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

Article abstract of DOI:10.1016/j.molstruc.2006.11.050

The infrared spectra (3500-50 cm^-1) of the gas and solid and the Raman spectrum (3200-30 cm^-1) of liquid germacyclopentane, c-C₄H₈GeH₂, have been recorded. Additionally, the infrared spectra (3200-400 cm^-1) of liquid xenon solutions have been recorded at -65 and -95 °C. In all of the physical states only the twisted C₂ conformer was detected. 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 up to 6-311G(2d,2p). All of these calculations predict only the twisted conformer as the stable form with an average barrier to planarity of 2756 cm^-1 (32.97 kJ/mol) from the MP2 calculations and a significantly lower value of 2128 cm^-1 (25.45 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions. The C_s conformer (envelope) has a lower energy of 700 cm^-1 (MP2) and 400 cm^-1 (B3LYP) than the planar form. Thus, the path between the two identical C₂ conformers is by a pseudorotational motion rather than through the planar form. From the isolated GeH frequency from the GeHD isotopomer the GeH distance was calculated to be 1.532(2) A?. By utilizing the previously reported microwave rotational constants for four isotopomers (⁷⁰Ge, ⁷²Ge, ⁷³Ge, ⁷⁴Ge) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r₀ structural parameters were obtained. The determined heavy atom distances are: r₀(GeC) = 1.972(5); r₀(C₂C₄) and (C₃C₅) = 1.541(5); and r₀(C₄C₅) = 1.533(5) A? and the angles in degrees: ∠CGeC = 93.6(5)°; ∠GeCC = 102.6(5)°; ∠CCC = 109.4(5)° with the two dihedral angles ∠GeCCC = -38.4(3)° and ∠CCCC = 53.7(3)°. A complete vibrational assignment is given for the twisted (C₂) conformer for the normal species and the GeD₂ isotopomer which 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 some similar molecules.

Full text of DOI:10.1016/j.molstruc.2006.11.050

Journal of Molecular Structure 834–836 (2007) 17–29
www.elsevier.com/locate/molstruc
Conformational stability, r0 structural parameters, ab initio
calculations, and vibrational assignment for germacyclopentane
q
a
b
Gamil A. Guirgis , Ahmed M. El Defrawy b,1, Todor K. Gounev ,
Mamdouh S. Soliman b,2, James R. Durig
b,
*
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
Received 6 November 2006; accepted 20 November 2006
Available online 6 March 2007
Abstract
The infrared spectra (3500–50 cmꢀ1) of the gas and solid and the Raman spectrum (3200–30 cmꢀ1) of liquid germacyclopentane, c-
C4H8GeH2, have been recorded. Additionally, the infrared spectra (3200–400 cmꢀ1) of liquid xenon solutions have been recorded at ꢀ65
and ꢀ95 °C. In all of the physical states only the twisted C2 conformer was detected. The conformational energetics have been calculated
with the Møller–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 up to 6-311G(2d,2p). All of these calculations predict only the twisted conformer as the stable
form with an average barrier to planarity of 2756 cmꢀ1 (32.97 kJ/mol) from the MP2 calculations and a significantly lower value of
2128 cmꢀ1 (25.45 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions.
The Cs conformer (envelope) has a lower energy of 700 cmꢀ1 (MP2) and 400 cmꢀ1 (B3LYP) than the planar form. Thus, the path
between the two identical C2 conformers is by a pseudorotational motion rather than through the planar form. From the isolated
˚
GeH frequency from the GeHD isotopomer the GeH distance was calculated to be 1.532(2) A. By utilizing the previously reported
microwave rotational constants for four isotopomers (70Ge, 72Ge, 73Ge, 74Ge) combined with the structural parameters predicted from
the MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters were obtained. The determined heavy atom distances are:
˚
r0(GeC) = 1.972(5); r0(C2C4) and (C3C5) = 1.541(5); and r0(C4C5) = 1.533(5) A and the angles in degrees: \CGeC = 93.6(5)°;
\GeCC = 102.6(5)°; \CCC = 109.4(5)° with the two dihedral angles \GeCCC = ꢀ38.4(3)° and \CCCC = 53.7(3)°. A complete vibra-
tional assignment is given for the twisted (C2) conformer for the normal species and the GeD2 isotopomer which are supported by normal
coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The results of these spectroscopic and theo-
retical studies are discussed and compared to the corresponding results for some similar molecules.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Conformational stability; r0 parameters; ab initio calculations; Germacyclopentane
1. Introduction
Five-membered rings have been of interest to structural
q
physicist and chemist because they may have any one of the
Presented in preliminary form as part of an invited presentation (PL1)
three different stable conformers, i.e., planar, twisted and
envelope forms, or possibly more than one of these forms.
Heteroatom within the ring or substituents on the ring
plays a major role in determining which structure is the sta-
ble one for a particular molecule. In addition to the interest
in the conformational stability of these molecules, there has
at the 28th EUMOS meeting, Istanbul, Turkey.
*
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
1
submitted in partial fulfillment of the Ph.D. degree.
2
Present address: Department of Chemistry, Mansoura University,
Mansoura, Egypt.
0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.11.050

18
G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
also been interest in the pathway and barriers the molecule
transverses on going from one conformer to another. When
the pathway is through a series of twisted forms the motion
looks like rotation as the individual atoms move out of the
hypothetical plane for four of the atoms. This motion has
been called pseudorotation [1–4] and was first observed
experimentally for cyclopentane [4] where the barrier is
very small. The size of the barriers to this motion varies
greatly and there is still questions concerning the barrier
heights for several molecules of interest as well as for some
there is a question which form is the most stable conform-
er. As a continuation of our studies on such molecules [5–8]
where we have utilizing theoretical predications and vibra-
tional studies of rare gas solutions, we have investigated the
germacyclopentane, c-C4H8GeH2, molecule for the purpos-
es to answer these questions.
An extensive vibrational analysis of germacyclopentane
was reported [9] where the twist C2 conformer was deter-
mined to be the stable form with the ring bending and ring
twisting modes observed at 113 and 273 cmꢀ1, respectively.
Studies were also reported on the barrier to pseudorotation
of germacylopentane [9] where the experimentally deter-
mined barrier to pseudorotation was found to be
5.9 0.1 kcal/mol (2061 cmꢀ1). By utilizing the two dimen-
sional treatment [10] with the observed frequencies for the
fundamentals and the ‘‘ hot bands ’’ of the two low frequen-
cy ring modes the barrier to interconversion to the two
equivalent C2 forms was found to be ꢁ1450 cmꢀ1. Addi-
tionally, it was shown that the interconversion between
these two equivalent twisted configurations is not entirely
along the twist axis but passes through a slightly bent con-
figuration. In addition to the vibrational studies, the micro-
wave spectra of four of the Ge isotopic species (70Ge, 72Ge,
73Ge, 74Ge) of germacyclopentane have been investigated in
the region 8–22 GHz [11]. The molecule was found to be in
the twisted C2 conformation with a twist angle of 18°.
To determine whether a second conformer is present for
germacyclopnetane as well as to determine the structural
parameters, we have utilized spectral studies of xenon solu-
tions. These studies have been supported by carrying out
ab initio calculations at the MP2 level with full electron cor-
relation by the perturbation method [12] 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 also carried out
density functional theory (DFT) calculations 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 depolariza-
tion ratios. The results of these spectroscopic and theoret-
ical studies are reported herein and they are compared to
similar quantities of some corresponding molecules.
prepared as follows: a Grignard reagent was prepared by
adding 46 g (0.20 mol) of 1,4-dibromobutane dissolved in
200 ml of anhydrous ethyl ether to 11 g (0.45 mol) of mag-
nesium turnings. The resulting solution was transferred
under a nitrogen atmosphere to a 300-ml dropping funnel.
The dropping funnel was attached to a three neck flask
which was equipped with a refluxing condenser cooled to
4 °C using ISOTEMP 1013S cooling system. The dropping
funnel and condenser were protected from the atmosphere
by calcium chloride-soda lime tube. The flask contained
500-ml was anhydrous ethyl ether and 38 g (0.18 mol) of
germanium tetrachloride. The Grignard reagent was added
dropwise with vigorous stirring over 1 h. White precipitate
was formed and the solution started refluxing and contin-
ued for 6 h. The mixture was then cooled to room temper-
ature and stirred under a nitrogen blanket for 20 h. The salt
was separated by filtration and washed with dry ether. The
ether was removed first by distillation under vacuum at
30 °C and then at 80 °C. The sample 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 [9].
The mid-infrared spectrum of the gas (Fig. 1) was
obtained from 3500 to 230 cmꢀ1 on a Perkin-Elmer model
2000 Fourier transform spectrometer equipped with a Ge/
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 cmꢀ1. Sixty-four
2. Experimental
Fig. 1. Comparison of experimental and calculated infrared spectra of
germacyclopentane: (A) observed infrared spectrum in gas; (B) observed
infrared spectrum in liquid xenon solution at ꢀ95 °C; (C) simulated
infrared spectrum of twisted (C2) germacyclopentane.
The sample of germacyclopentane was prepared in a two
step reaction. First, 1,1-dichlorogermacyclopentane was

G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
19
interferograms were added and transformed with a boxcar
truncation function.
spectral resolution of 0.5 cmꢀ1 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 trunca-
tion function. All of the bands observed in the various
infrared and Raman spectra which were chosen as
fundamentals are listed in Table 1 for c-C4H8GeH2 and
in Table 2 for the c-C4H8GeD2 isotopomer.
The mid-infrared spectra (3500–400 cmꢀ1) of the sample
dissolved in liquified xenon at two temperatures (ꢀ65 °C,
ꢀ95 °C Fig. 2) 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 were collected at a 1.0 cmꢀ1
resolution, averaged and transformed with a boxcar trun-
cation function. For these studies, a specially designed
cryostat cell was used. It consists of a copper cell with a
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 the 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
3. Ab initio calculations
The LCAO-MO-SCF restricted Hartree–Fock calcula-
tions were performed with the Gaussian-03 program [13]
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 [14]. The 6-31G(d),
6-311G(d,p), and 6-311G(2d,2p) basis sets as well as the
corresponding ones with diffuse functions were employed
with the Møller–Plesset perturbation method [12] to the
second order (MP2(full)) as well as with the density
functional theory by the B3LYP method. The predicted
conformational energy differences are listed in Table 3.
In order to obtain a complete description of the molec-
ular motions involved in the fundamental modes of germa-
cyclopentane, a normal coordinate analysis has been
carried out. The force field in Cartesian coordinates was
obtained with the Gaussian 03 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 given in Table
4 with the atomic numbering shown in Fig. 3. By using
Fig. 2. Mid-infrared spectra of (400–1500 cmꢀ1): (A) liquid xenon solution at ꢀ65 °C; (B) liquid xenon solution at ꢀ95 °C for germacyclopentane.

Table 1
Calculateda and observed frequencies (cmꢀ1) for germacyclopentane twisted (C2) form
Sym.
block
Vib.
No.
Approx. description
Ab
initio
Fixed
scaledb
IR
int.
Raman
act.
dp
ratio
Infrared
Gas
Raman Infrared PEDc
Liquid Solid
Band Contour
Xe
A
B
C
soln.
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
m1
m2
m3
m4
m5
m6
m7
m8
a-CH2 antisymmetric stretch
b-CH2 antisymmetric stretch
a-CH2 symmetric stretch
b-CH2 symmetric stretch
GeH2 symmetric stretch
b-CH2 deformation
a-CH2 deformation
b-CH2 wag
b-CH2 twist
a-CH2 twist
a-CH2 wag
Ring deformation
b-CH2 rock
GeH2 deformation
Ring breathing
a-CH2 rock
Ring deformation
GeH2 twist
Ring deformation
Ring twist
a-CH2 antisymmetric stretch
b-CH2 antisymmetric stretch
a-CH2 symmetric stretch
b-CH2 symmetric stretch
GeH2 antisymmetric stretch
b-CH2 deformation
a-CH2 deformation
b-CH2 wag
3191
3145
3123
3086
2059
1562
1514
1403
1321
1276
1143
1087
1006
903
892
796
670
564
359
293
3191
3150
3124
3088
2067
1553
1519
1384
1325
1224
1090
997
2993
2950
2930
2895
2059
1482
1436
1331
1253
1211
1085
1031
958
901
851
765
640
550
347
278
2993
2955
2930
2897
2067
1473
1442
1313
1257
1161
1034
945
0.7
28.1
4.6
21.8
133.5
0.1
0.0
3.0
0.05
1.6
18.0
0.8
0.02
67.7
0.1
4.6
0.5
2.1
5.0
132.6
159.8
141.0
147.5
170.2
2.4
23.8
3.6
7.3
2.9
9.4
5.8
3.8
15.5
6.3
6.7
17.7
7.5
3.3
0.64
0.23
0.28
0.08
0.09
0.41
0.75
0.73
0.47
0.14
0.65
0.64
0.68
0.75
0.11
0.50
0.11
0.35
0.34
0.52
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
–
2955
2969
2932
2904
2853
2054
–
1415
1319
1241
1196
1074
1025
–
878
848
761
635
–
345
273
–
2958
2951
2906
2850
2054
1463
1413
1320
1243
1195
1072
1029
966
878
850
762
631
547
345
–
2974
2926
2919
2862
2054
1447
1418
1306
1251
1146
1038
948
94S1
82S2, 10S4
87S3
89S4, 11S2
100S5
89S6, 10S7
89S7, 10S6
59S8, 27S9
45S9, 32S11
40S10, 11S11, 31S13
41S11, 20S9, 11S17
43S12, 22S17, 21S8
39S13, 40S10
99S14
43S15, 22S17, 18S12
46S16, 27S18, 16S13
39S17, 27S15, 18S18
45S18, 33S16
79S19, 11S20
82S20, 12S19
93S21
92S22
89S23
99S24
100S25
99S26
98S27
86S28
52S29, 20S31, 16S30
60S30, 13S37
59S31, 32S29
35S32, 18S30, 18S36
51S33, 31S36
59S34, 11S37, 10S35
82S35
63S36, 27S32, 11S33
59S37, 17S32, 14S38, 13S39
12S33
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
2945 2930
ꢁ2920 2909
2863 2854
2066 2058
1465 1468
1423 1418
1325 1321
1246 1241
1201 1198
1077 1073
1026 1027
m9
m10
m11
m12
m13
m14
m15
m16
m17
m18
m19
m20
m21
m22
m23
m24
m25
m26
m27
m28
m29
m30
m31
m32
m33
m34
m35
m36
m37
–
973
880
847
760
629
545
881
840
762
633
–
–
345
–
–
0.2
0.1
7.8
21.7
36.5
12.3
28.5
131.9
7.6
8.2
0.7
0.8
1.2
9.6
1.9
8.7
49.1
67.0
14.3
11.3
2976 2967
2934 2930
2925 2924
2880 2869
2066 2058
1455 1448
21
43
78
21
1
7
63
7
76
53
1
95
90
61
90
71
41
79
53.6
5.2
33.1
77.9
18.7
1.9
1.1
10.4
2.1
1.3
5.9
1.3
0.9
2932
2922
2874
2054
1448
–
57
22
79
99
93
37
93
24
47
99
5
10
39
10
29
59
1429
–
1312 1309
1251 1253
1150 1145
1033 1031
–
b-CH2 twist
a-CH2 wag
a-CH2 twist
Ring deformation
b-CH2 rock
a-CH2 rock
GeH2 wag
Ring deformation
Ring deformation
1250
1144
1030
946
–
947
860
805
695
588
479
947
867
802
693
586
480
918
844
707
627
874
807
702
595
868
802
692
588
–
4.4
5.4
1.0
695
588
481
501
478
484
,
B
B
a
m38
m39
GeH2 rock
Ring puckering
416
103
410
98
12.1
0.9
0.6
0.0
0.75
0.75
430
435
–
438
–
441
–
67S38, 19S34, 10S37
81S39, 12S37
–
4
100
96
113d
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 (PEDs).
Scaled frequencies with scaling factors of 0.88 for CH stretchs, 1.0 for GeH stretch and bends and 0.90 for all other modes.
Symmetry coordinates with PED contribution less than 10% are omitted.
b
c
d
Value taken from Ref. [9].

Table 2
Calculateda and observed frequencies (cmꢀ1) for 1, 1-d2 germacyclopentane twisted (C2) form
Sym. block Vib. No. Approx. description
Ab initio Fixed scaledb IR int. Raman act. dp ratio Infrard Raman PEDd
Band Contour
Gasc
Liquidc
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
m1
m2
m3
m4
m5
m6
m7
m8
a-CH2 antisymmetric stretch 3191
b-CH2 antisymmetric stretch 3145
2993
2950
2930
2895
1463
1483
1435
1331
1252
1210
1084
1031
946
642
851
736
621
436
345
275
2993
2955
2930
2897
1470
1476
1441
1312
1256
1161
1032
945
868
790
517
598
466
330
91
0.7
28.3
4.5
22.0
64.6
4.6
1.1
3.7
0.3
2.0
16.9
1.2
0.2
38.2
0.4
2.9
0.1
0.7
4.3
0.2
21.3
36.4
12.3
28.5
58.0
19.1
8.0
0.5
1.1
1.1
7.3
132.6
159.6
140.9
147.2
69.9
8.8
29.5
3.9
7.5
3.1
9.7
5.9
2.0
6.5
6.1
5.2
18.7
5.2
3.4
0.2
7.8
53.6
5.3
33.1
51.2
5.2
1.9
1.2
10.9
2.1
1.2
5.6
0.7
0.7
5.1
3.1
0.3
0.6
0.0
0.65
0.23
0.28
0.08
0.08
0.10
0.69
0.71
0.38
0.10
0.63
0.63
0.60
0.72
0.11
0.25
0.10
0.63
0.35
0.62
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
2978
2934
–
2864
1485
1453
–
1323
1247
1197
1077
1028
–
631
858
735
–
2967
2933
2900
2853
1477
1447
1415
1320
1243
1195
1075
1027
929
–
847
732
617
428
342
267
2967
–
–
2875
–
1447
–
–
1251
1142
–
945
–
94S1
82S2, 10S4
87S3
89S4, 11S2
93S5
84S6, 11S7
87S7
59S8, 27S9
45S9, 32S11
40S10, 31S13, 11S11
42S11, 20S9, 11S17
44S12, 23S17, 21S8
43S13, 43S10
97S14
44S15, 22S17, 18S12
61S16, 11S13
38S17, 32S15
79S18, 12S16
75S19, 14S20
79S20, 12S19
93S21
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
a-CH2 symmetric stretch
b-CH2 symmetric stretch
GeD2 symmetric stretch
b-CH2 deformation
a-CH2 deformation
b-CH2 wag
b-CH2 twist
a-CH2 twist
a-CH2 wag
Ring deformation
b-CH2 rock
3123
3086
1464
1563
1514
1403
1320
1276
1143
1087
996
m9
m10
m11
m12
m13
m14
m15
m16
m17
m18
m19
m20
m21
m22
m23
m24
m25
m26
m27
m28
m29
m30
m31
m32
m33
m34
m35
m36
m37
m38
m39
GeD2 deformation
Ring breathing
a-CH2 rock
654
894
772
Ring deformation
GeD2 twist
640
440
427
340
–
Ring deformation
Ring twist
357
289
a-CH2 antisymmetric stretch 3191
b-CH2 antisymmetric stretch 3150
2978
2944
2938
2880
1489
1453
1425
1311
1251
1137
1031
942
–
789
514
594
469
340
–
21
43
79
22
2
–
64
–
93
47
1
94
67
43
94
84
88
–
79
57
21
78
98
100
36
100
7
53
99
6
33
57
6
92S22
89S23
99S24
54S25, 46S26
54S26, 46S25
98S27
a-CH2 symmetric stretch
b-CH2 symmetric stretch
GeD2 antisymmetric stretch
b-CH2 deformation
a-CH2 deformation
b-CH2 wag
b-CH2 twist
a-CH2 wag
a-CH2 twist
Ring deformation
b-CH2 rock
3124
3088
1473
1553
1519
1383
1324
1224
1087
996
86S28
52S29, 20S31, 16S30
60S30, 14S37
59S31, 33S29
35S32, 18S30, 17S36
54S33, 33S36
67S34, 12S37
81S35
55S36, 27S32, 13S33
58S37, 15S32, 12S33, 12S35, 12S39
79S38, 11S34
77S39, 11S37, 11S38
1.4
2.0
914
a-CH2 rock
831
29.5
23.4
30.3
12.2
9.4
–
GeD2 wag
523
511
587
468
342
–
Ring deformation
Ring deformation
GeD2 rock
629
486
333
16
12
100
96
Ring puckering
96
0.9
4
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 (PEDs).
b
c
Scaled frequencies with scaling factors of 0.88 for CH stretches, 1.0 for GeH stretch and bends and 0.90 for all other modes.
Ref. [9].
d
Symmetry coordinates with PED contribution less than 10% are omitted.

22
G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
Table 3
Calculated electronic energies (hartree) and energy differences (cmꢀ1) for the twisted C2, planar C2v (TS)a and envelope Cs (TS)a of germacyclopentane
Method/basis set
Twist (C2)
Envelop (CS)
ꢀDE(C2 ꢀ CS)
Planar (C2v
)
ꢀDE(C2 ꢀ C2v
)
RHF/6-31G(d)
RHF/6-31+G(d)
ꢀ2230.539795
ꢀ2230.567647
ꢀ2231.199047
ꢀ2231.233704
ꢀ2233.829559
ꢀ2233.832392
ꢀ2233.899362
ꢀ2233.901439
ꢀ2233.434299
ꢀ2233.464848
ꢀ2235.500209
ꢀ2235.501031
ꢀ2235.506426
ꢀ2235.507120
ꢀ2230.531267
ꢀ2230.556514
ꢀ2231.190413
ꢀ2231.221918
ꢀ2233.820743
ꢀ2233.823650
ꢀ2233.890744
ꢀ2233.892875
ꢀ2233.426430
ꢀ2233.454849
ꢀ2235.492749
ꢀ2235.493554
ꢀ2235.499054
ꢀ2235.499760
1872
2443
1895
2587
1935
1919
1891
1880
1727
2195
1637
1641
1618
1615
ꢀ2230.527680
ꢀ2230.555891
ꢀ2231.184932
ꢀ2231.220514
ꢀ2233.817146
ꢀ2233.820103
ꢀ2233.887026
ꢀ2233.889176
ꢀ2233.423532
ꢀ2233.454621
ꢀ2235.490783
ꢀ2235.491640
ꢀ2235.497244
ꢀ2235.497949
2659
2580
3098
2895
2724
2697
2707
2691
2363
2245
2069
2061
2015
2013
MP2(full)/6-31G(d)
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.
the B matrix [15], the force field in Cartesian coordinates
was converted to a force field in internal coordinates. Sub-
sequently, scaling factors of 0.88 for CH, 1.0 for GeH2
stretches and bends, and 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
coordinates was used (Table 5) to determine the corre-
sponding potential energy distributions (PEDs). A compar-
ison between the observed and calculated wavenumbers,
along with the calculated infrared intensities, Raman activ-
ities, depolarization ratios and potential energy distribu-
tions for twisted germacyclopentane and the GeD2
isotopomer, are listed in Tables 1 and 2, respectively.
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-
mined from MP2(Full)/6-31G(d) calculations were
obtained based on the dipole moment derivatives with
respect to Cartesian coordinates. The derivatives were
developed [16–19]. The activity Sj can be expressed as:
Sj = gj ð45a2j þ 7bj2Þ, where gj is the degeneracy of the vibra-
tional mode j, aj is the derivative of the isotropic polariz-
ability, and bj is that of the anisotropic polarizability. To
obtain the polarized Raman scattering cross sections, the
polarizabilities are incorporated into Sj by multiplying Sj
with (1 ꢀ qj)/(1 + qj), where qj is the depolarization ratio
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 spectra of the liquid and the
calculated one are shown in Fig. 4A and B and those for
the GeD2 isotopomer in Fig. 4C and D, respectively.
4. Conformational stability and vibrational assignment
From the microwave study [11] the twist form with C2
symmetry was identified as the stable conformer so the
major effort was a search for another conformer. The spec-
tra of the sample dissolved in xenon was recorded at vari-
ous temperatures beginning at ꢀ65 °C to see if any changes
could be observed as the temperatures was lowered. The
spectra at ꢀ65 °C and ꢀ95 °C are shown in Fig. 2 and
the bands become narrower at the lower temperatures
but there is no significant change in the relative intensities
of the peaks. This experimental observation indicates that
there is only one conformer present or if more than one
the enthalpy difference is near zero. The ab initio calcula-
tions support the presence of a single conformer where
the MP2 calculations predict the envelope (Cs) to be on
an average of 2018 cmꢀ1 higher energy than the twisted
(C2) form and additionally it is a transition state. Similar
predictions are also made from the B3LYP calculations
but the energy difference is smaller at 1740 cmꢀ1. However,
both of these values are significantly lower than the predict-
ed energy of the planar form (lower by 700 cmꢀ1 for MP2
and 400 cmꢀ1 for B3LYP) which is also a transition state.
transformed with respect to normal coordinates by (olu/
P
oQi) =
(olu/oXj)Lij, where Qi is the ith normal coordi-
j
nate, Xj is the jth Cartesian displacement coordinate, and
Lij is the transformation matrix between the Cartesian dis-
placement coordinates and the normal coordinates. The
infrared intensities were then calculated by (Np)/(3c2)
[(olx/oQi)2 + (oly/oQi)2 + (olz/oQi)2]. In Fig. 1, a compar-
ison of experimental and simulated infrared spectra of ger-
mayclopentane 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 for supporting the vibrational
assignment.
Also 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

Table 4
Structural parameters (A and degree), rotational constants (MHz) and dipole moment (debye) for germayclopentane and silacyclopentane in the twisted (C2) form
˚
Structural Parameters
Internal coordinates
X=Ge
X=Si
MP2(full)/6-311+G(d,p)
MWa
Adjusted Parameters r0
MP2(full)/6-311+G(d,p)
EDb
1.892(2)
Adjusted Parameters r0
r (X–C2,3
r (C2,3-C4,5
r (C4-C5)
)
X1, X2
Y1, Y2
Z
1.969
1.540
1.536
1.524
1.095
1.092
1.098
1.096
93.2
1.95c
1.53c
1.53c
1.53c
1.09c
1.09c
1.09c
1.09c
98
1.972(5)
1.541(5)
1.533(3)
1.532(2)
1.095(2)
1.092(2)
1.098(2)
1.096(2)
93.6(5)
1.895
1.544
1.535
1.480
1.096
1.093
1.098
1.095
95.6
1.893(5)
1.549(5)
1.534(5)
1.486(3)
1.096(2)
1.093(2)
1.098(2)
1.095(2)
96.0(5)
)
1.535(5)
1.580(5)
1.497(8)
1.112(6)
1.112(6)
1.112(6)
1.112(6)
96.3(3)
r (X–H6,7
r (C–H8,9
r (C–H10,11
r (C–H12,13
r (C–H14,15
\ C3 X C2
\X C2,3 C4,5
\ C2,3 C4,5 C5,4
\ H6XH7
)
)
r1, r2
r3, r4
r5, r6
r7, r8
r9, r10
h
w1, w2
u1, u2
a
)
)
)
102.9
108.9
109.0
107.9
107.1
113.4
113.6
109.4
113.6
110.3
112.6
108.6
111.0
109.5
111.6
ꢀ39.1
54.4
106
115
111c
109c
109c
102.6(5)
109.4(5)
109.0(5)
107.9(5)
107.1(5)
113.4(5)
113.5(5)
109.4(5)
113.6(5)
110.4(5)
112.8(5)
108.4(5)
110.7(5)
109.5(5)
111.6(5)
ꢀ38.4(3)
53.7(3)
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
103.6(3)
108.4(7)
112.3(29)
–
–
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)
102.9(5)
108.3(5)
107.9(5)
107.3(5)
107.2(5)
113.1(5)
113.2(5)
109.9(5)
114.6(5)
109.7(5)
112.5(5)
108.5(5)
111.3(5)
109.6(5)
111.9(5)
ꢀ37.7(3)
51.6(3)
\ H8,9C2,3H10,11
\ H12,13 C5,4 H14,15
\ H6,7XC2,3
\ H6,7XC3,2
\ H8,9 C2,3 Ge
\ H10,11 C2,3 Ge
\ H8,9 C2,3 C4,5
\ H10,11 C2,3 C4,5
\ H12,13 C5,4 C4,5
\ H14,15 C5,4 C4,5
\ H12,13 C5,4 C3,2
\ H14,15 C5,4 C3,2
s XCCC
c1, c2
e1, e2
b1, b4
b2, b3
p1, p5
p3, p7
p2, p6
p4, p8
r6, r2
r8, r4
r5, r1
r7, r3
s1, s2
–
s CCCC
49.7(14)
A
B
C
5352.0
2836.8
2044.6
0.530
5323.7
2842.2
2041.5
0.665(7)
5323.8
2842.1
2041.5
5492.7
4134.4
2666.9
0.563
5477. 8
4137. 9
2661.5
5473.8
4137.5
2661.7
jlaj = jltj
a
Values taken from Ref. [11].
Values taken from Ref. [24].
b
c
Values marked with asterisk were assumed and other values determined; twist angle of 18° was reported; adjusted r0 parameters gives twist angle of 28°.

24
G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
The initial vibrational assignment [9] was proposed
based on the infrared band contours and determined
Raman depolarization values with the 20 fundamentals
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. How-
ever, band contours are not always easily discerned and
by recording the infrared spectrum of a xenon solution
the bands become narrower and the overlapping funda-
mentals can frequently be observed. Additionally the depo-
larization values for a symmetric vibration (A symmetry)
may be so near to the value of 0.75 (see Table 1, m7, m8
and m14) that the band could be misassigned as a vibration
of B-symmetry. In the current study the ab initio calcula-
tions provided estimated frequencies, hybrid band contours
for the B species fundamentals, depolarization values, and
infrared and Raman band intensities which significantly
aid in the vibrational assignment. Additionally, infrared
spectral data in the rare gas (xenon) solution in the finger
Fig. 3. Twisted conformer of germacyclopentane showing atomic
numbering.
Since these energy values are relatively large there can be
little doubt that there is a single stable conformer in both
fluid states and it is the twisted form with C2 symmetry.
Table 5
Symmetry coordinates of germacyclopentane
Description
Symmetry coordinatea
A a-CH2 antisymmetric stretch
A b-CH2 antisymmetric stretch
A a-CH2 symmetric stretch
A b-CH2 symmetric stretch
A GeH2 symmetric stretch
A b-CH2 deformation
A a-CH2 deformation
A b-CH2 wag
A b-CH2 twist
A a-CH2 twist
A a-CH2 wag
A ring deformation
A b-CH2 rock
A GeH2 deformation
A ring breathing
A a-CH2 rock
A ring deformation
A GeH2 twist
A ring deformation
A ring twist
S1 = r3 ꢀ r5 + r4 ꢀ r6
S2 = r7 ꢀ r9 + r8 ꢀ r10
S3 = r3 + r5 + r4 + r6
S4 = r7 + r9 + r8 + r10
S5 = r1 + r2
S6 = 4e1 ꢀ r1 ꢀ r2 ꢀ r3 ꢀ r4 + 4e2 ꢀ r5 ꢀ r6 ꢀ r7 ꢀ r8
S7 = 4c1 ꢀ p1 ꢀ p2 ꢀ p3 ꢀ p4 + 4c2 ꢀ p5 ꢀ p6 ꢀ p7 ꢀ p8
S8 = r1 ꢀ r2 + r3 ꢀ r4 + r5 ꢀ r6 + r7 ꢀ r8
S9 = r1 ꢀ r2 ꢀ r3 + r4 + r5 ꢀ r6 ꢀ r7 + r8
S10 = p1 ꢀ p2 ꢀ p3 + p4 + p5 ꢀ p6 ꢀ p7 + p8
S11 = p1 ꢀ p2 + p3 ꢀ p4 + p5 ꢀ p6 + p7 ꢀ p8
S12 = 4Z ꢀ X1 ꢀ X2 ꢀ Y1 ꢀ Y2
S13 = r1 + r2 ꢀ r3 ꢀ r4 + r5 + r6 ꢀ r7 ꢀ r8
S14 = 4a ꢀ b1 ꢀ b2 ꢀ b3 ꢀ b4
S15 = Z + X1 + X2 + Y1 + Y2
S16 = p1 + p2 ꢀ p3 ꢀ p4 + p5 + p6 ꢀ p7 ꢀ p8
S17 = X1 + X2 ꢀ Y1 ꢀ Y2
S18 = b1 ꢀ b2 ꢀ b3 + b4
S19 = 4h ꢀ w1 ꢀ w2 ꢀ u1 ꢀ u2
S20 = s1 + s2
B a-CH2 antisymmetric stretch
B b-CH2 antisymmetric stretch
B a-CH2 symmetric stretch
B b-CH2 symmetric stretch
B GeH2 antisymmetric stretch
B b-CH2 deformation
B a-CH2 deformation
B b-CH2 wag
B b-CH2 twist
B a-CH2 wag
B a-CH2 twist
B ring deformation
B b-CH2 rock
S21 = r3 ꢀ r5 ꢀ r4 + r6
S22 = r7 ꢀ r9 ꢀ r8 + r10
S23 = r3 + r5 ꢀ r4 ꢀ r6
S24 = r7 + r9 ꢀ r8 ꢀ r10
S25 = r1 ꢀ r2
S26 = 4e1 ꢀ r1 ꢀ r2 ꢀ r3 ꢀ r4 ꢀ 4e2 + r5 + r6 + r7 + r8
S27 = 4c1 ꢀ p1 ꢀ p2 ꢀ p3 ꢀ p4 ꢀ 4c2 + p5 + p6 + p7 + p8
S28 = r1 ꢀ r2 + r3 ꢀ r4 ꢀ r5 + r6 ꢀ r7 + r8
S29 = r1 ꢀ r2 ꢀ r3 + r4 ꢀ r5 + r6 + r7 ꢀ r8
S30 = p1 ꢀ p2 + p3 ꢀ p4 ꢀ p5 + p6 ꢀ p7 + p8
S31 = p1 ꢀ p2 ꢀ p3 + p4 ꢀ p5 + p6 + p7 ꢀ p8
S32 = X1 ꢀ X2 ꢀ Y1 + Y2
S33 = r1 + r2 ꢀ r3 ꢀ r4 ꢀ r5 ꢀ r6 + r7 + r8
S34 = p1 + p2 ꢀ p3 ꢀ p4 ꢀ p5 ꢀ p6 + p7 + p8
S35 = b1 ꢀ b2 + b3 ꢀ b4
B a -CH2 rock
B GeH2 wag
B ring deformation
B ring deformation
B GeH2 rock
S36 = X1 ꢀ X2 + Y1 ꢀ Y2
S37 = w1 ꢀ w2 + u1 ꢀ u2
S38 = b1 + b2 ꢀ b3 ꢀ b4
B ring puckering
S39 = s1 ꢀ s2
a
Not normalized.

G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
25
Fig. 5. Mid-infrared spectra of (400–1500 cmꢀ1): (A) gas in transmittance;
(B) liquid xenon solution at ꢀ95 °C in absorbance for germacyclopentane.
band had been assigned at 1057 cmꢀ1 in the B block which
was found in our study in the gas but only as a weak shoul-
der at 1061 cmꢀ1 in the xenon spectrum but not in the spec-
trum of the solid. Thus, this band was not considered to be
a fundamental Fig. 6.
In the carbon–hydrogen stretching region the eight CH
stretches are predicted to be in four pairs with the frequen-
cies in each pair being almost degenerate (Table 1). For m1
the predicted infrared intensity is only 0.7 km/mol whereas
all four CH stretching fundamentals in the A block are pre-
dict to give rise to strong Raman lines. The splitting of the
highest frequency mode is observed as a doublet in the
infrared spectrum of the xenon solution whereas the infra-
red spectrum of the gas shows a single strong Q branch for
m21 with a predicted contour of 21% B/79%C. The spectra
in this spectral region differ considerably with change of
Fig. 4. Comparison of experimental and calculated Raman spectra of
germacyclopentane: (A) observed of liquid; (B) simulated spectrum of
germacyclopentane; (C) observed of liquid; (D) simulated spectrum of 1,1-
d2-germacyclopentane.
print region at ꢀ95 °C (Fig. 2) are usually relatively sharp
so it was expected that it might be possible to separate
some of the bands that were previously not observed or
assigned [9] to two fundamentals. Therefore, by using the
data from the ab initio calculations along with more exper-
imental data, i.e., infrared spectra of the solution and solid,
it might be possible to provide a more convincing assign-
ment than the one given earlier [9]. Nevertheless since the
A species modes have both the band contour and depolar-
ization values as indicators the earlier selection [9] of the
fundamentals are nearly the same as those listed in Table
1 from the current study. There are two significant excep-
tions with one in each symmetry block and some minor
changes in the assignments of the CH stretching modes.
Fig. 5
For the A block there were two fundamentals proposed
at 762 and 785 cmꢀ1 in the initial study [9] with the latter
one estimated. However, the ab initio predictions have only
one fundamental at 761 cmꢀ1 in this region but two modes
near 950 cmꢀ1 with one in each symmetry species. Thus,
the predicted 958 cmꢀ1 fundamental which has
a
0.02 km/mol predicted infrared intensity is assigned to
the very weak band at 973 cmꢀ1. In the B block a fundꢀa1-
mental is predicted at 874 cmꢀ1 and assigned at 860 cm
which is the fourth fundamental in the 800–890 cmꢀ1
region whereas in the earlier study [9] only three fundamen-
tals were assigned in this spectral region. Previously, a
Fig. 6. Mid-infrared spectra of (400–1000 cmꢀ1): (A) amorphous; (B)
annealed solid of germayclopentane.

26
G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
state from gas, xenon solution, amorphous to polycrystal-
line solid (Fig. 7).
makes it possible to evaluate the quality of the predictions
when they are compared to observed frequencies, whereas
with multiple scaling factors to fit the observed frequencies
there is a superficial indication of a quality fit.
The potential energy distributions are relatively simple
with only one mode having contributions from five symme-
try coordinates but the main contribution is still 59% (m37).
There are 11 vibrations with contributions from three sym-
metry coordinates but the major one is more than 40%
except for three modes which have values of 35, 39, and
39% for the main contribution. For the GeD2 isotopomer
the PEDs are even purer with only eight modes having con-
tributions from three symmetry coordinates (Table 2).
Similar differences in the assignment for two of the fun-
damentals for the GeD2 isotopomer (Table 2) were also
made. Again there is a band previously assigned at
766 cmꢀ1 where we have no corresponding fundamental.
Our observed band at 929 cmꢀ1 which was not previously
assigned [9] as a fundamental is assigned as m13 in our
study. Therefore, there is remarkable good correspondence
between our assignments and those previously given for
these two isotopomers of germacyclopentane.
The ab initio predicted frequencies in the A block for the
normal molecule has an average difference between the
observed values of 8.2 cmꢀ1 which represent 0.6% error.
For the B symmetry block the difference is 9.4 cmꢀ1 which
is a 0.7% error. For the GeD2 compound the average error
for the A symmetry block is 8 cmꢀ1 and for the B symmetry
block the average difference is 9.2 cmꢀ1 which are the same
percent error as those for the normal molecule. These small
errors with only two scaling factors of 0.88 and 0.9 clearly
show the quality of ab initio predictions with the very small
basis set of 6-31G(d). By using very few scaling factors
5. Structural parameters
We [20] have recently shown that ab initio MP2/6-
311+G(d,p) calculations predict the r0 structural parame-
ters for more than fifty carbon–hydrogen distances better
˚
than 0.002 A compared to the experimentally determined
values from isolated CH stretching frequencies which were
compared [21] to previously determined values from micro-
wave studies. The GeH distance can be obtained from the
frequency of the isolated GeH stretch from the GeHD iso-
topomer which is an impurity in the GeD2 sample. It has
also been shown [22] that similar ab initio calculations pre-
dict the GeC distances for three organogermanes to match
exactly the experimental value where for two of the mole-
cules the listed experimental values had error limits of
˚
0.002 A. Finally, we have found [23] that we can obtain
good structural parameters by adjusting the structural
parameters obtained from the ab initio calculations to fit
the rotational constants obtained from the microwave
experimental data. In order to reduce the number of inde-
pendent 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 germacyclopentane. Also, bond angles
and torsional angles belonging to the same set keep their
differences in degrees. This assumption is based on the fact
that the errors from ab initio calculations are systematic.
Therefore, it should be possible to obtain ‘‘adjusted r0’’
structural parameters for germacyclopentane utilizing the
microwave rotational constants from the twelve reported
values [11] for the four isotopomers 70Ge, 72Ge, 73Ge, 74Ge.
First, the GeH distance was obtained [9] from the GeH
stretching frequency of 2066 cmꢀ1 observed in the spec-
trum of the GeHD isotopomer which was found as an
impurity in the GeD2 isotopomer. This value gives a
˚
GeH distance of 1.532(2) A from the previously reported
[27] equation r0 = 1.8625ꢀ0.000160misGeH. The CH distanc-
es were fixed at the values predicted by the ab initio MP2/6-
311+G(d,p) calculations so only the distances for the heavy
atoms were adjusted along with the angles. The predicted
parameters from the MP2/6-311+G(d,p) theoretical calcu-
lation are listed in Table 4 along with the previously report-
ed [11] values from the microwave study and the adjusted r0
Fig. 7. Comparison of the observed infrared spectra of (3000–2800 cmꢀ1):
(A) gas; (B) liquid xenon; (C) unannealed solid; and (D) annealed solid of
germacyclopentane.

G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
27
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 germacyclopentane
Isotopomer
Rotational constant
MP2(full)/6-311+G(d,p)
Experimentala
Adjusted r0
jDj
c-12C4H870GeH2
A
B
C
A
B
C
A
B
C
A
B
C
5352.1
2885.0
2069.5
5352.1
2860.4
2056.8
5352.1
2848.5
2050.6
5352.0
2836.8
2044.6
5323.90
2890.14
2066.11
5323.90
2865.61
2053.56
5323.80
2853.74
2047.46
5323.70
2842.20
2041.52
5323.82
2890.22
2066.19
5323.82
2865.61
2053.58
5323.82
2853.70
2047.46
5323.82
2842.12
2041.49
0.08
0.08
0.08
0.08
0.00
0.02
0.02
0.04
0.00
0.12
0.08
0.03
c-12C4H872GeH2
c-12C4H873GeH2
c-12C4H874GeH2
a
Ref. [11].
parameters. The fit of the twelve rotational constants is giv-
en in Table 6 with eleven of the twelve fit to 0.08 MHz or
better. The fit of the A rotational constant for the 74Ge iso-
topomer is 0.12 MHz. These results are excellent fits for all
twelve constant values which gives confidence that the
adjusted r0 parameters are well determined.
quencies from isolated CH and GeH stretches [26,27]. Also,
the listed uncertainties for the angles are consistent with
those obtained for several substituted hydrocarbons [20].
Thus, we believe the reported adjusted r0 parameters with
the listed uncertainties for germacyclopentane are as accu-
rate as or possibly more accurate than could be obtained
from additional microwave studies to obtain rs parameters
or from an electron diffraction investigation.
6. Discussion
In Table 4 structural parameters for silacyclopentane
[28], c-C4H8SiH2, are listed for comparison to those listed
for the germanium compound. Of particular note is the
bond distance for the C4–C5 atoms which is nearly the
same value for the two molecules. Also it should be noted
that the CH distances for germacyclopentane vary only
In the structural and conformational study of germacyc-
lopertane by microwave spectroscopy [11], the investigators
transfered the corresponding determined parameters from
propane and methylgermane and then determined the four
ring angles for the C2 conformer since the other two possi-
ble conformers could clearly be ruled out on the basis of
the observed Ia + Ib ꢀ Ic value along with the observed val-
ues for Ia which were the same for all four germanium iso-
topomers. However, all three of the previously reported
[11] internal ring angles (Table 4) are much larger by 3–
5° than the adjusted r0 values determined for these param-
eters in the current study. The major reason for the larger
angles is the value utilized for the two GeC distances of
˚
within 0.001 A from the similar CH distances in silacyl-
opentane. It is probable that good correlation among
structural parameters could be found between other five-
membered rings in similar rows.
In the microwave study [11] of germacyclopentane, none
of the centrifugal distortion constants were determined.
Therefore, we utilize the 19 assigned transitions of the
74Ge species and calculated three of the distortion con-
stants i.e., DJ, dJ, DJK while holding the ab initio predicted
values for the DK, dk constants at the predicted values. The
results for the determined values from the experimental fre-
quencies 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
considering the limited data with only seven different J val-
ues and nine different K values. Since the number of the
transitions assigned and measured for the 70Ge and the
72Ge isotopomers was the same and the predicted values
are nearly identical one expects the experimental values
to be quite similar. However, when the frequencies for
the transitions of the 70Ge and 72Ge isotopomer were used
for distortion constant determinations the results was quite
poor. Since the transitions for the later two isotopomers
will be much weaker because of the lower percentage of
these isotopomers, it is expected that the measured fre-
quencies were not as accurate. Also, it should be noted that
˚
˚
1.950 A which was taken from the value of 1.9453(5) A
from the distance of the GeC bond in CH3GeH3 [25].
The predicted value for the distance from the ab initio
˚
MP2(full)/6-311+G(d,p) calculation is 1.949 A which is
˚
exactly the value (1.949(1) A) reported later from adjust-
ments to the GeH distance from the isolated GeH frequen-
cy from the CH3GeHD2 molecule [26]. Thus, the ab initio
predicted values from the MP2(full)/6-311+G(d,p) calcula-
tions are expected to provide excellent predicted values for
GeC distances and for germacyclopentane, the two GeC
˚
distances are predicted to be 0.020 A longer than this bond
in methylgermane. Also, the two equivalent CC distances
˚
are predicted to be 0.010 A longer than the previously esti-
mated parameters. These four longer distances required
significantly smaller angles to fit the twelve experimentally
determined rotational constants.
The listed uncertainties for the adjusted r0 values reflect
those found from the parameters obtained from the fre-

28
G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
Table 7
Centrifugal distortion constants (kHz) of germacyclopentane in the ground vibrational state
Distortion constants
Predicted valuea
70Ge
Experimental valueb,c
74Ge
72Ge
0.39
1.37
ꢀ0.26
0.111
0.32
74Ge
0.39
1.37
ꢀ0.26
0.108
0.32
DJ
DK
DJK
dJ
0.40
1.36
ꢀ0.26
0.111
0.32
0.57(12)
1.37
ꢀ0.86(27)
0.092(14)
0.32
dK
a
This study: centrifugal distortion constants obtained from the MP2(full)/6-31G(d) calculations for the 70Ge, 72Ge and 74Ge species.
Calculated from the reported frequencies of the assigned transitions from microwave spectrum Ref. [11].
Values without uncertainties are the ab initio predicted values.
b
c
the frequency fit was not as good. Nevertheless, the ab initio
HF and the B3LYP computations with the aug-cc-pVQz
basis set, it was concluded that the N–H in the equato-
rial position is the most stable conformer of pyrrolidine
with estimated energy difference of 80–100 cmꢀ1. Never-
theless, the energy difference should be determined exper-
imentally which should be readily possible by using
variable temperature infrared spectra of rare gas solu-
tions for a confident determination of the conformational
stability and energy difference between the two stable
conformers.
Another five-membered ring where there is uncertainty
concerning the lowest energy form is the cyclopentylfluo-
ride molecule where from a vibrational study [33] it was
concluded that the molecule exist in the fluid states as a
mixture of the envelope forms with the fluorine atom in
both the axial and the equatorial positions. However, from
some theoretical calculations [34] it was predicted that the
only stable conformer is the envelope-equatorial form. We
have carried out some preliminary reinvestigation of the
vibrational spectrum [35] where the data indicate that only
one conformer exist in the fluid phases but it is probably
the twisted form with C1 symmetry rather than the enve-
lope conformers which were found for the corresponding
chloride [8] and bromide [7].
Another five-membered ring where there has been some
contradictions concerning the conformational stability is
the tetrahydrofuran (THF) molecule. From a microwave
study [36] of THF, it was showed that the five-membered
ring has a planar structure. However, from an electron dif-
fraction study [37] it was concluded that the ring is not planar
but it was also mentioned the possibility that the molecule
existing in only one conformation could not be ruled out
completely. However an ab initio study [38] of THF predict-
ed that the twisted C2 conformation has the minimum energy
and the envelope Cs conformation is a transition state with a
higher energy of 0.3 kcal/mol (105 cmꢀ1). Also, these calcu-
lations indicated that the planar C2V form is an energy max-
imum with an energy of 4.7 kcal/mol (1644 cmꢀ1). A more
recent ab initio study [39] suggests that the equilibrium
conformation of THF is an envelope Cs structure. Clearly
more studies are needed to be carried out to determine the
structural parameters and a conclusive experimental deter-
mination of the conformer stability.
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.
There has been considerable controversy concerning
the most stable conformer for several five-membered
rings. For example pyrrolidine, c-C4H8NH, it was initial-
ly concluded [29] from a microwave study of the normal
and N-D isotopic species that the molecule exists in an
envelope conformation with the imino H in the axial
position. This conclusion was later supported [30] by
an ab initio and electron diffraction study where the
envelope conformation with the imino H in the axial
direction has the lowest energy. These investigators [30]
calculated the energy profile for pseudorotation which
resulted in a second energy minimum for the envelope
conformer with the imino H in the equatorial position.
The energy difference between these two conformers
was predicted to be about ꢁ1 kcal/mol (ꢁ350 cmꢀ1) with
barrier to pseudorotation of 1.66 kcal/mol (580 cmꢀ1).
Structural parameters were reported [30] but the micro-
wave determined rotational constants could not be used
as constraints for the electron diffraction determined
structural parameters. However in the most recent micro-
wave investigation [31] of pyrrolidine by utilizing a free
jet which results in most of the molecules being in the
ground vibrational state, the data were interpreted with
the imino H in the equatorial position as the most stable
form rather than the axial conformation. However, these
investigators [31] obtained by assuming values for jlaj
and jlcj by vector compositions of bond moments the
energy difference between the envelope-axial and equato-
rial (EaxꢀEeq) forms to be ꢀ80 300 cmꢀ1 where the
very large uncertainty indicates the result to be undeter-
mined. Nevertheless, from pressure studies it was con-
cluded that the envelope-equatorial form was the more
stable conformer with the energy difference of
0 < E00 < 220 cmꢀ1. In a theoretical study [32] where
the profile of pseudorotation was predicted from ab initio
computations that both post Hartree–Fock and DFT
methods predict opposite energy difference between the
N–H axial and N–H equatorial conformers depending
on the size of the basis set. However, according to the

G.A. Guirgis et al. / Journal of Molecular Structure 834–836 (2007) 17–29
29
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, C. Adamo, J. Jaramillo, R.
Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P.
Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D.
Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S.
Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-
Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill,
B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople,
Gaussian 03, Revision B.04, Gaussian, Inc., Pittsburgh, PA, 2003.
[14] P. Pulay, Mol. Phys. 17 (1969) 197.
Unlike tetrahydrofuran, there has been unanimous con-
clusion that the corresponding sulfur molecule, tetrahydro-
thiophene, c-C4H8S, has the twisted C2 form as the only
stable conformer [40–42]. Nevertheless, there are several
five-membered ring molecules where the conformational
stability or energy difference between two stable conform-
ers has not been confidently determined. Because of the
very small energy difference for some of the conformer
pairs it is not possible to utilize ab initio calculations to
determine the conformational stability. The allyl fluoride
(3-fluoropropene) molecule provides an excellent example
where the ab initio MP2/aug-cc-pVDZ calculations predict-
[15] G.A. Guirgis, X. Zhu, Z. Yu, J.R. Durig, J. Phys. Chem. A 104 (2000)
4383.
ed [43] the gauche conformer to be more stable (55 cmꢀ1
)
than the cis form, whereas the experimentally determined
value in rare gas solutions clearly shows the cis conformer
to be the more stable form. Similar calculations at the
QCISD level with the 6-311++G(d,p) basis set also predict
the gauche form to be more stable (50 cmꢀ1) than the cis
conformer. Conformational equilibria with low energy dif-
ference are usually difficult to predict [44] especially in
quantitative terms. Thus, further experimental studies are
still needed to determine conformational stabilities for sev-
eral five-membered rings which are of considerable scientif-
ic and chemical interest.
[16] M.J. Frisch, Y. Yamaguchi, J.F. Gaw, H.F. Schaefer III, J.S. Binkley,
J. Chem. Phys. 84 (1986) 531.
[17] R.D. Amos, Chem. Phys. Lett. 124 (1986) 376.
[18] P.L. Polavarapu, J. Phys. Chem. 94 (1990) 8106.
[19] G.W. Chantry, in: The Raman Effect, A. Anderson, (Ed.), vol. 1,
Chapter 2, Marcel Dekker, New York, NY (1971).
[20] J.R. Durig, K.W. Ng, C. Zheng, S. Shen, Struct. Chem. 15 (2004) 147.
[21] J.L. Duncan, J.L. Harvie, D.C. McKean, J. Mol. Struct. 145 (1986)
225.
[22] J.R. Durig, C. Pan, G.A. Guirgis, J. Mol. Struct. 607 (2002) 117.
[23] B.J. van der Veken, W.A. Herrebout, D.T. Durig, W. Zhao, J.R.
Durig, J. Phys. Chem. A 103 (1999) 1976.
[24] Q. Shen, R.L. Hilderbrandt, V.S. Mastryukov, J. Mol. Struct. 54
(1979) 121.
[25] V.W. Laurie, J. Chem. Phys. 30 (1959) 1210.
[26] D.C. McKean, M.W. Mackenzie, A.R. Morrison, J. Mol. Struct. 116
(1984) 331.
Acknowledgment
JRD acknowledges the University of Missouri–Kansas
City for a Faculty Research Grant for partial financial sup-
port of the research.
[27] D.C. McKean, J. Chem. Phys. 79 (1983) 2095.
[28] G.A. Guirgis, A.M. El Defrawy, T.K. Gounev, M.S. Soliman, J.R.
Durig, J. Mol. Struct. 832 (2006) 73.
[29] W. Caminati, H. Oberhammer, G. Pfafferott, R.R. Filgueira, C.H.
Gomez, J. Mol. Spectrosc. 106 (1984) 217.
References
[30] G. Pfafferott, H. Oberhammer, J.E. Boggs, W. Caminati, J. Am.
Chem. Soc. 107 (1985) 2305.
[31] W. Caminati, A. Dell’ Erba, G. Maccaferri, P.G. Favero, J. Mol.
Spectrosc. 191 (1998) 45.
[32] L. Carballeira, I. Perez-Juste, C. Van Alsenoy, J. Phys. Chem. A 106
(2002) 3873.
[33] I.O.C. Ekejiuba, H.E. Hallam, Spectrochim. Acta 26 (1970) 59.
[34] L.A. Carreira, J. Chem. Phys. 55 (1971) 181.
[35] Study in progress.
[36] K.S. Murty, Proc. Int. Conf. Spectrosc., 1st, Bombay (1967) 396.
[37] A. Almenningen, H.M. Seip, T. Willadsen, Acta Chem. Scand. 23
(1969) 2748.
[1] K.S. Pitzer, W.E. Donath, J. Am. Chem. Soc. 81 (1959) 3213.
[2] J. Laane, in: J.R. Durig, (Ed.), Vibrational Spectra and Structure, vol.
1, Marcel Dekker, New York, NY (1972) Chapter 2.
[3] D.O. Harris, G.G. Engerholm, C.A. Tolman, A.C. Luntz, R.A.
Keller, H. Kim, W.D. Gwinn, J. Chem. Phys. 50 (1969) 2438.
[4] J.R. Durig, D.W. Wertz, J. Chem. Phys. 49 (1968) 675.
[5] C. Zheng, M.R. Yazidi, V.F. Kalasinsky, J.R. Durig, J. Raman
Spectrosc. 37 (2006) 52.
[6] J.R. Durig, W. Zhao, X. Zhu, J. Mol. Struct. 521 (2000) 25.
[7] H.M. Badawi, W.A. Herrebout, T.A. Mohamed, B.J. van der Veken,
J.F. Sullivan, D.T. Durig, C. Zheng, K.S. Kalasinsky, J.R. Durig, J.
Mol. Struct. 645 (2003) 89.
[38] B. Cadioli, E. Gallinella, C. Coulombeau, H. Jobic, G. Berthier, J.
Phys. Chem. 97 (1993) 7844.
[39] V.M. Rayon, J.A. Sordo, J. Chem. Phys. 122 (2005) 2043031.
[40] T.M. El Gogary, Spectrochim. Acta 57 (2001) 1405.
[41] Z. Nahlovska, B. Nahlovsky, H.M. Seip, Acta Chem. Scand. 23
(1969) 3534.
[42] J.R. Durig, J.F. Sullivan, T.S. Little, D.T. Durig, J. Mol. Struct. 142
(1986) 53.
[8] H.M. Badawi, W.A. Herrebout, C. Zheng, T.A. Mohamed, B.J. van
der Veken, J.R. Durig, Struct. Chem. 14 (2003) 617.
[9] J.R. Durig, J.N. Willis Jr., JR. J. Chem. Phys. 52 (1970) 6108.
[10] J.R. Durig, Y.S. Li, L.A. Carrira, J. Chem. Phys. 58 (1973) 2393.
[11] E.C. Thomas, V.W. Laurie, J. Chem. Phys. 51 (1969) 4327.
[12] C. Møller, M.S. Plesset, Phys. Rev. 46 (1934) 618.
[43] B. Galabov, J.P. Kenny, H.F. Schaefer III, J.R. Durig, J. Phys.
Chem. A 106 (2002) 3625.
[44] A.G. Csaszar, W.D. Allew, H.F. Schaefer III, J. Chem. Phys. 108
(1998) 9751.
[13] 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.