 scissors (2,3) 1596
1566
1510
1400
1330
1265
1532
1480
1371
1299
1234
1505
1451
1351
1286
1222
1467
1414
1318
1257
1194
1462
1408
1316
1253
1196
1.5
0.6
0.3
12.5
0.3
1.2
9.7
1.6
3.5
0.7
0.65
0.75
0.73
0.70
0.04
96S5
95S6
61S7, 28S8
47S8, 31S10
40S9, 33S12
11S15
CH2 wag (4,5)
CH2 twist (4,5)
CH2 twist (2,3)
1495
1417
1339
,
m10 CH2 wag (2,3)
m11 Ring breathing
m12 CH2 rock (4,5)
m13 SiF2 symm
stretch
1229
1103
1011
960
1150
1079
964
1120
1034
937
1107
1019
930
1081
1016
907
1078
1017.5
917
55.0
15.7
0.3
1.7
4.7
0.3
4.2
0.47
0.61
0.69
0.33
49S10, 19S7, 188
84S11
50S12, 40S9
67S13, 29S16
923
901
872
871
871
70.7
m14 Ring breathing
m15 CH2 rock (2,3)
905
780
887
749
852
725
841
721
838
710
848
697
20.2
10.8
9.5
5.5
0.05
0.04
49S14, 29S16
48S15, 13S16
13S17
,
m16 Ring breathing
677
656
632
629
622
608
0.5
11.6
0.03
21S16, 28S15
24S14, 11S13
59S17, 22S19
56S18, 20S19
15S17
54S19, 19S18
16S20
77S20, 19S19
,
m17 Ring deformation
m18 SiF2 scissors
470
295
445
295
437
274
436
269
435
268
16.3
7.4
2.4
0.6
0.15
0.41
272
229
174
,
,
m19 Ring pucker
m20 SiF2 twist
284
192
272
183
265
177
258
176
258
176
2.1
0.8
0.7
0.59
0.75
0.000
B
m21 CH antisymm
stretch (2,3)
m22 CH antisymm
stretch (4,5)
m23 CH antisymm
stretch (2,3)
m24 CH antisymm
stretch (4,5)
3271
3254
3207
3200
3188
3164
3117
3101
3120
3093
3056
3042
3099
3070
3037
3022
2940
2913
2881
2867
2947
2904
20.8
38.4
10.0
32.5
14.4
56.5
13.0
24.8
0.75
0.75
0.75
0.75
83S21, 15S23
65S22, 28S24
81S23, 13S21
70S24, 28S22
m25 CH2 scissors (4,5) 1642
m26 CH2 scissors (2,3) 1598
1559
1513
1385
1328
1523
1481
1359
1301
1498
1451
1347
1290
1460
1415
1314
1258
1453
1408
5.5
18.1
2.3
9.7
0.7
0.75
0.75
100S25
100S26
91S27
53S28, 14S29
21S30
m27 CH2 wag (4,5)
m28 CH2 twist (4,5)
1480
1409
0.007 0.75
1.5
3.7
1.3
0.75
0.75
,
,
m29 CH2 wag (2,3)
1321
1238
1201
1187
1164
1156
3.0
58S29, 16S36
11S35
m30 CH2 twist (2,3)
m31 CH2 rock (4,5)
1145
1030
1087
996
1066
965
1056
956
1031
944
26.5
1.2
0.3
3.9
0.75
0.75
58S30, 31S28
20S31, 22S35
21S29, 15S36
67S32, 18S34
944
877
,
m32 SiF2 antisymm
stretch
1010
972
953
908
902
50.6
1.2
0.75
m33 Ring breathing
m34 CH2 rock (2,3)
938
870
911
833
876
810
866
797
858
786
34.4
36.8
0.8
1.8
0.75
0.75
69S33, 26S31
32S34, 23S32
14S31
44S35, 20S34
12S31
65S36, 18S31
88S37
66S38, 18S39
12S34
780
697
492
,
,
m35 Ring breathing
761
741
711
707
700
48.6
1.0
0.75
m36 Ring deformation
m37 SiF2 wag
m38 SiF2 rock
528
337
285
507
321
277
494
312
268
495
310
264
490
310
263
2.7
30.5
2.1
0.7
1.0
0.3
0.75
0.75
0.75
,
m39 Ring pucker
80
74
77
71
70
1.1
0.3
0.75
76S39, 24S38
a
b
c
Order corresponding to vibrational modes from B3LYP/6-311 + G(2d,p). The numbers in parentheses correspond to the numbering of C atoms used.
Polarization functions P = (2d,p) in usual Gaussian notation.
Scale factors for force constants: 0.9 for C–H stretches only, 0.95 for all CH2 bends, 1.0 for other stretches, other bends, ring distortions, and twists.
Experimental from this study.
d
e
The intensities and activities are from Gaussian output: infrared intensities in km/mol, Raman activities in Å4/amu, DP ratios, and PED taken from B3LYP/6-311 + G(2d,p)
unscaled.
In order to obtain a more complete description of the molecular
motion involved in the fundamental modes of vibration of the sila-
cyclopentane derivatives, a normal coordinate analysis was carried
out. The internal coordinates used to calculate the Wilson G and B
matrices [9] are given together with the atomic numbering in
Fig. 5. By using the B matrix, the force field in Cartesian coordinates

S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
5
Table 3
Calculated energies (hartree) for the twisted C2 form and energy differences in cmꢀ1 relative to the C2 form for the envelope-axial Cs and planar C2v forms of the 1,1-dihalo-1-
silacyclopentane molecules. All MP2 entries are MP2(full).
Method/basis set
C4H8SiF2
C4H8SiCl2
a
b
C2
Cs
C2v
C2
Cs
C2v
rhf/6-31G(d)
rhf/6-31 + G(d)
mp2/6-31G(d)
mp2/6-31 + G(d)
0.9181527
0.9087651
1678
1666
1817
1742
1706
1685
1780
1657
1720
1703
1714
37
1507
1484
1389
1402
1429
1378
1398
1393
1394
1422
48
2080
2013
2492
2286
2308
2297
2576
2369
2458
2434
2403
105
1883
1746
1685
1688
1663
1649
1692
1681
1680
1707
71
0.8619427
0.8692962
1499
1482
1364
1336
1469
1319
1366
1338
1260
1216
1334
75
1355
1314
1275
1273
1232
1240
1260
1268
1263
1276
36
2165
2143
2628
2463
2504
2457
2572
2574
2554
2630
2548
67
1907
1866
1811
1815
1784
1790
1778
1780
1779
1790
42
ꢀ0.0657890
ꢀ0.0929549
ꢀ0.5226688
ꢀ0.5355096
ꢀ0.6558473
ꢀ0.6646147
ꢀ0.7854127
ꢀ0.7938305
ꢀ0.0477172
ꢀ0.0636588
ꢀ0.5027129
ꢀ0.5132538
ꢀ0.7930265
ꢀ0.6504059
ꢀ0.7986253
ꢀ0.7864725
mp2/6-311G(d,p)
mp2/6-311 + G(d,p)
mp2/6-311G(2d,2p)
mp2/6-311 + G(2d,2p)
mp2/6-311G(2df,2pd)
mp2/6-311 + G(2df,2pd)
mp2(full) average
Standard deviation
B3LYP/6-31G(d)
B3LYP/6-31 + G(d)
B3LYP/6-311G(d,p)
B3LYP/6-311 + G(d,p)
B3LYP/6-311G(2d,2p)
B3LYP/6-311 + G(2d,2p)
B3LYP/6-311G(2df,2pd)
B3LYP/6-311 + G(2df,2pd)
B3LYP/aug-cc-pVTZ
B3LYP average
ꢀ1.5674468
ꢀ1.5874504
ꢀ1.6961591
ꢀ1.7033426
ꢀ1.7206734
ꢀ1.7257924
ꢀ1.7313775
ꢀ1.7369628
ꢀ1.7508761
ꢀ2.2534280
ꢀ2.2600981
ꢀ2.3652696
ꢀ2.3794344
ꢀ2.3941791
ꢀ2.3963702
ꢀ2.4057786
ꢀ2.4078193
ꢀ2.4231432
Standard deviation
a
E(HF) = entry ꢀ 645 hartree. The first entry is 8644.0818473 = ꢀ645 + .9181527.
E(HF) = entry ꢀ 1365 hartree. The first entry is ꢀ1364.1380573 = ꢀ1365 + .8619427.
b
Cl
r1
δ Cl'
1
r1'
γ
β1'
1 Si1
R4
R5
α1
β2
β3'
r2'
H'
H
H'
δ2
Hδ
α3
C2
r2
3
C3
α2
γ3
γ2
R2
R3
β4'
α4
Cα5
5
C4
R1
γ4
r5
H'
H
H'
H
Fig. 6. Comparison between: (A) simulated infrared spectrum and (B) experimental
infrared spectrum of 1,1-dichloro-1-silacyclopentane.
Fig. 5. The structure of 1,1-dichloro-1-silacyclopentane with parameter symbols
used in the assignment of vibrational frequencies. The primes refer to the hydrogen
atoms away from the observer and their parameters. The torsional angles (s1
–s5)
correspond with the C–C bond lengths of the ring (R1–R5).
taken from the B3LYP/6-311 + G(2d,p) calculation was converted
to a force field in internal coordinates. Subsequently, scaling fac-
tors of 0.88 for C–H stretches, 1.0 for SiX2 stretches and bends,
and 0.9 for all other coordinates were applied. The geometric aver-
age of interacting scaling factors was used to scale interaction force
constants. The resultant fixed scaled force field produced wave-
numbers somewhat more closely related to those observed exper-
imentally. The potential energy distribution (PED) resulted from
the set of internal symmetry coordinates chosen. A comparison be-
tween the observed and calculated wavenumbers, along with the
calculated infrared intensities, Raman activities, depolarization ra-
tios and potential energy distribution for twisted 1,1-dihalo-1-sila-
cyclopentane is given in Tables 1 and 2.
Fig. 7. Comparison between: (A) simulated infrared spectrum and (B) experimental
The infrared spectra were predicted from the MP2(Full)/6-
31G(d) calculations. The predicted scaled frequencies were used
infrared spectrum of 1,1-dichloro-1-silacyclopentane in the region below
1600 cmꢀ1
.

6
S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
jth vibration and b2 is the anharmonicity calculated from the
changes in the six polarizability components calculated by
Gaussian for that vibration. In terms of the six components of
polarizability change,
Table 4
Symmetry coordinates for vibrations of 1,1-dichloro-1-silacyclopentane in the order
obtained by the ab initio calculation B3LYP/6-311 + G(2d,p).
Species Description
A CH symm stretch (2,3)
Coordinate
S1 = r02 + r3
S2 = r4 + r05
S3 = r2 + r03
S4 = r04 + r5
S5 = d4 + d5
S6 = d2 + d3
S7 = b4 + b04
S8 = b4 ꢀ b04
S9 = b2 ꢀ b02
S10 = b2 + b02
1
3
CH symm stretch (4,5)
CH symm stretch (2,3)
CH symm stretch (4,5)
CH2 scissors (4,5)
CH2 scissors (2,3)
CH2 wag (4,5)
CH2 twist (4,5)
CH2 twist (2,3)
CH2 wag (2,3)
Ring breathing
a
¼
ð
axx
þ
ayy
þ
azz
Þ
ꢀ
ꢁ
b2 ¼ a2xx
þ
a2yy
þ
a2zz
ꢀ
axxayy
ꢀ
axxazz
ꢀ
ayyazz þ 3 a2xy
þ
a2xz
þ
a2yz
ꢀ
c4
c4
c2
ꢀ
+
+
ꢀ
c04 + b5 + b05
c04 + b5 ꢀ b05
c02 + b3 ꢀ b03
ꢀ
c5
c5
c3
–
+
+
ꢀ
c40
c50
c3
The Raman scattering cross section is given for the jth vibration
by the relation [10]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
c2
c02 + b3 + b03
ꢀ
c3
c3
4
@
@
r
X
24p4
ð
m0
ꢀ
mj
Þ
h
S11 = R1 ꢀ 0.809 (R2 + R3) + 0.309
(R4 + R5)
¼
Sj
m
j=kT
8p
2cmj
45ð1 ꢀ eꢀhc
Þ
S12 = b4 ꢀ b04
+
c4
ꢀ
c04 + b5 ꢀ b05
+
c5
c3
ꢀ
ꢀ
c40
c3
CH2 rock (4,5)
Ring breathing
CH2 rock (2,3)
Ring breathing
S13 = R1 + R2 + R3 + R4 + R5
In the normal coordinate program used to do the calculations,
S14 = b2 ꢀ b02
+
c2
ꢀ
c04 + b3 ꢀ b03
+
the Raman scattering cross sections in cm2 were calculated using
this relation from the Raman scattering activities converted to
Å4/amu units by the introduction of suitable conversion factors.
The activities give a more realistic Raman spectrum than the scat-
tering cross sections, especially in the very low frequency region,
and the simulated spectrum presented in Fig. 7 uses the activities
rather than the cross sections.
S15 = R1 + 0.309 (R2 + R3) ꢀ 0.809
(R4 + R5)
Ring deformation
S16
=
+
a1 ꢀ 0.809 (a2
a5
+ a3) + 0.309
(a4
)
SiCl2 symm stretch
Ring pucker
SiCl2 scissors
SiCl2 twist
S17 = r1 + r01
S18
=
s1 ꢀ 0.809 (s2
+
s3) + 0.309(s4
+
s5
)
S19 = d1
S20 = b1 ꢀ b01
ꢀ
c1
+
c40
B
CH antisymm stretch
(2,3)
CH antisymm stretch
(4,5)
CH antisymm stretch
(2,3)
CH antisymm stretch
(4,5)
CH2 scissors (4,5)
CH2 scissors (2,3)
CH2 wag (4,5)
CH2 twist (4,5)
CH2 wag (2,3)
CH2 twist (2,3)
CH2 rock (4,5)
Ring breathing
CH2 rock (2,3)
Ring breathing
SiCl2 antisymm stretch
Ring deformation
SiCl2 rock
S21 = r02 ꢀ r3
S22 = r4 ꢀ r05
S23 = r2 ꢀ r03
S24 = r04 ꢀ r5
6. Discussion
There has been considerable interest concerning the most stable
conformer for silicon and germanium containing five-membered
rings. Guirgis et al. [1,3] reported that the 1-silacyclopentane and
1-germacyclopentane molecules exist in twisted form. The Laane
[2] laboratory previously showed the molecule to be twisted with
C2 symmetry but based their assignments and force constant cal-
culations on the easier to visualize planar C2v model. The results
of the Gaussian calculations confirm that the C2 conformer is the
most stable of the three possible conformers, and that the Cs con-
former lies above the C2 in energy. Both lie well below the planar
C2v form, which appears as a barrier or transition state according
to the Gaussian vibrational calculation.
The frequencies observed in the infrared spectrum of the gas
and the Raman spectrum of the liquid both at room temperature
and at ꢀ70 °C are collected together with the calculated frequen-
cies, both unscaled ab initio and scaled, in Tables 1 and 2 for 1,1-
dichloro-1-silacyclopentane and 1,1-difluoro-1-silacyclopentane
respectively. The symmetry coordinates used are collected in
Table 4.
S25 = d4 ꢀ d5
S26 = d2 ꢀ d3
S27 = b4 + b04
S28 = b4 ꢀ b0
ꢀ
c4
c4
ꢀ
c04 ꢀ b5 ꢀ b05
c04 ꢀ b5 + b05
c02 ꢀ b3 ꢀ b03
c02 ꢀ b3 + b0
+
+
+
+
c5
c5
c3
c3
+
ꢀ
+
ꢀ
c50
c50
c3
c3
ꢀ
+
ꢀ
+
S29 = b2 + b04ꢀ c2
2
S30 = b2 ꢀ b02
ꢀ
c2
ꢀ
S31 = b4 ꢀ b04
+
c4
c02 ꢀ b5 + b0 3ꢀ c5
+
c20
5
S32 = 1.118 (R3 ꢀ R2) + 0.691 (R5 ꢀ R4)
S33 = b2 ꢀ b02
+
c2
ꢀ
c02 + b3 ꢀ b03
+ c3 ꢀ c3
S34 = 1.118 (R2 ꢀ R3) + 1.809 (R5 ꢀ R4)
S35 = r1 ꢀ r01
S36 = 1.118 (a3
ꢀ
a
2) + 1.809 (a5
c20
c20
3) ꢀ 1.809(s4
ꢀ
a4
)
S37 = b1 ꢀ b0
+
c1
ꢀ
S38 = b1 + b01ꢀ c1
ꢀ
SiCl2 wag
Ring pucker
1
S39 = 1.118 (s2
ꢀ
s
ꢀ
s5)
For reference, the geometries calculated for 1,1-difluorosilacycl-
opentane and 1,1-dichlorosilacyclopentane are collected in Tables
5 and 6.
The assignments made in this work are compared with those of
Philen, Chao and Laane [6] in Table 7.
to obtain the calculated spectra. Infrared intensities determined
from MP2(Full)/6-31G(d) calculations were obtained based on
the dipole moment derivatives with respect to Cartesian coordi-
nates. The derivatives were transformed with respect to normal
P
coordinates by (d
l
/dQi) = j(d
l
/Xj)Lij, where Qi is the ith normal
coordinates, Xj is the jth Cartesian displacement coordinate, and
Lij is the transformation matrix between the Cartesian displace-
ment coordinates and the normal coordinates. The infrared inten-
7. Conclusion
The infrared spectrum of gas and Raman spectrum of liquid 1,1-
difluoro-1-silacyclopentane and of 1,1-dichloro-1-sulacyclopen-
tane are reported at room temperature. When the liquid was
cooled to ꢀ70 °C, there was no change in the relative intensities
of any of the Raman bands, strongly suggesting that only one con-
former was present. The vibrational frequencies were calculated
for several basis function selections and computational methods,
and the results collected together with the best assignment of fun-
damentals that the data allowed. The assignment is not complete,
as several bands were evidently insufficiently intense to appear
under the conditions it was possible to obtain the spectra. None-
theless, substantial agreement with the calculated values was
sities were then calculated by (Np l
/3c2) [d x/dQi)2]. In Fig. 6, a
comparison of the experimental and the simulated infrared spec-
trum of 1,1-dichloro-1-silacyclopentane is shown. The predicted
spectrum is in fairly good agreement with the observed.
To further support the vibrational assignments, we have simu-
lated the Raman spectra from the ab initio MP2(Full)/6-31G(d) re-
sults. The evaluation of the Raman activity by using the analytical
gradient methods is routinely included in the Gaussian 03 calcula-
tions. The activity Sj can be expresses as: Sj = gj(45a
2 + 7b2), where
gj is the degeneracy of the jth Raman active vibration, a2 is the
square of the spherical part of the polarizability change for the

S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
7
Table 5
Geometrical parameters of 1,1-difluoro-1-silacyclopentane by various ab initio and B3LYP calculation methods.a
Internal coord
Coord symbol
HF 6-31G(d)
MP2 6-31G(d)
B3LYP 6-31G(d)
B3LYP 6-311 + GP
SiC
C2C4
C4C5
SiF
R4 = R5
R2 = R3
R1
1.871
1.550
1.542
1.590
1.088
1.084
1.088
1.085
1.863
1.544
1.538
1.615
1.096
1.093
1.097
1.095
1.877
1.554
1.548
1.613
1.098
1.094
1.099
1.096
1.867
1.552
1.546
1.607
1.095
1.090
1.095
1.092
A
r1 = r01
r2
C2H
C2H0
C4H0
C4H
r20
r40
r4
C2SiC3
SiC2C4
C2C4C5
a1
a2
a4
b1
c1
d1
b1
b10
d2
b40
b4
d4
98.24
98.49
98.56
98.72
102.15
108.95
113.25
113.61
105.16
109.86
115.03
107.24
109.32
111.66
106.78
12.77
120.18
ꢀ116.87
122.04
118.96
ꢀ123.12
101.56
108.44
113.27
113.23
105.58
110.08
114.86
107.41
109.49
111.89
106.89
13.36
119.89
ꢀ116.54
122.09
118.52
ꢀ123.17
101.92
109.09
113.14
113.44
105.39
109.74
114.79
107.26
109.26
111.76
106.71
12.72
120.12
ꢀ116.97
122.16
118.93
ꢀ123.20
102.19
109.18
113.11
113.62
104.99
109.44
114.61
107.28
109.26
111.66
106.69
12.47
120.40
ꢀ117.13
122.33
119.08
ꢀ123.14
A
C2SiF
A
C2SiF0
A
FSiF0
SiC2H
SiC2H0
A
HC2H0
C2C4H0
C2C4H
HC4H0
A
C2SiC3C5
F07SiC3C2
H8C2SiC4
H010C2SiC4
H
12C5C3C4
H014C5C3C4
Rot constsb
A
B
C
3354.6
1701.1
1612.8
3320.2
1701.5
1618.3
3289.9
2682.5
1598.1
3320.9
1689.1
1605.8
a
b
A
1,1-Difluoro-1-sila-cyclopentane has C2 symmetry and the coordinates are given for only the atoms on the right side as shown in Fig. 1.
Calculated values of the rotational constants in MHz.
Coordinates not used in geometric optimization of the structure of 1,1-difluoro-1-sila-cyclopentane but included for completeness.
Table 6
Geometrical parameters of 1,1-dichloro-1-silacyclopentane by various ab initio and B3LYP calculation methods.a
Internal coord
Coord symbol
HF 6-31G(d)
MP2 6-31G(d)
B3LYP 6-31G(d)
B3LYP 6-311 + GP
SiC
C2C4
C4C5
SiCl
R4 = R5
R2 = R3
R1
1.880
1.549
1.540
2.066
1.087
1.084
1.088
1.085
1.877
1.544
1.536
2.062
1.097
1.094
1.098
1.095
1.887
1.552
1.543
2.085
1.097
1.094
1.098
1.096
1.877
1.549
1.544
2.082
1.094
1.090
1.095
1.093
A
r1 = r01
r2
C2H
C2H0
C4H0
C4H
r20
r40
r4
C2SiC3
SiC2C4
C2C4C5
a1
a2
a4
b1
c1
d1
b1
b10
d2
b40
b4
d4
97.73
97.74
97.90
97.95
102.18
108.90
112.86
113.04
107.30
109.69
114.61
107.44
109.45
111.32
106.84
12.93
119.07
ꢀ116.97
122.09
119.20
ꢀ122.92
101.66
108.30
112.86
112.65
107.95
109.86
114.42
107.59
109.61
111.57
106.98
13.58
118.63
ꢀ116.67
122.21
118.71
ꢀ122.98
102.09
109.10
112.74
112.98
107.46
109.31
114.25
107.51
109.40
111.35
106.78
12.81
119.05
ꢀ117.18
122.33
119.24
ꢀ122.96
102.35
109.05
112.77
113.18
107.03
109.02
113.98
107.58
109.39
111.41
106.77
12.70
119.31
ꢀ117.30
122.50
119.23
ꢀ122.96
A
C2SiCl
A
C2SiCl0
A
ClSiCl0
SiC2H
SiC2H0
A
HC2H0
C2C4H0
C2C4H
HC4H0
A
C2SiC3C5
C0l7SiC3C2
H8C2SiC4
H0l0C2SiC4
H
12C5C3C4
H0l4C5C3C4
Rot constsb
A
B
C
1781.3
1206.1
931.2
1776.3
1221.3
936.6
1750.5
1195.3
919.5
1765.5
1198.5
923.5
a
b
A
1,1-Dichloro-1-sila-cyclopentane has C2 symmetry and the coordinates are given for only the atoms on the right side as shown in Fig. 1.
Calculated values of the rotational constants in MHz.
Coordinates not used in geometric optimization of the structure of 1,1-dichloro-1-sila-cyclopentane but included for completeness.

8
S. Bell et al. / Journal of Molecular Structure 992 (2011) 1–8
Table 7
achieved. It is evident from the relative energies that the con-
former stable at room temperature is the twisted structure of C2
symmetry.
Comparison of assignments for (C4H8)SiX2 made in this work with those suggested by
Philen, Chao and Laane [5].
Species
A
Description
Fa
Cla
Fb
Clb
Acknowledgements
CH symm str (2,3)
CH symm str (4,5)
CH sym str (2,3)
CH sym str (4,5)
CH scissors (4,5)
CH scissors (2,3)
CH2 wag (4,5)
CH2 twist (4,5)
CH2 twist (2,3)
CH2 wag (2,3)
CH2 rock (4,5)
CH2 rock (2,3)
Ring breathing
Ring breathing
Ring breathing
Ring deformation
Ring pucker
2947
2904
2870
2863
1462
1408
1316
1253
1196
1078
917
2949
2910
2953
2911
2881
2874
1464
1410
1267
1017
1193
1310
712
864
1098
855
629
434
2947
2909
2909
2872
1481
1403
1267
1016
1197
1320
708
[868]
1082
846
735
515
Gamil A. Guirgis would like to acknowledge the College of
Charleston Office of Undergraduate Research and Creative Activi-
ties for the SURF Grant (SU2010-0029) which supported the re-
search of Ya Ying Zheng. Also the authors thank Prof. Jaan Laane
for his constructive remarks.
2867
1451
1404
1317
1250
1191
1076
915
735
1013
844
667
516
References
697
1017.5
848
[1] G.A. Guirgis, A.M. El Defrawy, T.K. Gounev, M.S. Soliman, J.R. Durig, J. Mol.
Struct. 832 (2007) 73–83.
[2] L.F. Colegrove, J.C. Wells, J. Laane, J. Chem. Phys. 93 (1990) 6291–6302.
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Struct. 832 (2007) 17–29.
[4] R. West, J. Am. Chem. Soc. 76 (1954) 6012–6014.
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Spectrochim. Acta A 61 (2005) 1547–1557.
608
229?
871
272
174
230
287
877
280
186
316
349
168
171
SiX2 sym str
SiX2 scissors
SiX2 twist
346.5
169.5
150
[6] D.L. Philen, T.H. Chao, J. Laane, J. Mol. Struct. 16 (1973) 417–431.
[7] Gaussian 03, Revision D.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E.
Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin,
J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M.
Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J.
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[8] P. Pulay, in: H.F. Schaefer III (Ed.), Modern Theoretical Chemistry, vol. 4,
Plenum Press, New York, 1977, p. 153;
B
CH antisymm str (2,3)
CH antisymm str (4,5)
CH antisymm str (2,3)
CH antisymm str (4,5)
CH scissors (4,5)
CH scissors (2,3)
CH2 wag (4,5)
2947
2904
2949
2910
2961
2932
2916
2881
1464
1410
1317
1256
1162
1–37
796
[646]
944
[678]
490
970
326
2947
2947
2909
2865
1455
1403
1300
1252
1160
1031
809
674
949
674
484
550
230
230
112?
2867
1459
1453
1408
CH2 wag (2,3)
1156
1155
CH2 twist (4,5)
CH2 twist (2,3)
CH2 rock (4,5)
CH2 rock (2,3)
Ring breathing
Ring breathing
Ring deformation
SiX2 str
1020
942
804
944
780
697
492
877
705
484
542
228
P. Pulay, Mol. Phys. 17 (1969) 197.
[9] E.B. Wilson Jr., J.C. Decius, P.C. Cross, Molecular Vibrations, McGraw-Hill, New
York, NY, 1955.
SiX2 wag
SiX2 rock
Ring pucker
287
[10] P.L. Polavarapu, J. Phys. Chem. 94 (1990) 8106–8112.
a
Frequencies assigned in this work.
Assignments given in Ref. [6].
b