chitosan by density functional calculation

Journal of Molecular Structure: THEOCHEM 860 (2008) 80–85

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Comparative study on interaction between copper (II) and chitin/chitosan by density functional calculation Renqing Lü *, Zuogang Cao, Guoping Shen College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Dongying, 257061 Shandong Province, PR China

a r t i c l e

i n f o

Article history: Received 9 November 2007 Received in revised form 25 February 2008 Accepted 17 March 2008 Available online 25 March 2008 Keywords: Chitin Chitosan Cu2+ Interaction Density functional theory (DFT)

a b s t r a c t The DMol3 calculations, based on density functional theory (DFT), have been employed to investigate the interactions between Cu2+ and chitin/chitosan residues. The possible initial conformations were optimized at the generalized gradient approximation (GGA) level, with spin unrestricted approach, symmetric unrestriction, doublet multiplicity and BLYP/DND methods. For all initial complexes considered, the Cu2+ was completed with H2O and/or OH groups to neutralize the initial complexes with hexacoordination geometries. The tendency of ligands to coordinate with Cu2+ is ANH2 > C3AOH > H2O > ANHCOCH3, suggesting that amine groups (ANH2) on chitosan prefer to bind Cu2+ and acetamide groups (ANHCOCH3) on chitin lose their coordination with Cu2+ in aqueous solution. The geometries of bridge and pendant models have been comparatively analyzed. The results show that bridge model is more favorable than pendant model. In terms of the optimized geometries, the initial hexacoordination structures of Cu2+ designed seem more reasonable than initial tetracoordination ones designed. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The increasing demand for new and economic processes for the recovery of metal ions from industrial effluents has led many researchers to investigate the possibility of using chitosan for metal uptake [1–4]. Chitosan is obtained by treating crude chitin with aqueous 40–50% sodium hydroxide. Chitin and chitosan are linear polysaccharides containing 2-acetamide-2-deoxy-D-glucopyranose and 2-amino-2-deoy-D-glucopyranose units joined by b(1 ? 4) glycosidic bonds [3]. Interactions between chitin/chitosan and metal ions are of great interest and are important in the field of depollution. It is accepted that the sorption of a metal may involve chelation mechanism and electrostatic attraction mechanism depending on the pH and composition of solution [5–7]. The free electron doublet on nitrogen (ANH2) may bind metal cations at pH close to neutrality by chelation mechanism [5]. The protonation of amine groups in acidic solutions ligates metal anions by electrostatic attraction mechanism, since the protonation of amine groups (ANH3+) gives the polymer a cationic behavior and consequently the potential for attracting metal anions such as Cr2O72, MoO42 etc. [6,7]. But in strong basic solution, the metallic ion adsorption on chitin/chitosan is not taken into account due to the precipitation of metallic hydroxides [8]. Most studies of metal uptake behavior and chelation mechanism by chitosan have been focused on Cu2+. Two models were proposed to elucidate the chelation

* Corresponding author. Tel.: +86 546 8392287; fax: +86 546 8391971. E-mail address: [email protected] (R. Lü). 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.03.013

mechanism [9,10]. The first one is called bridge model, which supposed that metal ions are bound with several amine groups from the same chain or from different chains, via intra- or intermolecular complexation [9]. The second one, called pendant model, considers that the metal ion is bound to an amine group in a pendant fashion [10]. The computational quantum chemical methods were invoked to investigate the sorption of lead and mercury on chitosan and pectic acid [11,12], equilibrium uptake of the Pb, Hg and Cd adsorption on chitosan–pectin pellets [13], binding form between lithium atom and sugar rings [14], complexation of Fe3+ with chitosan using Fe(III)(NH3)2(H2O)(OH)3 model [15]. The interaction between biopolymer chitosan and transition metal Cu2+ has been simulated by density functional theory (DFT) [16]. The results indicate that Cu2+ coordination to the chitosan takes place in the vicinity of the glycosidic oxygen and includes interactions with nitrogen and hydroxyl oxygen atoms, but gas phase mass spectroscopy experiments carried out on chitosan di-, tri- and tetra-saccharide are not in agreement with these calculations [17]. The geometries and interaction between Cu2+ and chitosan have also been studied with DFT [18]. The pendant model and bridge model were compared and the more stable sorption model is confirmed to be pendant model by means of all initial complexes designed as tetracoordination. The initial geometries of Cu2+ adsorption on chitin/chitosan are designed as hexacoordination. The aims of this paper are to compare the coordination potential of ANH2, AOH, ANHCOCH3 groups of chitin/chitosan and H2O molecules, and to select the more favorable sorption model from bridge model and pendant model.

R. Lü et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 80–85

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2. Computational methods

4. Results and discussion

The quantum chemical calculations were carried out by density functional theory (DFT). All the DFT calculations were performed with DMol3 method [19–21]. This method is based on accurate and efficient local density functional calculations (LDF), which uses fast convergent three-dimensional numerical integrations to calculate the matrix elements occurring in the Ritz variation method. The above method has many basic merits. First, it can be applied to the ground state of highly correlated transition metallic complexes. Second, it has small basis set superposition error (BSSE). Third, the growth of computational effort with basis set size N is proportional to N3. Fourth, it includes also an efficient, exact approach for calculating the electrostatic potential. The localized numerical orbitals used as basis sets are designed to give a maximum of accuracy for a given basis set size, which is done by treating the separated atom limit exactly with the numerical atomic basis functions. The atomic response to the molecular or solid environment can be handled robustly to an excellent approximation by a relatively small number of additional numerical functions. Basis sets with short tails significantly speed up calculations for large molecules and solids. Typically a doubled numerical basis set (DND) is used for molecular and solids calculations. The DND basis set, which is a double numerical basis set with polarizaition functions on atoms other than hydrogen, was employed and is equivalent to the 6-31G* basis set in ab initio methods. The generalized gradient corrected functionals GGA by BLYP was used for all geometry optimizations. The convergence criteria for these optimizations consisted of threshold values of 2  105 Ha, 0.004 Ha/Å and 0.005 Å for energy, gradient and displacement convergence, respectively, while a selfconsistent field (SCF) density convergence threshold value of 1  105 Ha was specified. All calculations employed a method based on Pulay’s direct inversion of iterative subspace (DIIS) technique to accelerate SCF convergence. All structure properties identified as stationary points were subjected to full frequency analysis to verify their classification as minimum points without imaginary frequencies or saddle points with imaginary frequencies.

The symmetric unrestriction and GGA/BLYP/DND methods were used to optimize the initial geometries and frequency analyses show that all stationary points are minimum points since no imaginary frequency appeared. The optimized geometries and partial bond lengths in angstroms are depicted in Fig. 2. It can be seen that the O30 AH18 bond length of chitin residue is 0.009 Å longer than that of chitosan residue, while the N20 AH8 bond length of chitin residue decreased 0.005 Å from that of chitosan residue. This calculation demonstrates that the deacetylation can influence chemical environment of coordinative groups.

3. Specification of initial geometries The nitrogen atom containing free electron doublet may be the reactive atom to bind metal cations, oxygen atoms of hydroxyl groups especially in the C-3 position may contribute to metal sorption. So the possible geometries formed between Cu2+ and nitrogen of acetamide/amine groups, Cu2+ and oxygen of C-3 position were specified. Because chitin/chitosan are polymers, the residues of chitin/chitosan are invoked to represent their coordinative sites, and the dangling oxygen atoms are terminated with hydrogen atoms [22]. The structures and atomic numbering [23] of chitin/ chitosan residues are displayed in Fig. 1a and b. The four-coordination complexation of Cu2+ and chitosan was supposed by ESR and FTIR results [24,25]. But the supposed complexes have +2 charge, so two OH groups are added to form neutral complexes with hexacoordination. Although the neutral Cu–chitosan complexes with tetracoordination were proposed by potentiometric and spectrophotometric results [8], the possible competitive coordination of Cu2+ with H2O may be neglected. The initial complexes were designed between Cu2+ and chitin/chitosan residues, with two hydroxide ions as ligands to neutralize complexes. Two/three water molecules were selected as ligands to attain hexa-coordinated bridge/pendant complexes. All initial geometries were exhibited in Fig. 1.

4.1. Pendant complexes between Cu2+ and chitin/chitosan residues The initial pendant models of Cu2+ adsorption on chitin acetamide group (ANHCOCH3) and C3-hydroxyl group (C3AOH) with hexacoordination were designated as Pendant-Chitin-N-Cu(OH)2(H2O)3 (I) and Pendant-Chitin-O-Cu-(OH)2(H2O)3 (II). The Cu2+AOH distances of structure I are 1.917 and 1.892 Å, while the corresponding distances of structure II are 1.903 and 1.906 Å. The shorter distances of Cu2+AOH in contrast to the other two Cu2+AH2O distances may be assigned to the strong electrostatic interactions between Cu2+ and negative OH groups. The separation between Cu2+ and nitrogen of acetamide group for optimized structure I is 4.162 Å, one H2O molecule is expelled outside to form hydrogen bonds with 1.654 and 1.631 Å distances. The O2ACuAO11, O11ACuAO3, O3ACuAO14, O14ACuAO2 bond angles are 102.4°, 74.7°, 92.0°, 91.0°. The copper (II) interacting structure is a tetracoordinated complex. The pronounced difference between optimized structures I and II is that the two H2O molecules are expelled outside to form hydrogen bonds in optimized structure II. Also, Cu2+ interacts with the C3-hydroxyl oxygen of chitin residue by distance of 2.235 Å. The O2ACuAO3, O14ACuAO17 bond angles are 169.9° and 144.8°, resulting in deformed tetragonal structure II. The structure II is more favorable than structure I in terms of total energy. Chitin acetamide groups cannot coordinate with Cu2+ in structure I, C3-hydroxyl groups can interact with Cu2+ in structure II. The pendant models of Cu2+ adsorption on chitosan amine groups and C3-hydroxyl groups with hexacoordination are designated as Pendant-chitosan-N-Cu-(OH)2(H2O)3 (III) and Pendantchitosan-O-Cu-(OH)2(H2O)3 (IV). The similarity between optimized structure III and IV is that two H2O molecules are expelled outside to form hydrogen bonding with other ligands, and central Cu2+ ion interacts with ligands by tetracoordination. The distances of CuAN20 of structure III, CuAO30 of structure IV are 2.112 and 2.220 Å respectively. The distance of CuAN20 is about 0.108 Å shorter than that of CuAO30 . Because two H2O molecules are expelled, it is concluded that Cu2+ can be adsorbed on both amine groups and C3-hydroxyl groups in aqueous solution. Structure III is more stable than structure IV in terms of total energy. The coordination tendency is ANH2 > C3AOH > H2O > ANHCOCH3. 4.2. Bridge models between Cu2+ and chitin/chitosan residues The bridge models of Cu2+ adsorption on chitin/chitosan were compared. The initial structure of Cu2+ binding to two acetamide groups, two H2O and two OH was designated as Bridge-chitinN(OH)2(H2O)2 (V). It can be found that the two acetamide groups of optimized structure V are expelled by separation of 4.076 and 4.415 Å, respectively. This calculation illustrates that Cu2+ is not adsorbed on chitin acetamide groups. The C3-hydroxyl groups of two chitin residues, two H2O and two OH were designed to coordinate with Cu2+, and Bridge-chitin-O(OH)2(H2O)2 (VI) can be ac-

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H

HO O

H

6' H OH

OH

6' H OH

HO

4 H2C

H

O5'

5

4' HO

4 H2C

2

3' OH

H

H

H

O

H

OH2 OH2

c

HO

H

Cu

HO

NH2

H

HO

H

OH2

Cu

H2O

OH2

HO

H2O

d

OH2

Cu

H2 O HO

H2 O

H

H H OH

OH

HO

H

H

O

H

H2O

e

HO

H

O

f

HO O

H

OH

H

HO

H

HO H

H 2O

OH

H2O

C H N

H

O OH

H H

OH

H

O

H

H

O

H OH

g H

OH

HO

H OH

H

OH

h H

HO

H

i

HO H

H

OH

H

H2N

OH

HO

O

OH

Cu

HO

OH2

HN

OH

H H

H 2O

OH2

NH2

H

HO

H

CH3

C

H

OH

H

H

Cu

HO

O

H

HO

C

H

OH2

CH3

NH

H H3C

HO

O

H3C

Cu

HO

HO

HO

H C

O

OH

H

N

O

H

OH HO

NH2

H

O

H OH

H2 O

HO

C

OH

OH

NH

H H3C

O

H

OH

HO

HO

O

H

HO

OH

H

b

HO

O

H3C

Cu

HO

H3COC

a H

1' OH

1

H

H2 O

2' NH

H

2' NH

3' OH

C

3

H

H

O5'

2

1' OH

1 H

H 5

4' HO

3

H

6

N

H

HO

6

H

H

O

HO H

O

O OH

OH OH

HO HO

N

H

HO

NH2

H

HO

C H

H 2O

Cu

HO

OH2 CH3

HO

H

O OH

H

H2N

H

OH

OH

O

OH

j

OH

H

H H

N

H

OH

OH

H

H2 N

OH

OH

HO

O

H

Cu

HO

HO

HO

H H

C H

H

NH2

H

HO

O

H3C

OH

Cu

HO

H

H H

H

H

O

H

OH

k

H

H

OH

H

l

Fig. 1. The initial geometries (a) chitin residue, (b) chitosan residue, (c) Pendant-Chitin-N-Cu-(OH)2(H2O)3 (I), (d) Pendant-Chitin-O-Cu-(OH)2(H2O)3 (II), (e) PendantChitosan-N-Cu-(OH)2(H2O)3 (III), (f) Pendant-Chitosan-O-Cu-(OH)2(H2O)3 (IV), (g) Bridge-chitin-N-(OH)2(H2O)2 (V), (h) Bridge-chitin-O-(OH)2(H2O)2 (VI), (i) Bridge-chitosanN-(OH)2(H2O)2 (VII), (j) Bridge-chitosan-O-(OH)2(H2O)2 (VIII), (k) Bridge-chitin-N-O-(OH)2 (IX), (l) Bridge-chitosan-N-O-(OH)2 (X).

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R. Lü et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 80–85

H18

H18 0.986 O3'

0.995 O3' 1.437

Ring

1.447 N2'1.022H 8

1.374 O C 1.224

H15 2.092 0.985 O 14 1.033 H16 1.639O 1.003 H 0.993

H

N2' 1.004 H18

2.235

H8

O 1.759 O3 H

H22

N2'

1.495

O3'

H 1.019

1.032 H21

2.112

Cu 2.121

1.979

H15 O14 1.000 1.013 H16

H 1.010

0.982 H

0.983 1.731 O3

H18

1.938

1.906 0.982 O2 H4 1.788

1.889 O

Ring

1.037

H5 0.983

0.982 H

H O2 0.9834 1.748

O

H 1.016

H 1.028

O

c

COCH3 H

1.631

1.031 H16 1.654

b

Ring

Cu

H15

N2' 1.027 H8

O2 H4 0.980

O14

H21

1.460 O3' 1.903

1.892

2.111 0.984

1.029

1.522

Cu

N2'

1.475

H13

2.204

H8

a

0.983 O3

Ring

Ring

CH3

H5

H3COC

1.433

H12 1.041 O11 0.985

H5 0.989 O3 1.917

H5

H18 0.991

1.946

2.220 Cu

O

H21

Ring O3' 1.484

N2' H8

0.983H

O

1.904 1.007 1.730 2.112 1.825H O O 2 14 H15 0.983 1.017 0.988 H4 H16

0.982 H

0.983 H

d

H5 0.983 O3 4.076 1.934

H3COC Ring

N2'

Cu

2.158

H8

0.999O

14

H15

1.005

e

H12 1.005 O11 0.994 H13 2.148 4.415 1.889 O2 0.984 H4

f

H12

COCH3 Ring

N2'

H10

O11 H5 0.981 H13 H18 O3 1.917 0.988 2.244 2.297 Cu O3' Ring

H10

N2' H8

H16

1.467

2.313

1.020 O14 H 0.984 4 H15 H16 COCH3

g

COCH3

N2' Ring O3' 0.996 H20

1.914 O2 0.988

H12 0.982 H13 0.996 O11 2.294 H5 0.983 H8 O3 H10 1.034 1.900 1.031 2.134 2.190 1.491 1.484 Ring N2' Cu N2' Ring 1.032 H21 H15 0.981

H22

0.979

1.622

H4

H16 O14 1.030

h

1.031

1.943 O2

i

H12 0.983 O11

H18 1.023 1.455 O3' Ring

1.019 H13 1.745

1.917

2.148

H5 0.985 O3

N2'

Cu

N2'

2.195 1.910

H21 1.777 H16 1.005

O2 0.980 H4

0.997 1.479 O3’ Ring 2.178

H22

H20

O2 0.983 H4

COCH3

O14

0.982 H15

Ring

2.189 1.914

N2' H8

COCH3

Cu

3.735

1.465 Ring O3' 0.994

H5 H10 0.982 O3 N2' 1.873 3.860

H18 H10

O3’ 1.489 Ring

O3’

1.458

1.031 H21

H5 1.029 H 18 1.633 0.980 O3

N2'

2.144 1.031 H8

Cu

k

1.030 2.189

N2'

1.03 1.484

1.942

0.997 H20

0.982 H4

j

H10

1.933

O2

O3’ 0.987

Ring 1.439

H20

l

Fig. 2. The optimized geometries (a) chitin residue, (b) chitosan residue, (c) Pendant-Chitin-N-Cu-(OH)2(H2O)3 (I), (d) Pendant-Chitin-O-Cu-(OH)2(H2O)3 (II), (e) PendantChitosan-N-Cu-(OH)2(H2O)3 (III), (f) Pendant-Chitosan-O-Cu-(OH)2(H2O)3 (IV), (g) Bridge-chitin-N-(OH)2(H2O)2 (V), (h) Bridge-chitin-O-(OH)2(H2O)2 (VI), (i) Bridge-chitosanN-(OH)2(H2O)2 (VII), (j) Bridge-chitosan-O-(OH)2(H2O)2 (VIII), (k) Bridge-chitin-N-O-(OH)2 (IX), (l) Bridge-chitosan-N-O-(OH)2 (X).

quired. The CuAO distances of optimized structure VI are 1.914, 1.917, 2.244, 2.297, and 2.313 Å. The five ligands bind to Cu2+ to

form triangular dipyramid structure. The coordination potency of ligands with Cu2+ is C3AOH > H2O > ANHCOCH3.

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R. Lü et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 80–85

Similarly, the two initial geometries of Bridge-chitosan-N(OH)2(H2O)2 (VII) and Bridge-chitosan-O-(OH)2(H2O)2 (VIII) were obtained. The optimized results exhibit that the two H2O molecules of optimized structures VII and VIII do not coordinate with Cu2+, resulting in tetracoordinated structures with twisted square. The distances of CuAN20 for structure VII and CuAO30 for structure VIII are 2.190, 2.134 Å and 2.195, 2.148 Å, respectively. Structure VII is more stable than structure VIII. The amine and C3-hydroxyl groups are not expelled in the optimized structures VII and VIII, this verifies that the more stable adsorption model is bridge model. The computational results disagree with proposed conclusions that pendant model is more favorable than bridge model [18], which was drawn by DFT and based on initial tetracoordinated Cu2+-chitin/chitosan complexes and differences of interaction energy (DE). 4.3. Bridge models between Cu2+ and chitin/chitosan residues as bidentate ligands The chitin/chitosan residues as bidentate ligands coordinate with Cu2+ to form initial Bridge-chitin-O-N-(OH)2 (IX) and Bridge-chitosan-O-N-(OH)2 (X). The calculated distances demonstrate that acetamide groups do not coordinate with Cu2+ in structure IX, while C3-hydroxyl groups (C3AOH) do not coordinate with Cu2+ in structure X. The two CuAOH distances of structure IX are shorter than corresponding distances of structure X. This may be ascribed to the longer distances of CuAO30 in structure IX than distances of CuAN20 in structure X, which is in agreement with the principle of bond order conservation [22,26,27]. According to the interaction distances and total energy, it can be concluded that the preference of Cu2+ binding to chitosan is ANH2 > C3AOH. The arithmetic mean of the calculated CuAO distances can be compared with the sums of ionic radii (rCu2+ + rO2). The calculated ionic radii of Cu2+ in optimized structures are 0.63–0.69 Å based on the estimate of the O2 radius 1.40 Å [28], while the ionic radius of Cu2+ in six-coordination is 0.73 Å. The different size may be attributed to the use of the fixed anion radius regardless of the bond length change accompanying coordination number change. The CuAO distances 2.35 Å for [Cu(H2O)6]+ and 2.13 Å for [Cu(H2O)6]2+ have been obtained by the theoretical study on nonadditivity in interactions of Cu+/Cu2+ and water [29]. While the mean CuAO distances of optimized structures are 2.03–2.05 Å, the CuAO distances for optimized and four-coordinated Cu2+ complexes are shorter than that of six-coordinated Cu2+, which is in accordance with the principle of bond order conservation [22,26,27]. The optimized Cu2+ ionic radii and CuAO distances are different from the reported data, due to the change of copper coordination number instead of the change of copper oxidation number. ESR results have also confirmed that copper oxidation number does not change with complexation [24,25,30]. The electronic configuration of Cu2+ is d9. It is generally assumed that the five d orbitals of Cu2+ are split into two groups in octahedral ligand fields, a triply degenerate t2g set and a doubly degenerate eg set, of which the latter contains the orbitals oriented toward the ligands, dx2y2 and dz2. The hydrated Cu2+ is a typical example of system with strong Jahn–Teller effect which gives rise to distortion of 6fold coordination. There are many experimental data related to the structure of the Cu2+ complex in aqueous solution using X-ray diffraction [31,32], extended X-ray absorption fine structures (EXAFS) [33], X-ray adsorption near-edge structure (XANES) [34] and isotopic substitution in neutron diffraction [35], proposing a configuration of the [Cu(H2O)6]2+ complex with equatorial CuAO distances 1.95– 2.04 Å and axial CuAO distances 2.29–2.60 Å. The 6-fold coordinated Cu2+ was also simulated by theoretical study [29,36–41]. Recent experimental and theoretical investigation showed that hydrated Cu2+ is 5-fold coordination [42–44]. Chaboy et al. suggested that there is not clearly preferred structure among those including 4-,

5-, and 6-fold coordinated Cu2+ ions [45,46]. Four, five, six-coordinated Cu2+ complexes have been synthesized [47]. A supramolecular sandwich consisting of two crown ether molecules and a trigonal– bipyramidal [Cu(H2O)5]2+ complex has been reported [48]. In comparison to the reported results, the preference of optimized Cu2+-chitin/chitosan complexes for tetracoordination instead of hexacoordination may be assigned to the following reasons. Firstly, the initial conformations were optimized without symmetric restriction. Secondly, the initial conformations were designed as neutral complexes, and OH was selected to neutralize the complexes. Jahn–Teller effect of OH neutralized Cu2+ complexes is more remarkable than that of hydrated Cu2+, so the two H2O molecules are expelled from the first hydration shell. The stronger interaction between Cu2+ and OH than that between Cu2+ and H2O may be confirmed by the shorter Cu2+AOH distances than the Cu2+AH2O distances. Thirdly, in order to get socalled correct 6-fold coordination number of Cu2+ in aqueous solution, the second hydration sphere was taken into consideration as solvent effect [36,37], which is less possible for Cu2+ anchoring on chitin/chitosan biopolymers than hydrated Cu2+. Therefore, the tetracoordinated structures of Cu2+-chitin/chitosan complexes may be possible. The Cu2+ coordination numbers change with different environment because of Jahn–Teller effect. 5. Conclusions Theoretical calculations were employed to distinguish copper (II) binding sites of chitin/chitosan. The adsorption model was selected according to the optimized structure and total energy. The initial interaction structures were designed as hexacoordinative complexes and the optimized structures were tetracoordination due to strong Jahn–Teller effect. The trend of coordination with copper (II) is ANH2 > C3AOH > H2O > ANHCOCH3. Analyzing the optimized structure VII, it can be concluded that the bridge model is prior to the pendant model. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2008.03.013. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

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