Complexes of carboxyl-containing polymer and monosubstituted bipyridinium salts

Article abstract of DOI:10.1016/j.molstruc.2013.05.049

Semi-empirical PM3 method for the quantum calculations of molecular electronic structure based on NDDO integral approximation is used to investigate the complex formation of monosubstituted 4,4'-bipyridinium salts BpyR (Hal) containing a halide anion interacting with the quaternary nitrogen atom and carboxylic group of the two-units construct. Significant effect of the BpyR (Hal) electronic structure is unveiled that contributes in two different structures of these salts, namely, partial charge transfer complex and ion pair structure, both having stable energy minima. We demonstrate that (i) the structure of the N-substituent modulates the energy and electronic characteristics of monosubstituted salts BpyR with chlorine and bromine anions and (ii) the coulomb interactions between quaternary N-atom, halogen anion, and the proton of carboxylic group stimulate the transformation of the charge transfer complex into the ion pair structure. Results of calculations are compared with the experimental FTIR spectra of blends of BpyR(Hal) with Eudragit copolymer.

Full text of DOI:10.1016/j.molstruc.2013.05.049

Journal of Molecular Structure 1048 (2013) 230–237
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Complexes of carboxyl-containing polymer and monosubstituted
bipyridinium salts
N.D. Merekalova a, G.N. Bondarenko a, D.W. Krylsky b, M.I. Zakirov b, R.V. Talroze a,
⇑
a AV Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 119991 Moscow, Russia
b Dubna International University of Nature, Society, and Man, 19 University Street, Moscow Region, 141980 Dubna, Russia
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Monosubstituted 4,40-bipyridinium halides form two energetically stable structures.
We calculated the partial charge transfer system to be energetically more favorable.
Coulomb interactions stabilize complexes between BpyR(Hal) and COOHA group.
Water molecules play an important role in complex formation.
The complex formation leads to change from partial charge transfer to ion pair.
a r t i c l e i n f o
a b s t r a c t
Article history:
Semi-empirical PM3 method for the quantum calculations of molecular electronic structure based on
NDDO integral approximation is used to investigate the complex formation of monosubstituted 4,40-bipy-
ridinium salts BpyR (Hal) containing a halide anion interacting with the quaternary nitrogen atom and
carboxylic group of the two-units construct. Significant effect of the BpyR (Hal) electronic structure is
unveiled that contributes in two different structures of these salts, namely, partial charge transfer com-
plex and ion pair structure, both having stable energy minima. We demonstrate that (i) the structure of
the N-substituent modulates the energy and electronic characteristics of monosubstituted salts BpyR
with chlorine and bromine anions and (ii) the coulomb interactions between quaternary N-atom, halogen
anion, and the proton of carboxylic group stimulate the transformation of the charge transfer complex
into the ion pair structure. Results of calculations are compared with the experimental FTIR spectra of
blends of BpyR(Hal) with Eudragit copolymer.
Received 14 March 2013
Received in revised form 16 May 2013
Accepted 21 May 2013
Available online 30 May 2013
Keywords:
Complex
PM3 method
Bipyridilium salt
Ionic interaction
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
[3]. However the IR spectra did not provide a conclusive evidence
for the proton transfer responsible for the complex formation,
In order to create new electrochromic systems, the effect of
non-covalent interactions between different organic species was
utilized in our previous work [1]. The system studied was made
of blends of carboxyl-containing copolymers and monosubstituted
salt of 4,40-bipyridinium that did not undergo any phase separation
even at 1:1 mol. ratio. The above salts are described in [2]. We ex-
pected that the reason for the blend stability in our system was the
formation of the complex in which carboxylic groups of metha-
crylic acid units in the amount of 50 mol.% (in a copolymer with
ethyl acrylate) or 29 mol.% (in a triple copolymer with tert-butyl
acrylate and ethyl acrylate) interacted with monosubstituted
bipyridinium salt due to either partial or total proton transfer
although the formation of the complex with high electrochromic
effectiveness was proven. Possible schematics of complexes were
discussed in which either quaternary or tertiary nitrogen atom of
a monosubstituted bipyridyl molecule could contribute in the
complex formation.
The major goal of the current research is to find the most prob-
able structure of complexes based on monosubstituted bipyridini-
um salts by applying the semi-empirical PM3 method for the
quantum calculation of molecular electronic structure based on
NDDO (the Neglect of Differential Diatomic Overlap) integral
approximation [4] and to confirm the calculated structures with
the experimental FTIR spectra.
Mono-substituted halide salts of 4,40-bipyridinium BpyR (Hal)
having the following chemical structure were chosen as the model
systems.
⇑
Corresponding author.
E-mail address: rtalroze@ips.ac.ru (R.V. Talroze).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.05.049

N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
231
where R = CH3 (M), CH2ACH@CH2 (A), CH2AC6H5 (B).
We made a choice for Eudragit as a polymer component used in
experimental studies having the following structure
Fig. 1. Possible conformers of two units construct.
It is an anionic copolymer based on methacrylic acid and ethyl
acrylate. The ratio of the free carboxyl groups to the ester groups is
approximately 1:1. To simulate the structure of that copolymer we
have used constructs composed of methacrylic acid and ethyl acry-
late units in two different conformations, namely, eclipsed (E) or
staggered (S) (Fig. 1).
Fig. 2. The distribution of charges in structures 1 (a) and 2 (b) with A1(Cl) and
A2(Cl) as examples: size of spheres (atoms) is proportional to sign value and red
color indicates positive and blue – negative charges. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
2. Experimental section
2.1. Materials and methods
2.2.2. BpyA(Br)
Unless otherwise stated, all starting materials were obtained
from commercial suppliers and used without further purification.
Eudragit copolymer containing 50 mol.% of MAA and 50 mol.% EA
monomer units (Mw = 320000) was purchased from Rohm Pharma
and used as received.
A solution of 4,40-bipyridine (4.68 g, 30 mmol) and allyl bro-
mide (3.81 g, 30 mmol) in toluene was mixed at room temperature
for 48 h. The precipitated salt was collected and filtered. Recrystal-
lisation from isopropanol gave the monoquaternary salt which was
dried in vacuo. The yield of BpyA(Br) pink powder was 50%. 1H
NMR (600 MHz, D2O): 8,88 (d,2H); 8,65 (d,2H); (d, 2H); 8,31 (d,
2H); 7,81 (d, 2H); 6,10 (m, 1H); 5,48 (m, 2H); 5, 20 (d, 2H); m/z
197.
1H NMR spectra were recorded in D2O using AvanceTM 600
Bruker spectrometer. Mass-spectra were run with Bruker Autoflex
II.
Thermo Scientific ‘‘Nicolet’’ 6700 FT-IR spectrometer equipped
with attenuated total reflectance device ‘‘Smart Orbit’’ was used
to obtain FTIR spectra. The spectra were analyzed with Thermo Sci-
entific OMNIC Array Automation software. IR spectra were run for
salts, matrix polymer and their blends in the form of solid powders
prepared as described below.
2.2.3. BpyB(Cl)
A solution of 4,40-bipyridine (4.68 g, 30 mmol) and benzyl chlo-
ride (3.81 g, 30 mmol) in toluene was heated at reflux for 4 h. Then
the reaction mixture was cooled down to room temperature. The
precipitated salt was collected and filtered. Recrystallisation from
toluene/isopropanol gave the monoquaternary salt which was
dried in vacuo. The yield of BpyB(Cl) white powder was 50%. 1H
NMR (600 MHz, D2O): 9,02 (d, 2H); 8,76 (d, 2H); 8,40 (d, 2H);
7,89 (d, 2H); 7,53 (s, 5H); 5,87 (s, 2H); m/z 247.
2.2. Synthesis of BpyR (Hal)
Monosubstituted halide salts of 4,40-bipyridinium, namely, 1-
methyl-4,40-bipyridinium iodide (BpyM(I)), 1-allyl-4,40-bipyridini-
um bromide (BpyA(Br)) and 1-benzyl-4,40-bipyridinium chloride
(BpyB(Cl)) were synthesized following the procedures described
in [5,6] with some modification. The description of synthesis is gi-
ven below.
2.3. Blends preparation
BpyR(Hal) and Eudragit (1:1 mol.%) were dissolved in a com-
mon solvent, 96% ethanol, by mechanical stirring for 8 h at 40 °C
until the solution became homogeneous. The solvent was vac-
uum-evaporated at 40 °C and the blend samples were dried under
vacuum at 60 °C for 24 h to remove any residual solvent.
2.2.1. BpyM(I)
A solution of 4,40-bipyridine (6.24 g, 40 mmol) an methyl iodide
(2.73 g, 44 mmol)in dry acetone was mixed at room temperature
for 24 h. The precipitated salt was collected and filtered. Recrystal-
lisation from isopropanol/water gave the monoquaternary salt
which was dried in vacuo. The yield of BpyM(I) yellow powder
was 75%:8.95 (d, 2H), 8.68 (d, 2H), 8.29 (d, 2H), 7,89 (d, 2H);
4,39 (s, 3H);; m/z: 171.
2.4. Calculations
Geometries as well as energies were calculated with Chem3D
Ultra 8.0 package and MOPAC2000 software. After optimization
and analysis of electronic, structural and energy characteristics of

232
N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
Table 1
Energy and electronic characteristics of monosubstituted salts BpyR.
BpyR (Hal) R – Structure
D
E (kJ/mol)
Charge (e)
N
Distance (Å) N+–Halꢁ
N+
Halꢁ
ꢁ17.50
ꢁ0.10
0.02
ꢁ0.20
2.73
ꢁ0.08
0.49
ꢁ0.73
3.57
ꢁ9.02
ꢁ0.10
ꢁ0.08
ꢁ0.09
ꢁ0.08
0.02
0.48
0.13
0.48
ꢁ0.21
ꢁ0.76
ꢁ0.41
ꢁ0.81
2.75
3.52
2.81
3.63
ꢁ12.22
ꢁ5.07
ꢁ0.10
ꢁ0.08
0.04
0.48
ꢁ0.22
ꢁ0.76
2.73
3.56
M2(I)
A2(I)
B2(I)
–
–
–
ꢁ0.08
ꢁ0.08
ꢁ0.08
0.51
0.50
0.51
ꢁ0.80
ꢁ0.81
ꢁ0.80
2.90
2.91
2.90
BpyR(Hal), the semi-empirical method PM3 based on NDDO inte-
gral approximation was used to explore the complex formation.
(Table 1). Thus, structure 1 may be considered as a complex with
a partial charge transfer whereas structure 2 is an ion pair. It can
be frequently seen when organic and inorganic moeties interact
[7,8]. The characteristic electronic density distribution in both
structures of BpyR(Hal) is given in Table 2 with R = A1 and A2 as
examples. There is an effect of the strong delocalization of electron
density across the aromatic part of the molecules, although it de-
pends on the specific structure (A1 or A2). When comparing two
pyridinium cores, the major contrast between 1 and 2 is seen in
the N+-substituted pyridinium core, whereas it is much less visible
in the non-substituted one. As one can see, structures 1 are more
energetically favorable in comparison with structures 2 although
3. Results and discussion
3.1. Structure of monosubstituted bipyridinium salts BpyR (Hal)
The PM3 calculations applied to monosubstituted bipyridinium
salts containing R substituents (M, A and B groups) and chloride (or
bromide) anion show two local minima of the total energy. BpyR
(Hal) may exist as two different structures that have stable energy
minima. Fig. 2 presents the images of these two structures (1 and
2) in which the atoms are shown as spheres. Structure 1 are char-
acterized by low charges on N+ and Halꢁ and a small distance be-
tween Halꢁ and N+ whereas structure 2 represent N+ and Halꢁ
atoms with high charges and a bigger distance between them
relatively low
DE value between structures 1 and 2 particularly
for benzyl (B) and allyl (A) substituents show that ion pairs may
also exist. The charge distribution in both bipyridinium core and
allyl substituent essentially depends on the counterion (anion).

N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
233
Table 2
The distribution of charges in structures 1 and 2.
Atoms
A(Cl) (e)
A(Br) (e)
A1
A2
A1
A2
N
2 C
3 C
4 C
5 C
6 C
7 C
ꢁ0.10
ꢁ0.14
ꢁ0.24
ꢁ0.01
ꢁ0.24
ꢁ0.14
0.03
ꢁ0.08
ꢁ0.13
ꢁ0.23
ꢁ0.04
ꢁ0.22
ꢁ0.13
0.00
ꢁ0.09
ꢁ0.14
ꢁ0.23
ꢁ0.02
ꢁ0.24
ꢁ0.14
0.04
ꢁ0.08
ꢁ0.13
ꢁ0.23
ꢁ0.05
ꢁ0.22
ꢁ0.13
0.01
Fig. 3. The images of conformers E (a) and S (b) when water molecule is included.
8 C
9 C
ꢁ0.30
ꢁ0.29
ꢁ0.16
0.02
ꢁ0.17
ꢁ0.18
ꢁ0.33
0.48
ꢁ0.42
ꢁ0.28
ꢁ0.33
ꢁ0.25
ꢁ0.76
ꢁ0.27
ꢁ0.28
ꢁ0.18
0.13
ꢁ0.01
ꢁ0.20
ꢁ0.27
ꢁ0.29
ꢁ0.41
ꢁ0.18
ꢁ0.18
ꢁ0.35
0.48
ꢁ0.32
ꢁ0.28
ꢁ0.34
ꢁ0.23
ꢁ0.81
10 C
11 N
12 C
13 C
14 C
15 C
Hal
mol). Nevertheless, the difference between the energy levels of
these two constructs with water is negligible (0.5 kJ/mol) which
means that both conformers could be taken in consideration for
the simulation of salt–polymer interactions. Interestingly, the for-
mation of both conformers containing water is accompanied by a
very short distance between hydrogen atoms of water and car-
bonyl groups in carboxylic and/or ester groups (Table 3). It also
proves the strong interaction between the construct and water
molecules (Fig. 3).
0.00
ꢁ0.17
ꢁ0.29
ꢁ0.30
ꢁ0.21
The use of the iodine anion changes the picture described above
because the calculated energies give just one minimum which can
be interpreted as related to the ion pair structure 2 in accordance
with high charges on N+ and Halꢁ atoms. However contrary to clo-
ride and bromide salt structures in the form of ion pair 2 the dis-
tance between N+ and Iꢁ atoms is more comparable with that of
the form 1 (Table 1).
3.3. Complex formation
The next step is to make a choice for the construct which pro-
vides the most energetically favorable complex with BpyR(Hal).
We have run the calculations of energy of the formation of E and
S complexes with BpyR. Taking into account the prevailing ener-
getic role of structures 1 (BpyR1) in the form of the charge transfer
structure shown in Table 1, the first simulations of complexes have
been done with structures BpyR1. We have analyzed the probabil-
ity of the contribution of both N atoms, the tertial and quaternary
ones, in BpyR(Hal) interaction with the carboxylic group of a mod-
el construct. The designation (Table 4) of R1-I complexes is related
to the complexes in which just tertiary N-atom has contributed
(Fig. 4a), whereas R1-II corresponds to the formation of complexes
in which the quaternary N-atom is involved (Fig. 4b). As it comes
from Table 4, the interaction of (E + H2O) with BpyR1(Hal) gives
very ambiguous results and the formation of BpyR1(Hal)-I does
not seem to be preferable, whereas the interaction S + H2O with
BpyR1(Hal) resulting in the formation of BpyR1(Hal)-II complexes
3.2. Conformation of the model two-units construct
To make the choice for the most probable conformation of the
model copolymer structure we have calculated the
the total energy of E and S conformers (Fig. 1, Table 3). The energy
gain is in favor of S conformer (
ESꢁE = ꢁ10.62 kJ/mol). However,
DE between
D
according to our previous experimental observations [1] that water
also plays an important role in the complex formation, the compar-
ison between the energies of (E + H2O) and (S + H2O) systems is
carried out. Fig. 2 shows the structure images of (E + H2O) and
(S + H2O). The calculations show that if water is involved in the for-
mation of (E + H2O), the system becomes energetically more favor-
able (ꢁ25.29 kJ/mol) than the formation of (S + H2O) (ꢁ14.18 kJ/
Table 3
Model conformation of the two units construct.
Abbreviation
Construct
D
E (kJ/mol)
D
ESꢁE (kJ/mol)
D
E(S+H2O)ꢁ(E+H2O) (kJ/mol)
Distance (Å) (COꢂ ꢂ ꢂH2O)
E
ꢁ25.29
ꢁ10.62
+0.5
1.81
E + H2O
S
ꢁ14.18
1.85
1.85
S + H2O

234
N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
Table 4
Energy change (kJ/mol) R1-I and R1-II complexes formation for E and S conformers with water.
Sample
R1-I
R1-II
ES
R (Hal)
D
ES+H20
D
EE+H20
D
ES+H20
ꢁ
D
EE+H20
D
D
ES+H20
D
ES+H20
ꢁ
D
ES
D
EE+H20
D
ES+H20
ꢁ
DEE+H20
M1(Cl)
A1(Cl)
B1(Cl)
A1(Br)
ꢁ40.41
ꢁ34.14
ꢁ28.33
ꢁ32.87
ꢁ51.48
ꢁ29.41
ꢁ35.85
ꢁ37.75
11.07
ꢁ4.73
7.52
ꢁ32.63
ꢁ57.29
ꢁ64.38
ꢁ32.02
ꢁ86.86
ꢁ92.00
ꢁ96.11
ꢁ70.76
ꢁ54.23
ꢁ34.71
ꢁ31.73
ꢁ38.74
ꢁ61.40
ꢁ55.55
ꢁ86.46
ꢁ58.60
ꢁ25.46
ꢁ36.45
ꢁ9.65
4.88
ꢁ12.16
Cl–
+
(a)
(b)
(c)
(d)
Fig. 4. Model images of complexes based on R(Cl):R1-I (a), R1-II (b), R1-III (c), R1-IV (d).
is energetically more favorable. That is why in our further calcula-
tions and data analysis we have used just S + H2O to contribute in
complex formation.
In addition to R1-I and R1-II complexes already mentioned in
Table 4, we have also considered the case when both N atoms
contribute in the complex formation resulting in the complex of
1 salt molecule per 2 molecules of the construct (R1-III) (Table 5).
As it is seen, the formation of complexes of all types (when either
tertiary or quaternary or both N atoms are involved) (Fig. 4a–c) is
an energetically favorable process.

N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
235
Table 5
highest energy gain and complex stability among all complexes
under consideration. Moreover, it means that the most energeti-
cally favorable complex involves primarily the N+ side of a mono-
substituted salt of BpyR together with an Halꢁ anion interacting
with 2 carboxylic groups (equal to 2 construct molecules), whereas
nonsubstituted N atom in BpyR(Hal) molecule is not directly in-
volved in the complex formation. The latter is clearly seen with
the important interatomic distances summarized in Table 6.
Energetically favorable R1-II and R1-IV are characterized by two
(or three) interatomic distances that have to be analyzed, namely,
one or two Halꢁꢂ ꢂ ꢂHconstruct distances (1.70–1.77 Å) and one
Halꢁꢂ ꢂ ꢂN+ distance (2.87–3.62 Å). It means that COOH group (or
two of them) of the construct is located in a close proximity of
the Hal anion indicating the strong interaction of the construct
molecule (molecules) with the BpyR(Hal) via anion. If we now
compare the electronic characteristics using as an example com-
plexes based on A1(Hal) (Table 7) with original structures
A1 and A2 structures (Tables 1 and 2) then one can conclude that
upon A1(Hal)-II and A1(Hal)-IV complexes formation the elec-
tronic density in the whole BpyR (Hal) molecule changes and the
charge distribution in BpyR1(Hal) molecule becomes very similar
to that in BpyR2(Hal). The charge values on N+ and Halꢁ become
much higher indicating the formation of the structure which is
much similar to an ion pair. This conclusion is also proved by the
increase in the Halꢁꢂ ꢂ ꢂN+ distance in BpyR1(Hal) complexes with
(S + H2O) II and IV which also becomes similar to that in
BpyR2(Hal). This is confirmed for all R-systems studied. One more
detail to point out is the change in the energy of complex IV forma-
tion with the variation of the R structure. The major energy gain is
calculated for the BpyA1(Hal) IV (ꢁ186.95 kJ/mol).
Energy of the complex formation between monosubstituted salts BpyR1(Cl) and
S + H2O construct.
R
DE (kJ/mol)
R1(Cl)-I
R1 (Cl)-II
R1 (Cl)-III
R1 (Cl)-IV
M
A
B
ꢁ40.41
ꢁ34.14
ꢁ28.33
ꢁ86.86
ꢁ92.00
ꢁ96.11
ꢁ118.33
ꢁ125.05
ꢁ120.28
ꢁ168.02
ꢁ186.95
ꢁ180.55
Table 6
Interatomic distances calculated for complexes BpyR (Hal).
Complexes
R1(Hal)
Distance (Å)
Nꢂ ꢂ ꢂHpoly
1.78
–
1.79
–
Distance (Å)
Distance (Å)
Halꢁꢂ ꢂ ꢂHconstruct
Halꢁꢂ ꢂ ꢂN+
M1(Cl)-I
–
2.74
2.87
3.58
3.59
M1(Cl)-II
M1(Cl)-III
M1(Cl)-IV
1.70
1.73
1.76
1.77
–
1.73
1.73
1.77
1.77
–
A1(Cl)-I
1.79
–
1.79
–
2.73
3.57
3.57
3.61
A1(Cl)-II
A1Cl)-III
A1(Cl)–IV
B1(Cl)-I
1.78
–
1.79
–
2.74
3.57
3.57
3.62
B1(Cl)-II
B1(Cl)-III
B1(Cl)–IV
1.73
1.74
1.76
1.76
Table 7
If we apply the similar approach to iodide salts BpyR (I) which
are forming just ion pair type of structure 2 (Table 1) one can see
the formation of complexes of the same type. As an example the
comparison of the values of energy of the formation of complexes
of type II and IV is given in Table 8. The energy gain of complex for-
mation by BpyR (I) is always lower than that of complexes formed
with chloride and bromide salts which may be resulted from the
much bigger size of Iꢁ anion.
Electronic characteristics calculated for BpyA1(Cl) and BpyA1(Br) structures in
complexes with S + H2O construct.
Atoms A1(Cl)_I A1(Cl)_II A1(Cl)_III A1(Cl)_IV A1(Br)_II A1(Br)_IV
1 N
2 C
3 C
4 C
5 C
6 C
7 C
8 C
ꢁ0.12
ꢁ0.14
ꢁ0.23
0.00
ꢁ0.24
ꢁ0.14
0.03
ꢁ0.08
ꢁ0.13
ꢁ0.23
ꢁ0.04
ꢁ0.23
ꢁ0.13
0.01
ꢁ0.12
ꢁ0.14
ꢁ0.23
ꢁ0.03
ꢁ0.22
ꢁ0.13
0.00
ꢁ0.18
ꢁ0.17
ꢁ0.39
0.52
ꢁ0.29
ꢁ0.31
ꢁ0.32
ꢁ0.26
ꢁ0.82
ꢁ0.08
ꢁ0.14
ꢁ0.23
ꢁ0.06
ꢁ0.22
ꢁ0.13
0.03
ꢁ0.08
ꢁ0.13
ꢁ0.22
ꢁ0.04
ꢁ0.23
ꢁ0.13
0.03
ꢁ0.09
ꢁ0.14
ꢁ0.23
ꢁ0.05
ꢁ0.22
ꢁ0.14
0.04
ꢁ0.17
ꢁ0.18
ꢁ0.22
0.55
ꢁ0.31
ꢁ0.31
ꢁ0.30
ꢁ0.27
ꢁ0.97
3.4. Experimental FTIR spectra and comparison with PM3 results
ꢁ0.30
ꢁ0.29
ꢁ0.16
0.03
ꢁ0.19
ꢁ0.17
ꢁ0.39
0.52
ꢁ0.18
ꢁ0.15
ꢁ0.35
0.50
ꢁ0.19
ꢁ0.19
ꢁ0.20
0.55
9 C
10 C
11 N
12 C
13 C
14 C
15 C
Hal
The next step is to compare PM3 calculations of BpyR(Hal) and
their complexes with a two-unit construct with the experimental
FTIR spectra obtained for salts and their complexes with Eudragit
copolymer. As an example, FTIR spectra of BpyM(I) and BpyA(Br),
the Eudragit copolymer and their 1:1 blends are presented in Fig. 5.
ꢁ0.01
ꢁ0.18
ꢁ0.25
ꢁ0.30
ꢁ0.21
ꢁ0.29
ꢁ0.31
ꢁ0.32
ꢁ0.26
ꢁ0.83
ꢁ0.32
ꢁ0.30
ꢁ0.29
ꢁ0.28
ꢁ0.86
ꢁ0.32
ꢁ0.31
ꢁ0.29
ꢁ0.28
ꢁ0.93
First of all, note that there is an intense
m(OH) band at
3437 cmꢁ1 in the spectra of the blends (Fig. 5c and d) that confirms
the presence of coordinated water which is considered in PM3 cal-
culations as a necessary tool for the complexes formation. The
spectra of initial components contain fairly broad bands in that
spectral range. However, the spectral bands of the blends are char-
acterized by the strong increase in the relative intensity and a
slight shift of mOH to the longer wavelength. It serves as a direct
indication of the additional association of AOH bonds of water,
and this is in good agreement with the results of quantum-chem-
ical calculations. The experimental IR spectrum of Eudragit copoly-
mer contains an intense spectral band at 1695 cmꢁ1 and a shoulder
at 1725 cmꢁ1 (Fig. 5). Both of them correspond to the mC@O of
the carboxylic (1695 cmꢁ1) and ester (1725 cmꢁ1) groups [9]. The
spectral band at 1725 cmꢁ1 remains practically unchanged in
the spectra of 1:1 copolymer blends with mono-N-substituted
4,40-bipyridinium halogenated salts indicating that ester groups
are not involved in any interaction with salts studied. As for mC@O
at 1695 cmꢁ1 corresponding to the carboxylic group, it either
nearly disappears in spectra of blends with M and A – substituted
Table 8
Energy of the complexes II and IV formation between monosubstituted salts BpyR
(Hal) and S + H2O construct.
R
Energy change, (kJ/mol)
R1(Cl)-II
R1(Cl)-IV
R1(Br)-II
R1(Br)-IV
R2(I)-II
R2(I)-IV
M
A
B
ꢁ86.86
ꢁ92.00
ꢁ96.11
ꢁ168.02
ꢁ186.95
ꢁ180.55
ꢁ79.92
ꢁ70.76
ꢁ67.20
ꢁ135.02
ꢁ143.53
ꢁ145.75
ꢁ51.49
ꢁ54.38
ꢁ64.98
ꢁ91.05
ꢁ99.67
ꢁ82.03
The total energy change under complex formation shows the in-
crease in the complex stability in the following range: R1-I < R1-
II < R1-III. However the simulation of the structure of R1-III
complex during the optimization of Nꢂ ꢂ ꢂH distance shows that
R1-III cannot be stabilized and it undergoes a transformation into
R1-IV complex structure (Fig. 4d). This R1-IV complex shows the

236
N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
Fig. 5. FTIR spectra of 4,40-bipyridinium salts (-..-): M (I) – (a and b), A(Br) – (c and d) and B(Cl) (e and f); Eudragit copolymer (ꢂ ꢂ ꢂ) and their 1:1 complexes (___).
salts (Fig. 4a and b) or its intensity decreases in the spectrum of the
blend with a B-substituted salt. These data prove the participation
of COOH-groups of the copolymer in a complex formation. In addi-
tion to the changes in the carboxylic group bands, there are some
changes in the spectra of monosubstituted 4,40-bipyridinium salts.
By comparison of FTIR spectra of free salts and 1:1 blends with
Eudragit one can see that the prominent aromatic core skeletal
band mas at 1630 cmꢁ1 shifts to 1637 cmꢁ1 and ms at1595 cmꢁ1
moves up to 1601 cmꢁ1 due to the interaction. The spectral change
can be also seen in the spectral range of the pyridine core (1300
and 880 cmꢁ1): dCAH stretching mode at 809 cmꢁ1 shifts up to
815 cmꢁ1
.

N.D. Merekalova et al. / Journal of Molecular Structure 1048 (2013) 230–237
237
It means that the whole system of conjugated bonds in the salts
molecules rearranges its structure due to an interaction with
copolymer molecules which results in the formation of polymer–
bipyridilium salt complex. The fact of the experimentally observed
change of the distribution of electron density along the whole salt
molecule is in good agreement with the calculated distribution of
electron density that strongly changes when BpyR1(Hal)-II com-
plexes are formed. On the other hand, FTIR spectra do not show
any sign of the COOH-group proton transfer and carboxylate anion
formation.
between proton of the carboxylic group and halogen anion. Due
to a much higher electrophilicity of the proton in comparison with
the quaternary nitrogen atom, the electron density distribution in
the initial partial charge transfer complex BpyR1(Hal) changes. The
halogen anion acquires a high negative charge, whereas its interac-
tion with proton provides the energy gain and Halꢂ ꢂ ꢂHOOC dis-
tance becomes close to that of a strong hydrogen bond (Table 6).
During the formation of that complex BpyR1(Hal)-II the interac-
tion N+ꢂ ꢂ ꢂHalꢁ reaches the level of coulomb interaction in such a
way that halogen anion ends up in an electrostatic trap comprised
of two (complex II) or three atoms (complex IV). In addition, water
molecules play an essential role in stabilization of complexes con-
sidered in this study. The results of calculations are in a very good
agreement with experimental FTIR spectra data.
4. Conclusions
PM3 calculations show that monosubstituted 4,40-bipyridinium
halides exist in two different energetically favorable structures:
the first structure is a partial charge transfer complex BpyR1
(Hal) that includes both organic and inorganic (halogen) moieties
and the second one is an ion pair BpyR2 (Hal). The charge transfer
system is energetically more favorable. The additive character of
energy values given in Table 5 allows to hypothesize the coulomb
mechanism of interactions between the halogenated monosubsti-
tuted 4,40-bipyridinium salts and COOH-containing polymers lead-
ing to a complex formation.
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However, it is much different from the idea about proton trans-
fer and contribution of the tertiary nitrogen in the process of the
complex formation. The basic role is played by the interaction