Journal of The Electrochemical Society, 159 (11) G151-G159 (2012) G151
0013-4651/2012/159(11)/G151/9/$28.00 © The Electrochemical Society
The Electrochemical Oxidation of 6-Aminoquinoline:
Computational and Voltammetric Study
Milica C. Stevi´ c,
a, z
Gordana
´
Ciri´ c-Marjanovi´ c,
a
Budimir Marjanovi´ c,
b
Ljubiˇ sa M. Ignjatovi´ c,
a
and Dragan Manojlovi´ c
c
a
Faculty of Physical Chemistry, University of Belgrade, 11158 Belgrade, Serbia
b
Centrohem, 22300 Stara Pazova, Serbia
c
Faculty of Chemistry, University of Belgrade, 11158 Belgrade, Serbia
The theoretical study of the 6-aminoquinoline (6-QNH
2
) electrochemical oxidation mechanism, based on the semi-empirical quantum
chemical computations of the heat of formation, ionization energy, and spin density of reaction intermediates, taking into account
the influence of pH and solvation effects, has been conducted. Two possible 6-QNH
2
electro-oxidation pathways are investigated,
namely, the initial single-electron oxidation of 6-QNH
2
at lower electrode potentials, leading to the formation of cation radicals
[6-QNH
2
]
•+
in acidic solutions and neutral radicals [6-QNH]
•
in alkaline solutions, as well as the two-electron oxidation of
6-QNH
2
leading to the initial formation of nitrenium cations [6-QNH]
+
at higher electrode potentials. The regioselectivity of
6-QNH
2
dimerization reactions, which follow both the single- and two-electron transfer reactions, is computationally studied. Cyclic
voltammetry experiments, conducted at a glassy carbon paste electrode in Britton-Robinson buffer/methanol media, are correlated
with the computationally predicted 6-QNH
2
electro-oxidation mechanism. Differential pulse voltammetric and adsorptive stripping
differential pulse voltammetric analyses of 6-QNH
2
oxidation have also been performed.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.004212jes] All rights reserved.
Manuscript submitted July 30, 2012; revised manuscript received August 21, 2012. Published September 21, 2012.
Electrochemical oxidations of heterocyclic aromatic compounds,
such as pyrrole and thiophene, as well as the electro-oxidations of
carbocyclic arylamines, such as aniline, aminonaphthalenes, etc.,
have been the subject of numerous studies during the decades.
1
Special attention has been paid to the electrochemical oxidative
polymerizations of heterocyclic aromatic compounds and carbocyclic
arylamines, which lead to the formation of corresponding conducting
polymers (polypyrrole, polythiophene, polyaniline, etc.).
2
Much
less attention, both from the experimental and theoretical points
of view, has been focused on the electrochemical oxidation of hete-
rocyclic arylamines, e.g. aminopyridines,
3–8
aminoimidazoles,
9
aminothiophenes,
10–14
aminoindoles,
15
aminoacridines,
16
aminophenazines,
17
aminopurines,
18–20
aminothiazoles,
21,22
and
aminobenzothiazoles.
23
Aminoquinolines are heterocyclic arylamines with considerable
industrial use, especially as intermediates in the pharmaceutical
industry.
24
The electrochemical oxidation of aminoquinolines has
scarcely been investigated.
25–29
Based on various electroanalytical
techniques, 2,2
′
-azoquinoline was proposed as the major product of
the oxidation of 2-aminoquinoline at a stationary pyrolytic graphite
electrode in methanol-phosphate buffer.
25
No products were isolated
from the attempted electrochemical oxidation of 3-aminoquinoline
(3-QNH
2
) in 75−80% sulfuric acid solution at a platinum electrode.
26
Poly(5-aminoquinoline) films were prepared on gold electrodes by
anodic polymerization of 5-aminoquinoline (5-QNH
2
) in acetonitrile
using cyclic voltammetry and potential step methods.
27
Voltammetric
determination of 3-QNH
2
, 5-QNH
2
, 6-QNH
2
, and 8-QNH
2
, separately
or in their mixtures, by differential pulse voltammetry and adsorptive
stripping differential pulse voltammetry on carbon paste electrode was
reported by Zima et al.
28,29
Recently, we have successfully applied a combined computa-
tional and experimental approach in order to obtain new insights
into the mechanism of chemical oxidative polymerization of hetero-
cyclic arylamines such as aminophenazines
30
and aminoacridines,
31
as well as in order to determine unambiguously the main structural
units of the obtained oligomeric and polymeric oxidation products.
We have recently used successfully a combined computational and
experimental approach in the case of electrochemical oxidation of
hydroxyquinolines.
32,33
In the present communication we reported
a semi-empirical quantum chemical study of the electro-oxidation
of 6-QNH
2
(3, Scheme 1) in aqueous solution. To our knowledge,
there has been no computational study of the electrochemical ox-
z
E-mail: milica@ffh.bg.ac.rs
idation of aminoquinolines. We correlated a theoretically proposed
6-QNH
2
electro-oxidation mechanism with cyclovoltammetric (CV)
behavior of 6-QNH
2
at a glassy carbon paste (GCPE) working elec-
trode, in a supporting electrolyte of mixtures of Britton-Robinson
buffer/methanol. GCPE exhibits a very low background current, has a
wide potential range,
34
combines the favorable electron-transfer kinet-
ics of glassy carbon (GC) with the attractive advantages of composite
paste electrode materials,
35
and is compatible with a high content
of methanol.
36,37
Differential pulse voltammetry (DPV) was used for
establishing the optimum conditions for 6-QNH
2
determination and
finding the limits of detection (LOD). Adsorptive stripping differential
pulse voltammetry (AdSDPV) has also been explored as a technique
for sensitive quantitative analysis of the investigated substance.
Computational Methods
A semi-empirical AM1 method
38
(included in the molecular or-
bital package MOPAC 97,
39
a part of the Chem3D Pro 5.0 pack-
age, CambridgeSoft Corporation) has been used to obtain the heat
of formation (H
f
), and spin density of investigated species. This
method is accurate enough to have useful predictive power and it is
fast enough to allow the processing of oligomers.
32,33,40–45
The ion-
ization energy (E
i
) of 6-QNH
2
in different acid-base forms, as well as
E
i
of their oxidation products was computed by using RM1 method
46
(improved/reparameterized version of AM1 included in the MOPAC
2009, a part of the ChemBio3D Ultra 12.0, CambridgeSoft Corpo-
ration). Solvation effects were taken into account using COSMO
47
(the conductor-like screening model) to approximate the effect of sol-
vent surrounding the analyzed molecules, ions and free radicals. Con-
formational analysis of reaction intermediates was done. The steric
energy was minimized using MM2 molecular mechanics force-field
method.
48
Input files for the semi-empirical quantum-chemical com-
putations of dimeric 6-QNH
2
intermediates were the most stable con-
formers of the investigated molecular structures. The geometrical op-
timization was performed by the eigenvector following procedure.
49
The restricted Hartree−Fock method has been used for the inves-
tigated molecules and ions, while the unrestricted Hartree−Fock
method has been used for free radical species.
Experimental
Reagents and chemicals.— 6-QNH
2
(95%) and glassy carbon
powder, comprising spherical particles with diameters in the range
2−12 μm, were supplied by Sigma−Aldrich (Poole, UK). Methanol
) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 144.82.108.120 Downloaded on 2014-07-07 to IP