LEAD-210 SEPARATIONS. MOLECULAR SYMMETRY OF SOME MOLECULES WITH POLYETHER RINGS

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LEAD-210 SEPARATIONS. MOLECULAR SYMMETRY OF SOME
MOLECULES WITH POLYETHER RINGS
*

CORINA ANCA SIMION, ROMUL MARGINEANU, DIANA CHIPER, CATALINA BARNA
National Institute for Research & Development of Physics and Nuclear Engineering,
407 Atomistilor Street, P.O.Box MG 6 – Magurele, 077125 Bucharest, Romania;
E-mail: corina.simion@rdslink.ro
Received December 21, 2004
The molecular symmetry of Dibenzo-12(to 24)-crown-4(to 8) ethers was
analyzed using HyperChem 5.02. Computational Chemistry Model Building Program.
The results are in agreement with those obtained with earlier experimental data and
molecular models built with Fisher-Hirsch-Welder-Taylor atomic models in the crown
ethers series, and will be a helpful method to predict the most appropriate structure of
selective crown ether for lead-210 separations.
INTRODUCTION
In 1967, thirty-three cyclic polyethers, derived from aromatic vicinal diols
and containing from 9 to 60 atoms including 3 to 20 oxygen atoms in the ring, have
been synthesized by C.J. Pedersen [1]. Many of those containing five to ten oxygen
atoms form stable complexes with some or all of the cations of: Li, Na, NH
4
,
RNH
3
, K, Rb, Cs, Ag, Au, Ca, Sr, Ba, Cd, Hg, La, Ce, and Pb. The resulted
products appear to be salt-polyether complexes formed by ion-dipole interaction
between the cation and the negatively charged oxygen atoms of the polyether ring.
The stoichiometry of the complexes is one molecule of polyether per single ion
regardless of the valence, and the complex conserves the crown ether structure with
the cation placed in the center of the polyether ring. The complexation reaction is
widely used for the separation and preconcentration of some of these cations from
biological, environmental, and nuclear waste samples by extraction
chromatography techniques [2, 3, 4]. Figure 1 shows a cyclic polyether containing
six oxygen atoms in the crown ether structure [cis (to the left) and trans (to the
right) isomers, depending on the relative positions of the two aromatic rings]:

*

Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July
2004, Constanţa, Romania.
Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 263–268, Bucharest, 2006

Corina Anca Simion et al. 2
264
O
O
O
O
O
O
O
O
O
O
O
O
Fig. 1 – 4,4'(5')-dibenzo-18-crown-6 (cis form) or DBz18C6 (cis)
, and 4,4'(5')-dibenzo-18-crown-6
(trans form) or DBz18C6 (trans).
The complex is not stable if the ion is too large or too small to lie in the hole
of the polyether ring. The holes correspond to the diameters of the largest spheres
which can pass through them calculated by means of molecular models built with
Fisher-Hirsch-Welder-Taylor atomic models. A complex is even more stable, the
greater the number of oxygen atoms, provided the oxygens are coplanar and
symmetrically distributed in the polyether ring. An oxygen atom is considered to
be coplanar if it lies in the same plane as all the other oxygens in the ring, and the
apex of the C O C angle is centrally directed in the same plane. Symmetry is at
a maximum when all the oxygen atoms are evenly spaced in a circle. When seven
or eight oxygens are present in the polyether ring, they cannot arrange themselves
in a coplanar configuration, but they can arrange themselves around the surface of
a right circular cylinder with apices of the C O C angles pointed towards the
center of the cylinder. This configuration, termed cylindrically symmetrical,
permits the formation of salt complexes. Steric hindrance in the polyether ring
prevents the formation of complexes.

So, the conditions for the formation and the factors influencing the stability
of the complexes include:
(1) the relative sizes of the ion and the hole in the polyether ring
(2) the number of oxygen atoms in the polyether ring
(3) the coplanarity of the oxygen atoms
(4) the symmetrical placement of the oxygen atoms
(5) the basicity of the oxygen atoms
(6) steric hindrance in the polyether ring
(7) the tendency of the ion to associate with the solvent
(8) the electrical charge of the ion.
This paper is focused on the more accurate theoretical study of some of these
factors determining the molecular symmetry of the crown ethers using
HyperChem 5.02. Model Building Program [5] than the earlier models built with
Fisher-Hirsch-Welder-Taylor atomic models, in order to explain and predict the
stability and selectivity of the complexes for future applications.
3 Lead-210 separations
265
RESULTS AND DISCUSSION
The molecular symmetry is one of the most important factors determining the
stability of the complexes. A systematic study of the molecular symmetry in the
cyclic polyether ring series has not been reported. Our theoretical study starts with
the determination of the molecular properties in the crown ethers serie 12(to 24)-
crown-4(to 8).
The next step is the modelling of the corresponding dibenzo- compounds in
order to explain how the aromatic substitution at the polyether ring can influence
the molecular symmetry and properties of the compounds.
The simulation strategy on HyperChem 5.02. Program includes two stages of
molecular orbital calculation (MO calculation) one using MM
+
, Polak-Ribière
optimizer, RMS (Gradient) of 0.1 kcal/mol.Å, and another using semi-empirical
AM1 method, Polak-Ribière optimizer, RMS (Gradient) of 0.1 kcal/mol.Å (then
0.01 kcal/mol.Å for complexes) in both single point and geometry optimization
stages. Use of “restraints…”technique is required on O C (belonging to benzo
moiety)  C (belonging to benzo moiety of the same ring)  O torsion angle
and/or C (belonging to benzo moiety)  O...O C (belonging to benzo moiety of
another ring) improper torsion angle. Finally, a vibrational analysis is developed in
order to calculate the normal modes of vibration (all normal modes of vibration
must have only positive values for a molecule reaching minima on EPS).
The results of our MO calculations for Dibenzo-12(to 24)-crown-4(to 8) (cis
and trans isomers) are listed and commented in the table together with previous
calculations on precursors, and those of Pedersen’s group. The values included in
brackets correspond to the unsubstituted crown ether:


Compound
Heat of
formation
[kj/mol]
Molecular
Point
Group*
The
relative
size of
the hole,
[Å]
The steric
hindrance,
[Å]**
The
number of
O atoms
The
coplanarity
and
symmetrical
placement
of O atoms
The coplanarity
and symmetrical
placement of O
atoms according
to Fisher-
Hirsch-Welder-
Taylor atomic
models [1]
DBz12C4
(cis)
-82.81 C
2

(C
2
)
3.3
(4.2)
1.5; yes
(2.3)
4 yes
(yes)
symmetrical
DBz12C4
(trans)
-84.31 C
I

(C
2
)
3.3
(4.2)
1.5; yes
(2.3)
4 yes
(yes)
symmetrical
DBz15C5
(cis)
-125.82 C
1

(C
1
)
4.2
(5.1)
2.5; yes
(2.8)
5 yes
(yes)
symmetrical
DBz15C5
(trans)
-125.83 C
1

(C
1
)
3.8
(5.1)
3.1; yes
(2.8)
5
no
(yes)
asymmetrical
DBz18C6
(cis)
-174.98 C
1

(C
I
)
5.8
(6.3)
3.1; yes
(1.8)
6 yes
(yes)
symmetrical
DBz18C6
(trans)
-173.21 C
S

(C
I
)
5.9
(6.3)
2.9; yes
(1.8)
6 yes
(yes)
symmetrical
Corina Anca Simion et al. 4
266
continued
DBz21C7
(cis)
-221.37 C
1

(?)
7.9
(7.9)
5.1; yes
(5.0)
7 yes
(yes)
cylindrically
symmetrical
DBz21C7
(trans)
-223.31 C
1

(?)
7.2
(7.9)
2.5; yes
(5.0)
7 yes
(yes)
cylindrically
symmetrical
DBz24C8
(cis)
-269.82 C
1

(C
1
)
8.1
(8.1)
4.9; yes
(6.1)
8 yes
(yes)
cylindrically
symmetrical
DBz24C8
(trans)
-267.13 C
2H

(C
1
)
8.2
(8.1)
4.9; yes
(6.1)
8 yes
(yes)
cylindrically
symmetrical
* The Molecular Point Group symmetry class is defined taking account that the POINT is the
center of mass of the whole molecular system.
** The steric hindrance is described by the relative diameter hole of the atoms belonging to the
upper and lower planes towards to the oxygen ring plane.


All the molecules are stable and are included in the C class symmetry. These
molecules do not possess any element of symmetry except of C axis that
corresponds with the direction of the cation complexation process. For the cis –
trans correspondence, the lower the value for the heat of formation is, the higher
the class of symmetry is required.
The coplanarity and symmetrical placement of O atoms, except for DBz15C5
(trans), are present and the results are in agreement with the earlier reported [1].
For DBz18C6, the Figure 2 describes the optimized structures of its cis and trans
isomers together with the inertial axes system:

5 Lead-210 separations
267
Fig. 2 – The optimized structures of DBz18C6 cis and DBz18C6 trans isomers.
In all cases, the relative diameter of the hole describing the polyether ring is
significantly greater than of the imaginary holes characterizing the atoms belonging
to the upper and lower neighboring planes. Obviously, this is the reason for the
steric hindrance in the crown ether’s structure. For the compounds having five to
seven oxygen atoms in molecules, the steric hindrance in the dibenzo- series avoid
the complexation with cations having diameters up to 2.50 Å, and no greater than
5.8 Å. Due to this distribution, a several substituents in the aromatic rings is
required in order to restrain the domain to only few cations (to only one cation, if
possible) related to the same crown ether structure.
CONCLUSIONS
The Pedersen’s group that evaluated the diameter of the hole in polyether
rings in the early 60’s does not taken into account a different model for cis and
trans isomers as well as the steric hindrance of the another atoms (especially H
atoms) undergoing to a significantly reduced diameter of the hole for O atoms. An
improved access may be accomplished only by the introduction of the substituents
(like dibenzo-) in the crown ether’s molecule.
Our calculations also denoted that, for the cis – trans correspondence, the
lower the value for the heat of formation is, the higher the class of symmetry is
required, as well as, for the unsubstituted – substituted crown ethers series, the
substitution does not essentially influence the class of symmetry. The C class still
remains.
Corina Anca Simion et al. 6
268
Dibenzo- substituents play an important role to restrain the domain of the
steric hindrance to the polyether ring from 1.8…6.1 Å to 1.5…5.1 Å, enhancing the
selectivity for cation complexation.
With a calculated steric hindrance of ~ 3.0 Å and a diameter of the hole in the
polyether ring of ~ 5.8 Å, DBz18C6 remains the most appropiated compound of
dibenzo- series for lead (II) complexation with both cis and trans isomers. In this
order, the saturated di-t-butylcyclohexano-18-crown-6 ether (DCH18C6) derived
from DBz18C6 is an improved technical solution for the complexation of lead (II)
(diameter of the divalent cation being 2.40 Å) [6, 7]. This experimental conclusion
is in agreement with our MO calculation. In this case, the diameter of the hole of
oxygen atoms ring is 5.8 Å, and the diameter of the hole steric hindrance is 3.8 Å
in both cis and trans isomers.

Acknowledgments: The authors express their gratitude to IAEA-Vienna for it’s permanently
support. The investigation was carried out in the frame of the IAEA Coordinated Research Program
No. 12328/RO entitled "Using fallout radionuclides to evaluate the effectiveness of soil conservation
measures for sustainable crop production".
REFERENCES
1. Pedersen C.J., J. Am. Chem. Soc., 1967, 89(26), 7017–7036.
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3. Vajda N., La Rosa J., Zeiser R., Kis-Benedek Gy., J. Environ. Radioact., 1997, 37(3), 355–372.
4. Horwitz E.Ph., Dietz M.L., Rhoads S., Felinto C., Gale N.H., Houghton J., Anal. Chim. Acta, 1994,
292 (3), 263–273.
5. HyperChem; Release 5.02. For Windows 95/NT. Molecular Modeling System, Hypercube, Inc.,
1997.
6. Moyer B.A., Alexandratos S.D., Chiarizia R., Dietz M.L., Hay B.P., Sachleben R.A., Design and
Synthesis of the next generation of crown ethers for waste separations:An inter-laboratory
comprehensive proposal, U.S. Department of Energy, Final Report, 2000.
7. Izatt R.M., Bruening R.L., Clark G.A., Lamb J.D., Christensen J.J., Sep Sci. Technol., 1987, 22
(2-3), 661–675.
8. Simion C.A., Margineanu, R., Lead-210 separations. A theoretical study of 18C6 derivatives and
lead complexes, 12-th IIS-CED Meeting, Bad Soden, Germany, June 17–18 2004.