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: igm@xanum.uam.mx (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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