Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for silacyclopentane

Article abstract of DOI:10.1016/j.molstruc.2006.07.041

The infrared spectra (3500-50 cm^-1) of the gas and solid and the Raman spectrum (3200-30 cm^-1) of liquid silacyclopentane, c-C₄H₈SiH₂, have been recorded. Additionally the infrared spectrum (3200-400 cm^-1) of liquid xenon solutions has been recorded at -65 and -95 °C. In all of the physical states only one conformer was detected which is the twisted C₂ form. The conformational energetics have been calculated with the M?ller-Plesset perturbation method to the second order, (MP2(full)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. All of these calculations predict only one stable conformer i.e. the C₂ form with an average barrier to planarity of 2558 cm^-1 (30.60 kJ/mol) from the MP2 calculations and a significantly lower value of 1918 cm^-1 (22.95 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions. From the isolated SiH frequency from the SiHD isotopomer the SiH distance was calculated to be 1.486(3) A?. By utilizing the previously reported microwave rotational constants for three isotopomers (²⁸Si,²⁹Si, and ²⁸Si-d₂) combined with the structural parameters predicted from the MP2(full)/6-311 + G(d,p) calculations, adjusted r₀ structural parameters have been obtained. The determined heavy atom distances are: r₀(SiC) = 1.890(5); r₀(C₂C₄) and (C₃C₅) = 1.547(5); r₀(C₄C₅) = 1. 535(5) A? and the angles in degrees: ∠CSiC = 95.9(5)°; ∠SiCC = 103.3(5)°; ∠CCC = 107.9(5)° with the two dihedral angles ∠SiCCC = -38.3(3)° and ∠CCCC = 52.4(3)°. A complete vibrational assignment is given for the twisted C₂ conformer for the normal species and the Si-d₂ isotopomer which are supported by normal coordinate calculations utilizing scaled force constants from ab initio MP2(full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Full text of DOI:10.1016/j.molstruc.2006.07.041

Journal of Molecular Structure 832 (2007) 73–83
www.elsevier.com/locate/molstruc
Conformational stability, r structural parameters, ab initio
0
calculations, and vibrational assignment for silacyclopentane
a
b,1
b
Gamil A. Guirgis , Ahmed M. El Defrawy , Todor K. Gounev ,
c
b,
*
Mamdouh S. Soliman , James R. Durig
a
Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
b
c
Department of Chemistry, Mansoura University, Mansoura, Egypt
Received 28 June 2006; accepted 17 July 2006
Available online 10 October 2006
Abstract
The infrared spectra (3500–50 cm ) of the gas and solid and the Raman spectrum (3200–30 cm ) of liquid silacyclopentane,
ꢀ
1
ꢀ1
ꢀ
1
c-C H SiH , have been recorded. Additionally the infrared spectrum (3200–400 cm ) of liquid xenon solutions has been recorded at
4
8
2
ꢀ
65 and ꢀ95 °C. In all of the physical states only one conformer was detected which is the twisted C
getics have been calculated with the Møller–Plesset perturbation method to the second order, (MP2(full)) as well as with density func-
tional theory by the B3LYP method utilizing a variety of basis sets. All of these calculations predict only one stable conformer i.e. the C
2
form. The conformational ener-
2
ꢀ
1
form with an average barrier to planarity of 2558 cm (30.60 kJ/mol) from the MP2 calculations and a significantly lower value of
ꢀ
1
1
918 cm (22.95 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions.
˚
From the isolated SiH frequency from the SiHD isotopomer the SiH distance was calculated to be 1.486(3) A. By utilizing the previously
reported microwave rotational constants for three isotopomers ( Si, Si, and Si-d ) combined with the structural parameters predicted
2
8
29
28
2
from the MP2(full)/6-311 + G(d,p) calculations, adjusted r
tances are: r (SiC) = 1.890(5); r (C ) and (C ) = 1.547(5); r
SiCC = 103.3(5)°; \CCC = 107.9(5)° with the two dihedral angles \SiCCC = ꢀ38.3(3)° and \CCCC = 52.4(3)°. A complete vibra-
tional assignment is given for the twisted C conformer for the normal species and the Si-d isotopomer which are supported by normal
0
structural parameters have been obtained. The determined heavy atom dis-
˚
0
0
2
C
4
3
C
5
0
(C
4
C
5
) = 1. 535(5) A and the angles in degrees: \CSiC = 95.9(5)°;
\
2
2
coordinate calculations utilizing scaled force constants from ab initio MP2(full)/6-31G(d) calculations. The results of these spectroscopic
and theoretical studies are discussed and compared to the corresponding results for some similar molecules.
Ó 2006 Elsevier B.V. All rights reserved.
0
Keywords: Conformational stability; r structural parameters; Ab initio calculations; Silacyclopentane
1
. Introduction
Five-membered ring molecules are structurally interest-
having more than one form stable. Additionally, the energy
differences among the forms may be so small that they
readily interconvert with the motion appropriately called
and treated as pseudorotation [1–4]. Therefore, we [5]
recently investigated the conformational stability of 2,5-
disila-1-oxacyclopentane (c-OSiH CH CH CH SiH ) and
ing since potentially there are three different conformers,
i.e. planar, twisted, and envelope forms, any of which can
be the most stable one or the only one present, as well as
2
2
2
2
2
found the twisted C conformer as the most stable form
2
with the planar C2V conformer a transition state and the
*
Corresponding author. Tel.: +1 816 235 6038; fax: +1 816 235 2290.
E-mail address: durigj@umkc.edu (J.R. Durig).
Taken in part from the thesis of A.M.El Defrawy, which will be
envelope C conformer not a stable form. Ab initio and
s
1
density functional theory calculations with several basis
sets including diffuse functions can frequently be used to
submitted in partial fulfillment of the Ph.D. degree.
0
022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.07.041

74
G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
predict the conformational stabilities as well as the barriers
between stable conformers or the identification of transi-
tion states. Furthermore, the predicted frequencies for the
vibrational fundamentals for different conformers are
usually sufficient for the experimental determination of
the conformers present (as an example, 2,2,5,5-tetramethyl-
2
,5-disila-1-oxacyclopentane, c-OSi(CH ) CH CH Si(CH )
3 2 2 2 3 2
[5] at ambient temperature as well as the determination of
the enthalpy difference from temperature studies when more
than one conformer is present.
As a continuation of our recent studies [6–8] of the struc-
ture and conformational studies of some substituted five-
membered rings, we have again investigated the infrared
spectra of gaseous, solid, and xenon solutions of silacycl-
opentane. To support the vibrational study we have carried
out ab initio calculations at the MP2 level with full electron
correlation by the perturbation method [9] utilizing a variety
of basis sets up to 6-311G(2d,2p) as well as those with the
diffuse functions, i.e. 6-311 + G(2d,2p). We found that
there is not a significant difference in the predicted stabilities
of the c-C H SiH molecule with the size of the basis set,
particularly with the inclusion of the diffuse functions. We
also carried out density functional theory (DFT) calcula-
tions by the B3LYP method with the corresponding basis
sets used for the MP2 calculations. We have calculated the
optimized geometries, conformational stabilities, harmonic
force fields, infrared intensities, Raman activities, and
depolarization ratios. The results of these spectroscopic
and theoretical studies are reported herein.
Fig. 1. Comparison of experimental and calculated infrared spectra of
silacyclopentane: (A) observed infrared spectrum in gas; (B) observed
infrared spectrum in liquid xenon solution at ꢀ65 °C; (C) simulated
4
8
2
2
infrared spectrum of twisted C silacyclopentane.
olution, averaged and transformed with a boxcar trunca-
tion function. For these studies, a specially designed
cryostat cell was used. It consists of a copper cell with a
ꢀ
1
path length of 4 cm with wedged silicon windows sealed
to the cell with indium gaskets. The copper cell was
enclosed in an evacuated chamber fitted with KBr win-
dows. The temperature was maintained with boiling liquid
nitrogen and monitored by two Pt thermoresistors. The
complete cell was connected to a pressure manifold, allow-
ing the filling and evacuation of the system. After cooling
to the designated temperature, a small amount of the sam-
ple was condensed into the cell. Next, the system was then
pressurized with the noble gas, which immediately started
to condense in the cell, allowing the compound to dissolve.
The far infrared spectrum of gaseous sample was
recorded on the previously described Perkin-Elmer 2000
spectrometer. A grid beam splitter and a cryostat cell with
polyethylene windows was used to record the spectrum of
the solid with sample being deposited on a silicon substrate
at 77 K and multiple annealings were performed in order to
obtain a good polycrystalline solid. For the spectrum of the
gas, the sample was contained in a 12 cm cell equipped with
polyethylene windows. The spectra were recorded at a
1
.1. Experimental
The silacyclopentane sample was prepared by first pre-
paring 1,1-dichloro-1-silacyclopentane compound (bp
50 °C) by the method of West [10]. The 1,1-dichloro-1-
1
silacyclopentane was then purified using trap-to-trap distil-
lation, then reduced by using lithium aluminum hydride in
dry n-butyl ether to produce the corresponding hydride. It
was further purified by means of a low-temperature, low-
pressure fractionation column and the purity was checked
by NMR and a comparison of the infrared spectrum to
that previously reported.
The mid-infrared spectrum of gas (Fig. 1A) was
ꢀ
1
obtained from 3500 to 230 cm on a Perkin-Elmer model
000 Fourier transform spectrometer equipped with a Ge/
2
CsI beamsplitter and a DTGS detector. Atmospheric water
vapor was removed from the spectrometer housing by
purging with dry nitrogen. The theoretical resolution used
to obtain the spectrum of the gas was 0.5 cm . Sixty-four
interferograms were added and transformed with a boxcar
truncation function.
ꢀ1
ꢀ1
spectral resolution of 0.5 cm , and typically, 256 scans
were used for both the sample and the reference data to give
a satisfactory signal-to-noise ratio. The interferograms were
averaged and then transformed with a boxcar truncation
function. All of the bands of significant intensity observed
in the various infrared and Raman spectra which were
chosen as fundamentals are listed in Table 1 for c-C H SiH
ꢀ1
The mid-infrared spectra (3500–400 cm ) of the sample
dissolved in liquified xenon at two temperatures (ꢀ65 °C,
Fig. 1B, ꢀ95 °C) were recorded on a Bruker model IFS-
6
6 Fourier transform spectrometer equipped with a globar
source, a Ge/KBr beamsplitter and a DTGS detector. In all
cases, 100 interferograms were collected at a 1.0 cm res-
4
8
2
ꢀ
1
and those for the c-C H SiD isotopomer in Table 2.
4
8
2

Table 1
Calculated and observed frequencies (cm ) for silacyclopentane twisted C
a
ꢀ1
2
form
c
Symmetry Vib. no. Approx. description
block
Ab initio Fixed
IR int. Raman act. dp ratio Infrared
Gas
Raman Infrared P.E.D.
Band contour
b
scaled
Xe solu. Liquid
Solid
A
B
C
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
1
a-CH
b-CH
a-CH
b-CH
2
2
2
2
antisymmetric stretch 3180
antisymmetric stretch 3146
2983
2951
2921
2899
2166
1484
1439
1328
1258
1208
1090
1028
956
947
852
767
668
568
405
268
2983
2957
2921
2901
2171
1476
1442
1311
1258
1167
1033
946
889
842
757
647
510
448
105
1.3
28.7
4.1
22.9
160.8
0.2
0.2
2.2
2.6
0.1
38.2
1.5
73.2
44.9
0.1
4.2
2.3
4.0
5.9
134.9
143.2
148.8
147.2
151.5
2.9
25.5
3.5
7.1
2.1
6.3
6.2
10.8
8.4
0.64
0.28
0.21
0.09
0.09
0.45
0.75
0.73
0.61
0.31
0.72
0.67
0.70
0.75
0.11
0.50
0.10
0.46
0.50
0.65
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
2948 2936
2923 2907
2878 2874
2866 2866
2148 2143
1458 1456
1420 1419
1317 1315
1250 1247
1198 1200
1079 1077
1021 1020
2940
2909
2875
2860
2144
1452
1412
1316
1248
1199
1079
1022
956
946
852
768
689
576
413
264
2957
–
2946
2907
2892
2866
2138
1457
1421
1304
1247
1199
1074
1022
956
–
853
767
682
576
412
–
2960
2927
2917
2854
2132
1449
1396
1304
1257
1146
1039
921
886
832
758
662
530
466
93S
85S
90S
89S
1
2
3
4
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
,11S
4
3
symmetric stretch
symmetric stretch
3114
3090
2283
1565
1517
1401
1326
1274
1149
1084
1008
998
4
, 11S
2
5
SiH
2
symmetric stretch
100S
5
6
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
twist
92S
92S
61S
45S
6
7
8
9
7
8
, 23S
, 37S11
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
38S10, 31S14, 10S11
39S11,21S ,11S17
46S12,24S17,20S
64S13,13S14,13S10
27S14,34S13,27S10
44S15,23S17,16S12
43S16,30S18,15S14
38S17,35S15,11S18
46S18,31S16
wag
9
ring deformation
SiH deformation
b-CH rock
Ring breathing
a-CH rock
Ring deformation
SiH twist
8
2
960
945
956
941
2
898
808
704
599
428
284
6.5
7.5
18.8
6.7
2.9
–
–
2
767
684
572
411
767
685
575
410
2
Ring deformation
Ring twist
80S19
0.04
0.04
–
–
88S20
91S21
a-CH
b-CH
a-CH
b-CH
2
2
2
2
antisymmetric stretch 3180
antisymmetric stretch 3152
27.1
35.6
14.9
29.5
182.2
6.6
12.9
1.4
0.2
0.6
12.0
0.7
72.7
67.7
83.4
2.8
16.6
4.7
11.4
52.1
5.1
32.1
63.4
18.7
1.7
1.0
10.4
2.9
2.1
6.6
3.2
1.2
2.8
9.8
0.6
0.4
2963 2955
2954
16 84
46 54
71 29
24 76
–
92S22
symmetric stretch
symmetric stretch
3114
3092
2288
1556
1520
1383
1326
1230
1089
997
2934 2926
2857 2856
2920
–
–
91S23
98S24
SiH
2
antisymmetric stretch
2151
–
100S25
1
99
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
wag
1454 1450
1420 1419
1310 1305
1258 1260
1151 1148
1037 1033
1452
1412
1316
1255
1150
1037
–
890
832
–
99S26
12 88
70 30
56 44
11 89
6
3
93
99
72 28
78 22
45 55
13 87
99S27
87S28
49S29,20S30,20S31
56S30,15S37
94
97
7
twist
58S31,32S29
Ring deformation
931
891
833
760
661
529
455
925
887
833
757
661
521
457
35S32,18S30,12S33,12S36,12S37
27S33,30S35,44S36
43S34,16S33,12S35,11S38
46S35,19S34,12S33,11S32
56S36,27S32,11S35
40S37,37S38,12S33,12S39
38S38,27S37,25S34
82S39,11S37
b-CH
a-CH
2
rock
rock
937
888
1
2
SiH
2
wag
798
682
538
473
Ring deformation
Ring deformation
665
525
462
–
SiH
2
rock
–
–
2
8
98
92
Ring puckering
112
0.8
0.02
–
–
a
˚
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (A /u), depolarization ratios (dp), and potential energy distributions (P.E.D.s).
Scaled frequencies with scaling factors of 0.88 for CH and SiH stretches, with 0.90 for all other modes.
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.
b
c

Table 2
Calculated and observed frequencies (cm ) for silacyclopentane Si-d
a
ꢀ1
2
2
twisted C form
b
d
Symmetry block Vib. No. Approx. description Ab initio Fixed scaled
IR int. Raman act. dp ratio Infrared Raman P.E.D.
Band contour
c
c
Gas
Liquid
A
B
C
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
1
a-CH
b-CH
a-CH
b-CH
2
2
2
2
antisymmetric stretch 3180
antisymmetric stretch 3146
2983
2951
2921
2899
1551
1485
1439
1329
1258
1208
1089
1028
702
937
852
735
638
456
391
267
2983
2957
2921
2901
1569
1476
1442
1311
1257
1167
1028
946
868
815
543
687
489
380
98
1.3
29.1
4.1
22.9
97.2
0.0
0.0
2.8
1.4
0.2
30.5
2.3
76.6
0.3
0.4
0.0
2.2
2.0
3.0
0.0
26.4
35.4
14.8
29.6
105.4
7.4
13.1
0.8
0.1
0.7
7.2
0.6
9.2
48.1
25.4
70.2
7.4
8.7
0.7
134.8
143.5
148.2
146.8
71.3
2.5
26.4
3.7
7.1
2.0
7.1
6.4
6.4
0.64
0.28
0.22
0.09
0.09
0.56
0.75
0.72
0.56
0.26
0.71
0.67
0.27
0.61
0.09
0.18
0.19
0.60
0.56
0.72
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
2937
–
2931
2902
2869
2856
1553
1450
1409
1316
1246
1199
1075
1019
–
941
849
729
652
465
–
93S
85S
90S
89S
99S
92S
92S
61S
45S
1
2
3
4
5
6
7
8
9
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
,11S
4
2
3
symmetric stretch
symmetric stretch
3114
3090
1635
1565
1517
1401
1326
1273
1148
1084
740
2868
2860
1560
1456
1409
1319
1254
–
1077
1021
715
940
–
4
,11S
5
SiD
2
symmetric stretch
6
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
twist
7
8
,23S
9
9
,36S11
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
38S10,31S14,10S11
39S11,21S ,11S17
wag
9
Ring deformation
SiD deformation
b-CH rock
Ring breathing
a-CH rock
Ring deformation
SiD twist
47S12,24S17,20S
73S13
8
2
2
988
898
775
672
481
412
281
1.9
6.8
6.8
16.6
5.7
46S14,42S10
47S15,22S17,16S12
52S16,11S15
26S17,27S15,22S13
63S18,25S19
58S19,19S18,13S16
86S20
2
733
–
–
–
–
–
2
460
394
259
2965
2950
2924
2909
1568
1456
1409
1307
1261
–
1033
–
861
–
561
705
497
405
–
Ring deformation
Ring twist
3.0
0.1
–
a-CH
b-CH
a-CH
b-CH
2
2
2
2
antisymmetric stretch 3180
antisymmetric stretch 3153
11.4
52.4
5.1
31.6
29.2
19.3
1.7
1.2
10.8
3.0
2.0
6.6
0.8
1.2
8.3
1.2
0.4
0.6
2954
–
2916
2909
–
1450
1409
–
91S21
92S22
17
46
72
24
1
10
69
42
41
21
1
93
90
64
98
88
36
–
83
54
28
76
99
90
31
58
59
79
99
7
10
36
2
12
64
100
92
symmetric stretch
symmetric stretch
3114
3092
1654
1556
1520
1382
1325
1230
1048
997
91S23
98S24
SiD
2
antisymmetric stretch
100S25
99S26
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
wag
99S27
87S28
–
49S29,20S30,20S31
56S30,15S37
58S31,33S29
35S32,18S30,12S33,12S36,12S37
45S33,47S36
61S34
1147
–
–
–
–
556
701
498
399
–
twist
Ring deformation
b-CH
a-CH
2
rock
rock
915
859
2
SiD
2
wag
572
724
515
401
75S35,19S36
24S36,32S32,16S35,13S33
59S37,18S33,15S39
73S38,17S34
79S39,11S37
Ring deformation
Ring deformation
SiD
2
rock
–
–
Ring puckering
103
0.0
8
a
˚
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (A /u), depolarization ratios (dp) and potential energy distributions (P.E.D.s).
Scaled frequencies with scaling factors of 0.88 for CH and SiH stretches, with 0.90 for all other modes.
Reference [16].
b
c
d
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.

G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
77
1
.2. Ab initio calculations
mined from MP2(Full)/6-31G(d) calculations were
obtained based on the dipole moment derivatives with
respect to Cartesian coordinates. The derivatives were
The LCAO-MO-SCF restricted Hartree–Fock calcula-
tions were performed with the Gaussian 03 program [11]
using Gaussian-type basis functions. The energy minima
with respect to nuclear coordinates were obtained by the
simultaneous relaxation of all geometric parameters using
the gradient method of Pulay [12]. The 6-31G(d), 6-
transformed with respect to normal coordinates by
P
ðol =oQ Þ ¼
ðol =oX
j
ÞLij, where Q is the ith normal
u
i
j
u
i
coordinate, X is the jth Cartesian displacement coordinate,
j
and L is the transformation matrix between the Cartesian
ij
displacement coordinates and the normal coordinates. The
2
3
11G(d,p), and 6-311G(2d,2p) basis sets as well as the cor-
infrared intensities were then calculated by (Np)/(3c )
2
2
2
responding ones with diffuse functions were employed with
the Møller–Plesset perturbation method [9] to the second
order (MP2(full)) as well as with the density functional the-
ory by the B3LYP method. The predicted conformational
energy differences are listed in Table 3. In order to obtain
a complete description of the molecular motions involved
in the fundamental modes of silacyclopentane, a normal
coordinate analysis has been carried out. The force field
in Cartesian coordinates was obtained with the Gaussian
[(ol /oQ ) + (ol /oQ ) + (ol /oQ ) ]. In Fig. 1, a compar-
x
i
y
i
z
i
ison of experimental and simulated infrared spectra of sila-
cyclopentane is shown. The predicted spectrum is in good
agreement with the experimental spectrum which indicates
the utility of the scaled predicted frequencies and predicted
intensities.
To further support the vibrational assignments, we
have simulated the Raman spectra from the ab initio
MP2(Full)/6-31G(d) results. The evaluation of Raman
activity by using the analytical gradient methods has been
0
3 program at the MP2(Full) level with the 6-31G(d) basis
set. The internal coordinates used to calculate the G and B
matrices are shown in Table 4 with the atomic numbering
shown in Fig. 2. By using the B matrix [13], the force field
in Cartesian coordinates was converted to a force field in
internal coordinates. Subsequently, scaling factors of 0.88
for CH stretches, 0.9 for all other coordinates were applied,
along with the geometric average of scaling factors for
interaction force constants, to obtain the fixed scaled force
field and resultant wavenumbers. A set of symmetry coor-
dinates was used (Table 5) to determine the corresponding
potential energy distributions (P.E.D.s). A comparison
between the observed and calculated wavenumbers, along
with the calculated infrared intensities, Raman activities,
depolarization ratios and potential energy distributions
developed [14,15]. The activity S can be expressed as:
j
2
2
S
j
¼ g ð45a þ 7b Þ, where g is the degeneracy of the vibra-
j
j
j
j
tional mode j, a is the derivative of the isotropic polariz-
j
ability, and b is that of the anisotropic polarizability. To
j
obtain the polarized Raman scattering cross sections, the
polarizabilities are incorporated into S by multiplying S
j
j
with (1 ꢀ q )/(1 + q ), where q is the depolarization ratio
j
j
j
of the jth normal mode. The Raman scattering cross sec-
tions and calculated wavenumbers obtained from the
Gaussian 03 program were used together with a Lorentzian
function to obtain the simulated Raman spectra. Compar-
ison of experimental Raman spectrum of the liquid and the
calculated one is shown in Fig. 3 and that for the Si-d iso-
2
topomer in Fig. 4.
for twisted silacyclopentane and the Si-d isotopomer, are
2
listed in Tables 1 and 2, respectively.
1.3. Conformational stability
The infrared spectra were predicted from the
MP2(Full)/6-31G(d) calculations. The predicted scaled fre-
quencies were used together with a Lorentzian function to
obtain the calculated spectra. Infrared intensities deter-
In the initial vibrational study [16] of silacyclopentane
the infrared spectra of the gas and liquid were utilized
along with the Raman spectrum of the liquid with the
Table 3
ꢀ1
a
a
Calculated electronic energies (hartree) and energy differences (cm ) for the twisted C
2
, planar C2v (TS) and envelope C
s
(TS) of silacyclopentane
Method/basis set
Twist (C
2
)
Envelop (C
S
)
ꢀDE(C ꢀ C Planar (C2v
2
S
)
)
ꢀDE(C
2
ꢀ C2v
)
RHF/6-31G(d)
RHF/6-31 + G(d)
MP2(full)/6-31G(d)
ꢀ446.213045
ꢀ446.215535
ꢀ446.846557
ꢀ446.856188
ꢀ447.180920
ꢀ447.183961
ꢀ447.248223
ꢀ447.249997
ꢀ447.945842
ꢀ447.952030
ꢀ448.016753
ꢀ448.017657
ꢀ448.026410
ꢀ448.026976
ꢀ446.205950
ꢀ446.208508
ꢀ446.839136
ꢀ446.849054
ꢀ447.173670
ꢀ447.176807
ꢀ447.241243
ꢀ447.243060
ꢀ447.939502
ꢀ447.945765
ꢀ448.010718
ꢀ448.011605
ꢀ448.020542
ꢀ448.021122
1557
1542
1629
1566
1591
1570
1532
1523
1392
1375
1325
1328
1288
1285
ꢀ446.202812
ꢀ446.205357
ꢀ446.834480
ꢀ446.844751
ꢀ447.169568
ꢀ447.172682
ꢀ447.236270
ꢀ447.238150
ꢀ447.936665
ꢀ447.943065
ꢀ448.008098
ꢀ448.009012
ꢀ448.017918
ꢀ448.018489
2246
2234
2651
2510
2491
2475
2623
2600
2014
1968
1900
1897
1864
1863
MP2(full)/6-31 + G(d)
MP2(full)/6-311G(d,p)
MP2(full)/6-311 + G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311 + G(2d,2p)
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)
a
Transition state.

78
G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
Table 4
˚
Structural parameters (A and degree), rotational constants (MHz) and dipole moment (Debye) for silacyclopentane twisted C
2
form
a
Structural parameters
r (Si-C2,3
Internal coordinates
MP2(full)/ 6-311 + G(d,p)
ED + MW
Adjusted parameters r
0
)
X
Y
Z
1
1
, X
, Y
2
1.895
1.544
1.535
1.480
1.096
1.093
1.098
1.095
95.6
103.1
108.0
107.9
107.3
107.2
113.1
113.4
109.9
114.6
109.6
112.4
108.6
111.5
109.6
111.9
ꢀ38.3
52.3
1.892(2)
1.535(5)
1.580(5)
1.497(8)
1.112(6)
1.112(6)
1.112(6)
1.112(6)
96.3(3)
103.6(3)
108.4(7)
112.3(29)
–
1.890(5)
1.547(5)
1.535(5)
1.486(3)
1.096(2)
1.093(2)
1.098(2)
1.095(2)
95.9(5)
103.3(5)
107.9(5)
107.9(5)
107.3(5)
107.2(5)
113.1(5)
113.4(5)
109.9(5)
114.6(5)
109.6(5)
112.4(5)
108.6(5)
111.5(5)
109.6(5)
111.9(5)
ꢀ38.3(3)
52.4(3)
r (C2,3 ꢀ C4,5
)
2
r (C
4
ꢀ C
5
)
r (Si-H6,7
r (C-H8,9
)
r
1
r
3
r
5
r
7
r
9
, r
, r
, r
, r
2
4
6
8
)
r (C-H10,11
r (C-H12,13
r (C-H14,15
)
)
)
, r10
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
C
3
Si C
Si C2,3
2,3 4,5
2
h
C
4,5
5,4
w
u
a
1
, w
, u
2
2
C
C
C
1
H
H
H
H
H
H
H
H
H
H
H
H
H
6 7
SiH
8,9
C
2,3
H
10,11
c
1
, c
, e
2
12,13
C
5,4
H
14,15
e
1
2
–
6,7SiC2,3
6,7SiC3,2
b
b
p
p
p
p
r
r
r
r
1
2
1
3
, b
, b
, p
, p
4
113.2(3)
113.2(3)
110.1(8)
110.1(8)
110.1(8)
110.1(8)
110.1(8)
110.1(8)
110.1(8)
110.1(8)
–
3
5
7
8,9
C
2,3 Si
2,3 Si
2,3 4,5
10,11
C
8,9
C
C
2, 6
p
10,11
12,13
14,15
12,13
14,15
C
C
C
C
C
2,3
5,4
C
C
C
C
C
4,5
4,5
4,5
3,2
3,2
4
, p
8
4
3
6, 2
r
8
, r
r
5, 1
5,4
5,4
5,4
7
, r
s SiCCC
1, 2
s s
s CCCC
49.7(14)
5477. 8
4137. 9
2661.5
A
B
C
5492.7
4134.4
2666.9
0.563
5473.9
4137.2
2661.7
b
jl
a
j = jl j = 0.726(6)
t
a
Values taken from Ref.[26].
b
Ref.[17].
second minor conformer present in the fluid phases. Since
neither the infrared nor Raman spectra of the solid was
recorded there is also a question whether the C conformer
2
is the form present in the solid.
The search for a second conformer began with the com-
parison of the infrared spectra of the sample in liquid
xenon at ꢀ65 and ꢀ95 °C (Fig. 5). There was no indication
that any band had significantly different intensity with the
variation in temperature. Therefore, no evidence was found
for a second conformer in the fluid phase. The infrared
spectra of the amorphous and the annealed solid were
recorded (Fig. 6) and all bands observed in the amorphous
phase were observed in the spectra of the polycrystalline
solid. However, three of the weaker bands which will be
shown (Fig. 6 marked by an asterisk) not to be fundamen-
tals appeared to decrease significantly in intensity but none
of them could be assigned to fundamentals of a second
conformer.
Fig. 2. Twisted conformer of silacyclopentane showing atomic
numbering.
depolarization data. With these data the fundamental
modes could be identified with considerable confidence
and it was concluded that the planar conformation (C )
2
v
or a molecule with a low barrier to pseudorotation could
be ruled out since only 31 of the 39 fundamentals are infra-
red active and only 12 fundamentals could give rise to
In support of the conclusion that there is only one stable
conformer of silacyclopentane from the experimental data
the ab initio calculations indicate that the planar (C2v)
polarized Raman lines, i.e. the molecule must have C or
2
C symmetry which could not be distinguished by the vibra-
s
and envelope (C ) forms are transition forms since each cal-
s
tional data. However, a microwave study [17] showed that
the major conformer present in the gas was the twisted C2
conformer. Therefore, the question is whether there is a
culation gives an imaginary frequency. Therefore, ab initio
calculations predict that only the twisted C conformer is a
2
stable conformer.

G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
79
Table 5
Symmetry coordinates of silacyclopentane
a
Description
a-CH antisymmetric stretch
b-CH
Symmetry coordinate
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
2
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
1
2
3
4
5
6
7
8
9
= r
= r
= r
= r
= r
= 4e
= 4c
= r
= r
3
7
3
7
1
ꢀ r
5
9
+ r
+ r
4
ꢀ r
ꢀ r10
+ r
+ r10
6
2
antisymmetric stretch
symmetric stretch
symmetric stretch
ꢀ r
8
a-CH
b-CH
2
+ r
+ r
+ r
5
9
2
+ r
+ r
4
6
2
8
2
SiH symmetric stretch
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
twist
1
ꢀ r
1
ꢀ r
ꢀ p
+ r
2
ꢀ r
3
ꢀ r
4
+ 4e
+ 4c
2
ꢀ r
5
ꢀ r
6
ꢀ r
7
ꢀ r
8
1
ꢀ p
1
2
ꢀ p
3
ꢀ p
4
2
ꢀ p
5
ꢀ p
6
ꢀ p
7
ꢀ p
8
1
ꢀ r
2
3
ꢀ r
4
+ r
+ r
5
ꢀ r
6
+ r
ꢀ r
7
ꢀ r
8
1
ꢀ r
2
ꢀ r
3
+ r
4
5
ꢀ r
6
7
+ r
8
10 = p
11 = p
1
ꢀ p
2
ꢀ p
3
3
+ p
4
+ p
5
ꢀ p
ꢀ p
6
ꢀ p
7
+ p
ꢀ p
8
8
wag
1
ꢀ p
2
+ p
ꢀ p
4
+ p
5
6
+ p
7
Ring deformation
SiH deformation
b-CH rock
Ring breathing
a-CH rock
Ring deformation
SiH twist
12 = 4Z ꢀ X
1
ꢀ X
2
ꢀ Y
1
ꢀ Y
2
2
13 = 4a ꢀ b
1
ꢀ b
2
ꢀ b
ꢀ r
+ Y
ꢀ p
ꢀ Y ꢀ Y
ꢀ b + b
ꢀ w ꢀ u
3
ꢀ b
+ r
+ Y
+ p
4
2
14 = r
15 = Z + X
16 = p + p
17 = X
18 = b
19 = 4h ꢀ w
20 = s + s
1
+ r
2
ꢀ r
3
4
5
+ r
6
6
ꢀ r
7
7
ꢀ r
ꢀ p
8
8
1
+ X
ꢀ p
2
1
2
2
1
2
3
4
5
+ p
ꢀ p
1
+ X
2
1
2
2
1
ꢀ b
2
3
4
Ring deformation
Ring twist
1
2
1
ꢀ u
2
1
2
a-CH
b-CH
a-CH
b-CH
2
2
2
2
antisymmetric stretch
antisymmetric stretch
symmetric stretch
21 = r
22 = r
23 = r
24 = r
25 = r
3
7
3
7
1
ꢀ r
ꢀ r
+ r
+ r
ꢀ r
5
9
5
9
2
ꢀ r
4
8
4
8
+ r
+ r10
ꢀ r
ꢀ r10
6
ꢀ r
ꢀ r
ꢀ r
6
symmetric stretch
2
SiH antisymmetric stretch
b-CH
a-CH
b-CH
b-CH
a-CH
a-CH
2
2
2
2
2
2
deformation
deformation
wag
twist
wag
26 = 4e
27 = 4c
28 = r
29 = r
30 = p
31 = p
1
ꢀ r
1
ꢀ r
2
ꢀ r
3
ꢀ r
4
ꢀ 4e
2
+ r
5
+ r
+ p
6
+ r
+ p
7
+ r
+ p
8
1
ꢀ p
1
ꢀ p
2
ꢀ p
3
ꢀ p
4
ꢀ 4c
2
+ p
ꢀ r
+ r
ꢀ p
+ p
5
6
7
8
1
1
1
1
ꢀ r
2
2
2
2
+ r
3
3
3
3
ꢀ r
4
4
4
4
ꢀ r
5
5
5
5
+ r
6
6
6
6
7
+ r
ꢀ r
+ p
ꢀ p
8
8
8
8
ꢀ r
ꢀ p
ꢀ p
ꢀ r
+ p
ꢀ p
+ r
ꢀ r
ꢀ p
ꢀ p
+ r
+ p
+ p
7
ꢀ p
+ p
+ Y
7
twist
7
Ring deformation
32 = X
1
ꢀ X
2
ꢀ Y
1
2
b-CH
a-CH
2
rock
rock
33 = r
34 = p
35 = b
36 = X
37 = w
38 = b
39 = s
1
+ r
+ p
2
ꢀ r
ꢀ p
+ b
3
3
3
ꢀ r
ꢀ p
ꢀ b
4
4
4
ꢀ r
5
ꢀ r
ꢀ p
6
6
+ r
+ p
7
+ r
+ p
8
8
2
1
2
ꢀ p
5
7
SiH
2
wag
1
ꢀ b
ꢀ X
ꢀ w
+ b
ꢀ s
2
Ring deformation
Ring deformation
1
2
+ Y
+ u
ꢀ b
1
ꢀ Y
2
1
2
1
ꢀ u
2
SiH
2
rock
1
2
3
ꢀ b
4
Ring puckering
1
2
a
Not normalized.
Fig. 3. Comparison of experimental and calculated Raman spectra of
silacyclopentane: (A) observed of liquid; (B) simulated spectrum.
Fig. 4. Comparison of experimental and calculated Raman spectra of
1,1-d -silacyclopentane: (A) observed of liquid; (B) simulated spectrum.
2

80
G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
ꢀ1
Fig. 5. Mid-infrared spectra of (400–1500 cm ): (A) liquid xenon solution at ꢀ65 °C; (B) liquid xenon solution at ꢀ95 °C for silacyclopentane.
ꢀ1
Fig. 6. Mid-infrared spectra of (400–1000 cm ): (A) amorphous; (B) annealed solid for silayclopentane. Bands with asterisk are not fundamentals.
1
.4. Structure
satisfactorily for a number of organosilanes although the
predicted values are usually too short by about 0.006 A.
˚
We [18] have recently shown that ab initio MP2/
-311 + G(d,p) calculations predict the r0 structural
Finally, we have found [25] that we can obtain good
structural parameters by adjusting the structural parame-
ters obtained from the ab initio calculations to fit the
rotational constants obtained from the microwave experi-
mental data. In order to reduce the number of independent
variables, the structural parameters are separated into sets
according to their types. Bond lengths in the same set keep
their relative ratio which results in only two heavy atom
distances for silacyclopentane. Also, the bond angles and
torsional angles in the same set keep the distances in
degrees. This assumption is based on the fact that the
6
parameters for more than 50 carbon–hydrogen distances
˚
better than 0.002 A compared to the experimentally deter-
mined values from isolated CH stretching frequencies
which were compared [19] to previously determined values
from earlier microwave studies. The SiH distance can be
obtained from the frequency of the isolated SiH stretch
from the Si-d isotopomer which is an impurity in the
1
Si-d2 sample [20]. It has also been shown [21–24] that
similar ab initio calculations predict the SiC distances

G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
81
errors from ab initio calculations are systematic. Therefore,
There is one major difference in the choice of the funda-
mentals from the earlier vibrational study [16] where two
it should be possible to obtain ‘‘adjusted r ’’ structural
0
ꢀ
1
parameters for silacyclopentane utilizing the microwave
rotational constants from the nine reported values [17]
bands were chosen between 930 and 970 cm
whereas
three fundamentals are predicted in this region from the
2
8
29
28
for the three isotopomers Si, Si, and Si-d2.
ab initio calculations. Since only two fundamentals were
ꢀ
1
First, the SiH distance was obtained [20] from the SiH
assigned in this 930–970 cm region resulted in assigning
ꢀ
1
ꢀ1
stretching frequency of 2150 cm which gives a SiH dis-
the 836 cm band into two fundamentals (m34 and m ) in
35
˚
tance of 1.486 ± 0.003 A. By utilizing the ab initio MP2/
the earlier study [16] with only one fundamental predicted.
ꢀ
1
6
-311 + G(d,p) predicted values for the CH distances
In the infrared spectrum of the gas, the band at 960 cm
leaves only the heavy atom parameters to be adjusted.
The predicted parameters from the various theoretical
calculations are listed in Table 4. The fit of the nine rota-
tional constants is given in Table 6 with eight of the nine
fit to 0.5 MHz or better. The fit of the A rotational for
clearly has an A-type contour and must be therefore
assigned to the SiH deformation (m ). On the low frequen-
2
13
cy shoulder of m13 there is a rather complex band which pre-
viously was assigned as one fundamental with two
ꢀ
1
Q-branches at 945 and 938 cm with the band center at
2
8
ꢀ1
the Si-d isotopomer is 1.01 MHz which is quite good
941 cm for the B-type band for m . The next lower fre-
2
32
ꢀ
1
when taking into account the slight differences in the SiH
and SiD distances.
quency band at 890 cm shows a very clear B-type band
for m33 (Fig. 7) with the ab initio predicted contour with
9
9% B-type; for m , the predicted contour (Table 1) is
32
1
.5. Vibrational assignment
93% B-type but it has a very small (0.7 km/mol) predicted
intensity. Therefore we have simply assigned it to the
ꢀ
1
The initial vibrational assignment [16] was proposed
shoulder a 931 cm (m ) but it could be in coincidence
32
ꢀ1
based on the infrared band contours and Raman deter-
mined depolarization values with the 20 fundamental of
the A species having A-type bands and polarized Raman
lines and the 19 fundamentals of the B species with B, C
or B–C hybrid contours and depolarized Raman lines.
However, because several of them are assigned as two fun-
damentals. Thus, band contours are not always easily dis-
cerned. In the current study the ab initio calculations
provided estimated frequencies, band contours for the
B species fundamentals, and infrared and Raman band
intensities which significantly aids in the vibrational assign-
ment. Additionally, infrared spectral data in the rare gas
with the 945 cm (m ) fundamental since there does not
14
appear to the expected corresponding Raman line for the
lower frequency. For the Si-d isotopomer, there is the pre-
2
dicted assignment of two fundamentals at 937 and
ꢀ1
946 cm where only one fundamental is assigned to band
ꢀ
1
at 940 cm . With the exception of this one fundamental,
the previous assignment [16] is in excellent agreement with
the predicted fundamental frequencies.
1.6. Discussion
At ambient temperature in the fluid phases, there is only
one conformer present which is the twisted form with C2
symmetry. The conformational energies have been calculat-
ed with a variety of basis sets at the level of MP2(full) ab ini-
tio calculations as well as by density functional theory
calculations by the B3LYP method. The MP2 predicted
barriers at the planar conformation range from a high val-
(
xenon) solution were recorded at two different tempera-
tures (ꢀ65 and ꢀ95 °C). Since the observed bands by this
technique are usually relatively sharp it was expected that
it should be possible to separate some of the bands that
were previously assigned to two fundamentals. Therefore,
by using the data from the ab initio calculations along with
more experimental data, i.e. infrared spectra of the solution
and solid, the original vibrational assignment have been
synchronized.
ꢀ
1
ꢀ1
ue of 2651 cm to a low value of 2475 cm with the aver-
ꢀ1
age value of 2558 cm . The predict barrier at the planar
position from the DFT calculation is much lower with an
Table 6
Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/6-311 + G(d,p) predictions from, microwave spectra and the
adjusted structural parameters for silacyclopentane
a
Isotopomer
Rotational constant
MP2(full)/6-311 + G(d,p)
Experimental
Adjusted r
0
jDj
1
1
1
2
2
2
28
28
29
c-
c-
c-
C
C
C
4
4
4
H
8
H
8
H
8
SiH
SiD
SiH
2
2
2
A
B
C
A
B
C
A
B
C
5492.7
4134.4
2667.0
5316.7
3772.8
2545.6
5492.8
4082.4
2645.3
5473.39
4137.01
2661.55
5308.25
3774.14
2542.43
5473.42
4085.26
2639.99
5473.91
4137.22
2661.67
5307.24
3773.91
2542.22
5473.91
4085.29
2640.08
0.52
0.21
0.12
1.01
0.23
0.21
0.49
0.03
0.09
a
Ref. [17].

82
G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
ꢀ1
Fig. 7. Mid-infrared spectra of (500–1500 cm ): (A) gas in transmittance; (B) liquid xenon solution at ꢀ65 °C in absorbance for silacyclopentane.
ꢀ
1
average value of 1918 cm . The barrier at the envelope
position is significantly lower with an average value of
1
rather than 0.88. Therefore, the MP2(full)/6-31G(d) calcu-
lations with two scaling factors are excellent for the funda-
mental frequency predictions and there is no need to use
several different values for the scaling factors.
ꢀ
1
ꢀ1
569 cm from the MP2 calculations and 1332 cm from
the DFT predictions. The barrier to pseudorotation was
obtained in the earlier vibrational study [16] from the far
infrared spectra where eight pronounced Q-branches were
observed on the ring puckering mode at 101 cm for the
normal species and five for this mode (94 cm ) for the
In the A symmetry block, the P.E.D. indicates signifi-
cant mixing of the b-CH rock with the SiH deformation
2
2
ꢀ
1
and the a-CH rock is extremely mixed with the SiH twist.
2 2
ꢀ
1
Also, the ring breathing and deformation are quite mixed
but most of the other modes for the A species have major
contributions from the indicated motions given for the
fundamental. In the B block, there are four vibrations that
have significant contributions from four symmetry coor-
dinates whereas one has contributions from five. Never-
theless, there is still one vibration, i.e. m33 where the
description is rather arbitrary.
Si-d isotopomer. By using the one-dimensional model bar-
2
ꢀ
1
riers of 1360 and 1470 cm were obtained, respectively,
from these Q-branches. About the same time a second far
infrared study was reported [26] and the barrier was report-
ꢀ1
ed as1362 ± 25 cm which is the same as we reported [16].
These values are in excellent agreement with the predicted
values associated with the envelope transition state.
In a later study [27] of the Raman spectrum of the gas
which provided frequencies for the ground and two excited
states of the twisting mode, a two-dimensional model was
utilized for obtaining the barrier to interconversion. The
resulting barrier with this more realistic model was
In the microwave study [17] of silacyclopentane, none of
the centrifugal distortion constants were determined.
Therefore we utilize the 29 assigned transitions of the nor-
2
8
mal species ( Si) and calculated three of the distortion con-
stants i.e D , d , d while holding the ab initio predicted
J
J
K
ꢀ
1
1
414 cm which is nearly the same value obtained from
values for the D , D constants at the adjusted values of
K JK
the one-dimensional model. Therefore, the interconversion
between the two equivalent twisted modes proceeds
through a puckered intermediate rather than the planar
form. However, the determined [27] two-dimensional
potential surface indicates that this puckered intermediate
0.95 of the predicted values of these constants. The results
for the determined values from the experimental frequen-
cies for the remaining three distortion constants are listed
in Table 7. The agreement of the predicted values with
the determined values is considered quite satisfactory con-
sidering the limited data with only six different J values and
even fewer different K values. Since the number of the tran-
ꢀ
1
is only about 150 cm more stable than the planar form
ꢀ
1
whereas the MP2 predicted value is nearly 1000 cm and
ꢀ
1
29
28
from the DFT calculations about 600 cm . It is expected
that these predicted values are too large and those obtained
from the spectral data too small.
sitions assigned and measured for the Si and the Si-d2
isotopomers was even fewer than for the normal species
only the D values were obtained for these two molecules
J
The scaled ab initio predicted frequencies for the funda-
mentals of the A species gives an average error of 1%
whereas those for the B species are predicted slightly better
with an average error of 0.9%. The predictions are satisfac-
tory with only two scaling factors and could be improved
to 0.5% by scaling the CH stretching frequencies by 0.86
with the other four normally determined constants held
to the ab initio predicted values. These results are also listed
in Table 7. Thus, the ab initio calculation at the MP2 level
with a relatively small basis set of 6-31G(d) provides good
predictions of the centrifugal distortion constants for these
size molecules.

G.A. Guirgis et al. / Journal of Molecular Structure 832 (2007) 73–83
83
Table 7
Acknowledgement
a
Centrifugal distortion constants (kHZ) of silacyclopentane in the ground
vibrational state
J.R.D. acknowledges the University of Missouri-Kansas
City, for a Faculty Research Grant for partial financial
support of this research.
b
c,d
Distortion
constants
Predicted value
Experimental value
a
28
28
29
28
Si
Si
Si
Si-d
2
D
D
D
J
0.68
1.54
0.94(14)
1.46*
0.83(9)
1.54
0.48(24)
1.31
K
*
References
JK
ꢀ0.65
ꢀ0.62
ꢀ0.64
ꢀ0.45
d
d
J
0.22
0.24(9)
0.56(24)
0.21
0.16
K
0.36
0.36
0.33
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Quadratic centrifugal distortion constants are defined according to
x
Wilson’s A-reduction for I .
b
This study: centrifugal distortion constants obtained from the
28
29
28
2
MP2(full)/6-31G(d) calculations for the Si, Si and Si-d species.
c
Calculated from the reported frequencies of the assigned transitions
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reported HSiH angles of 112.3(29)° from the electron dif-
fraction study is clearly too large and the longer reported
distance of 1.497(8) A for the SiH bonds is barely in agree-
ment with our value of 1.486(3) A for this bond. However
the most drastic difference are for the CC bond distances
with that for the unique C C bond of 1.580(5) A is certain-
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˚
S
0
˚
˚
˚
4
5
ly too long whereas this bond distance from our study is
.535(5) A. For the two equivalent CC bonds the previous-
ly reported values of 1.535(5) A is nearly in agreement with
our reported value of 1.547(5) A. The distances that should
˚
1
˚
˚
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383.
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much longer than the other two. There are several angles
which are quite different from those reported earlier [28]
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°. It is believed that the values reported herein are much
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1
91 (2000) 525.
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[
[
[
[