Practical aspects of cyclic voltammetry: How to estimate reduction potentials when irreversibility prevails

Article abstract of DOI:10.1149/2.0241905jes

What is the best approach for estimating standard electrochemical potentials, E(0), from voltammograms that exhibit chemical irreversibility? The lifetimes of the oxidized or reduced forms of the majority of known redox species are considerably shorter than the voltammetry acquisition times, resulting in irreversibility and making the answer to this question of outmost importance. Halfwave potentials, E(1/2), provide the best experimentally obtainable representation of E(0). Due to irreversible oxidation or reduction, however, the lack of cathodic or anodic peaks in cyclic voltammograms renders E(1/2) unattainable. Therefore, we evaluate how closely alternative potentials, readily obtainable from irreversible voltammograms, estimate E(0). Our analysis reveals that, when E(1/2) is not available, inflection-point potentials provide the best characterization of redox couples. While peak potentials are the most extensively used descriptor for irreversible systems, they deviate significantly from E(0), especially at high scan rates. Even for partially irreversible systems, when the cathodic peak is not as pronounced as the anodic one, the half-wave potentials still provide the best estimates for E(0). The importance of these findings extends beyond the realm of electrochemistry and impacts fields, such as materials engineering, photonics, cell biology, solar energy engineering and neuroscience, where cyclic voltammetry is a key tool.

Full text of DOI:10.1149/2.0241905jes

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
H3175
JES FOCUS ISSUE ON SEMICONDUCTOR ELECTROCHEMISTRY AND PHOTOELECTROCHEMISTRY IN HONOR OF KRISHNAN RAJESHWAR
Practical Aspects of Cyclic Voltammetry: How to Estimate
Reduction Potentials When Irreversibility Prevails
1
,=
2,=
2
3
1
Eli M. Espinoza, John A. Clark, Joey Soliman, James B. Derr, Maryann Morales,
1
,2,3,4,z
and Valentine I. Vullev
1
Department of Chemistry, University of California, Riverside, California 92521, USA
Department of Bioengineering, University of California, Riverside, California 92521, USA
Department of Biochemistry, University of California, Riverside, California 92521, USA
Materials Science and Engineering Program, University of California, Riverside, California 92521, USA
2
3
4
What is the best approach for estimating standard electrochemical potentials, E(0), from voltammograms that exhibit chemical
irreversibility? The lifetimes of the oxidized or reduced forms of the majority of known redox species are considerably shorter than
the voltammetry acquisition times, resulting in irreversibility and making the answer to this question of outmost importance. Half-
wave potentials, E( , provide the best experimentally obtainable representation of E . Due to irreversible oxidation or reduction,
1/2)
(0)
however, the lack of cathodic or anodic peaks in cyclic voltammograms renders E(1/2) unattainable. Therefore, we evaluate how
closely alternative potentials, readily obtainable from irreversible voltammograms, estimate E . Our analysis reveals that, when
(0)
(
1/2)
E
is not available, inflection-point potentials provide the best characterization of redox couples. While peak potentials are the
(0)
most extensively used descriptor for irreversible systems, they deviate significantly from E , especially at high scan rates. Even for
partially irreversible systems, when the cathodic peak is not as pronounced as the anodic one, the half-wave potentials still provide
the best estimates for E( . The importance of these findings extends beyond the realm of electrochemistry and impacts fields, such
as materials engineering, photonics, cell biology, solar energy engineering and neuroscience, where cyclic voltammetry is a key tool.
0)
©
The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0241905jes]
Manuscript submitted November 15, 2018; revised manuscript received January 14, 2019. Published January 28, 2019. This paper
is part of the JES Focus Issue on Semiconductor Electrochemistry and Photoelectrochemistry in Honor of Krishnan Rajeshwar.
Cyclic voltammetry (CV) is the hallmark of electrochemical anal-
ysis and it impacts on countless fields outside of chemistry, such as
materials science, photonics, cell biology, neuroscience, electrical en-
Chemical conversion, such as decomposition or dimerization, of
the species produced on the electrode surface upon oxidation or re-
duction is the source of the observed chemical irreversibility. Two
principal approaches allow for gaining reversibility: (1) increasing
the scan rates so that the acquisition of the voltammograms is much
faster than the lifetime of the formed oxidized or reduced species;
and (2) lowering the temperature in order to slow down the undesired
chemical conversions. The former approach produces voltammograms
1
–10
gineering and condensed-phase physics.
Voltammograms provide
a wealth of information about the charge-transfer and mass-transport
11–15
processes at the surfaces of the working electrodes.
The evolv-
ing voltammetry theories, based on Butler-Volmer and Marcus-Hush
formalisms, reveal key mechanistic insights about these interfacial
16–20
phenomena.
C
suffering from enormous capacitance currents (i ) that are inherent
(0)
Estimating standard electrochemical potentials (E ) of oxidation
and reduction encompasses one of the most widely spread uses of CV,
which makes this technique popular. The strong correlation of E(0)
with the energy levels of frontier orbitals and band edges defines the
invaluable importance of the electrochemical potentials for charac-
terization of electronic properties of molecular species and materials.
Combining the Butler-Volmer equation with Fick’s second law results
for large scan rates. At scan rates of kV/s and MV/s, faradaic sig-
nals can be orders of magnitude smaller than the background, and
decreasing the area of the working electrode, in order to decrease
C
i , decreases the signal-to-noise ratios. Conversely, lowering the tem-
perature sufficiently enough to attain reversibility limits the number
of usable solvents with acceptable electrochemical windows due to
their freezing points. Indeed, both of these approaches are quite in-
volved, and they have not gained popularity as routine methods for
electrochemical analysis.
in an expression of the faradaic current (i
F
) in terms of the applied
potential (E), where E( is one of the parameters. This expression of
as a function of E, however, is an integral equation and its solution
remain in the realm of numerical analysis, rendering it impractical
0)
i
F
When cyclic voltammograms show irreversible behavior, the most
common practice involves reporting the peak potentials (E( ), i.e.,
p)
(0)
for routine estimations of E from voltammograms. Therefore, half-
a c
anodic potentials (E ) for oxidation or cathodic potentials (E ) for
(
1/2)
wave potentials (E ) have become the preferred representation of
reduction (Figure 1a). Voltammograms of reversible processes, how-
ever, elucidate that the values of the peak potentials deviate quite a bit
from E( , especially for large scan rates.
(
0)
(1/2)
(0)
E , i.e., E
≈ E for reversible oxidation and reduction. The av-
1/2)
a c
erage between the anodic (E ) and the cathodic (E ) potentials define
(
1/2)
Another option involves the use of the edge potentials (E(e)) of the
E
, which are facile to extract from cyclic voltammograms mani-
11
(1/2)
(0)
festing reversibility (Figure 1a). This definition of E
also extends
anodic or cathodic waves as estimates of E of irreversible processes
(Figure 1). At the edges of the voltammogram waves, E( represents
e)
to quasireversible cases (i.e., chemically reversible and electrochem-
ically irreversible) when the rates of interfacial electron transfer are
slower that the rates of mass transport to and away from the electrode
(
e)
the potentials of the first detectable faradaic currents. Thus, E un-
derestimates E( of oxidation and overestimates E of reduction by
0)
(0)
1
1,21
and |E
a
– E
c
| increases with an increase in the scan rate.
a few k T/F (k T is the thermal energy and F is the Faraday constant).
B
B
(0)
(i)
TheconundrumishowtoestimateE fromvoltammograms show-
Conversely, we determined that the potentials, E (Figure 1), at
ing chemically irreversible oxidation and reduction (Figure 1b). An-
the inflection points of the anodic and cathodic waves are quite close
other question is how well E( estimates E in the cases of partial
1/2)
(0)
to E
timating E( of irreversible oxidation and reduction.
Nicewicz et al. proposed the use of the half-peak potentials (E
(1/2)
for reversible processes. Therefore, we employ E for es-
22
(i)
0)
23–27
chemical reversibility. For the rest of this discussion, we refer to
Recently,
(
p/2)
“
chemical reversibility” as “reversibility.”
)
(0)
for estimating E of cyclic voltammograms showing irreversibil-
28
=
z
ity (Figure 1). For symmetrical sigmoid curves progressing along
the ordinate, the inflection points are at their half-heights, but the
These authors contributed equally to this work.
E-mail: vullev@ucr.edu
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3176
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
Figure 1. Examples of cyclic voltammograms showing chemically reversible and irreversible oxidation with designated potentials: Ea = anodic potential, Ec =
cathodic potential, E( = inflection-point potential, E
i)
(p/2)
= half-peak potential, E = edge potential. (a) 1 mM ferrocene. Inset: the initiation of the scan showing
(e)
the rise of the faradaic current, iF, on the background of the capacitance current, iC, and the ohmic current, iO. (b) 1 mM 4Pip. (For both voltammograms: 200 mM
N(C4H9)4PF6 in MeCN, and v = 100 mV s .)
–1
voltammogram waves do not necessary have such a shape leading to
the peaks.
solution (100 mL). The organic layer was collected, dried over
Na SO , and concentrated in vacuo. The product was purified using
flash chromatography (column: 1” internal diameter, packed with
silica gel in hexanes, 6” to 8” height of the packed stationary phase).
The purification (stationary phase: silica gel; eluent gradient: from
2
4
Herein, we review E(1/2), E , E(p/2), E(p), and E(e) extracted from
(i)
cyclic voltammograms for reversible, irreversible and partially re-
29
versible electrochemical oxidation. ANOVA analysis provides sta-
(
i)
(p/2)
(p)
tistical quantification of how close the values of E , E , E , and
100% hexanes to 100% ethyl acetate) afforded 1.01 g (3.17 mmol,
(
e)
(1/2)
(i)
(i)
1
E
are to those of E
for different scan rates. The values of E
79% yield) of a yellow solid of 1. H NMR (600 MHz, CDCl
3
)
(
1/2)
(p/2)
are the closest to those of E , but E
fairs almost as well as E
δ/ppm: 7.34 (1 H, d, J = 2.56 Hz), 7.30 (1 H, d, J = 8.70 Hz), 7.00
(1 H, d, J = 7.68 Hz), 5.43 (1 H, s), 3.27 (4 H, m), 1.79 (2 H, q, J =
7.68 Hz), 1.67 (4 H, m), 1.62 (2H, m), 1.37 (s, 1 H), 0.91 (3 H, t,
in the statistical analysis. A straightforward differential analysis, i.e.,
the first and second derivatives of the cyclic voltammograms, provides
convenient ways for determining peak potentials and inflection points.
In addition, this differential approach proves immensely beneficial for
estimating the reversibility of the electrochemical processes. Regres-
1
3
J = 7.42 Hz). C NMR (150 MHz, CDCl
3
) δ/ppm: 165.87, 152.15,
148.25, 129.42, 122.43, 118.32, 109.84, 54.61, 49.22, 32.87, 26.19,
25.09, 24.00, 8.34. HRMS (m/z, ESI-TOF): calcd. for C17
320.1974 [M + H] ; found, 320.1956.
+
H
26
N
3
O
3
,
(
1/2)
+
sion analysis reveals that even for partially reversible system, E
still provides a good estimate for E .
(0)
N-(tert-pentyl)-4-(piperidin-1-yl)-2-(2-propylpentanamido)
benzamide (4Pip).—510 mg of 1 (1.6 mmol) and dicobalt octacar-
bonyl (1.1 g, 3.2 mmol) were placed in a 100 mL Schlenk tube
equipped with a magnetic stir bar. While purging with argon, 5 mL
of 1,2-dimethoxyethane and 2 drops of DI water were added. While
stirring, the pressure tube was immersed in a temperature-controlled
oil bath. The mixture was heated to 90 C and stirred for an hour. The
reaction mixture was filtered; the filtrate was collected, diluted with
Experimental
Materials and general synthetic considerations.—All reagents
and solvents were purchased from TCI America, Sigma-Aldrich and
Alfa Aesar and used as received. The reaction progress was moni-
tored by the means of thin layer chromatography (TLC), which was
performed with aluminum foil plates, covered with silica gel 60 F254
◦
(
Merck). The products were purified using column chromatography
2
5 mL DCM, and washed with water (100 mL). The organic layer was
1
13
packed with Kieselgel 60 (Merck). All reported H-NMR and C-
NMR spectra were recorded on a 600 MHz spectrometer. Chemical
shifts (δ/ppm) were determined using the solvent peaks as internal
references. High-resolution mass spectra were obtained using electro-
spray MS (ESI-MS).
collected, dried over Na SO , and concentrated in vacuo. The formed
2
4
amine was used for the next step without further purification. While
purging with argon, the resulting organic solid was transferred into a
Schlenk tube using 5 mL dry DCM. The Schlenk tube was immersed
in a dry ice/acetone bath, allowed to cool down, followed by addition
of pivaloyl chloride (500 μL, 4 mmol) and N-methylmorpholine
(750 μL, 6.8 mmol). The reaction mixture was allowed to warm up to
room temperature and was stirred overnight. The solution was diluted
with 25 mL of DCM, and washed with 5% HCL (2 × 100 mL) and
with a brine solution (100 mL). The organic layer was collected,
2
-nitro-N-(tert-pentyl)-4-(piperidin-1-yl)benzamide
Nitro-4-(piperidin-N-yl)-benzoic acid (1.0 g, 4 mmol), prepared
(1).—2-
30
as previously described, was placed in a 100 mL Schlenk tube
equipped with a magnetic stir bar. While purging with argon,
chloro-N,N,N ,N - tetramethylformamidinium hexafluorophosphate
2.0 g, 6 mmol) and 5 mL of dry dichloromethane (DCM) were
added. The Schlenk tube was immersed in a dry ice/acetone bath and
-methylbutan-2-amine (940 μL, 8 mmol) was slowly added to it
ꢀ
ꢀ
2 4
dried over Na SO , and concentrated in vacuo. The product was
(
purified using flash chromatography (column: 1” internal diameter,
was packed with silica gel in hexanes, 6” to 8” height of the packed
stationary phase). The purification (stationary phase: silica gel: eluent
2
followed by the addition of N-methylmorpholine (1.5 mL, 14 mmol).
The reaction mixture was allowed to warm up to room temperature
and was stirred overnight. The solution was diluted with 25 mL of
DCM, and washed with 5% HCL (2 × 100 mL) and with a brine
gradient: from 100% hexanes to 100% ethyl acetate) afforded 435
1
mg, (1.2 mmol, 73% yield) of 4Pip. H NMR (600 MHz, CDCl
3
)
δ/ppm: 11.51 (1 H, s), 8.29 (1 H, s), 7.21 (1 H, m), 6.47 (1 H, s),
5.68 (1 H, s), 3.27 (4 H, s), 1.80 (2 H, q, J = 7.17 Hz), 1.63 (4 H, s),
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
H3177
1
.58 (2 H, s), 1.36 (6 H, s), 1.29 (10 H, s), 0.86 (3 H, t, J = 7.42
connected to a three-electrode cell equipped with a glassy carbon
working electrode, an SCE reference electrode (connected to the cell
via a salt bridge), and a platinum counter electrode, as previously
13
Hz). C NMR (150 MHz, CDCl
3
) δ/ppm: 178.04, 168.99, 153.96,
1
41.31, 127.35, 111.18, 108.33, 106.54, 54.38, 48.94, 40.18, 32.72,
32,33
27.59, 26.52, 25.38, 24.30, 8.39. HRMS (m/z, ESI-TOF): calcd. for
described.
Anhydrous acetonitrile (MeCN) was employed with different concen-
trations of a supporting electrolyte, N(C PF , N(C BF and
LiClO . Prior to recording each voltammogram the sample is ex-
The salt bridge was filled with a saturated KCl solution.
+
+
C
22
H
36
N
3
O
3
, 390.2757 [M + H] ; found, 374.2814.
4
H
9
)
4
6
4
H
9
)
4
4
2
-nitro-N-(tert-pentyl)-5-(piperidin-1-yl)benzamide
(2).—2-
4
Nitro-5-(piperidin-N-yl)-benzoic acid (530 mg, 2.12 mmol), prepared
tensively purged with argon while maintaining its volume constant
by adding more of the anhydrous solvent. For each sample, a set of
voltammograms is recorded where the electrolyte concentration is in-
creased from 25 mM to 200 mM in increments of 25 mM, at scan rates,
v = 10, 20, 50, 100, 200 and 500 mV s . For each sample and at each
of the conditions and the scan settings, a triplicate of triplicates was
measured, and the reported error bars represent plus-minus one stan-
dard deviation. That is, the same sample was measured three times in
three different days, and at each measurement three voltammograms
were recorded.
31
as previously described, was placed in a baked 50 mL round bottom
flask equipped with a magnetic stir bar. While purging with argon,
ꢀ
ꢀ
chloro-N,N,N ,N -tetramethylformamidinium hexafluorophosphate
890 mg, 3.2 mmol) and 1,2-dichloroethane (5 mL) were added,
–1
(
and the reaction was cooled down in a dry ice/acetone bath. While
stirring, 4-heptylamine (380 μL, 2.5 mmol) and 1 mL triethylamine
(
7.2 mmol) were slowly added. The reaction mixture was allowed
◦
to warm up to room temperature and was stirred overnight at 60 C.
The solution was diluted with 25 mL of DCM, and washed with
5
% HCL (2 × 100 mL) and with brine (100 mL). The organic
layer was collected, dried over Na SO , and concentrated in vacuo.
The product was purified using flash chromatography (column, 1
Analysis of the voltammograms and the obtained electrochemical
potentials was carried out using Igor Pro, v. 7.02 (WaveMetrics, Inc.,
Lake Oswego, Oregon, U.S.A.). The half-wave potentials, E , are
2
4
ꢀ
ꢀ
(1/2)
internal diameter, packed with silica gel in hexanes, 6” to 8” height
of the packed stationary phase). The purification (stationary phase:
silica gel; eluent gradient: from 100% hexanes to 100% ethyl acetate)
determined from the midpoints between the cathodic and anodic peak
potentials, E and E , respectively. E and E are determined from
a c a c
the zero points of the first derivatives of the voltammograms, i.e., the
1
afforded 330 mg (0.95 mmol, 45% yield) of a yellow solid of 2. H
potentials where ∂i/∂E = 0 at ∂E/∂t = constant. The inflection point
(
i)
NMR (600 MHz, CDCl
H, dd, J = 9.5, 2.8 Hz), 6.68 (1 H, d, J = 3.1 Hz), 5.39 (1 H, d, J
9.2 Hz), 4.1 (1 H, m), 3.41 (4 H, m), 1.67 (6 H, m), 1.53 (2 H,
3
) δ/ppm: 8.02 (1 H, d, J = 9.2 Hz), 6.77 (1
potentials, E , are determined from the zero points of the second
derivatives at the rising spans of the anodic waves of the voltammo-
2
2
=
grams, i.e., the potentials where ∂ i/∂E = 0 at ∂E/∂t = constant.
1
3
m), 1.43 (6 H, m), 0.94 (6 H, t, J = 7.2 Hz). C NMR (150 MHz,
CDCl ) δ/ppm: 167.43, 153.88, 136.47, 134.04, 127.43, 112.82,
12.16, 49.61, 48.43, 36.91, 25.18, 24.05, 19.03, 14.11. HRMS (m/z,
When the signal-to-noise ratios of the second derivatives are not high
th
3
enough, they are smoothed using 4 order Savitzky-Golay algorithm.
1
Linear fits of the voltammogram sections after the initial capacitance
rise and before the faradaic wave provide the estimates for the base-
lines. Similar linear fits of the anodic waves after the beginning of the
initial faradaic rise and before the curvature leading to the peak yields
the anodic asymptotic lines. The edge potentials, E( , are estimates
from the points where these asymptotes cross the baselines. The peak
heights, p, is determined from the current difference between the an-
odic peak and inclined baseline at the peak potential. The potentials
at the points on the rising anodic wave that corresponds to p/2 provide
the estimates for E( . Functions, built in Igor Pro, were used for the
statistical tests that produced the p-values.
+
+
ESI-TOF): calcd. for C19
48.1938.
H
30
N
3
O
3
, 348.2287 [M + H] ; found,
3
e)
N-(heptan-4-yl)-5-(piperidin-1-yl)-2-(2-propylpentanamido)
benzamide (5Pip).—290 of 2 (0.84 mmol) was suspended in ethyl
acetate with 60 mg Pd/C (10%) in a 50 mL round bottom flask
equipped with a magnetic stir bar. The mixture was stirred overnight
under a hydrogen atmosphere at room temperature. The completion
of the reduction led to a color change from yellow to colorless and
appearance of blue fluorescence, which was monitored using TLC.
The catalyst on the support was filtered out and the ethyl acetate was
removed in vacuo. The solid was resuspended in 1,2-drichloroethane
p/2)
(
5 mL), blanked with continuous flow of Ar and placed in a dry
General Considerations
ice/acetone bath. Concurrently, 2-propylpentanoic acid (160 uL, 1
mmol) was converted to its acyl chloride form by treatment with
oxalyl chloride in a flask immersed in dry ice/acetone bath. The
thus obtained 2-propylpentanoyl chloride was added to the amine
solution dropwise followed by a dropwise addition of triethylamine
(
1/2)
For reversible processes, the CV-obtained values of E
offer ex-
(0)
cellent estimates for E . Irreversibility or partial reversibility, how-
ever, are significantly more prevalent than reversibility, especially
for organic and bioorganic redox couples, and for protic and other
potentially reactive media. Oxidative or reductive degradation, dimer-
ization, reactions with the solvent, relatively fast mass transport and
other processes that deplete the electrochemically produced species
at the surface of the working electrode, strongly affect the recorded
cyclic voltammograms making them “asymmetric” and even com-
pletely eliminating the anodic or the cathodic peak.
(
1 mL, 7.2 mmol). The reaction mixture was allowed to warm up to
◦
room temperature and was stirred overnight at 60 C. The solution
was diluted with 25 mL of DCM, and washed with 5% HCL (2 ×
1
00 mL) and with brine (100 mL). The organic layer was collected,
2 4
dried over Na SO , and concentrated in vacuo. The product was
purified using flash chromatography (column, 1” internal diameter,
was packed with silica gel in hexanes, 6” to 8” height of the packed
stationary phase). The purification (stationary phase: silica gel: eluent
The pressing question at hand is how well E , E(p/2), E(p), and
(
i)
E(e) can serve as estimates for E when E
(0)
(1/2)
is not available? The
gradient: from 100% hexanes to 100% ethyl acetate) afforded 110
shapes of the voltammogramic waves depend on the electron-transfer
kinetics, mass transport dynamics and scan rates, as well as on chem-
ical reactions that deplete the analyte from the surface of the working
electrode. For example, an increase in the scan rate can improve the
reversibility if the lifetimes of the electrocehcmially produces species
are comparable with the time spans between the beginning of the
1
mg (0.25 mmol, 30% yield) of 5Pip. H NMR (600 MHz, CDCl
3
)
δ/ppm: 10.45 (1 H, s), 8.36 (1 H, d, J = 9.2 Hz), 7.02 (2 H, m), 5.0
1 H, J = 9.2 Hz), 4.09 (1 H, m), 3.04 (4 H, m), 2.22 (1 H, dt, J =
.2, 4.6 Hz), 1.71 (4 H, dt, J = 11, 5.8 Hz), 1.63 (2 H, m), 1.53 (4 H,
(
9
13
m), 1.34 (12 H, m), 0.9 (6 H, p, J = 7.2 Hz), 0.86 (6H, m). C NMR
150 MHz, CDCl ) δ/ppm: 174.70, 168.83, 147.45, 131.84, 123.04,
22.77, 120.72, 115.54, 51.91, 49.22, 49.00, 37.50, 35.40, 25.66,
(
1
3
forward-scan wave and the end of the back-scan one. An increase in
the scan rates, however, pushes the peak potentials away from E(
1/2)
.
2
C
3.82, 20.74, 19.20, 14.08, 13.97. HRMS (m/z, ESI-TOF): calcd. for
Therefore, if reversibility is not achieved, increasing the scan rates
+
+
(p)
27
H
46
N
3
O
2
, 444.3590 [M + H] ; found, 444.3692.
can prove detrimental, especially when E provides the metrics for
(0)
E . Concurrently, reversing the direction of the scans at potentials
as soon as possible after the peak of the forward-scan waves can also
improve the reversibility of the cyclic voltammograms. Bringing the
Methods.—CV measurements were conducted using Reference
00 Potentiostat/Galvanostat/ZRA (Gamry Instruments, PA, U.S.A.),
6
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3178
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
potential of the scan reversal too close to the peak, however, can alter
(
p)
its shape and shift E
.
The values of E( are most frequently reported for potentials of
processes displaying irreversible voltammograms. The positons of the
peaks of the voltammogramic waves, however, strongly depend on the
scan rates, the sample concertation, and the kinetics of interfacial elec-
tron transfer especially when side reactions deplete the analyte from
the electrode surface. Hence, E( is not invariant to the experimental
p)
p)
(
1/2)
settings. Inherently, the peak potentials can never overlap with E
,
(0)
(p)
and with E . Therefore, two-dimensional extrapolation of E to zero
scan rate and zero sample concentrations from multiple measurements
is the way to obtain values from the peak potentials that are close to
Chart 1. Analytes used for this study.
(0)
E .
Conversely, E(e) represents the potential at which the Fermi level
The dynamics of mass transport to and from the electrode sur-
face also affects the recorded voltammograms. Agitating the solvent
(e.g., purging with inert gas during data acquisition) and moving the
electrode (e.g., using a rotating disc electrode) improve the efficiency
of bringing fresh analyte to the electrode surface. It also aids the re-
moval of the electrochemically generated species from the electrode
surface and compromise the reversed waves. Therefore, rotating disc
electrodes are invaluable for mechanistic studies of the dynamics of
interfacial processes employing linear voltammetry, rather than CV.
Similarly, using small electrodes (e.g., with area smaller than about
of the working electrode approaches the energy level of the frontier
orbital of the analyte and the interfacial electron transfer becomes
detectable. The edge potentials, therefore, are inherently sensitive to
temperature. An increase in the temperature of the electrode conduc-
tive material broadens the energy distribution of the electrons above
and the holes below the Fermi energy level. Despite it, the values
of E( are inherently invariant to most other experimental settings.
Nevertheless, E cannot overlap with, and even approach, E . Fur-
thermore, from a practical point of view, the capacitance and the
ohmic currents affect the baseline of the voltammograms needed for
e)
(
e)
(1/2)
2
100 μm ) enhances the edge effects, where the mass transport has
estimating E(
e)
.
multiple directions. Decreasing the working surface of an electrode
increases the average mass-transport rates per unit area. Employing
working electrodes with such small dimensions at relatively low scan
Overall, E( overestimates reduction potentials of oxidation and
p)
E(e) underestimates them. Conversely, the values of E and E
(i)
(p/2)
are
always between those of E and E . It is reasonable to hypothesize,
(
e)
(p)
–1
rates, e.g., < 1 V s , leads to cyclic voltammograms where the an-
(
i)
(p/2)
therefore, that E and E
offer the best readily accessible estimates
odic and cathodic waves practically overlap and do not show apparent
for E( when E
0)
(1/2)
is not attainable.
(p)
anodic and cathodic peaks, making it challenging to estimate E
(0)
(1/2)
The standard electrode potential, E , is a fundamental thermo-
dynamic characteristic of a redox couple and as such, its estimated
values should be invariant to the experimental settings. In condense
phase, indeed, the media affects the electronic properties of analytes.
The heterogeneous nature of electrochemical processes presents fur-
ther challenges. How do the electrode material and the composition
of the electrolyte solution near the electrode surface affect the sol-
vation energy of the charged components of a redox couple? While
extrapolation to zero salt concentration can eliminate the effects of the
supporting electrolytes and produce the potentials of the redox cou-
and E . For a working electrode, therefore, we use a glassy carbon
2
electrode with area of 7 mm .
Conducting CV measurements is relatively easy, which makes
this technique as popular as it is. Setting up the reference electrode,
however, can be a potential source for errors. While, for example,
pseudoreference electrodes, such as silver wires, have their role in
34–46
broadening the field,
they inherently have a range of shortcomings
that are beyond the scope of this discussion. Conversely, we employ
saturated calomel electrode (SCE) that, when not in use, we store in
saturated aqueous solution of KCl. To prevent cross-contamination
with the sample, we use a salt bridge for connecting the SCE elec-
trode with the cell. Ideally, the salt bridge should contain the same
solution with which the reference electrode is filled. It ensures that
the electrolyte composition inside the electrode remains unchanged,
and hence, prevents drifts of its potential during extended use.
When the reference electrode is water-based, however, such aque-
ous salt bridges present two practical issues for organic electrochem-
ical analysis: (1) huge junction potentials between the bridge and the
electrolyte solutions of the cell when using low-polarity solvents, such
as CHCl and CH Cl ; and (2) water leakage from the bridge into the
32
ples for neat solvents, it does not compensate for potential “artifacts”
that may originate from certain types of working electrodes.
The material of the working and the counter electrodes should
be inert. Thus, gold and platinum present good choices for such
electrodes, except when the samples contain sulfur at low oxidation
states. Thiols and other sulfur compounds have an immense affin-
ity for coinage metals and may affect the recorded voltammograms.
Concurrently, gold is a soft metal and routine mechanical chelating
may compromise the smoothness of its surface. These challenges
with noble metals, in addition to their cost, have made glassy car-
bon a preferred material for working electrodes for electrochemical
analysis.
3
2
2
cell, which may present challenges when the dryness of the organic
solution is important for the analysis. The use of a salt bridge that con-
tains electrolyte solution in a moderately polar water-miscible organic
solvent, such as acetonitrile and DMF, present a means for addressing
both of those issues. Such a setup splits the huge junction potential
into two moderate ones: at the bridge-electrode and the bridge-cell
interfaces. Also, leaking an aprotic organic solvent with a wide elec-
trochemical window into the sample solution is a better alternative
to leaking water. This setup, however, is not ideal, either. Although
it is slow, the diffusion of the component of the organic solution in
such bridges across the frit of the reference electrode can cause drifts
in the potential during extended use. Therefore, regular monitoring
of the performance of the reference electrode, using samples with
well-known potentials, is paramount for electrochemical analysis.
For analysis of reversible oxidation, we focus on ferrocene
(Chart 1), which is one of the most broadly studied compounds in ana-
lytical electrochemistry and its voltammograms manifest pronounced
The morphology of the electrode surface also affects the recorded
voltammograms, especially when sharp edges and roughness are es-
sential for the nucleation of a new phase. For example, the over-
potential for water reduction on mercury is huge because hydrogen
bubbles cannot nucleate easily on its atomically flat surface. There-
+
fore, mercury cathodes can readily reduce Na to metal sodium from
aqueous solutions to form liquid Na/Hg amalgam. The high solubility
of sodium metal in mercury also helps for driving this process. In ad-
dition, mercury always exposes new pristine surface when forming a
drop. While the Hg susceptibility to oxidation reduces its anodic win-
dow, dropping mercury electrode has been an indispensable choice
for analyzing reduction processes. Its toxicity, however, has dimin-
ished the enthusiasm for its use. The morphology of electrode surface
can control the selectivity of one process over another. Attaching a
catalyst to the surface of an electrode offers the best way for attaining
specificity of an interfacial process and observe thermodynamic limits
of of a redox reaction.
47–50
reversibility.
In voltammetry, therefore, ferrocene has gained pop-
ularity as a convenient internal standard and frequently potentials are
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
reported vs. its oxidation.51–53 As robust as the ferrocene-ferrocenium
H3179
redox couple is, its use for a reference should be approached with
caution. The reduction potential of ferrocenium strongly depends on
32
the solvent polarity and the electrolyte concentration. (According
to the accepted convention, E( represents the reduction potentials
of the oxidized components of redox couples, regardless whether the
voltammograms examine the reduction or oxidation propensity of the
0)
54,55
analyte).
To illustrate CV analysis of irreversible and partially reversible
oxidation, we focus on electron rich anthranilamide residues that we
developed as building blocks for hole-transfer bioinspired molecu-
56–60
lar electrets.
Placing an amine on position 5, such as in 5Pip
(
Chart 1), stabilizes the radical cation and leads to reversible elec-
22,25
trochemical behavior.
Moving the amine to position 4, however,
e.g., 4Pip (Chart 1), causes a positive shift of the anodic wave along
with irreversible behavior.22,25 We demonstrated that eliminating the
proton on the C-terminal amide of 4Pip and replacing it with an alkyl
•+
group stabilizes the radical cation, 4Pip , leading to voltammograms
showing partial reversibility and showing that the amide proton is
22
involved in the oxidative degradation of 4Pip. In this study, we test
if the α-protons on the aliphatic chains attached to the two amides
•+
are responsible for the decomposition of 4Pip . Therefore, we use a
derivative of 4Pip capped with tertiary alkyl substituents (Chart 1).
For a solvent, we employ dry acetonitrile (MeCN) because its mod-
erately high polarity minimizes the dependence of the measured po-
32,33
tentials on the electrolyte concentration, Cel
.
Nevertheless, we still
extrapolate the measured potentials to Cel = 0 (Figure 2), estimating
32,33
(1/2)
their values for the neat solvent.
The extrapolated values of E
for electrolyte-free media relate the electrochemical findings with re-
2
6,32,61
sults from optical studies,
which prove especially invaluable for
2
3,31,62
analyzing systems mediating photoinduced charge transfer.
The best way to validate the extrapolation of potential values to
C
el = 0 is to test the same analyte with different electrolytes in the
same solvent. Furthermore, the extrapolated potentials should be in-
variant to the sample concentration. For three different supporting
electrolytes and ferrocene concentration between 1 and 20 mM, the
(
1/2)
extrapolation to Cel = 0 yields values for E
that are the same
within experimental uncertainty (Figure 2). This finding validates the
(
1/2)
extrapolation approach for estimating E
for neat solvents.
Conversely, the trends of E( vs. Cel reveal two features that are
important for CV experimental designs. First, an increase in ferrocene
concentration increases the standard deviations of the extrapolated
potentials (Figure 2b). That is, increasing the analyte concentration
1/2)
increases the uncertainty of estimating the potentials for neat solvents,
which appears to originate from deviations of the E(
1/2)
values for
small Cel. This finding strongly suggests that the concentration of the
supporting electrolyte should exceed the concentration of the analyte
by at least a factor of 5 or 10.
Second, while lowering the electrolyte concentration converges
E(1/2) to the same value, there is a significant difference between the
potentials for perchlorate and the other two electrolytes (Figure 2a).
Figure 2. Extrapolation of the half-wave potentials of ferrocene to zero elec-
trolyte concentration using different electrolytes and different ferrocene con-
–1
centrations recorded at 50 mV s . (a) Dependence of the half-wave potentials
on the electrolyte concentration. The solid lines represent the data fits and the
dotted lines – extrapolation to zero. (b) Comparison between the half-wave
potentials obtained from the extrapolation to zero electrolyte concentration.
Also, the values of E(
1/2)
for LiClO
and BF salts. While increasing the concentra-
tion of the supporting electrolyte improves the conductance of electro-
chemical cells and decreases the ohmic current (i ), it also changes the
4
show stronger dependence on Cel
than those for the PF
6
4
O
dielectric constant of the solvent (ε). About ten years ago, we demon-
strated that this variation in the dielectric properties of the media is
responsible for the dependence of E(1/2) on Cel for electrolytes with
cannot singlehandedly provide an explanation for the observed trends
(Figure 2a).
32
bulky polarizable ions, such as N(C
4
H
9
)
4
PF
6
and N(C
4
H
9
)
4
BF
4
.
The
Recently, Miller et al. demonstrated the drastic effects that pairing
with the ions of the supporting electrolyte can have on electrochem-
results for LiClO
4
appear to suggest that this salt drastically changes
+
65
–
–
the properties of acetonitrile. Indeed, as a hard Lewis acid, Li can
ical potentials. The anions of the three electrolytes, PF
6
, BF
4
63
–
66
affect the structure of solvents such as acetonitrile. Still, it is un-
likely for the lithium ions of the electrolyte to increase the dielectric
constant of the solution to a large enough extent to induce 100-mV
and ClO
4
, have similar radii and comparable electronic properties.
Conversely, N(C
+ +
4
H
9
)
4
and Li are distinctly different. Unlike
+
+
+
N(C
4
H
9
)
4
, small alkaline ions, such as Li and Na , have a strong
(
1/2)
negative shifts of E
(Figure 2a). Considering the Born solvation
propensity for coordination with oxygen-containing ligands, such
as water. In organic solvents, which cannot provide such ligation,
these cations are often prone to aggregation with the analyte. As
energy, E( is proportional to (1 – ε ).
1/2)
–1 32,64
For polar media, there-
–1
fore, variations in ε will have quite smaller effect on ε (and on the
reduction potentials) than for non-polar solvents.
32,33
67–69
Hence, differ-
an electron-rich compound, ferrocene has nucleophilic properties
and a propensity for binding alkaline ions, especially Li .
70
+ 71–73
ences in the interactions between the electrolyte ions and the solvent
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3180
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
Binding lithium ions adds positive charges to the complexes with
ferrocene and should impede the extraction of electrons. Hence,
LiClO should cause a positive shift in the reduction potential, which
relationship E vs. v has a slope 0. The calculated p-values represent the
probability to obtain the measured potentials if H is correct. While
such analysis does not validate H , it can readily allow rejecting H
0
4
0
0
is contrary to what we observe (Figure 2a).
when p is a small number, i.e., when p < α, and α = 0.01, 0.05 or
While electrochemical potentials correlate with the energy levels
0.1 depending on the selected confidence interval. The immensely
74–81
(0)
(1/2)
(p)
(i)
(p/2)
of the frontier orbitals of the analyte,
FE and FE
of the
small p-values for E , E and E , extracted from voltammograms
oxidation and reduction do not measure the energies, respectively,
of the highest occupied molecular orbitals (HOMOs) and the low-
est unoccupied molecular orbitals (LUMOs). That is, the negative
of samples with different electrolyte concentrations (Table I), clearly
0
show that we can readily reject the H for these potentials. We can
e)
(p)
(i)
(e)
also reject H
E
0
for E( for α = 0.05 or larger. That is, E , E , E and
(
1/2)
(p/2)
E
shifts, induced by LiClO
4
(Figure 2a), do not indicate that the
show dependence on v with 95% confidence (Figure 4a). Exam-
(
1/2)
electrolyte raises the HOMO energy of ferrocene. Rather, FE
mea-
ining the scan-rate dependence of the extrapolated potentials for neat
solvent reveal the same trends (Figure 4b). While the extrapolation
sures energy differences between the oxidized and the reduced forms
of the analyte. Differences between the solvation energies of these
two forms further modulate E( . The change in the analyte charge
during oxidation or reduction is the principal contribution to these
solvation-energy differences.
increases the uncertainty and the p-values, we can still reject the H
0
1/2)
(p)
(i)
(p/2)
for E , E and E
with the same confidence of 95% (Table I).
(
e)
The edge potential, E , appears to show some invariance to v
(Figure 4). Despite the relatively small uncertainty (small standard
deviations) of its estimates, its p-values are not too small, i.e., p >
0.01 for Cel = 100 mM and p > 0.1 for a neat solvent. This finding
indicates that the inception of the faradaic signal does not have a
strong dependence on the scan rate, which is consistent with large
electron-transfer rates and with a relatively large amount of analyte
adsorbed on the surface of the working electrode.
+
Therefore, a loss of a lithium ion during the oxidation of Li -
ferrocene complexes can account for the observed trends (Figure 2a).
+
In addition to an entropic gain, oxidizing a Li -ferrocene complex
to a ferrocenium ion conserves the positive charge on the analyte
and minimizes the changes in the solvation energy, assuming that
–
the excess number of ClO
4
counterions can readily “assimilate” the
+
+
released Li ions. In this manner, Li interaction with ferrocene can
improve the ease of its oxidation and lower the reduction potential
Because of the lack of invariance with v for most of the potentials,
we test how close E( , E , E
p)
(i)
(p/2)
and E are to E
(e)
(1/2)
for each scan
(
Figure 2a).
rates. We employ a Welch’s analysis of variance (ANOVA) where H
states that the means of the compared potentials are the same, and the
0
Overall, the supporting electrolyte is not necessarily a passive
component of the samples. For the rest of the study, we employ
N(C PF , and limit the analyte concentration to 1 mM to en-
sure reliable extrapolation to Cel = 0.
p-values represent the probability of obtaining the observed values
4
H
9
)
4
6
when H
of H , ANOVA does not provide the basis for accepting H
p > α.
For data directly obtained from the measured voltammograms,
0
is correct. While small p-values, i.e., p < α, permit rejection
0
0
when
Results and Discussion
(e)
(p)
e.g., for Cel = 100 mM, we can clearly reject H
0
for E and E
Reversible oxidation.—The first and the second derivatives of
cyclic voltammograms are immensely instrumental for determining
some of the key potentials of the analyte (Figures 3a,3b). The zero
values of the first derivatives, ∂i/∂E = 0, yield the peak potentials,
with 95% confidence (Table II). That is, the peak potentials and the
(
1/2)
(0)
edge potentials are not a good representation of E , and of E for
that matter. For E( , only v = 200 and 500 mV s yield p larger
p/2)
–1
0
than about 0.05 and we can reject H with 95% confidence for most
E
/
a
and E
c
, essential for calculating E( , i.e., E
1/2)
(1/2)
= (E
a
+ E
c
)
of the examined scan rates (Table II). Hence, E
(p/2)
is not a good
representation of E( and of E for small scan rates.
1/2)
(0)
2 (Figure 3b). Also, for oxidation, the peak potentials of interest
(
p)
(i)
correspond to the anodic peaks, i.e., E = E
a
2
. The potentials at
The ANOVA tests for the inflection potential, E , appear most
2
which the second derivatives assume zero, ∂ i/∂E = 0, represent the
inflection points of the voltammograms and provide a straightforward
way for determining E( (Figure 3b).
encouraging. If we adopt a less conservative 90% confidence, we can
–1
perhaps reject H
however, we cannot truly reject H
While ANOVA does not validate the acceptance of H
0
only for 10 and 20 mV s . With 95% confidence,
i)
(i)
0
for any of the scan rates for E .
, these findings
The derivatives also reveal a range of subtle details that are not truly
apparent in the voltammogram. After all, differentiation eliminates the
0
(0)
imply that the inflection potentials provide the best estimates for E ,
when E( cannot be calculated due to irreversibility, for example.
The extrapolation of the different potentials to Cel = 0 adds un-
certainty in their estimates, which affects the outcome of the ANOVA
tests. While the trends appear the same, the p-values for the extrapo-
lated potentials are larger than the p-values for those obtained directly
from the voltammograms, e.g., for Cel = 100 mM (Table II). With
1/2)
C
offset induced by i and yields in a constant value for the small linear
rise due to i (Figures 1, 3). Visual inspection of the derivatives allows
R
for facile examination of the reversibility. If a process is reversible,
the first derivatives of its cyclic voltammograms have axial symmetry
(
1/2)
with the forward and back sweep crossing at E(1/2)
across E = E
(
Figure 3c). In addition, reversibility yields second derivatives that are
centrosymmetric around the point on the ordinate corresponding to
the half-wave potential, i.e., (E( ,0) (Figure 3d). As revealed by their
derivatives, all voltammograms of ferrocene manifest reversibility.
The cyclic voltammograms of irreversible oxidation does not show
cathodic peaks and cannot provide a means for estimating E( from
0
95% confidence we can still reject H for the edge potentials even at
1/2)
(e)
C
el = 0, confirming that E does not provide a good estimate for
(0)
E . We cannot reject H
0
for any of the other cases for Cel = 0, except
perhaps for E( at 500 mV s , assuming 90% confidence (Table II).
p)
–1
1/2)
(i)
Overall for oxidation of ferrocene, the inflection potentials, E ,
provides the best estimates for E (Table II). The half-peak potentials,
, also appear promising for representing E , especially for larger
scan rates. The peak potentials, E , are positively shifted from E
by 30 to 100 mV (Figure 4). With this level of uncertainty, however,
(0)
E
a
and E
c
. Therefore, using the characteristic of the anodic waves of
(
p/2)
(0)
reversibly oxidizable ferrocene provides the ideal means for exam-
E
(
1/2)
(p)
(1/2)
ining which of the potentials have values close to those of E . In
addition to E( and E , which we estimate from the derivatives of
i)
(p)
(
p/2)
(e)
(p)
(1/2)
the voltammograms, we also examined E
and E . The half-height
the peak potentials, E , could be representative of E
but only for
peak potential, E( , corresponds to the potential of the rising anodic
p/2)
(e)
small scan rates. In contrast, for all cases the edge potentials, E
,
underestimate E( by about 70–80 mV for all scan rates (Figure 4),
1/2)
wave that is in the middle between the peak and the baseline (Figures
(0)
1
, 3b). The crossing of the asymptotic lines of the baseline and the
and should not be used for representing E .
(
e)
(1/2)
(e)
rising anodic wave provides the estimates for the edge potentials E
While E
and E are invariant to v, an increase in the scan rate
(
p)
(p/2)
(i)
(
Figures 1, 3b).
causes positive shifts of E , E
and E (Figure 4). These shifts
Because E( is a fundamental characteristic, it is invariant to exper-
0)
(p)
(1/2)
(i)
bring the values of E away from E
and the values of E and
(
p/2)
(1/2)
0)
imental parameters, such as scan rate (v). To examine the invariance
of the different potentials on the scan rates (Figure 4), we resort to a
linear regression analysis with a null hypothesis (H ) that the linear
0
E
closer to E . While at truly small scan rates, peak potentials
(
(p)
can represent E with some uncertainty, the values of E can never
overlap with those of E( . Conversely, an increase in the scan rates
1/2)
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
H3181
Figure 3. Cyclic voltammogram and its derivatives of ferrocene (1 mM), recorded in the presence of 50 mM N(C4H9)4PF6 as a supporting electrolyte at
0 mV s– . (a) The voltammograms and its first and second derivatives where different colors represent the forward and the back scans as designated by the arrows.
1
5
(
(
b) Representation of the voltammogram and its derivatives, zoomed on the anodic and the cathodic waves, with designation of the different potentials of interest.
(
1/2)
c) Demonstration of the axial symmetry of the first derivative. The derivative (upper curve) and its representation inverted around E = E
(lower curve) overlap
perfectly (middle curves). (d) Demonstration of the centrosymmetric shape the second derivative. The derivative (upper curve) and its representation inverted
around E = E
(
1/2)
and around i = 0 (lower curve) overlap perfectly (middle curves).
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3182
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
Figure 4. Dependence of the different potentials of ferrocene (1 mM) on the scan rate. (a) The potentials are obtained from voltammograms recorded for samples
containing 100 mM N(C4H9)4PF6 as a supporting electrolyte. (b) The potentials are obtained from extrapolation to zero electrolyte concentration.
makes E(i) closely overlap with E
(1/2)
for v of 100 mV s–1 and larger
smaller than the anodic peak; and an additional small cathodic peak
at about 0.3 V vs. SCE becomes apparent (insets of Figures 5a, 5b).
The first derivatives of the cyclic voltammograms of 5Pip clearly
show a lack of symmetry for small scan rates, which is consistent with
deviation from reversibility (Figure 6). As the scan rates increase, the
first derivatives gradually gain symmetry and the additional cathodic
(
p/2)
(
Figure 4). The values of E
show a similar trend and an overlap
at 500 mV s . For the examined scan rates, however, the
half-peak potentials show larger underestimates of E
making the inflection potentials the preferred representation of E
(
1/2)
–1
with E
(
1/2)
(i)
than E ,
(0)
(
1/2)
when E
is inaccessible.
–1
peak at 0.3 V vs. SCE becomes less apparent. At 500 mV s , the
oxidation is practically reversible, i.e., the first derivative exhibits
axial symmetry across E( , and the area of the second cathodic peak
drops to less than 5% of the area under the anodic wave.
When partial irreversibility is not obvious.—Ferrocene-
ferrocenium is an extensively studied well-behave redox couple and
serves as an excellent model for testing concepts and ideas. To further
validate and expand the findings from the ferrocene studies, we focus
on an electron-rich amidated non-native amino acid, 5Pip (Chart 1),
the dipole of which can induce significant rectification of charge
1/2)
These findings are consistent with degradation of the oxidized
•+
analyte, 5Pip , within the timescales of acquisition at small scan
rates. The timespans between the beginning of the anodic and the
completion of the cathodic waves range from about 2 s, for 500 mV
31
transfer.
–
1
–1
s , to about 70 s, for 10 mV s (Figure 6c). Hence, the observed
The cyclic voltammograms of 5Pip show excellent reversibil-
ity in anhydrous media, especially for polarizable solvents, such as
We observe similar behavior of 5Pip for this setup with
aqueous salt bridge, but only at high scan rates (Figures 5a, 5b). At
•+
partial reversibility is consistent with a lifetime of 5Pip that is in
the order of 100 s under the conditions of the measurements.
Regardless the partial irreversibility, the presence of anodic and
cathodic peaks allows for estimating the half-wave potentials of 5Pip
for different scan rates and electrolyte concentrations. Extrapolation to
22,25
CH
2
Cl
2
.
–1
scan rates of 10 and 20 mV s , however, the cathodic wave appears
(
1/2)
C
el = 0 yields similar values of E
for the different scan rates (Figure
–1
(1/2)
5
c). Increasing v from 10 to 500 mV s increases E (Cel = 0) from
(1/2)
Table I. p-values from linear regression tests of the dependence of
about 0.73 to 0.75 V vs. SCE (Figure 7). The results for E
at 500 mV
the different potentials on the scan rate.a
s
–1
are the best estimates for E because of the reversibility at this scan
rate. Despite the induced irreversibility, decreasing the scan rates by a
(0)
factor of 50 changes FE( by less than k
1/2)
T. This finding has important
ferrocene
B
5
Pip
implications, demonstrating that even for partially reversible behavior,
0 mM b
0 mM b
(1/2)
(0)
potential
C el =
100 mM
0.14
E
still provides the best estimates for E .
The extrapolated potentials for a neat solvents show that E(1/2),
E (1/2)
0.27
0.014
6.8 × 10
3.3 × 10
0.11
0.054
E(i) and E
(p/2)
of 5Pip practically overlap (Figure 7), and the ANOVA
(
i)
–6
–4
E
E
5.1 × 10
2.5 × 10
1.7 × 10
0.023
4.2 × 10
5.6 × 10
3.7 × 10
0.20
(
1/2)
(i)
(p/2)
(
p/2)
p)
–9
–7
–3
–4
–5
tests yield large p-values when comparing E
with E and E
(
p)
(e)
E (
–3
(Table II). The values of E and E are distinctly separated from
(
1/2)
–1
(
e)
E
(Figure 7), and except for 10 mV s , all p-values from the
E
(
e)
(1/2)
ANOVA test comparing E with E( are smaller than 0.05 (Table II).
p)
(1/2)
aH0: The relation E vs. v has a slope 0. Small p-values permit rejection
of H0 and suggest that E depends on v.
From extrapolation to zero electrolyte concentration.
While the differences between E with E
are similar to those
between E( with E , the ANOVA p-values for E vs. E
e)
(1/2)
(p)
(1/2)
are
b
quite large due to uncertainty of estimating the peak potentials, which
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
Table II. p-values from Welch’s ANOVA tests comparing E(1/2) with the other potentials.a
H3183
sample
Cel / mM
v / mV s–1
(E (1/2), E (i)
)
(E (1/2), E (p/2)
)
(E (1/2), E (p)
)
(E (1/2), E (e)
)
–3
–3
ferrocene
100
10
0.050
0.070
0.19
0.42
0.75
0.40
0.019
0.027
0.043
0.022
0.049
0.39
8.2 × 10
0.013
0.011
0.015
0.017
0.022
1.8 × 10
–3
–3
–3
–3
2
5
0
0
1.9 × 10
3.9 × 10
3.8 × 10
3.6 × 10
0.022
1
2
5
00
00
00
0b
10
0.35
0.61
0.77
0.78
0.91
0.97
0.22
0.43
0.47
0.48
0.57
0.84
0.72
0.73
0.13
0.13
0.10
0.071
0.022
0.041
0.038
0.028
0.029
0.032
2
5
0
0
1
2
5
00
00
00
5Pip
0b
10
0.68
0.91
0.65
0.96
0.94
0.42
0.31
0.33
0.47
0.63
0.99
0.83
0.23
0.081
0.10
0.11
0.076
0.14
0.067
0.031
0.027
0.034
0.021
0.013
2
5
0
0
1
2
5
00
00
00
aH0: E(1/2) and E(x) have the same means, where x = i, p/2, p and e. Small p-values permit rejection of H0 and suggest that E
(1/2)
and E(x) have different
values.
b
From extrapolation to zero electrolyte concentration.
is similar to what we observe for ferrocene (Table II). While the
statistical analysis does not allow the rejection of H , as an indication
acid derivative, 4Pip (Chart 1), forming a radical cation that is unstable
22,25
0
under the CV conditions.
In anthranilamide (Aa) residues, moving the amine from 5 to 4
(
p)
(1/2)
th
th
that E vs. E
are significantly different, the large variations in the
estimates E( (Figures 4b, 7) renders the peak potentials unattractive
p)
position (i.e., 5Pip vs. 4Pip) makes the oxidation of these derivatives
completely irreversible due to a significant change in the spin-density
for quantification of molecular electronic characteristics.
22
distribution in their radical cations. Extending the positive charge of
•+
Aa moieties over their C-terminal amides leads to decomposition. A
loss of anamide protonis a possible pathforthis oxidative degradation.
As we showed, derivatizing 4Pip with a tertiary C-terminal amide,
Irreversible behavior.—For systems undergoing irreversible ox-
idation or reduction, E( (i.e., E
p)
a
or E
c
, respectively) is the most
22
2
i.e., -C(O)N(R )-, induces partial reversibility. It confirms that the
frequent choice for representing their electrochemical potentials. This
practice has a lot of shortcomings since the peak potentials are im-
mensely sensitive to experimental settings (e.g., scan rate, sample
concentration, and electrolyte concentration), and the inherent prop-
erties of the redox couples (e.g., rates of interfacial electron transfer
and diffusion). The studies with ferrocene and 5Pip show that inflec-
•+
C-terminal amide proton contributes to the instability of 4Pip but is
not solely responsible for the oxidative degradation.
Another possibility involves the alkyl α-protons next to the amides.
In all Aa derivatives, these α-protons are deshielded, with chemical
shifts downfield from those of the rest of the alkyl protons, which
may make them labile enough under oxidative conditions. To test
it, in this study we capped 4Pip amides with tertiary alkyl chains
(0)
tion and half-peak potentials provide good estimates of E , better
than the peak potentials. To illustrate how they compare for irre-
versible oxidation, we choose another electron-rich non-native amino
(
Chart 1). For all scan rates and electrolyte concentrations, the cyclic
Figure 5. Cyclic voltammograms of 5Pip (1 mM) recorded at different scan rates in the presence of different concentrations of N(C4H9)4PF6 as a supporting
electrolyte. (a,b) Voltammograms of samples containing 25 and 200 mM electrolyte. (c) Dependence of the half-wave potentials on the electrolyte concentration
for the different scan rates. The solid lines represent the data fits and the dotted lines – the extrapolation to zero electrolyte concentration.
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3184
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
Figure 6. First derivatives of cyclic voltammograms of 5Pip (1 mM) recorded at different scan rates in the presence of 25 and 200 mM of N(C4H9)4PF6 as a
supporting electrolyte. (a,b) the first derivatives. The obtained half-wave potentials are shown. (c) Dependence of the first derivatives on the scanning time showing
the durations of data acquisition for different scan rates.
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
H3185
Figure 7. Dependence of the different potentials of 5Pip (1 mM) on the scan
rate. The potentials are obtained from extrapolation to zero concentration of
N(C4H9)4PF6 (Figure 5c).
voltammograms of the t-alkyl capped 4Pip show complete irreversibil-
ity, with no detectable cathodic peaks. Hence, removal of the α-protons
next to the amides does not stabilize the radical cation of 4Pip for the
timescales of the CV data acquisition.
The first and second derivatives of the cyclic voltammograms of
the irreversible oxidation of 4Pip do not show symmetric features
(
Figure 8a). The derivatives of the anodic wave of 4Pip appear the
same as those of ferrocene and 5Pip. It is, however, the derivatives of
the cathodic waves that discern irreversible from reversible oxidation
(
Figures 3, 6, 8a).
Because the anodic waves of the voltammograms and their deriva-
tives are quite similar for analytes undergoing reversible and irre-
(
i)
(p/2)
versible oxidation, we can reliably use them for estimating E , E
and E( , in addition to E (Figure 8a), and extrapolate them to zero
electrolyte concentration. Similar to 5Pip, the scan-rate dependence
of the extrapolated potentials of 4Pip show clustering of the values for
e)
(p)
(
i)
(p/2)
(e)
(p)
(i)
E
and E . Conversely, E and E are well separated from E
and E( (Figure 8b).
p/2)
–1
(p)
As the scan rates vary from 10 to 500 mV s , E of 4Pip in-
creases with 150 mV, while E( and E
i)
(p/2)
increase with about 80 mV
(
Figure 8b). For 4Pip, this increase in the potentials is predominantly
–1
(i)
at the small scan rates. Hence, between 100 and 500 mV s , E and
(
p/2)
E
of 4Pip vary within about 40 mV (i.e., less than 2k
B
T), while
the E( of 4Pip varies over 100 mV (Figures 8b). Considering that
p)
(
1/2)
(i)
reversible oxidation results in the best overlap of E
E
is the range where E and E
with E and
(
p/2)
–1
at scan rates between 100 and 500 mV s (Figures 4, 7), which
(
i)
(p/2)
of 4Pip show the smallest dependence
on v, makes the inflection and half-peak potentials the best estimate
for E( of the irreversibly behaving systems, 4Pip.
0)
Figure 8. Electrochemical properties of 4Pip. (a) Cyclic voltammogram,
along with its first and second derivative, of 4Pip (1 mM) recorded at 50 mV s
Pros and cons of inflection potentials.—Our studies reveal that
inflection potentials present the best estimates for E(0) from voltam-
mograms showing irreversible behavior. Overall, differential analysis
–1
in the presence of 200 mM N(C4H9)4PF6. The different potentials obtainable
from the anodic wave are designated. (b) Dependence of the different poten-
tials of 4Pip on the scan rate. The potentials are obtained from extrapolation
to zero concentration of N(C H ) PF .
(
i)
(
needed for determining E ) has the power to reveal subtle features
of voltammograms (Figures 3, 6, 8). This property of differential
4
9 4
6
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

H3186
Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
analysis, however, is also responsible for amplifying the inherent ex-
perimental noise. While lowering the analyte concentration is essential
for reliability of the recorded voltammetric data (Figure 2a), it also
decreases the signal-to-noise ratios. To address this challenge, we
subject the first-derivatives curves to smoothing prior to the second
numerical differentiation, as stated in the Experimental section. Lim-
iting the span of shape smoothing to about 25 data points (at 1 mV per
a data-point increment) ensures that the uncertainties in the obtained
values for the potentials at the curve extrema are considerably smaller
References
1
2
. C. Costentin, J.-M. Saveant, and C. Tard, ACS Energy Lett., 3, 695 (2018).
. N. Arshad and S. I. Farooqi, Appl. Biochem. Biotechnol., 186, 1090 (2018).
3. J. B. Derr, J. Tamayo, E. M. Espinoza, J. A. Clark, and V. I. Vullev, Can. J. Chem.,
96, 843 (2018).
4
5
. A. J. Wierzba, A. Wincenciuk, M. Karczewski, V. I. Vullev, and D. Gryko, Chem.
Eur. J., 24, 10344 (2018).
. N. T. Rodeberg, S. G. Sandberg, J. A. Johnson, P. E. M. Phillips, and
R. M. Wightman, ACS Chem. Neurosci., 8, 221 (2017).
6. F. Harnisch and S. Freguia, Chem. Asian J., 7, 466 (2012).
. I. Hwang and K. Yong, ChemElectroChem, 2, 634 (2015).
. S. Gupta, M. R. Chatni, A. L. N. Rao, V. I. Vullev, L. V. Wang, and B. Anvari,
Nanoscale, 5, 1772 (2013).
9. C. E. Nebel, B. Rezek, D. Shin, and H. Watanabe, Physica Status Solidi A, 203, 3273
(2006).
0. N. Armaroli, G. Accorsi, F. Song, A. Palkar, L. Echegoyen, D. Bonifazi, and
F. Diederich, ChemPhysChem, 6, 732 (2005).
1. D. A. C. Brownson et al., “Interpreting Electrochemistry” in The Handbook of
Graphene Electrochemistry, D. A. C. Brownson and C. E. Banks Editors, p. 23,
Springer-Verlag London Ltd. (2014).
2. B. J. Adesokan, X. Quan, A. Evgrafov, A. Heiskanen, A. Boisen, and M. P. Soerensen,
J. Electroanal. Chem., 763, 141 (2016).
3. N. Elgrishi, K. J. Rountree, B. D. McCarthy, E. S. Rountree, T. T. Eisenhart, and
J. L. Dempsey, J. Chem. Educ., 95, 197 (2018).
14. N. V. Rees and R. G. Compton, Chem. Commun., 46, 4238 (2010).
5. D. H. Evans, Chem. Rev., 108, 2113 (2008).
6. J. A. V. Butler, Trans. Faraday Soc., 19, 734 (1924).
7. T. Erdey-Gruz and M. Volmer, Z. Physik. Chem., 150, 203 (1930).
18. R. A. Marcus, Angew. Chem. lnr. Ed. Engl., 32, 1111 (1993).
19. C. Batchelor-McAuley, E. Kaetelhoen, E. O. Barnes, R. G. Compton, E. Laborda,
and A. Molina, ChemistryOpen, 4, 224 (2015).
7
8
that the thermal energy, i.e., << Fk
B
T.
(
i)
The differential nature of the analysis producing E , provides its
most important advantage over E( . The half-peak potential, E
p/2)
(p/2)
,
depends only on three points: the baseline current, the peak current
1
and the potential on the curve at the midpoint between these currents.
Thus, E(
p/2)
cannot account for complex shapes of voltammogramic
1
waves. Conversely, differential analysis can reveal shoulders and over-
lapping waves, producing multiple inflection points. This feature can
prove invaluable for the analysis of: (1) multi-electron processes; and
1
(
2) samples comprising a mixture of conformers or aggregates ex-
1
hibiting close, but different, reduction potentials. For example, when
the potentials of two sequential electron-transfer steps are relatively
close, the voltammogramic waves can appear broad but they cannot
be discerned from those of single-step multi-electron processes.13,15
Increasing the difference between the potentials of sequential reduc-
tion or oxidation steps, first broadens the waves, then leads to the
appearance of a shoulder and finally results in two distinct peaks. Dif-
ferential analysis can prove invaluable for analysis of such complex
shapes and allow for selective evaluation of E( of the first step, for
example.
1
1
1
2
0. E. Laborda, M. C. Henstridge, C. Batchelor-McAuley, and R. G. Compton, Chem.
Soc. Rev., 42, 4894 (2013).
i)
21. R. M. Wightman, M. R. Deakin, P. M. Kovach, W. G. Kuhr, and K. J. Stutts,
J. Electrochem. Soc., 131, 1578 (1984).
2
2
2. E. M. Espinoza, J. M. Larsen, and V. I. Vullev, J. Phys. Chem. Lett., 7, 758 (2016).
3. A. Purc, E. M. Espinoza, R. Nazir, J. J. Romero, K. Skonieczny, A. Je z˙ ewski,
J. M. Larsen, D. T. Gryko, and V. I. Vullev, J. Am. Chem. Soc., 138, 12826
(2016).
In addition to the scan rates and other experimental settings, the
shapes of voltammograms depends on: (1) the rates of interfacial
(0)
electron transfer, characterized with k ; (2) the rates of diffusion
of the reduced and oxidized forms of the analyte, Dred and Dox
3) the charge transfer coefficient, α; and (4) the rates of degrada-
;
24. E. M. Espinoza, J. M. Larsen-Clinton, M. Krzeszewski, N. Darabedian, G. D. T., and
V. I. Vullev, Pure Appl. Chem.,89, 1777 (2017).
(
2
5. J. M. Larsen-Clinton, E. M. Espinoza, M. F. Mayther, J. Clark, C. Tao, D. Bao,
C. M. Larino, M. Wurch, S. Lara, and V. I. Vullev, Phys. Chem. Chem. Phys., 19,
7871 (2017).
tion or other chemical transformation that depletes the electrochem-
ically formed species, as characterized with k, for example. Differ-
(0)
ent k , k, Dox, Dred, and α result in voltammograms with different
26. E. M. Espinoza, J. A. Clark, J. B. Derr, D. Bao, B. Georgieva, F. H. Quina, and
(
p)
11
(e)
(i)
V. I. Vullev, ACS Omega, 3, 12857 (2018).
shapes where E varies the most. While E and E appear to
2
7. H. G. Ryu, M. F. Mayther, J. Tamayo, C. Azarias, E. M. Espinoza, M. Banasiewicz,
L. G. Lukasiewicz, Y. M. Poronik, A. Jezewski, J. Clark, J. B. Derr, K. H. Ahn,
D. T. Gryko, D. Jacquemin, and V. I. Vullev, J. Phys. Chem. C, 122, 13424
(2018).
8. H. G. Roth, N. A. Romero, and D. A. Nicewicz, Synlett, 27, 714 (2016).
9. M. S. Thomas, V. Nu n˜ ez, S. Upadhyayula, E. R. Zielins, D. Bao, J. M. Vasquez,
B. Bahmani, and V. I. Vullev, Langmuir, 26, 9756 (2010).
(
e)
be least susceptible to these multiparameter variations, E inher-
(0)
ently always underestimates and overestimates E of oxidation and
reduction, respectively. Therefore, E( is the closest estimate of E
i)
(0)
2
2
that is easy to obtained from voltammograms manifesting irreversible
behavior.
3
3
3
0. J. M. Larsen, E. M. Espinoza, J. D. Hartman, C.-K. Lin, M. Wurch, P. Maheshwari,
R. K. Kaushal, M. J. Marsella, G. J. O. Beran, and V. I. Vullev, Pure Appl. Chem.,
87, 779 (2015).
Conclusions
1. D. Bao, S. Upadhyayula, J. M. Larsen, B. Xia, B. Georgieva, V. Nunez,
E. M. Espinoza, J. D. Hartman, M. Wurch, A. Chang, C.-K. Lin, J. Larkin, K. Vasquez,
G. J. O. Beran, and V. I. Vullev, J. Am. Chem. Soc., 136, 12966 (2014).
2. D. Bao, B. Millare, W. Xia, B. G. Steyer, A. A. Gerasimenko, A. Ferreira,
A. Contreras, and V. I. Vullev, J. Phys. Chem. A, 113, 1259 (2009).
The half-wave potential, which are readily obtainable from cyclic
voltammograms, are by far the best representation for E(0) even in
cases manifesting partial irreversibility due to slow decomposition of
the produced species. Conversely, all evidence suggest that when E(
is not attainable due to complete chemical irreversibility, the inflection
potentials and the half-peak potentials provide the best estimates for
1/2)
33. D. Bao, S. Ramu, A. Contreras, S. Upadhyayula, J. M. Vasquez, G. Beran, and
V. I. Vullev, J. Phys. Chem. B, 114, 14467 (2010).
3
4. E. T. S. G. da Silva, S. Miserere, L. T. Kubota, and A. Merkoci, Anal. Chem., 86,
0531 (2014).
1
(
0)
–1
E , for moderate scan rates, e.g., 100 – 500 mV s . While in many
3
5. A. A. G. F. Beati, R. M. Reis, R. S. Rocha, and M. R. V. Lanza, Industr. Eng. Chem.
Res., 51, 5367 (2012).
cases E( and E
i)
(p/2)
perform almost the same, the values of E tend to
(i)
be inherently closer to E(
1/2)
(p/2)
36. M. Abdul Aziz, B.-K. Kim, M. Kim, S.-Y. Yang, H.-W. Lee, S. W. Han, Y. I. Kim,
than those of E . This report provides
S. Jon, and H. Yang, Electroanalysis, 23, 2042 (2011).
an important foundation and key evidence for transforming widely
accepted practices of analyzing electrochemical findings in order to
improve and broaden their utility.
3
3
7. G. Jones II, X. Zhou, and V. I. Vullev, Photochem. Photobiol. Sci., 2, 1080 (2003).
8. M. Diaz-Gonzalez, C. Fernandez-Sanchez, and A. Costa-Garcia, Electroanalysis, 14,
665 (2002).
39. G. Jones II and V. I. Vullev, Org. Lett., 4, 4001 (2002).
40. P. Hrncirova and F. Opekar, Sens. Actuat. B, 81, 329 (2002).
41. V. I. Vullev and G. Jones, Tetrahedr. Lett., 43, 8611 (2002).
Acknowledgments
42. D. K. Lee, H. J. Lee, G. S. Cha, H. Nam, and K. J. Paeng, J. Chromatogr. A, 902, 337
2000).
4
(
We extend our gratitude to the U.S.A. National Science Foundation
for supporting this research (grants CHE 1465284 and CHE 1800602).
3. G. Jones II, V. Vullev, E. H. Braswell, and D. Zhu, J. Am. Chem. Soc., 122, 388
(2000).
4. S. Taunier, C. Guery, and J. M. Tarascon, Electrochim. Acta, 44, 3219 (1999).
5. C. D. T. Bratten, P. H. Cobbold, and J. M. Cooper, Anal. Chem., 69, 253 (1997).
6. G. Jones II, L. N. Lu, V. Vullev, D. Gosztola, S. Greenfield, and M. Wasielewski,
Bioorg. Med. Chem. Lett., 5, 2385 (1995).
4
4
4
ORCID
4
7. E. Halls Jonathan, A. Hernan-Gomez, D. Burrows Andrew, and F. Marken, Dalton
Valentine I. Vullev https://orcid.org/0000-0002-3416-9686
Transact., 41, 1475 (2012).
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 166 (5) H3175-H3187 (2019)
H3187
4
4
5
8. Zheng, Evans, and Nelsen, J. Org. Chem., 65, 1793 (2000).
64. M. Born, Z. Phys., 1, 45 (1920).
65. M. J. Bird, T. Iyoda, N. Bonura, J. Bakalis, A. J. Ledbetter, and J. R. Miller,
9. B. Bry and B. Tremillon, J. Electroanal. Chem. Interfac. Electrochem., 46, 71 (1973).
0. S. P. Gubin, S. A. Smirnova, L. I. Denisovich, and A. A. Lubovich, J. Organometal.
Chem., 30, 243 (1971).
1. A. M. Bond, K. B. Oldham, and G. A. Snook, Anal. Chem., 72, 3492 (2000).
2. A. M. Bond, E. A. McLennan, R. S. Stojanovic, and F. G. Thomas, Anal. Chem., 59,
J. Electroanal. Chem., 804, 107 (2017).
66. T. Kakiuchi, J. Noguchi, and M. Senda, J. Electroanal. Chem., 327, 63 (1992).
67. M. Rosenblum, J. O. Santer, and W. G. Howells, J. Am. Chem. Soc., 85, 1450 (1963).
68. A. F. Cunningham Jr., J. Am. Chem. Soc., 113, 4864 (1991).
5
5
2853 (1987).
69. M. J. Mayor-Lopez, J. Weber, B. Mannfors, and A. F. Cunningham Jr.,
5
5
5
5
5
3. R. R. Gagne, C. A. Koval, and G. C. Lisensky, Inorg. Chem., 19, 2854 (1980).
4. S. Trasatti, Pure Appl. Chem., 58, 955 (1986).
5. W. M. Latimer, J. Am. Chem. Soc., 76, 1200 (1954).
6. J. M. Larsen, E. M. Espinoza, and V. I. Vullev, J. Photon. Energy., 5, 055598 (2015).
7. M. K. Ashraf, R. R. Pandey, R. K. Lake, B. Millare, A. A. Gerasimenko, D. Bao, and
V. I. Vullev, Biotechnol. Progr., 25, 915 (2009).
Organometallics, 17, 4983 (1998).
70. A. H. Ilkhechi, J. M. Mercero, I. Silanes, M. Bolte, M. Scheibitz, H.-W. Lerner,
J. M. Ugalde, and M. Wagner, J. Am. Chem. Soc., 127, 10656 (2005).
Rodr ´ı guez-Otero,
E. M.
Cabaleiro-Lago,
´
A.
Pena-Gallego,
˜
and
71. J.
M. Merced Montero-Campillo, Tetrahedron, 65, 2368 (2009).
72. A. Irigoras, J. M. Mercero, I. Silanes, and J. M. Ugalde, J. Am. Chem. Soc., 123, 5040
(2001).
73. M. Scheibitz, R. F. Winter, M. Bolte, H.-W. Lerner, and M. Wagner, Angew. Chem.
Int. Ed., 42, 924 (2003).
5
5
8. V. I. Vullev, J. Phys. Chem. Lett., 2, 503 (2011).
9. B. Xia, D. Bao, S. Upadhyayula, G. Jones, and V. I. Vullev, J. Org. Chem., 78, 1994
(2013).
6
0. S. Upadhyayula, D. Bao, B. Millare, S. S. Sylvia, K. M. M. Habib, K. Ashraf,
A. Ferreira, S. Bishop, R. Bonderer, S. Baqai, X. Jing, M. Penchev, M. Ozkan,
C. S. Ozkan, R. K. Lake, and V. I. Vullev, J. Phys. Chem. B, 115, 9473 (2011).
1. E. M. Espinoza, B. Xia, N. Darabedian, J. M. Larsen, V. Nunez, D. Bao, J. T. Mac,
F. Botero, M. Wurch, F. Zhou, and V. I. Vullev, Eur. J. Org. Chem., 2016, 343 (2016).
2. M. Krzeszewski, E. M. Espinoza, C. Cervinka, J. B. Derr, J. A. Clark, D. Borchardt,
G. J. O. Beran, D. T. Gryko, and V. I. Vullev, Angew. Chem., Int. Ed., 57, 12365
74. N. Grover, M. Sankar, Y. Song, and K. M. Kadish, Inorg. Chem., 55, 584 (2016).
75. M. Boulkroune, L. Ignatovich, V. Muravenko, J. Spura, A. Chibani, and V. Jouikov,
Chem. Heterocyclic Compounds, 49, 1579 (2014).
76. S. Zhu, Q. Song, S. Zhang, and Y. Ding, J. Molecul. Struct., 1035, 224 (2013).
77. T. Fujimoto, M. M. Matsushita, and K. Awaga, J. Phys. Chem. C, 117, 5552 (2013).
78. M. Schmittel, Q. Shu, and M. E. Cinar, Dalton Transact., 41, 6064 (2012).
79. K. R. Edelman and B. J. Holliday, Inorg. Chem., 49, 6787 (2010).
80. A. A. Popov, I. E. Kareev, N. B. Shustova, E. B. Stukalin, S. F. Lebedkin, K. Seppelt,
S. H. Strauss, O. V. Boltalina, and L. Dunsch, J. Am. Chem. Soc., 129, 11551 (2007).
81. G. V. Loukova and V. V. Strelets, Russ. Chem. Bull., 49, 1037 (2000).
6
6
(2018).
6
3. O. Borodin, S.-D. Han, J. S. Daubert, D. M. Seo, S.-H. Yun, and W. A. Henderson,
J. Electrochem. Soc., 162, A501 (2015).
Downloaded on 2019-01-29 to IP 91.200.82.104 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).