Journal of
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Journal of Electroanalytical Chemistry 573 (2004) 307–314
ElectroanalyticalChemistry
Electrochemical and ESR study on the transformation processes ofa-hydroxy-quinones q
Carlos Frontana a, Bernardo A. Frontana-Uribe b, Ignacio Gonzalez a,*
a UAM-Iztapalapa, Departamento de Quımica, Apartado Postal 55-534, 09340 Mexico D.F., Mexicob Instituto de Quımica, UNAM, Circuito Exterior Ciudad Universitaria, Coyoacan 04510 Mexico D.F., Mexico
Received 24 March 2004; received in revised form 15 July 2004; accepted 19 July 2004
Available online 23 September 2004
Abstract
The electrochemical analysis by cyclic voltammetry and double potential step chronoamperometry of two a-hydroxyquinones (2-hydroxy-1,4-naphthoquinone and perezone) in acetonitrile, reveals that in the first electron transfer process, self-protonation reac-
tions are present. One of the products of this reduction is the deprotonated original quinone. This last intermediate is reduced by a
monoelectronic process in the second reduction step, generating a radical dianion. The radical dianions formed can be detected by
EC-ESR coupled experiments and the spectra characteristics were explained in terms of the electron delocalization properties of the
analyzed compounds. Upon the addition of a base (tetrabutylammonium hydroxide), the ESR signal increases in intensity and
hyperfine coupling analysis is better resolved, proving the radical nature of such species. The oxidation properties of the deproto-
nated quinone of 2-hydroxy-1,4-naphthoquinone were also studied under these basic conditions. The results provide insights into
the proposal of a dimerization process occurring in this oxidation process.
� 2004 Elsevier B.V. All rights reserved.
Keywords: Quinones; Perezone; Self-protonation; Radical dianion; Chronoamperometry; Cyclic voltammetry; ESR
1. Introduction
Several naturally occurring quinones have a hydroxy
function (represented as Q–OH) in their structure [1].
The presence of this type of hydroxy functionality seems
to be related to the biological activity of this kind ofcompound [2–7], and the position of this functional
group can alter the typical redox behavior of the quinoid
moiety [8–12]. This is relevant, considering that most of
the biological activity of quinonoid systems is related to
their capacity to carry out free radical generation via
redox reactions. The electrogenerated radical anion
species (semiquinones) are capable of sustaining long
0022-0728/$ - see front matter � 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2004.07.017
q Contribution No. 2471 from Instituto de Quımica, UNAM.* Corresponding author. Tel.: +52 55 5804 4671 ext. 12; fax: +52 55
5804 4666.
E-mail address: [email protected] (I. Gonzalez).
half-life periods and ultimately lead to the generation
of reactive oxygen species (ROS, e.g. superoxide anion
radical and H2O2) in biological systems [13,14]. There-
fore, several electrochemical experiments have been per-
formed in order to interpret and rationalise how the
overall mechanistic sequence can affect the behavior ofQ–OH compounds.
While b-hydroxyquinones generate a stable semiqui-
none in the first electron transfer process [15], a-hydro-quinones present a more complicated electroreduction
mechanism. The conclusions obtained for the latter type
of compound are generally that simultaneous self-proto-
nation [16]/reduction processes are involved in the redox
behavior of these a-hydroxyquinones, with generationof an appreciable amount of deprotonated quinone
(Q–O�) (Eqs. (1)–(5)) [8–10].
Q–OHþ e�¡ðQ–OHÞ�� ð1Þ
308 C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314
ðQ–OHÞ�� þQ–OH ! ðHQ–OHÞ� þQ–O� ð2Þ
ðHQ–OHÞ� þ e�¡ðHQ–OHÞ� ð3Þ
ðHQ–OHÞ� þQ–OH ! H2Q–OHþQ–O� ð4Þ
3Q–OHþ 2e� ! H2Q–OHþ 2Q–O� ð5ÞAs many research groups conclude, Q–O� is responsible
for the appearance of the second quasi-reversible reduc-
tion process, but the nature of the electrochemical reac-
tion occurring at this potential level is still controversial.Some authors propose that the reduction proceeds by a
monoelectronic reduction of Q–O�, (Eq. (6)) [17–19].
Q–O� þ e�¡Q–O�¼ ð6ÞOn the other hand, Goulart and co-workers have pro-
posed that this reduction occurs by a bielectronic mech-
anism, either by an EE or an ECE pathway. This meansthat the radical dianions formed in Eq. (6) would be
immediately transformed by a second electron uptake
[10]. However, in this research, some of the experimental
conditions are quite different from those reported previ-
ously (Pt, GC vs Hg) and may suggest that the nature of
such an electrochemical reaction could depend on the
nature of the electrode.
It is interesting to note that only reaction 6 can be re-lated to the formation of a stable semiquinone-type rad-
ical species (Q–O�=), and reports on the stability of these
intermediates in the literature are few [20]. In this work,
the processes associated with the formation of this radical
dianion are discussed by studying a standard compound
(2-hydroxy-1,4-naphthoquinone 2, Fig. 1) by cyclic vol-
tammetry, double potential step chronoamperometry
and in situ electrochemical-ESR studies (EC-ESR), to
Fig. 1. Structures for the quinoid compounds investigated: 1: 1,4-
naphthoquinone; 2: 2-hydroxy-1,4-naphthoquinone; 3: perezone.
clarify the controversy previously stated. The results were
extended to perezone (3, Fig. 1), a naturally occurring
compound that also presents an a-hydroxy functionality,allowing a generalization of the mechanism.
2. Experimental
2.1. Substances
1,4-Naphthoquinone (1, Fig. 1) (Aldrich 98%, A.R.
grade), was resublimed prior to its use. 2-Hydroxy-1,4-
naphthoquinone 2 (Aldrich 98%, A.R. grade) was used
without further purification. The natural product pere-zone, 3, was extracted fromPerezia roots [21]. Tetrabutyl-
ammoniumhydroxide (TBAOH,Aldrich, 1M solution in
water), was used to prepare the basic medium employed
for this work. A 91 mM stock solution was prepared by
dissolving 100ll of this solution in 1ml of dry acetonitrile.
2.2. Solvent and supporting electrolyte
Anhydrous acetonitrile (CH3CN, Aldrich 98%) was
dried overnight with P2O5 and distilled prior to its use.
The distillate was received over oven-activated 3 A
molecular sieves (Merck) and kept in a desiccator. This
method is useful to obtain dry acetonitrile, characterized
by the absence of OH bands in the IR spectra. Tetraeth-
ylammonium tetrafluoroborate (Fluka Chemika, Electr-
ochemical grade, Et4NBF4) was used as the supportingelectrolyte. The salt was dried the night before use at
90 �C and 0.1 M solutions were prepared and used as
the supporting electrolyte.
2.3. Electrodes, apparatus and instrumentation
Cyclic voltammetry and double potential step chron-
oamperometry were performed with an AUTOLABPGSTAT 30 potentiostat/galvanostat. An IR drop cor-
rection was applied during all the experiments, using
Ru values obtained with the positive feedback technique
(82 X). A conventional three-electrode cell was used to
carry out these experiments, employing a platinum
microelectrode (BAS, surface: 0.03 cm2) as the working
electrode. This was polished using 0.05 lm alumina
(Buehler), sonicated in distilled water for 10 min andrinsed with acetone prior to its use. A platinum mesh
was used as the counter electrode (surface: 0.6 cm2).
The potential values were obtained against a reference
electrode (BAS) of Agj0.01 M AgNO3 0.1 M tetrabutyl-
ammonium perchlorate (TBAP) in acetonitrile, sepa-
rated from the medium by a Vycor membrane. Ep
values are reported vs the ferricinium/ferrocene couple
(Fc+/Fc), according to the IUPAC recommendation[22]. The potential of the Fc+/Fc couple against this ref-
erence electrode was 0.25 V. Solutions of the quinones
Fig. 2. Typical cyclic voltammograms of 3.9 mM 2-hydroxy-1,4-
naphthoquinone, obtained with a platinum microelectrode (0.03 cm2)
in 0.1 M Et4NBF4 + CH3CN. The potential scan was initiated from
�450 mV vs Fc+/Fc, in the negative direction ((a) 10 mV s�1; (b) 1000
mV s�1). Both cathodic and anodic peaks are indicated.
C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314 309
were prepared by dissolving the desired compound with
0.1 M Et4NBF4 (1: 1 mM; 2: 3.9 mM; 3: 3 mM). The
solution was deoxygenated for 30 min and the cell was
kept under a nitrogen atmosphere (grade 5, Praxair)
throughout the experiment. Periodic double potential
step experiments were performed using the methodologysuggested by Amatore and Saveant [23].
2.4. EC-ESR spectroscopy experiments
ESR spectra were recorded in the X band (9.85 GHz)
using a Jeol FA-300 instrument with a cylindrical cavity.
A commercially available spectroelectrochemical cell
(Wilmad) was used, employing a 0.02 mm platinum wire(3.1 cm2) as the working electrode, introduced in the flat
path of the cell. Another platinum wire was used as the
counter electrode (2.5 cm2). Agj0.01 M AgNO3j0.1 M
TBAP in acetonitrile, was employed as the reference
electrode. Potential sweep control was performed with
a 100 B voltammetric analyzer (BAS) interfaced with a
personal computer.
It is important to note that under the experimentalconditions, the thin layer EC-ESR cell can have some
problems with the potential control in the desired re-
gions [24]. To minimize this problem, low scan rates
were used, leading to good measurements. The tech-
nique proved to be fairly suitable, as the stable benzo-
quinone radical anion structure is obtained, and its
appearance corresponds exactly to that reported in the
literature [25]. Positive feedback can also be performedby the equipment, but Ru values are usually higher than
in typical voltammetric experiments, a typical value of
Ru being 350 X.Solutions of quinones were prepared by dissolving
the desired compound with 0.1 M Et4NBF4 (1: 0.5
mM; 2: 1 mM; 3: 1.5 mM). The solutions were deoxy-
genated for 30 min and the cell was kept under a nitro-
gen atmosphere (grade 5, Praxair) throughout theexperiment.
2.5. ESR simulations
PEST WinSim free software Version 0.96 (National
Institute of Environmental Health Sciences) was used
to measure hyperfine coupling constant values (a) and
to perform simulations of radical species.
3. Results and discussion
The voltammetric behavior of compound 2 is pre-
sented in Fig. 2. Reaction layer effects are manifested
by comparing the electrochemical response under differ-
ent scan rate conditions. The long time window (e.g. 10mV s�1, Fig. 2(a)), shows two main cathodic electron
transfer processes, presented as peaks Ic and IIc, the lat-
ter being associated with the formation of the stable sig-
nals IIa and IIa 0. When the scan rate is increased (1000mV s�1, Fig. 2(b)) signals Ic and IIc remain. Signal IIa
shows the presence of a shoulder that can be well re-
solved by increasing the scan rate (up to 100,000 mV
s�1). Signal Ia is associated with Ic but its current value
is higher when the potential sweep is reversed after peak
IIc. Under these time scale conditions, another signal
(IIIa) becomes visible, even when the scan potential is
reversed just after peak Ic. This comparison indicatesthat the species associated with the oxidation peaks Ia
and IIIa are formed during the reduction processes
and leave the interface when the time window is long.
These peaks must correspond to the oxidation of the
intermediates described in the set of reactions depicted
previously (Eqs. (3), (4), and (6)).
The electrochemical analysis of the cathodic peak Ic
for compounds 2 and 3 leads to slopes for EpIc = f(log v)in accordance with the previously reported value for
self-protonation mechanism depicted in Eqs. (1)–(3)
(Table 1 [16]). The lack of reversibility on this signal is
related to the instability of the first radical anion (Eq.
(1)), differing from the typical monoelectronic process
Table 1
Values obtained for electrochemical and ESR simulation analysis for: 1,4-naphthoquinone 1; 2-hydroxy-1,4-naphthoquinone 2 and perezone 3
Compound EpIc/Va EpIIc/V
a m EpIc = f(log v)/mV dec�1 b m EpIIc = f(log v)/mV dec�1 b g a/mT
1 �1.25 �1.68 �7 �3 2.0052 H2.3: 0.17
2 �1.11 �1.92 �30 �13 2.0049 �c
3 �1.16 �1.9 �30 �15 2.0073 H5: 0.82, H14q: 0.16, H7d: 0.108
Numbers in bold represent the potential values for semiquinone-type species appearance.a Obtained for 100 mV s�1.b Obtained for scan rates from 10 to 40,000 mV s�1 (theoretical value for m (slope) Ep = f(log v) in self-protonation pathways: �29.6 mV dec�1
[16].)c This spectrum could not be simulated satisfactorily. q and d subscripts represent quartet and doublet signals, respectively.
310 C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314
for quinones (with and without b-hydroxyquinones) inaprotic media. When double potential step chronoam-
perometric experiments are performed for the first
reduction process of compound 2, fixing the reverse
pulse potential just at the peak base, almost no product
is recovered (I(2h)/I(h) � 0, Fig. 3(a)). On the other
hand, setting an inversion potential just higher than
peak Ia leads to recovery of an electroactive productthat diminishes as the time is increased (I(2h)/I(h) � 0.294 maximum recovery Fig. 3(b)). This high
recovery obtained for this inversion condition shows
that the electronic stoichiometry of the oxidation reac-
tion is higher than that proposed for the reduction step
(2/3e� mol per quinone mol). A reasonable proposal in-
volves the monoelectronic electrochemical oxidation of
species (HQ–OH)� [10].Considering the obvious fast rate of disappearance of
the radical species appearing in Eqs. (1)–(3) [(Q–OH)��,
(HQ–OH)�], the study of their properties requires the use
of a coupled experimental technique. It has been pro-
posed that ESR spectra acquired under these potential
conditions (Fig. 2, Peak Ic), could give some insight into
the presence of these semiquinone species [26]. However,
Fig. 3. Variation of the quotient I(2h)/I(h), for different values of thepulse time (h) for 3.9 mM 2-hydroxy-1,4-naphthoquinone 2. (a) Edirect
pulse: �1.28 V, Einverse pulse: �0.815 V vs Fc+/Fc. (b) Edirect pulse: �1.28
V, Einverse pulse: �0.38 V vs Fc+/Fc. Dotted lined on top represents the
experimental limit obtained for a reversible monoelectronic system
(1,4-naphthoquinone, 0.277).
when ESR spectra are recorded using these potential
values in the spectroelectrochemical cell, no signal is ob-
tained, even when large electrolysis periods are used (as
long as 15 min). Again, the fast consumption rate of the
radical intermediates by the protonation reaction given
in Eq. (2), and the ease of reduction of such formed spe-
cies lead to short half-life periods, unattainable by the
acquisition time of the ESR spectrometer.Although the rate of the protonation reaction is very
high, some other reactions can compete with it, such as
oxygen interaction with the radical anion, as recently
been described. This particular result could show that
oxygen interferes in the self-protonation reaction [27].
In spite of this, the color of the solution contained in
the flat path of the EC-ESR cell changes from yellow
to red in 2-hydroxy-1,4-naphthoquinone, while for pere-zone, the color change is from pale yellow to purple.
These color changes are typical for enol-enolate com-
pounds [28], such as those obtained from the global
mechanistic sequence (Eq. (5)). On the other hand, in
the absence of the a-hydroxy function (e.g. 1,4-naphtho-
quinone 1), the reduction at the first cathodic peak leads
to a well-behaved ESR spectrum (Fig. 4(a)) [29] and its
corresponding g value is reported in Table 1.For the second electron transfer process, the slope
analysis of the function EpIIc = f(log v) shows a low va-
lue (Table 1), which allows us to discard a self-protona-
tion process. EC-ESR studies in the region of potential
values more negative than peak IIc for compounds 2
and 3 make it possible to identify the presence of a rad-
ical species (Fig. 4(b) and (c)). This confirms that the
electron transfer under these potential condition is asso-ciated with Eq. (6) and the radical dianion formed is sta-
ble enough in the aprotic medium employed and is easily
detected.
The spectroscopic characteristics of such species are
noticeable. Except for 1, the other quinones lack symme-
try centers and therefore provide several possible hyper-
fine coupling constants. The spectrum obtained for
compound 2 could not be simulated satisfactorily, prob-ably because of the presence of a dynamic process,
which affects the line width of the observed signals. This
is not the case for perezone, and the ESR spectrum is
adequately described by the simulation process, taking
Fig. 4. ESR spectra obtained for quinoid compounds at the potential
value signaled in bold in Table 1. (a) 1 mM 1,4-naphthoquinone
semiquinone; (b) 3 mM 2-hydroxy-1,4-naphthoquinone radical dian-
ion; (c) 1 mM perezone radical dianion. Dotted line represents
experimental spectrum. Solid line depicts simulated spectrum.
Fig. 5. Typical cyclic voltammograms of 3.9 mM 2-hydroxy-1,4-
naphthoquinone obtained with a platinum microelectrode (0.03 cm2)
in 0.1 M Et4NBF4 + CH3CN, in the presence (black lines), and absence
(grey lines) of TBAOH (1.1 equivalents). (a) Potential scan initiated
from �800 mV (black line) and �500 mV (grey line) vs Fc+/Fc, in the
negative direction. (b) Potential scan initiated from �800 mV (black
line) and �370 mV (grey line) vs Fc+/Fc, in the positive direction (500
mV s�1). Both cathodic and anodic peaks are indicated.
C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314 311
into account only the interactions between proton
groups H-5, H-7 and H-14 with the unpaired electron(Fig. 1). The 5-signal spectrum obtained is a result of
merging of the expected quartet signal of H-14 and the
doublet of H-7, as the experimental line width and mod-
ulation parameters do not allow a better resolution of
the different groups of signals.
Radical dianion species are known for hydroxyqui-
nones of biological importance [20] but, as far as weknow, no information regarding the electrochemically
mediated generation of such a species and its simultane-
ous spectroscopic identification, has been reported. In
order to validate further the proposal of Eq. (6), the
deprotonated quinone (Q–O�) was generated in situ
via the addition of a basic species (TBAOH) to the solu-
tion containing the a-hydroxy compound. Fig. 5(a) de-
picts the typical voltammetric behaviour forcompound 2 under these experimental conditions.
Increasing the amount of TBAOH, with the potential
scan initiated in the negative direction, results in a de-
crease of the current associated with peak Ic until its dis-
appearance upon the addition of a molar equivalent of
the basic compound. Peak IIc evolves into peak IIc 0,
appearing now at a less negative potential (DEp � 100
Fig. 6. Variation of the quotient I(2h)/I(h), for different values of thepulse time (h) for 3.9 mM 2-hydroxy-1,4-naphthoquinone 2 in the
presence of 1.1 M equivalents of TBAOH. (a) Edirect pulse: 0.5 V, Einverse
pulse: 0.1 V vs Fc+/Fc. (b) Edirect pulse: �1.92 V, Einverse pulse: �1.52 V vs
Fc+/Fc. Dotted lined on top represents the experimental limit obtained
for a reversible monoelectronic system (1,4-naphthoquinone, 0.277).
Fig. 7. Typical cyclic voltammograms of 3 mM perezone obtained
with a platinum microelectrode (0.03 cm2) in 0.1 M
Et4NBF4 + CH3CN, with (black lines), and without (grey lines) the
addition of TBAOH (1.1 equivalents). (a) Potential scan initiated from
�800 mV vs Fc+/Fc, in the negative direction. (b) Potential scan
initiated from �600 mV (black line) and �440 mV (grey line) vs Fc+/
Fc, in the positive direction. Scan rate: 500 mV s�1. Both cathodic and
anodic peaks are indicated.
312 C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314
mV), and becomes a reversible signal under scan rates
below 1 V s�1 ðIpIIc0=IpIIa0 ¼ 1; DEpðIIc–IIaÞ ¼ 0:059 VÞ.The same result is obtained in double potential stepchronoamperometry for the IIc 0/IIa 0 system (Fig. 6(b)).
Under these basic conditions the interference of the
reduction signal Ic is avoided.
The chronoamperometric analysis confirms the sta-
bility of system IIc 0/IIa 0 as a monoelectronic reversible
process (I(2h)/I(h) � 0.247, maximum recovery). The
missing recovery percentage can be explained by the fact
that only part of the faradaic current measured in theseexperiments is associated with reduction IIc 0; part is
associated with the prepeak irreversible signal which
diminishes the I(2h)/I(h) quotient. This indicates that
effectively this peak is associated with a one-electron
transfer as shown in Eq. (6) and invalidates the EE or
ECE mechanism proposed by other authors [10].
When the electrolysis was performed at potentials
more negative than that of IIc 0 in the EC-ESR cell forcompound 2 and using the basic medium of TBAOH
added, a more intense spectrum is acquired and hyper-
fine coupling constants are better resolved compared
to those obtained without the added base (Fig. 4(b)),
but no extra information concerning the structure of
the radical species was obtained. It is noticeable that
upon the addition of base, a considerable amount of
water is now present during the experiments (theTBAOH used is a 1 M aqueous solution). The stability
of the ESR signals confirms that the added water be-
haves as a very weak proton donor in acetonitrile, as
was previously described for DMSO [17].
Under the basic conditions previously described, the
behavior of perezone, differs from that of compound
2, as the signal group IIc 0 and IIa 0 does not evolve into
a reversible system in cyclic voltammetry
(IpIIc0=IpIIa0 ¼ 0:77; DEpðIIc–IIaÞ ¼ 0:3 V Fig. 7(a)). Never-
theless, the ESR spectrum depicted in Fig. 4(c) is still
observed and the radical species formed proves to be
more stable than compound 2 radical dianion (t1/2: pere-
zone, �25 min, t1/2: 2-hydroxy-1,4-naphthoquinone �30s). This interesting result could be the starting point for
biological activity correlations of this type of com-
pound, since it shows that the presence of the aromatic
moiety interferes with the stability of the radical species.
On the other hand, for compounds 2 and 3, studying
the oxidation properties under basic conditions (Figs.
5(b) and 7(b)), peak Ia diminishes after the first addition
of TBAOH and the irreversible anodic peak IIIa grows,indicating that this latter peak corresponds to the oxida-
tion of the deprotonated quinone (Q–O�), as other
authors claim [30]. Voltammetric analysis of this peak
reveals that the anodic peak potential shifts to more pos-
itive values when the scan rate increases (22 mV dec�1,
Fig. 8, Ep/2�Ep � 60 mV), evidence of the presence of
coupled chemical processes, probably a dimerization
Fig. 8. Variation of the potential of the oxidation peak IIIa of 2-
hydroxy-1,4-naphthoquinone with the scan rate in the presence of 1.1
equivalents of TBAOH. Slope value, 22 mV dec�1 (r2 = 0.9738).
Theoretical value for DIM2 processes (29.6 mV dec�1, Ep/2–Ep � 58.3
mV [31,32]).
C. Frontana et al. / Journal of Electroanalytical Chemistry 573 (2004) 307–314 313
process via DIM2 [31,32] as reported for phenolate oxi-
dation [33,34]. Also, the I(2h)/I(h) quotient for peak
IIIa, in the double step potential chronoamperometric
experiments shows no recovery signal, presumably due
to the very high rate of the homogeneous chemical reac-
tion (see Fig. 6(a)).
In order to deduce the nature of the chemical se-
quence occurring at peak IIIa, exhaustive electrolysiswas performed to measure simultaneously the total
charge associated with the electrochemical oxidation
and to try to isolate the products obtained. The electrol-
ysis results show that only one equivalent of charge per
mole of quinone was consumed. The product obtained
has an almost identical RF value in thin layer chroma-
tography compared with the former quinone and the
structural analysis by 1H and 13C NMR, shows thatgroup H-3 and C-3 of compound 2 (Fig. 1) still appear
in the electrolysis product but are displaced to lower
fields, which indicates the presence of a higher electro-
negative environment near such a signal. Unfortunately
the product obtained was not sufficiently stable to deter-
mine, by mass spectroscopy, which of the different pos-
sible types of dimer was present. This reaction is
currently under study and the complete results will be re-ported later.
4. Conclusions
The electrochemical analysis by cyclic voltammetry
and double potential step chronoamperometry of two
a-hydroxyquinones (2-hydroxy-1,4-naphthoquinone 2and perezone 3), in acetonitrile reveals that in the first
electron transfer process, self-protonation reactions are
present. One of the products of this reduction is the
deprotonated original quinone. This last intermediate
is reduced by a monoelectronic process at the second
reduction step, generating a radical dianion. The radical
dianions formed can be detected by EC-ESR coupled
experiments and the spectra characteristics were ex-
plained in terms of the electron delocalization properties
of the compounds analyzed. Upon the addition of a base
(TBAOH), the ESR signal increases in intensity and thehyperfine coupling analysis is better resolved, proving
the radical nature of such species. The oxidation proper-
ties of peak IIIa which appears as an stable signal under
the basic conditions for 2-hydroxy-1,4-naphthoquinone
were also studied. The results provide insights into the
proposal of a dimerization process occurring at this ano-
dic signal.
Acknowledgements
The authors kindly thank Virginia Gomez, Marıa de
las Nieves Zavala and Marıa de los Angeles Pena for
their technical assistance. C. Frontana thanks CONA-
CyT-Mexico for the scholarship granted.
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