Journal of Molecular Structure
Elsevier Publishing Company, Amsterdam. Printed in the Netherlands
MICROW AVE SPECTRUM, STRUCTURE, DIPOLE MOMENT, AND 35Cl
QUADRUPOLE COUPLING CONSTANT OF CYCLOPENTADIENYLBERYLLIUM CHLORIDE
ALF BJØRSETH, DOUGLAS A. DREW, K. M. MARSTOKK AND HARALD MØLLENDAL
Department of Chemistry, The University of Oslo, Blindern, Oslo 3 (Norway)
(Received 11 April 1972)
ABSTRACT
Microwave spectra of five isotopic species of cyclopentadienylberyllium
chloride, C5HsBeCI, are reported. An ro and a partial rs structure have been
determined and found to be in good agreement with electron diffraction results.
The dipole moment is 4.26:t0.16 D while for the 3sCI quadrupole coupling
constant a value of -22:t2 MHz was found. The chemical bonding ofthis molecule is considered in some detail.
INTRODUCTION
Cyclopentadienylberyllium chloride, CsHsBeCI, has recently been synthesised 1. An electron diffraction study2 yielded an accurate structure of the
moleculedemonstrating that it possessesCSv symmetry (see Fig. 1). The present
microwave study was undertaken in order to compare the structure determined
by electron diffraction to the one obtained by microwave spectroscopy, and,
moreover, to determine the dipole moment and the quadrupole coupling constant
of the chlorine nucleus. These parameters are important for an understanding
of chemical bonding of this compound.
EXPERIMENTAL
CsHsBeCI was synthesised as described elsewhere1. The spectrum was
measured with the brass absorption cells cooled to about - 30°C. Smallamounts
of the compound were allowed to condense in the cell, and the spectra were meaJ. Mol. Structure, 13 (1972)
233
Th
l
o =c
.
o
= Be
=H
~=CI
Fig. 1. Molecular model of cyclopentadienylberyllium
chloride.
sured as the molecule sublimed off the cell waIls. The compound reacted
readily under the se conditions, and the half life time was estimated to be at
minutes. Absorption lines presurnably belonging to cyc10pentadiene increai
intensity as cyc10pentadienylberyllium chloride decomposed. The decompo
was accompanied by a rapidly increasing pressure. In about 10 minutes the
sure was found to change from about 10 Il to about 100 Il. The decompo
products were therefore pumped off at about 10 minutes intervals.
The spectrometer was of the conventional Stark modulation type an
been described briefly before3. Measurements were perforrned in the 9-33
region.
SPECTRUM
The spectrum of CsHsBeCI is typical of a symmetric top and consi
groups of strong lines separated by constant intervals. Transitions in the
J = 2 ~ 3 to J = 9 ~ 10 were measured for the species CsHsB(
CsHsBe37CI, C/3CHsBe3sCl, C/3CHsBe37CI, and CsH4DBe3sCl in n:
abundance. The spectra are presented in Tables 1-5. The lines were broal
to the high dipole moment, unresolved quadrupole fine structure and n
increasing pressure. Hence, the measurement uncertainties are as large as 0.1~
MHz for the various isotopic species.
The measured ground vibrational state lines were !east squares fitted to
v
234
= 2Bo(J+
1)-4D}(J+
1)3
J; Mol. Structure, 13
T ABLE 1
MICROWAVESPECTRUM OF CsHsBe3SCl
J-transition
Vibrational
state
2--+3
F =
= 9/2
7/2 --+
--+ 11/2}
9/2
F
4--+5
6--+7
F
3/2 --+
F=
= 5/2
--+ 5/2
7/2}
7--+8
v = O
v
v
VI
VI
=
=
=
=
V
= O
Vll
= 1
VI
= 1
VI
=2
=3
=4
VI
VI
V
8--+9
9 --+10
O
O
1
2
V
= O
=O
Observeda
(MHz)
Calculated
(MHz)
9803.03
9802.64
9802.67
9802.28
16336.90
22871.48
22893.80
22916.23
26138.40
26079.87
26164.23
26189.78
26213.60
26238.91
29405.23
32672.26
16337.08
22871.46
22893.91
22916.29
26138.48
26079.87
26164.14
26189.72
26213.60
26238.91
29405.37
32672.11
v = o. Ground vibrational state.
VI = 1, 2, 3,4. Successively excited states of carbon ring-Be-Cl bending motion.
Vll = 1. First excited state of carbon ring bending motion.
a ::1:0.15MHz.
T ABLE 2
MICROWAVE
SPECTRUM OF CsHsBe37Cl
J-transition
4--+5
7--+8
8--+9
Observeda
Calculated
(MHz)
(MHz)
= O
15877.63
25403.05
25426.75
28578.68
31754.04
15877.35
25403.42
25426.75
28578.68
31753.89
V
V = O
VI = O
V = O
V = O
9 --+ 10
a
Vibrational
state
::1:0.15MHz.
T AULE 3
MICROWAVE
SPECTRUM
OF 13CC4HsBe3sCl
J-transition
Observeda
414 --+ 51s
413 --+ 514
4
--+ 5
616--+717
61s --+ 716
6
--+ 7
717--+818
716--+817
7
--+ 8
818 --+ 919
8 --+ 9
16206.03
16242.90
16225.00
22687.69
22741.10
22714.39
25928.58
25988.92
25959.13
29169.38
29203.49
(K =I=-l)
(K =I=-1)
(K =I=l)
(K =I=1)
(MHz)
Calculated
(MHz)
16205.55
16243.27
16224.53
22687.67
22740.47
22714.32
25728.69
25989.03
25959.22
29169.68
29204.11
a ::1:0.20 MHz.
J. Mol. Structure, 13 (1972)
235
TABLE 4
MICROWAVE SPECTRUMOF 13CC4HsBe37CI
J-transition
7 ->- 8 (K -=Fel)
8 ->- 9 (K -=Fel)
a
Observeda
Calculated
(MHz)
(MHz)
--
25227.61
28379.51
25227.61
28379.51
::1::0.25 MHz.
TAB LE 5
MICROWAVE
SPECTRUM
OF CsDH4Be35CI
J-transition
7 ->- 8 (K -=Fel)
8 ->- 9 (K -=Fel)
Observeda
Calculated
(MHz)
(MHz)
25784.70
29006.90
25784.70
29006.90
a 0.30 MHz.
but the centrifugal distortion effect was so
could not be obtained. Nor could a splitting
distortion constant D JK be detected even
CsHsBe3sCl. The results of the least squares
small that significant values (
of the lines due to the centri
for the J = 9 ~ 10 transitic
refinement are shown in Table
TABLE 6
MOLECULAR
Maleeule
Ao
Bo
(MHz)
(MHz)
PARAMETERS FOR CsHsBeCI
CsHsBe35CI
C sHsBe37 Cl
1633.74
::1::0.02
1587.75
::1::0.04
1633.74 ::1::0.02
-1.592::1::0.009
3.658::1::0.015
<0.3
<0.7
-22.0
::1::2.0
1587.75
-1.481
::1::0.04
::1::0.009
13CC4HsBe35CI
13CC4HsBe37
Cl
4460::1::21
1626.22 ::1::0.03
1577.05 ::I::O.04a
Co
(MHz)
0([
(MHz)
0(11 (MHz)
DJ (MHz)
DJK (MHz)
eqQ (MHz)
a
CsD
1611
1618.68::1::0.03
<0.3
<0.3
<0.7
<0.7
«
<0.3
<0.7
<C
This parameter is HBo+Co).
Each ground vibrational state line is followed by a very complicated p2
of excited vibrational state lines appearing at higher frequencies probably d
successively excited states of the cycIopentadienyl ring-beryllium-chl
bending vibration. These excited states are split into a complicated patten
to Coriolis interaction, but because of the high instability of the comp
complete analysis of this fine structure proved toa difficult. The "main" e~
state lines were measured and are also listed in Tables 1 and 2. These cou
fitted
236
to By
=
Bo - CWi with sufficient
accuracy.
This regular
variation
o
J. Mol. Structure, 13
rotation-vibration interaction constant rxindicates that the corresponding bending
vibration is quite harmonic with no potential banier at the fivefold axis of symmetry. Crude relative intensity measurements yield a value of 250:!:75 cm-l for
this bending motion. Another excited state is observed at lower frequencies
relative to the ground state line. This is tentatively assigned to a low energy ring
bending motion.
DIPOLE MOMENT
The J = 4 -+ 5 transition was usedto determinethe dipole moment.A small
d.c. voltage was applied between the Stark septum and the cell, with the modulating
square wave voltage superimposed. The Stark components were broad and hard to
measure accurately. Measurements were perforrned on several M-components for
several splittings ranging from about 4 MHz to about 25 MHz. The d.c. voltage
was measured with a digital voltmeter having an accuracy of 0.025 %. The Stark
splittings were analysed utilizing the first order theory. A dipole moment of 4.26
D with a standard deviation of 0.16 D was obtained. In the calculation of the first
order Stark effect, influence of the quadrupole moment was neglected because of
the small quadrupole coupling constant. OCS was used to calibrate the cell
spaeing employing the dipole moment of OCS reported by Muenter4.
QUADRUPOLE
COUPLING
CONSTANT
The quadrupole coupling constant was determined for the 3sCI-nudeus of
the CsHsBe3sCl speeies from the M = Ocomponent of the J = 2 -+ 3 transition.
This was the lowest accessible transition on our speetrometer. The M =1=
OStark
components were removed by applying a small d.c. voltage on the Stark septum.
A splitting of 1.42:!:0.10 MHz was observed yielding eqQ = 22::!:2 MHz for the
3sCl quadrupole coupling constant.
STRUCTURE
The rotational constants in Table 6 yield sufficient information to calculate
the rs coordinatesS,6 ofthe chlorine and the carbon atoms. The results are presented
in Table 7. Structural parameters involving only these two atoms can thus be
calculated with high accuracy and are expected to be dose to the equilibrium
values6. Unfortunately, this is not the case for the structural coordinates of beryllium and hydrogen. The former atom has no stable isotopes and cannot be located
in away analogous to the one used for chlorine and carbon. Assuming the
J. Mol. Structure, 13 (1972)
237
T ABLE 7
KRAITCHMAN'S
COORDINATES
FOR CsHsBeCI
Atom
z (A)
y (A)
CI
C
Be"
Hb
-2.1375
0.0
(10)
+ 1.1952 (47)
-0.33
(3)
+ 1.25 (4)
1.2113 (32)
0.0
2.30
(3)
" Calculated assuming the hydrogen atoms co plan ar with the carbon atoms.
b Calculated
assuming B-C = 26.83:1::0.50 MHz for the CsH4DBe35CI speeies.
TABLE 8
STRUCTURAL
PARAMETERS"
OF CsHsBeCI
C-He
C-C
Be-CI
Be-ring (h)
CI'"
C
Microwave
Microwave
rs
ro
Electron diffractionb
rg
1.09 (3)
1.424 (3)
1.81 (3)
1.52 (3)
3.546 (5)
l. 090
1.424
1.839
1.485
3.538
1.097
1.424
1.837
1.484
3.535
(4)
(1)
(7)
(7)
(5)
" See text and comments to Table 7.
b
e
Taken from ref. 2.
Hydrogen atoms assumed to be coplanar with the carbon atoms.
hydrogen atoms to be coplanar with the carbons, beryllium was located emplo:
Imizi = O. The beryllium coordinate obtained in this way is 0.33::!::0.03Å.
large standard deviation is due to the larger mass of the five carbon atoms.
determination of the structural coordinates of the hydrogen atoms must alsc
based on assumptions. CsH4DBe3sCl yielded B+ C whereas B-C could no
determined owing to the small intensity of the K = 1 doublets. Considera
of reasonable structural models suggests that B-C = 26.83::!::0.50 MHz. Tt
rotational constant is likewise estimated to be 4220.00::!::20.00MHz. The hydn
coordinates depend strongly on B and C, but very weakly on A. We obta
y = 2.30::!::0.03 Å and z = 1.25::!::
0.04 Å. The results are summarized in Tab
and the rg-like structure determined from these Cartesian coordinates is show
Table 8.
In addition, an ro structure was determined by fitting the structural pal
eters to the observed rotational constants. The resulting structure which reprod
all the rotational
constants
better than 0.1
% is
inc1uded in Table 8 together .
the electron diffraction structure2.
DISCUSSION
The microwave spectrum reveals that CsHsBeCI is a Csv symmetri c
molecule. The rg and the ro structures given in Table 8 are in excellent agreeI
with the electron diffraction rg structure. Both methods yield Be-CI dista
238
J. Mol. Structure, 13 (
distinctly longer than 1.75 Å reported for gaseous beryllium chloride 7. The
hydrogen atoms are coplanar or very nearly coplanar with the car bon atoms.
The high dipole moment and particularly the small quadrupole coupling constant
is compatible with a fairly ionic Be-CI bond. This is supported by simple arguments.
lf the ionic, ie, and n characters, ne, are computed as described in ref. 8, ie = 0.65
and ne = 0.10 are obtained. The ionic character is considerably higher than 0.35
found for CH3CI, 0.38 for SiH3CI, and 0.57 for GeH3CI, for example8. The
comparatively long Be-CI distance of CsHsBeCI, the small quadrupole coupling
constant, and high dipole moment suggest that the beryllium-chlorine bond is a
polar single bond apparently with little pn-dn beryllium-chlorine back bonding.
Our results are in agreement with the bonding scheme proposed by Drew and
Haaland9 for this and similar beryllium-cyclopentadienyl compounds. They assurne that the Be atom is sp hybridised using one such orbital in bonding to the CI
and the other in bonding to the aen orbital of the ring. Two more Be-ring bonds
form by combination of the two degenerate Ben orbitals of the ring with the two
unhybridised 2p beryllium orbitals. The beryllium atom then has an "octet" of
electrons. lnvolvement of the 2p orbitals of Be in bonding with the ring reduces
the possibility for dative n bonding between Be and Cl. Therefore this bond is
longer than in monomeric beryllium dichloride 7.
ACKNOWLEDGEMENT
We are grateful to the Norwegian Research Council for Science and the
Humanities for financial support. D.A.D. is indebted to the National Science
Foundation for a grant (GP 24090).
REFERENCES
l
2
3
4
5
6
7
8
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J. S. MUENTER, J. Chem. Phys., 48 (1968) 4544.
J. KRAITCHMAN, Amer. J. Phys., 21 (1953) 17.
C. C. COSTAIN, J. Chem. Phys., 29 (1958) 864.
P. A. AKISHIN AND U. P. SPIRIDINOV, Kristallografiya,
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W. GORDY AND R. L. COOK, in W. WEST (editor), Chemical Applications
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239