Molecular structure of propargylgermane (2-propynylgermane) determined by gas-phase electron diffraction and quantum chemical calculations

Article abstract of DOI:10.1016/j.molstruc.2005.06.047

The molecular structure of propargylgermane, HC≡CCH ₂GeH₃, has been determined by gas-phase electron diffraction. The electron-diffraction investigation has been supported by density functional theory and ab initio calculations. The

Full text of DOI:10.1016/j.molstruc.2005.06.047

Journal of Molecular Structure 780–781 (2006) 157–162
www.elsevier.com/locate/molstruc
Molecular structure of propargylgermane (2-propynylgermane)
determined by gas-phase electron diffraction and quantum
chemical calculations
a
Tatyana Strenalyuk , Svein Samdal , Harald Møllendal , Jean-Claude Guillemin
a,
a
b
*
aDepartment of Chemistry, University of Oslo, PO Box 1033 Blindern, NO-0315 Oslo, Norway
` ´
Laboratoire de Synthese et Activation de Biomolecules, UMR CNRS 6052, Institut de Chimie de Rennes, ENSCR, F-35700 Rennes, France
b
Received 29 April 2005; accepted 7 June 2005
Available online 26 August 2005
Dedicated to Jean Demaison on the occasion of his retirement
Abstract
The molecular structure of propargylgermane, HCbCCH2GeH3, has been determined by gas-phase electron diffraction. The electron-
diffraction investigation has been supported by density functional theory and ab initio calculations. The ra value of the bond lengths (pm) are:
r(C–Ge)Z197.2(1); r(C–C)Z143.9(2); r(CbC)Z123.1(1); r(H–Cacetylene)Z108.5(3); r(C–H)Z111.6(3) and r(Ge–Haverage)Z153.7(2).
The Ge–C–C angle is 111.7(1)8 and the C–CbC angle is 178.3(4)8. The uncertainties are one standard deviation from the least-squares
refinement.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Propargylgermane; Electron diffraction; Molecular structure; Quantum chemical calculations
1. Introduction
Gas-phase electron diffraction (GED) can be used to
determine an accurate structure of propargylgermane. It is
fortunate that the degree to which electrons are scattered
by the germanium atom is high, which means that the
bond lengths of atoms attached to germanium can be
accurately determined. The present work was undertaken
to extend the knowledge of the structures of compounds
containing the germyl group. The electron-diffraction
work has been supplemented by quantum chemical
calculations.
The structures of relatively few gaseous compounds
possessing the germyl (GeH3) group have been reported.
Accurate structures have so far been determined for
germane (GeH4) [1,2] the germyl halides (GeH3X, XZF
[3], Cl [4,5], Br [5,6] and I [5,6]), germylacetylene (HCbC–
GeH3) [7], methylgermane (CH3GeH3) [8], ethylgermane
(CH3CH2GeH3) [9,10] and cyclopropylgermane (C3H5
GeH3) [11].
Some structural information is available for vinylger-
mane (H2CaCHGeH3) [12] (fluoromethyl)germane (CH2
FGeH3) [13] and (chloromethyl)germane (CH2ClGeH3)
[14]. Ab initio calculations at a relatively high level of
theory (MP2/aug-cc-pVTZ) have been performed for
allylgermane (H2CaCHCH2GeH3) [15]. A review of
germane and the germyl halides has been given by Bu¨rger
and Rahner [16].
2. Synthesis
2.1. Materials
Germanium tetrachloride, 1,2-propadienyltri-n-butyl-
stannane, tetraethylene glycol dimethyl ether (tetraglyme)
and lithium aluminum hydride were purchased from Aldrich
and were used without further purification.
*
Corresponding author. Tel.: C47 2285 5458; fax: C47 2285 5441.
E-mail address: svein.samdal@kjemi.uio.no (S. Samdal).
Caution! Propargylgermane is pyrophoric and potentially
toxic. All reactions and handling should be carried out in
a well-ventilated hood.
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.06.047

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T. Strenalyuk et al. / Journal of Molecular Structure 780–781 (2006) 157–162
Propargylgermane was prepared in a two-step reaction,
3. Quantum chemical calculations
starting with germanium tetrachloride and 1,2-propadienyl-
tributylstannane, followed by the chemoselective reduction
of the formed propargyltrichlorogermane [17].
Two rotamers, denoted eclipsed and staggered, are
possible for propargylgermane. These two forms are shown
in Fig. 1, where the atom numbering is also indicated.
Density Functional theory (DFT) calculations for the
eclipsed and the staggered conformers were first performed
using the GAUSSIAN 03 suite of programs [19]. The main
objectives of these calculations were to predict a force field
for the title compound and to explore its conformational
composition. The functional of Becke [20] (B3LYP) was
used in conjunction with the 6-311G** basis set. Full
geometry optimizations were conducted for the eclipsed and
staggered forms. No negative vibrational frequencies were
computed for staggered rotamer indicating that this form is a
minimum on the potential energy hypersurface [21].
One imaginary frequency (K132 cmK1) was calculated
for eclipsed isomer, which suggests that this geometry is not
true minimum, but a first-order transition state [21]. The
atomic movements associated with this imaginary vibration
showed that it represents a rotation around the Ge1–C2 axis.
The total energy of the hypothetical eclipsed rotamer was
GeCl4
+
-(Bu3SnCl)
SnBu3
GeCl3
GeH3
LiAlH4
tetraglyme
.
2.2. Propargyltrichlorogermane
Into a two-necked flask equipped with a nitrogen inlet
and a magnetic stirring bar was introduced germanium
tetrachloride (4.3 g, 20 mmol). The propadienyltri-n-butyl-
stannane (6.6 g, 20 mmol) was slowly added at room
temperature and the mixture was stirred for 30 min at
50 8C. The flask was then attached to a vacuum line and
the propargyltrichlorogermane was purified by selective
condensation in a trap cooled at K30 8C. The product was
kept in a freezer (K20 8C). Yield: 2.3 g (53%). bp: 25 8C
4
(0.1 mm Hg). 1H NMR (CDCl3): d 2.33 (t, 1H, JHH
Z
4
2.9 Hz, CH); 2.93 (d, 2H, JHHZ2.9 Hz, CH2). 13C NMR
1
2
(CDCl3): d 20.9 (t, JCHZ141.0 Hz, CH2), 73.0 (d, JCH
Z
1
51.1 Hz, HCbC); 73.7 (d, JCHZ254.0 Hz, CH).
2.3. Propargylgermane
The apparatus was similar to that described for the
preparation of 2-propynylphosphine [18]. In a 250 mL two-
necked flask the reducing agent (LiAlH4, 1.14 g, 30 mmol)
and tetraglyme (50 mL) were introduced. The flask was
attached to a vacuum line, immersed in a cold bath (0 8C) and
degassed. The propargyltrichlorogermane (2.2 g, 10 mmol)
in tetraglyme (10 mL) was slowly added via a flex-needle
through the septum over the course of 5 min. During and
after the addition, the formed propargylgermane was
distilled off in vacuo from the reaction mixture. A cold trap
(K80 8C) selectively removed less volatile products and
propargylgermane was condensed in a second cold trap
(K120 8C) to remove the most volatile products (mainly
GeH4). After disconnecting from the vacuum line by
stopcocks, the product was kept at low temperature
(!K50 8C) before analysis. The propargylgermane was
thus obtained in a 78% yield (0.90 g). bp: zK90 8C
4
(0.1 mmHg). 1H NMR (CDCl3): d 1.95 (t, 1H, JHH
Z
4
3
2.9 Hz, CH); 1.87 (dq, 2H, JHHZ2.9 Hz, JHHZ3.2 Hz,
CH2); 3.87 (t, 3H, 3JHHZ3.2 Hz, GeH3). 13C NMR (CDCl3):
dK3.7 (t, 1JCHZ135.3 Hz, CH2); 67.4 (d, 1JCHZ248.7 Hz,
CH); 82.8 (d, 2JCHZ48.6 Hz, HCbC). HRMS: m/z calcd for
C3H5Ge [MKH]C, 114.9603; found, 114.959.
Fig. 1. Models of the staggered (top) and eclipsed (bottom) forms of
propargylgermane with atom numbering.

T. Strenalyuk et al. / Journal of Molecular Structure 780–781 (2006) 157–162
159
Table 1
Calculated and experimental structural parametersa of propargylgermane
Parameter
Eclipsed B3LYPb
Staggered
B3LYPb
GED
MP2c
ra
rg
ucalc
uexp
Ge1-C2
C2-C3
200.6
145.0
120.3
106.2
153.3
153.8
109.3
199.5
145.0
120.3
106.2
153.9
153.5
109.4
195.5
145.0
121.7
106.2
151.8
151.5
109.0
197.2(1)
143.9(2)
123.1(1)
108.5(3)
154.0(2)
153.6(2)
111.6(3)
197.4
144.0
123.2
109.0
154.6
154.2
112.1
5.4
4.7
3.5
7.3
9.0
8.9
7.7
5.8(1)
4.3(2)
3.9(2)
7.3
C3bC4
C4–H5
Ge1–H10
9.5(2)
9.4(2)
7.7
d
Ge1–H8
d
C2–H6
Angles
:Ge1C2C3
113.1
178.5
111.9
178.0
110.1
177.3
179.6
108.9
109.3
108.8
110.1
110.6
180.0
111.7(1)
178.3(4)
180.0
e
:C2C3C4
:H5C4C3
e
179.3
179.1
:Ge1C2H6
:C2Ge1H10
:C2Ge1H8
107.5
107.8
104.9(3)
110.9(9)
111.6
110.1
108.7
e
109.2
109.4
e
:H8Ge1H9
108.7
109.9
111.8
e
:C3C2H6
C3C2Ge1H10
110.6
110.9
108.0
0.0
180.0
180.0
E0
K2194.82562
5.9
K2194.82788
Rf,Z5.85
DE
a
Distances in pm, angles in degrees, parenthesized values are the least-squares standard deviations in units of the last digit, energies in Hartree, rotational
barrier, DE, in kJ molK1
.
b
B3LYP/6-311G**.
c
MP2/aug-cc-pVTZ.
d
These distances were calculated as Ge1–H8ZGe1–H10–0.45, and C2–H6ZC4–H5C3.18;-dependent parameter; see text.
The dihedral angle Ge1C2C3C4 is 08, with the C4 atom bent towards the Ge atom. The dihedral angle C2C3C4H5 is 1808, with the H5 atom bent away from
e
the Ge atom.
calculated to be 5.9 kJ molK1 higher than the energy of the
staggered form. Selected results of the B3LYP calculations
of the two rotamers are collected in Table 1.
5. Electron diffraction
5.1. Experimental
It has been shown that a second order Møller-Plesset
perturbation treatment of electron correlation [22] employ-
ing a relatively large basis set predicts an accurate
equilibrium structure [23]. It is of interest to compare the
approximate equilibrium structure obtained in this manner
with the GED structure. Dunning’s correlation-consistent,
triple-z basis function with polarized valence electrons and
diffuse functions was chosen in the present case. Calcu-
lations were only carried out for the staggered confor-
mation. The results of the MP2/aug-cc-pVTZ optimization
made for this rotamer are listed in Table 1.
The GED data were recorded using a Balzers KD-G2 unit
[25]. The experimental data were recorded on BAS-III
image plates, which were scanned using a BAS-1800II
scanner. Both the image plates and the scanner are
manufactured by FujiFilm. The image plates are more
sensitive, have a higher resolution, a much higher linear
response and dynamic range than photographic plates.
Owing to the high linear response of the image plates, no
blackness correction is needed.
Each image plate was divided into four sectors, two in
x-direction (left and right), and two in y-direction (up and
down). Data for each sector were treated separately and the
two sectors in x-direction were averaged to give one
modified intensity curve in the x-direction and similarly,
one modified intensity curve in y-direction. The data range
in x-and y-direction is slightly different as a consequence of
the rectangular shape of the image plate. This procedure
applies for both camera distances, and gave four curves as
shown in Fig. 3. These four curves were used in the least-
squares structure analysis. The raw data were further
processed as described in the experimental section of
Ref. [26].
4. Microwave experiment
Attempts were made to obtain the microwave spectrum
of the title compound using the Oslo spectrometer [24].
However, no spectrum was observed. It is believed that this
is a consequence of a small electric dipole moment of the
molecule. This is in accord with the B3LYP/6-311G**
quantum chemical calculations described above, which
predict the principal-axes dipole moment components to be
maZ0.38, mbZ0.53 and mcZ0 Debye. It is the experience of
these authors that the theoretical dipole moments are in
general larger than the experimental ones.
The necessary modification and scattering functions
were computed from tabulated atomic scattering factors

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T. Strenalyuk et al. / Journal of Molecular Structure 780–781 (2006) 157–162
5.2. Structure refinement
According to the quantum chemical calculations above,
the staggered rotamer with Cs symmetry is the most stable.
For a complete description of the molecular geometry
eighteen parameters are needed, and those selected are the
bond distances: r(Ge1C2), r(C2C3), r(C3C4), r(C4H5),
r(C2H6)Zr(C2H7), r(Ge1H8)Zr(Ge1H9), r(Ge1H10); the
bond angles :(Ge1C2C3), :(Ge1C2H6)Z:(Ge1C2H7),
:(C3C2H6)Z: (C3C2H7), :(C2C3C4), :(C3C4H5),
:(C2Ge1H8)Z: (C2Ge1H9), :(C2Ge1H10), :(H8Ge1
H10)Z:(H9Ge1H10); and the dihedral angles:
f(C3C2Ge1H10), f(Ge1C2C3C4), f(C2C3C4H5). The dihe-
dral angles are all fixed at values of 0 or 1808.
Owing to the low scattering power of the H atoms, it is
often not possible to determine all of the structural
parameters involving H atoms. Therefore constraints have
to be imposed on the molecular geometry and the choices
were:
Fig. 2. Radial distribution curve. Interatomic distances are indicated.
[27] for the proper wavelength and s-values. The
experimental backgrounds were computed using the
program KCED12 [28], where the coefficients of a chosen
degree of a polynomial function are determined by the least-
squares method by minimizing the differences between the
total experimental intensity and the molecular intensity
calculated from the current best geometrical model.
The average experimental intensities were modified by
s/jf0Cf0Gej, where f0 denotes the coherent scattering factors.
The nozzle temperature was 22 8C. The molecular
intensities were obtained in the s ranges 17.5–117.5 nmK1
and 20.0–145.0 nmK1 for y and x direction respectively,
with DsZ1.25 nmK1 and a camera distance of 498.65 mm,
and correspondingly 40.0–247.5 nmK1 and 40.0–
285.0 nmK1 with DsZ2.50 nmK1 and a camera distance
of 248.88 mm. The experimental and theoretical radial
distribution and intensity curves for the best model are
shown in Figs. 2 and 3, respectively. The electron wave
length is 5.820 pm.
r(Ge1H8)Zr(Ge1H9)Zr(Ge1H10)CD1, where D1 is the
difference between r(Ge1H8) and r(Ge1H10) taken from the
B3LYP quantum chemical calculations. Analogously:
rðC2–H6ÞZrðC2–H7ÞZrðC4–H5ÞCD2
:ðC3C2H6ÞZ:ðC3C2H7ÞZ:ðC1C2H6ÞCD3
:ðC2Ge1H8ÞZ:ðC2Ge1H9ÞZ:ðC2Ge1H10ÞCD4
:ðH10Ge1H8ÞZ:ðH10Ge1H9ÞZ:ðC2Ge1H10ÞCD5:
The D0s are adjustable parameters or fixed to their
B3LYP/6-311G** values which are 0.45 pm, 3.18 pm,
3.078, 0.658, 0.928, for D1–5 respectively.
Root-mean-square amplitudes of vibration (u-values)
and perpendicular correction coefficients (D-values) were
derived from the B3LYP/6-311G** force field using the
ASYM program [29,30]. Some of the calculated u-values
are given together with the refined structural parameters
in Table 1.
Attempts were made to refine the f(C3C2Ge1H10)
dihedral angle in order to ascertain whether it would be
possible to determine the barrier to internal rotation of the
germyl group. However, owing to the small contributions to
the total scattering from the C3/HGe, and C4/HGe
distances, this angle could not be determined with sufficient
accuracy. The standard deviation of this parameter was
about 10–138, its value was close to 1808 and it was not
significantly influenced by the other parameters. It was
therefore not possible to determine the barrier to internal
rotation of the germyl group by including a dynamic model
in the electron-diffraction analysis. Furthermore, the
f(C3C2Ge1H10) dihedral angle was kept constant at 1808
in the final refinement. The nearly linear C3C4H5 angle
could not be determined for the same reasons as mentioned
above. Its value was therefore fixed to 1808.
The experimental structure and its B3LYP and MP2
counterparts are given in Table 1.
Fig. 3. Intensity curves; see text for details.

T. Strenalyuk et al. / Journal of Molecular Structure 780–781 (2006) 157–162
161
6. Discussion
calculations, while the GED gives a significant shorter
bond distance (143.9(2) pm).
In Table 1, the B3LYP, MP2, ra and rg structures of
propargylgermane are listed. The MP2 and B3LYP
structures are approximations of the equilibrium (re)
structure. They agree rather well with two exceptions, viz.
the Ge–C and Ge–H bond lengths. The Ge–C bond is 4 pm
longer in the B3LYP structure than in the MP2 structure. A
similar tendency is found in the case of the Ge–H bond
lengths (Table 1). The difference between these values
would appear to be a result of the quantum chemical
treatment of the large number of electrons belonging to the
germanium atom (32).
Table 2 lists structural parameters, barriers to internal
rotation of the germyl group and dipole moments of selected
germanes. The germyl group is attached to carbon atoms
that are in different states of hybridization. The distance
between the carbon atom and the attached germyl group is
expected to be a function of the hybridization of the carbon
atom. The carbon atom is sp3-hybridized in propargylger-
mane. Comparison of the C–Ge ra distance of this
compound (197.2(1) pm; Table 1) with its counterparts in
methylgermane [8] (194.53(5) pm) and ethylgermane [10]
(195.9(5) pm; Table 2) shows that the bond length in the
title compound is perhaps slightly longer than in the two
other cases. However, the bond lengths in methyl-and
ethylgermane have been determined from microwave data.
A direct comparison of bond lengths determined by the two
different methods should therefore be viewed with some
caution.
Comparison of the approximate re structures obtained in
the MP2 and B3LYP calculations with the ra and rg GED
structures (defined in Ref. [31,32]), which are not
equilibrium structure, is not straightforward. The ra and rg
bond lengths are always longer than the re bonds by
definition [31,32].
Comparison of ra and rg bond lengths in Table 1 with the
corresponding bond lengths obtained in the MP2 calcu-
lations, which is assumed to be close to the equilibrium
structure [23], is in accord with this theory with the
exception of the C2–C3 bond length. This bond length is
predicted to 145.0 pm both in the DFT and MP2
However, it is interesting to note that the MP2/aug-cc-
pVTZ calculations predict the C-Ge bond lengths both in
propargylgermane (Table 1) and in the anticlinal conformer
of allylgermane (Table 2) [15] to be about 195 pm,
practically the same as in methyl- and ethylgermane
(Table 2).
Table 2
Important structureal data, rotational barriers and dipole moments of selcted germyl compoundsa
Compound
Structural parameters
Rotational barrier
Dipole moment
H3GeCbCH [7] germylacetylene
r(Ge–C)Z189.6(1)
r(CbC)Z120.8(1)
r(Ge–H)Z152.1(1)
:HGeHZ109.9(10)
r(Ge–C)Z194.53(5)
r(Ge–H)Z152.9(5)
:HGeHZ109.3(5)
r(Ge–C)Z195.9(5)
r(C–C)Z153.2(5)
r(Ge–H)Z152.5(5)
:CCGeZ112.3(5)
r(Ge-C)Z192.6(12)
r(CaC)Z134.7(15)
r(Ge–H)Z152.1(5)
:CGeHZ109.7(6)
r(Ge–C)Z191.8(1)
r(C–C)Z152.0(1)
r(Ge–H)avZ153.5(1)
mtotalZ0.136(2)
CH3GeH3 [8] methylgermane
DEZ5.2(1)
DEZ5.9(1)
mtotalZ0.635(6)
CH3CH2GeH3 [10] ethylgermane
maZ0.71(1)
mbZ0.27(1)
mtotalZ0.76(2)
CH2CHGeH3 [12] Vinylgermane
DEZ5.2(3)
maZ0.49(2)
mbZ0.12(2)
mtotalZ0.50(3)
C3H5GeH3 [11] cyclopropylgermane
CH2FGeH3 [13] (fluoromethyl)germane
CH2ClGeH3 [14] (chloromethyl)germane
CH2ZCHCH2GeH3 Allylgermaneb [15]
DEZ5.58(20)
DEZ5.8(2)
DEZ7.3(1)
maZ0.684(7)
mcZ0.261(8)
mtotalZ0.739(9)
maZ1.00(3)
mbZ1.30(3)
mtotalZ1.64(3)
r(Ge–C)Z196.1
r(Ge–H)Z151.7
:HGeCZ107.7
r(Ge–C)Z195.1
–(CaC)Z133.7
r(Ge–H)avZ151.8
r(C–C)Z149.0
maZ0.04
mbZ0.51
mbZ0.02
mtotalZ0.51
a
Bond lenghts in pm, angles in degree, barriers in kJ molK1 and dipole moments in debye.
b
Anticlinal conformer. MP2/aug-cc-pVTZ values; see Ref. [15].

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T. Strenalyuk et al. / Journal of Molecular Structure 780–781 (2006) 157–162
[14] J. Nakagawa, M. Hayashi, Bull. Chem. Soc. Jpn 49 (1976) 3441.
The Ge–H bond length is another interesting parameter.
[15] A. Horn, H. Mollendal, J. Demaison, D. Petitprez, J.R.A. Moreno,
A. Benidar, J.-C. Guillemin, J. Phys. Chem. A ACS ASAP.
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[18] J. Demaison, J.-C. Guillemin, H. Møllendal, Inorg. Chem. 40 (2001)
3719.
It can be seen in Table 2 that the values of this parameter
lie between 151.7 pm in (chloromethyl)germane [14] and
152.5(3) pm in ethylgermane [10]. This is somewhat shorter
than 154.0(2) pm and 153.6(2) pm obtained in our case
(Table 1). The values of this bond length, as determined by
spectroscopy, is similar to the MP2 results of the title
compound and of the anticlinal form of allylgermane (about
151.8 pm).
[19] MS, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
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Acknowledgements
T.S. thanks the International Student Quota Program for
financial support. The Aurora exchange program between
France and Norway is gratefully acknowledged for grants
to H. M. and J-C. G. The Research Council of Norway
(Programme for Supercomputing) is thanked for a grant
of computer time. J-C. G. thanks PCMI(INSU-CNRS) for
financial support. George C. Cole is thanked for his
thorough reading and correction of the manuscript.
[20] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[21] W.J. Hehre, L. Radom, P.v.R. Schleyer, Ab Initio Molecular Orbital
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