Lead ion selective electrodes from dibenzo-18-crown-6 derivatives: An exploratory study

Article abstract of DOI:10.1016/j.molstruc.2020.129575

Dibenzo-18-crown-6 (DB18C6) and three of its derivatives (-COCH₃, -Br, -NO₂), are investigated via Density Functional Theoretical (DFT) modelling, Fourier Transform Infrared (FT-IR) and absorption spectroscopies, Differential Pulse Anodic Stripping (DPASV), Cyclic (CV) and Square Wave (SWV) voltammetries, as possible materials for preparing plasticiser free lead(II) ion selective electrodes. The spontaneous, entropy driven, interactions between lead(II) ions and DB18C6 derivatives are such that they form 1:1 complexes via coordination with the high electron density open ether cavity, except for the brominated derivative where the metal: ligand stoichiometry is 2:1 due to exo-cavity coordination via the high electron density bromine atoms. Monolayers resulting from electropolymerization of some derivatives (-H, -COCH₃, -Br) and chemisorption of the -NO₂ derivative, allows quantification of lead(II) ions at concentrations below 10 mg L^?1 with minimal interference from other metal ions except Hg²⁺ and Al³⁺.

Full text of DOI:10.1016/j.molstruc.2020.129575

Journal Pre-proof
Lead Ion Selective Electrodes from Dibenzo-18-crown-6 derivatives:
An exploratory study
Deneikah T. Jackson , Peter N. Nelson , Irvin N. Booysen
PII:
S0022-2860(20)31889-5
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129575
MOLSTR 129575
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
7 September 2020
31 October 2020
2 November 2020
Please cite this article as: Deneikah T. Jackson , Peter N. Nelson , Irvin N. Booysen , Lead Ion Se-
lective Electrodes from Dibenzo-18-crown-6 derivatives: An exploratory study, Journal of Molecular
Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.129575
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Highlights


Dibenzo-18-crown-6 and its derivatives spontaneously coordinate with lead(II) ions.
All derivatives form stable electropolymerized films or chemisorbed monolayers on
the electrode surface.


Aromatic ring chemistry influences the lead(II) sensing behaviour of all electrodes.
All modified electrodes allow quantification of lead(II) ions below 10 ppm.
1

Lead Ion Selective Electrodes from Dibenzo-18-crown-6 derivatives: An exploratory
study
a
a
b
Deneikah T. Jackson, Peter N. Nelson*, Irvin N. Booysen
a
University of the West Indies Mona, Department of Chemistry, Faculty of Science and
Technology, Kingston 7, St. Andrew, Jamaica.
b
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01,
Scottsville, Pietermaritzburg, 3209, South Africa.
Abstract
Dibenzo-18-crown-6 (DB18C6) and three of its derivatives (-COCH , -Br, -NO ), are
3
2
investigated via Density Functional Theoretical (DFT) modelling, Fourier Transform Infrared
(
FT-IR) and absorption spectroscopies, Differential Pulse Anodic Stripping (DPASV), Cyclic
CV) and Square Wave (SWV) voltammetries, as possible materials for preparing plasticiser
(
free lead(II) ion selective electrodes. The spontaneous, entropy driven, interactions between
lead(II) ions and DB18C6 derivatives are such that they form 1:1 complexes via coordination
with the high electron density open ether cavity, except for the brominated derivative where
the metal: ligand stoichiometry is 2:1 due to exo-cavity coordination via the high electron
density bromine atoms. Monolayers resulting from electropolymerization of some derivatives
(
-H, -COCH , -Br) and chemisorption of the -NO derivative, allows quantification of lead(II)
3
2
-1
ions at concentrations below 10 mg L with minimal interference from other metal ions
2
+
3+
except Hg and Al .
Keywords: Lead(II); Electropolymerization, Dibenzo-crown; Density-Functional-Theory.
Send correspondences to Peter Nelson
e-mail: peter.nelson02@uwimona.edu.jm
Tel: (876) 927 - 1910
2

1
Introduction
Metal ions are environmentally ubiquitous, arising from numerous natural as well as
synthetic/ man-made processes. For instance, increased industrialization, coupled with the
continuous advancement in technology, over the last three decades, have encouraged an
astronomical increase in manning activities, leading to increase exposure of ground water,
flora and fauna to toxic heavy metals such as lead, arsenic, cadmium, mercury, chromium,
copper, chromium and others [1,2,3,4]. The toxicity of such species highlights the need for
highly sensitive methodologies, offering cheap and convenient detection of same. Hence,
compleximetric techniques, applying various ligand systems, have been investigated by
numerous researchers for the colorimetric and electrochemical detection of various heavy
metals [5,6,7,8]. Host-guest complexes of macrocyclic organic molecules such as calixarenes
[
9,10,11,12]. Cryptands [13,14,15,16] and crown ethers [17,18,19,20,21] have been
investigated as possible complexing agents for the detection, quantification and sequestration
of heavy metals. However, though the reports on these systems indicate some level of
success, poor selectivity and low sensitivity remains a challenge. Nonetheless, it has been
demonstrated that the type of coordinating atoms can significantly affect the selectivity of
these systems. For instance, it has been demonstrated that for the detection of lead(II) ion via
compleximetry, sulphur atoms, single or double bonded, allows tremendous selectivity
[
22,23,24,25,25b,26]. The charge density (hardness or softness) of the analyte ion relative to
the donor atom, has also been demonstrated to be a useful parameter for controlling
selectivity [27].
Crown ether type systems such as dibenzo-18-crown-6 (DB18C6) represent an interesting
starting point for development of high selectivity compleximetric reagents because of their
ability to size bind and also to alter their molecular structure to accommodate ions that are
larger than their cavity size [28,29,30,31]. However, though various studies have been carried
out on crown ethers and DB18C6 in particular, modification of the electronic structure by
manipulating the substituents on the phenyl rings remains unrealized. Hence, the effects of
such modification on the suitability of DB18C6 for electrode surface modification and thus
use as an electrochemical sensor, remains unprobed. Furthermore, though an interesting
computational study on the metal binding selectivity of DB18C6 towards alkali metals was
reported by Kim and co-workers [32]. Substituent effects on metal ion selectivity was not
investigated. However, the results presented therein, not only indicated that the high
selectivity for potassium was a result of competition between the solvent and the cavity but
3

also that cheap computational techniques are capable of producing reasonable data on the
metal binding behaviour of these systems.
Hence, in this paper, DB18C6 and three of its derivatives will be investigated for their ability
to form stable complexes with lead(II) ions, both computationally and experimentally.
Furthermore, application of DB18C6 derivatives in the surface modification of electrodes for
the electrochemical quantification of lead(II) ions, as presented in this paper, represent the
use of an experienced method with a novel tool for the sensing of this heavy metal which has
been a major cause for concern in many countries such as the United States of America
[
33,33b,34] and Canada [35], in recent times.
4

2
Experimental
2
.1 Synthesis
All compounds were synthesized as depicted in scheme 1.
Scheme 1. Synthetic scheme for DB18C6 and three of its derivatives.
Dibenzo-18-crown-6 (DB18C6)
To an n-butanol solution of catechol (11.9926 g, 0.1089 mol), containing sodium hydroxide
(4.4943 g, 0.1124 mol), an n-butanol solution of 2, 2-dichlorodiethyl ether (8.0508 g, 0.0563
mol) was added drop-wise with continuous stirring and heating over a 2 hrs. period, prior to
the mixture being cooled to 90 °C and more sodium hydroxide (4.4471 g, 0.1112 mol) added.
The reaction mixture was then refluxed for 16 hrs. after which the liquor was cooled then
5

acidified. Acetone (100 mL) was subsequently added with stirring and the resulting
precipitate collected by vacuum filtration and recrystallized from acetonitrile to give an off
white powder. M.Pt. 163 – 165 °C. Yield of 3.47 %. Success of the synthesis was confirmed
1
13
by H, C NMR and FT-IR measurements (See S1). However, it is worthy of note that other
authors [36,37], whose work was not found during the period of synthesis for this paper, were
able to achieve yields of up to 48 %.
2
0, 25-Diethanonedibenzo-18-crown-6 (DB18C6diacetate)
Anhydrous iron (III) chloride (0.179 g, 0.0011 mol) was combined with acetyl chloride
0.8593 g, 0.0109 mol) in dichloromethane (DCM), in a 50 mL round bottom flask, to which
(
a DCM solution of 0.2 g dibenzo-18-crown-6 (0.0005 mol) was added drop-wise over a 5
min. period. The reaction mixture was stirred at room temperature for 2 hrs., then quenched
with water (5 mL) and allowed to stir for an additional 5 minutes, followed by extraction with
DCM and aqueous sodium hydroxide (50 mL, 0.5 M). The solid precipitate was then
collected via vacuum filtration. M.Pt. 196 - 199 °C. Yield of 42.6 %. Success of the synthesis
1
13
was confirmed by H, C NMR and FT-IR measurements (See S1).
Tetranitrodibenzo-18-crown-6 (TetranitroB18C6)
Concentrated sulphuric acid (5 mL) and nitric acid (5 mL) were both added to a DCM
solution of Dibenzo-18-crown-6 (0.3540 g, 0.0009 mol). The resultant mixture was stirred at
room temperature for 12 hrs., following which it was neutralized with sodium hydroxide
solution (8 M, ~50 mL) and the pale yellow precipitate collected via vacuum filtration. M.Pt.
1
13
2
20 - 224 °C. Yield 100 %. Success of the synthesis was confirmed by H, C NMR and FT-
IR measurements (See S1).
2
0,21,24,25 Tetrabromodibenzo-18-crown-6 (TetrabromoDB18C6)
Liquid bromine (2 mL, 0.0776 mol) was added to a mixture of Dibenzo-18-crown-6, (0.8974
g, 0.0027 mol), glacial acetic acid (30 mL) and water (25 mL), in a round bottom flask, and
6

o
the resulting mixture heated to reflux for 12 hrs. The mother liquor was then cooled to ~28 C
and the resulting white precipitate collected by vacuum filtration and washed with ~20 mL of
diethyl ether. Recrystallization of the precipitate was carried out in chloroform. M.Pt. 222 –
1
13
2
23 °C. Yield 100 %. Success of the synthesis was confirmed by H, C NMR and FT-IR
measurements (See S1).
2
.2 Molecular Modelling
DFT calculations, conducted using a Gaussian 16 software package [38], were carried out at
the 6-311++G(d,p), 6-311++G(2d,p), 6-311G(d,p) levels of theory, where Becke‘s three
parameter hybrid function (B3) [39] was employed for fermion exchange in addition to Lee,
Yang and Parr‘s (LYP) correlation function: B3LYP [40], augmented for long range
interactions via a Coulomb Attenuating Method: CAM-B3LYP [41]. Solution phase
calculations were effected via the Conductor-like Polarisable Continuum Model: C-PCM
[
42,43], where the solute is surrounded by a dielectric of permittivity ε, representing the
solvent. Calculations involving lead(II) were carried out via Las Almos National
Laboratory‘s double zeta Effective Core Potential (ECP): LANL2DZ [44,45].
2
.3 Cyclic voltammetry
All electrochemical studies were carried using a CorrTest CS350 potentiostat/ Galvanostat
workstation, controlled by CS studio Software package. The applied three electrode set-up
was composed of a silver-silver chloride reference whereas platinum was used for the
working and counter electrodes. All measurements were carried out under a static nitrogen
atmosphere.
3
Results and Discussion
3
.1 Molecular Modelling and Infrared Spectroscopy
Minimum structures, calculated at the 6-311++G(d,p)/RB3LYP level of theory, indicated by
the absence of imaginary frequencies in the Hessian, show that all neighbouring methylene
groups of the ether chains adopt a nearly perfect all-trans arrangement (Fig. 1); the most
stable configuration for such groups [46,47,48].
7

Fig. 1. Optimized structures for (a) DB18C6, (b) DB18C6diacetate, (c) tetrabromoDB18C6
and (d) tetranitroDB18C6.
However, though all CH groups are arranged in this low energy configuration, the two ether
2
chains connecting the two phenyl rings, differ considerably in their overall structure. For
instance, the -OCH CH O- fragments, two of which combine to form a single ether chain,
2
2
adopt different configurations (Fig. 2) in both chains of the ether ring; that is, for DB18C6, in
o
one chain the dihedrals are -49.97 and -175.19 whereas for the other chain they are 71.80
o
and 72.29 ; hence, these minimum structures are such that the ether cavity is open (Fig. 1
and Fig. 2). Indeed, similar such angles are calculated for the other derivatives; hence, the
same open cavity structure: cavities as wide as ca. 4.74 Å, are calculated for them. Calculated
methylene C-H distances, in the range of 1.097 - 1.100 Å, are close to that expected for
hydrogens attached to aliphatic carbons [49], however, the slightly shorter distances
calculated here, are most likely caused by the electron withdrawing nature of the oxygen
atoms. Additionally, since the calculated ether C-O distances: 1.422 Å, are typical for such
systems [50], it is clear that the applied modelling chemistry is of sufficient accuracy to allow
rationalization of the data presented herein. However, the C-O bond orders, where the ether
chains attach to the phenyl rings, are 1.5 for all compounds; that is, these bond distances are
in the range of 1.364 - 1.377 Å. This is due to conjugation between the phenyl pi-systems and
8

the n-electrons of the attached oxygen atoms, a clear indication that the electronic structure of
the phenyl ring is influential where electron density inside the ether cavity is concerned.
Fig. 2. Optimized structure depicting structural differences between the ether chains.
Calculated C-C distances for the phenyl rings reveal reduced symmetry. For example, in the
case of DB18C6, the phenyl C-C distance between carbons to which the ether chains are
attached (C42-C43 and C4 –C3, Fig. 2) are ca. 1.409 Å, whereas the other analogous C-C
bond lengths are ca. 1.391 Å. Indeed, similar results were obtained for the other derivatives.
Interestingly, all compounds show reduced symmetry due to elongation of the aromatic C-C
bonds attached to the substituent baring carbons, highlighting the severity of substituent
effects on the molecular structure of these compounds. Expectedly, concomitant with such
variations in bond lengths, divergence between expected and calculated phenyl bond angles
are observed [51]. For example, the internal angles of the phenyl rings vary from 120.49 to
o
1
19.85 , for all compounds. However, since the force on all atoms is zero, it is clear that
angle strain resulting from such variations are cancelled by variations in other regions of the
rings. Surely, such peculiar structural features should be reflected in their spectroscopic
properties
Hence, experimental infrared spectra were collected at ambient temperature in the
solid state, for all compounds. They consist of low intensity bands in the high frequency
-1
region of 3500 - 2800 cm and medium to high intensity bands in the low frequency region
-1
of 1700 - 500 cm (Fig. 3).
9

Fig. 3. Solid state FT-IR spectra for DB18C6diacetate, exhibiting features typical for
dibenzo-18-crown-6 derivatives.
-1
The broad medium intensity band, observed at ~ 3500 cm , characteristic of the –OH
stretching vibration of water molecules, is indicative of hydrated ether cavities. In fact,
hydration of the ether cavity in such systems is not surprising since the oxygen atoms can
facilitate hydrogen bonding, as reported elsewhere [52]. These results suggest that the
samples, prepared herein, are hydrates, hence, the observed divergence between theoretical
and experimentally determined elemental composition (See S2). Furthermore, since the
relative intensities of the OH vibration is not identical for all compounds, different hydration
numbers are indicated; that is, the true composition of the samples are DB18C6.0.5H O,
2
DB18C6diacetate.0.25H O, tetrabromoDB18C6.3H O and tetranitroDB18C6.2H O. This
2
2
2
high frequency region is also characterized by the aromatic carbon-hydrogen symmetric
-1
stretching modes (ힶ C-H), at ca. 3067 cm , as expected [53], with the methylene group
s
asymmetric (ힶas) and symmetric stretching vibrations of the ether rings at ca. 2954 and ca.
-1
2
854 cm , respectively. However, the broad and complex nature of the methylene stretching
band indicates that the CH groups of the ether chains are not equivalent; a proposal
2
supported by the asymmetric nature of the minimize structures (Fig. 1); hence, the ힶ C-H and
s
ힶasC-H modes for the ether chain segments are observed at slightly different frequencies,
leading to band broadening. Calculated infrared spectra, scaled by various factors (See S3),
10

such that the experimental and calculated C=C stretching vibrations coincide, supports the
hypothesis of vibrational non-equivalence among the ether CH population; that is, multiple
2
-1
peaks are calculated in the region of 2850 to 3100 cm , all associated with the symmetric and
asymmetric C-H stretching of different regions of the ether chains, as indicated by the
animated calculated spectra. Since the C=O stretching vibration for DB18C6diacetate is
-1
-1
observed at 1674 cm but calculated, for the isolated molecular, to be 1701 cm , strong
intermolecular interactions, in the solid state, is indicated, hence, the observed high melting
point, relative to DB18C6, for this derivative. Furthermore, the asymmetric nature of both the
calculated and observed ν C=O vibration band suggest non-equivalence of the two acetate
s
substituents on each of the phenyl rings. Indeed, animation of the calculated spectra indicates
-1
that the peak, calculated at 1701 but observed at 1674 cm , is composed of two energetically
similar vibrations, associated with individual C=O groups. Similar effects are also observed
for both the calculated and experimental ν C=C vibration, confirming non-equivalence of the
s
phenyl rings; in all derivatives. For the tetranitro compound, the N=O stretching vibration is
observed and calculated as a broad strong band (Full width at half maximum, FWHM, ~45
-1
cm ). Such band broadening is due to non-degeneracy between the N=O vibrational energy
levels caused by the low overall molecular symmetry. However, over estimation of the N=O
-1
-1
vibrational energy: calculated at ca. 1608 – 1602 cm but observed at 1535 cm , is not only
due to the harmonic nature of the calculations; partially accounted for via the aforementioned
scaling regime, but is mostly due to the calculated N=O bond order; recognized as 1.5 but
calculated as full double bonds. Interestingly, for the tetrabromo derivative, the aromatic C=C
vibrations for each ring are significantly separated; that is, they are observed at 1583 and
-1
-1
1
568 cm but calculated at 1587 and 1581 cm , without scaling. Overall, these results show
that the calculated and experimental spectra are tremendously similar, hence, the scaling
factors are in the region of 0 – 0.996; at testament of the appropriateness of the applied
modelling parameters.
3
.2 Uv-vis spectroscopy
Absorption spectra, collected for all compounds at ambient temperature in
dimethylsulphoxide, shows up to two clear bands. For instance, the spectra for
tetrabromoDB18C6 shows a single broad (FWHM = 25 nm) asymmetric absorption at 298
nm, most likely associated with a combination of the n to π* transitions between the aromatic
rings and the bromine atoms, in addition to the π to π* transitions of the phenyl rings as
11

indicated by the calculated frontier molecular orbital surfaces (Fig. 4). However, for the
tetranitro derivative, these two transitions are observed as individual bands; that is, two broad
absorptions are observed at 275 and 305 nm, due to π to π* transition of the aromatic rings in
addition to n to π* transitions between the aromatic rings and the nitro substituents,
respectively.
Fig. 4. Calculated frontier molecular orbital surfaces (HOMO and LUMO) for all
compounds.
Similarly, for the diacetate derivative, two absorptions are obvious at 305 and 276 nm,
associated with the n to π* transition due to electron density migration from the acetate
oxygen into the conjugated aromatic - acetate π-system in addition to the higher energy π to
π* transitions of the phenyl and C=O pi-systems, respectively. For the unmodified DB18C6,
two transitions are obvious: 277 and 306 nm, emanating from the inter-aromatic ring π to π*
charge transfer and the lower energy n to π* transitions between the ether oxygens and the
aromatic π-system. Interestingly, for all compounds, the HOMO to LUMO transition is
associated with charge transfer between the aromatic rings; that is, from one side of the
molecule to the other (Fig. 4), indicating that their structural asymmetry is also reflected in
their electronic structure. Indeed, electrostatic potential maps (ESPM) confirm their highly
12

uneven charge distribution; that is, the cavity of the either ring maintains relatively high
electron density, in all cases (Fig. 5), except for the tetranitro derivative where the electron
withdrawing effect of the nitro groups cause relatively low electron density in the ether
cavity.
Fig. 5. Electrostatic potential maps for (a) DB18C6, (b) DB18C6diacetate, (c) tetrabromo
DB18C6, (d) tetranitroDB18C6.
These results indicate that lead(II) ions are most likely to coordinate with the ether cavity of
all systems except for the tetranitro derivative, where coordination is more probable at the
nitro groups. The high electron density of the carbonyl oxygens, relative to the ether cavity of
the diacetate compound (Fig. 5(b)), suggest possible exo-cavity coordination. However, the
―well-like‖ shape of the ether cavity should facilitate the formation of a more stable complex
2
+
with Pb relative to that possible due to coordination at the C=O moieties. Hence, endo-
cavity coordination is expected to be most probable. Similarly, high electron density on the
bromine atoms, of the tetrabromo derivative, suggest that there could be exo-cavity metal
binding or even a combination of exo- and endo- cavity binding. However, for a more
2
+
comprehensive understanding of the Pb binding behaviour of these systems, additional data
is required.
13

Therefore, lead(II) nitrate was added in a step-wise manner to solutions of all
compounds, whilst collecting the absorption spectra of the resulting solutions (uv-vis
titration). Hence, a plot of absorbance versus metal: ligand ratio (mole ratio plot) was derived
in all cases (See S4), revealing a 2:1 metal: ligand binding ratio for the tetrabromo derivative
but a 1:1 ratio for the other compounds (Table 1).
Table 1. Lead(II) complexation thermodynamic parameters for DB18C6 derivatives at 298 K.
o
ΔG /
ΔH /
ΔS /R
as
as
as
Compound
Ratio
Log Kas
-1
-1
(
kJ mol )
R (K )
DB18C6
Tetrabromo-
Tetranitro-
Diacetate-
1:1
2:1
1:1
1:1
3.56 ± 0.01
5.17 ± 0.08
2.79 ± 0.06
3.84 ± 0.03
-20.32
-29.49
-16.02
-21.89
18462.5 ± 610.8
25777.4 ± 2922.2
3648.6 ± 313.5
70.1 ± 2.1
98.8 ± 9.8
18.6 ± 1.1
80.8 ± 10.7
21487.5 ± 3198.0
From the mole ratio plots, Hill plots were derived for all compounds (See S4), allowing
2+
determination of their association constants (K ) with Pb and hence, the free energy change
as
o
as
(
ΔG ) associated with such interactions (Table 1). Since all complexation potentials are
exergonic; spontaneous binding of lead(II) ions with all derivatives is indicated. Expectedly
however, the least negative value is calculated for the tetranitro derivative, indicating weak
metal-ligand interactions relative to the other compounds. This is ascribed to complexation
occurring via coordination at the nitro groups which are weakly basic; that is, poorly electron
donating. Interestingly, whereas the lead(II) complexation potentials for DB18C6 and the
2+
diacetate derivative are comparable, formation of the Pb -tetrabromo DB18C6 complex is
2+
most probable (Table 1). Optimization of Pb -DB18C6 complex in acetonitrile, using the 6-
2
+
3
11++G(d,p)/RB3LYP model chemistry, for all atoms except Pb , where the LANL2DZ
o
ECP was applied, allowed calculation of the complexation potential via ΔG as = Gcom – G –
g
2+
G where G , G and G , represent the absolute free energies of the complex, Pb (guest)
h
com
g
h
-1
and the DB18C6 (host), respectively. The calculated association potential: -140 kJ mol ,
though attractive (exergonic), is significantly less than that adduced experimentally (Table
14

1
). Interestingly, this value is similar to that reported for the complexation of alkali metals:
+
+
Li to Cs , with DB18C6 [32]. However, despite exploration of several other model
chemistries: 6-311G(d,p)/CAM-RB3LYP, 6-311++G(2d,p)/CAM-RB3LYP, 6-
1++G(d,p)/CAM-RB3LYP, no improvement in this value was achieved. Such under
3
estimation of the complexation potential is most likely due to the absence of direct solvent
coordination effects, a factor well known to be highly influential in host-guest interactions
[
54]. Furthermore, since the guest is charged, stabilization from solvation effects are most
likely significant.
The effect of temperature on the Kas values was also investigated via variable
temperature uv-vis titrations and the resulting values applied via Van ‗t Hoff plots (See S5),
in calculating the association enthalpy (ΔH ) and entropy (ΔS ) changes for all compounds.
as
as
Unexpectedly, positive entropy values are observed for all systems (Table 1), suggesting that
solvent rearrangement, as the separately hydrated host and guess species combine, plays a
2
+
critical role in the complexation thermodynamics and hence, Pb complexation probability
of these ether systems. Hence, greater solvent rearrangement for the tetrabromo complexes
where two metal ions are involved, leads to significant disorder in the solvent, hence, ΔS is
as
most favourable (Table 1). That the formation of all complexes is endothermic, yet
2
+
exergonic, the dominance of entropy in determining their Pb absorption behaviour is
highlighted. For the tetranitro derivative, coordination at the nitro groups probably requires
minimal solvent rearrangement, hence, ΔS is least positive for this compound, thus, its
as
formation is less probable.
3
.3 Electrochemical behaviour
Cyclic and square wave voltammograms, collected for all compounds in acetonitrile (HPLC
grade ≥ 99.9), show at least one anodic peak in the region of 1.4 to 2.2 V for both oxidatively
(O) and reductively (R) initiated scans (Fig. 6). For instance, the cyclic voltammogram for
DB18C6 shows a single oxidation at ca. 1.65 V with no corresponding reduction wave, even
at very high or low scan rates (ν), indicating that this electron transfer (E) is followed by a
very fast chemical process(C): EC mechanism, hence, this oxidation is irreversible.
1
/2
2
Furthermore, since a plot of current (I) versus ν is linear (R = 0.98, See S6), it is
reasonable to conclude that the processes associated with this oxidation are diffusion
controlled, as suggested by the appropriate form of the popular Randles-Ševčík model
[
55,56]. Additionally, since the square wave voltammogram reveal a peak area which is
15

basically identical to that for ferrocene, under similar conditions, it can be reasonably
concluded that the charge involved in this process is similar to that associated with the
oxidation of ferrocene; a well-known one electron process [57].
Fig. 6. Oxidatively initiated cyclic voltammograms for all compounds [Conc. 1 mM].
Calculated ground state MO surfaces (Fig. 4), for DB18C6, show that the HOMO is localized
over the pi-system of a single phenyl ring, in addition to the oxygen atoms which are directly
attached to that ring. Hence, reaction with a solvent molecule subsequent to the formation of
a radical: EC mechanism, is most probable at the phenyl ring or at the directly attached ether
oxygens. The presence of an acetate group on the phenyl rings result in an additional one
electron diffusion controlled, irreversible oxidation at 1.91 V (Fig. 6), due to the loss of an n-
electron from the acetate oxygen, as suggested by the calculated MO surfaces. Similarly, for
the tetrabromo derivative, two oxidations are obvious: 2.03 and 1.87 V (Fig. 6), due to the
loss of three electrons, most likely from the aromatic rings, since the n-electrons of the high
electron density halogen atoms tend to conjugate with the aromatic pi-system, thereby
increasing the electron density therein. In addition to a single diffusion controlled irreversible
oxidation at 2.21 V, associated with electron loss from the aromatic pi-system of a single
phenyl ring, the voltammogram for the tetranitro derivative is characterized by two
successive quazi-reversible one electron reductions at: -0.87 and -1.49 V; for which the
corresponding oxidations are observed at -0.98 and -0.74 V, respectively, due to the redox
activity of the NO group, as is typical for o-dinitrobenzene compounds [58]. These results
2
16

indicate that the redox behaviour of DB18C6 derivatives is highly dependent of the chemistry
of the aromatic rings, effects which might affect their redox behaviour in the presence of
lead(II) ions.
Hence, electropolymerization was attempted, for all compounds, via repeated CV
scans (cycling). The repetitive voltammetric scans show significant changes on each cycle,
for all compounds, except for the tetranitro derivative. For example, the oxidation, originally
observed at 1.65 V for DB18C6, as discussed in the foregoing, shows significant reduction in
current after the first cycle (Fig. 7); subsequent cycles resulting in a continuous and uniform
current decay, concomitant with a shift to lower energy: 1.4 V.
Fig. 7. Repeated cyclic voltammetry scans (30 scans at ν = 200 mV/ s) used to effect
polymerization of DB18C6 [Conc. 1 mM in acetonitrile].
Interestingly, these changes are simultaneous with the emergence and progressive growth of
two reductions at ca. -1.40 and ca. -0.99 V (Fig. 7); the emergence of which is indicative of
the formation of a new electro active material, increasing in concentration to a point of
saturation whilst the parent material: DB18C6 in this case, is depleted at the electrode surface
due to its progressing electro-transformation. Such changes are consistent with
electropolymerization [59]. For the other derivatives, similar changes, albeit more subtle, are
obvious (See S7). The voltammogram for the tetrabromo derivative, for instance, shows two
new reduction bands: -1.36 and -0.94 V, in addition to a new oxidation at 0.39 V, which
shows saturation after ~6 to 10 cycles. Similarly, polymerization of the diacetate derivative
on the electrode surface is suggested by significant reduction in the intensities of both
17

oxidation bands, originally observed at 1.65 and 1.91 V (vide supra) and their replacement by
two new oxidations at 1.56 and 1.79 V (See S7). However, for tetranitro derivative, though
there are slight changes in the position of the original peaks, no new peaks are observed (See
S7), nonetheless, the slight current decay for the original peaks, on each cycle, is indicative of
progressive electrode surface passivation. These observations indicate that instead of
polymerization, the tetranitro derivative probably forms a stable chemisorbed coating on the
electrode surface. Surely, such differences in the nature of the electrode surface passivation
2
+
could significantly affect its response to the presence of Pb , hence, electrode surface
coverage (Γ) for each modified electrode is investigated based on
ꢃ
ꢃ
ꢂ ꢄ AΓ(ν)
ꢁ
ꢀ
ꢅꢆꢇ
where, Ip, n and F represents the peak current, the number of electrons transferred on
oxidation/ reduction of the analyte (Analyte = ferrocene where n = 1) and Faraday‘s constant,
respectively. A is the electrode surface area whereas R and T represent the universal gas
constant and the temperature. For all compounds, plots of Ip versus ν are linear, as expected
-11
-12
-2
(
R2 ~ 0.99 – 0.97), and the calculated Γ values are in the range of 10 – 10 mol cm ,
-11
similar to values reported for monolayer coverage [60,61]. Since a Γ value of 1.22 x 10 mol
-2
cm was determined experimentally for the tetranitro derivative whereas, that calculated
-10
-2
based on the area of the optimized structure is 2.169 x 10 mol cm , it is clear that the area
of this compound, when adsorbed to the electrode surface, is larger than that estimated from
molecular modelling (vide supra). This is most likely due to increase planarity of the
structure on the electrode surface caused by strong interactions between the electrode surface
and the nitro groups; a surface driven shift in the equilibrium molecular geometry. For the
-12
other compounds, experimentally determined Γ values are of the order 10 , also indicating
significant differences in molecular orientation on the electrode surface. Indeed, this is
expected since, the majority of evidence suggest that they undergo electro-polymerization,
resulting in an immobilized polymeric coating on the electrode surface. Surely, such
modification of the electrode surface should affect its ability to interact with lead(II) ions.
Furthermore, the structural difference in these coating materials should be reflected in the
2
+
behaviour of these electrodes, where the quantification of Pb is concerned.
18

Therefore, differential pulse anodic stripping voltammetric (DPASV) studies, a very
experienced and sensitive method, was carried out to assess the usefulness of these modified
2
+
electrodes for the quantification of Pb in aqueous solution. DPASV scans with a DB18C6-
functionalized platinum working electrode show a well-defined peak at ca. -0.72 V in the
2
+
presence of lead(II) ions (Fig. 8(a)), whose intensity (current) increases linearly with Pb
concentration (Fig. 8(b)), as opposed to the bare electrode where a very asymmetric band is
observed at ca. 0.79 V, in addition to another broad band at ca. 0.46 V; none of which show a
2
+
clear trend with Pb concentration.
2+
2+
Fig. 8. (a) DPASV scan for a solution will [Pb ] = 0 (dashed line) and [Pb ] = 1 mM (solid
line). (b) Calibration curve derived from the DPASV results for DB18C6. The pH was
maintained by an acetate-acetic acid buffer system.
2
+
Therefore, current response as function of [Pb ] was measured using all modified electrodes
See S8) at a pH value: 4.5, where current response is maximum. The linear regression
method was then applied in determining the limits of detection (LOD) and quantification
LOQ) as well as the linear ranges (LR) for all modified electrodes (Table 2). Interestingly,
(
(
the DB18C6 electrode offers the lowest LOQ and LOD, followed by the tetrabromo
derivative which also offers a slightly wider linear range than the other electrodes. The wider
linear range observed for the tetrabromo DB18C6 electrode might be a result of its ability to
2
+
interact with Pb in 2:1 metal: ligand ratio. Expectedly, the electron withdrawing nature of
the nitro groups which prevent interactions between lead ions and the ether cavity, in addition
to the weak metal-nitro group interactions, not only result in one of the narrowest linear
ranges but also the highest LOD and LOQ (Table 2), clear indications of its inferior
sensitivity.
19

Table 2. Linear range, limits of detection and quantification adduced from the calibration
2
+
curves of current versus [Pb ] for the different functionalized electrodes.
LR/
LOD/
LOQ/
Compound
-1
-1
-1
mg L
mg L
mg L
DB18C6
Diacetate
Tetrabromo
Tetranitro
8.49 – 56.60
5.68 – 59.14
2.85 – 59.14
2.85 – 54.05
3.06
5.27
9.27
15.97
13.38
33.65
4.56
11.11
These results clearly show that the compounds investigated in this study holds real potential
for application in the preparation of modified electrodes for the electrochemical detection and
quantification of lead(II) ions in aqueous solution. However, ―real-life‖ samples are often
contaminated with other electrolytes which can significantly impact the results adduced from
such electrodes. Therefore, DPASV scans in the presence of a mixture of various other metals
ions were collected at ambient temperature, for all electrodes: interference study. The
presence of other metal ions resulted in slight potential shifts of the lead peak, however, since
2
+
current is the parameter applied in the quantification of Pb , such slight changes in peak
potential are inconsequential. Changes in the current on addition of each metal ion to the
mixture are obvious (Fig. 9), however, all changes are such that the current values are of the
2+
2+
same order of magnitude as that for Pb in pure water, hence, the Pb concentration
adduced, in most cases, will not be significantly affected. Though the presence of other metal
-5
-5
ions leads to current fluctuation between 3.18 x 10 and 2.59 x 10 : minor changes, the
presence on mercury(II) ions leads to significant changes (increase) in the current for
DB18C6, DB18C6diacetate and the tetranitroDB18C6 electrodes (Fig. 9(a)). This indicates
that these systems would suffer from a mercury error. Such effects might be due to significant
2
+
2+
coordination competition between Hg and Pb , resulting from entropic effects associated
with solvent rearrangement energetics in the complexation process, in addition to the higher
2
+
charge density of Hg which allows stronger interactions with the hard donor sites of these
ether systems.
20

Fig. 9. Effects of presence of other metal ions [Conc. 10 ppm] on the current response
obtained for lead [Conc. 50 ppm] when the electrode surface is modified with (a) DB18C6,
(b) DB18C6diacetate, (c) TetrabromoDB18C6 and (d) TetranitroDB18C6. Solutions where
buffered by sodium acetate-acetic acid mixture to maintain a pH of 4.5.
2
+
Interestingly, for electrodes prepared from the tetrabromo derivative, addition of Hg ions
3
+
does not cause a significant change in the current, however, the presence of Al result in a
noteworthy current spike (Fig. 9 (c), albeit without a change in the order of magnitude of the
2
+
3+
signal. This is indicative of significant coordination competition between Pb and Al , most
3
+
likely due to the significantly higher charge density of Al , coupled with the entropic effects
associated with solvent rearrangement during complexation.
4
Conclusion
Dibenzo-18-crown-6 and its diacetate, tetrabromo and tetranitro derivatives are able to
spontaneously coordinate with lead(II) ions in their free monomeric form; a process
controlled by entropic effects. Their ability to passivate a platinum electrode surface, via
electropolymerization or chemisorption in the case of the tetranitro derivative, allows the
preparation of modified electrodes, capable of quantifying lead(II) ion at concentrations
-1
3+
below 10 mg L with minimal errors in the presence of other metal ions, except for Al and
21

2
+
3+
Hg which offers significant coordination competition to lead ions. Hence, where Al and
2
+
Hg concentrations are significant, the results adduced from these modified electrodes will
require correction. Nonetheless, they represent real prospects for fast and relatively cheap
2
+
designing of plasticizer free Pb ion selective electrodes for operation in aqueous media.
Acknowledgement
National Commission on Science and Technology, Jamaica for research funds provided to
this research group.
Credit author statement
I hereby declare that the work presented in this manuscript was conducted solely by the three
authors presented in the list of authors on the cover of the paper. The lead researcher Ms
Deneikah Jackson who is a graduate student was responsible for collecting and analysing
experimental data. She was also instrumental in preparing the manuscript. This work was
carried out under the supervision of Peter Nattaniel Nelson who is the corresponding author.
He was responsible for designing the project, analysing data, guiding the experimental
process and preparing the manuscript for publication. The third author, Irvin Booysen was
instrumental in analysing data, guiding the experimental process and helping to prepare the
final manuscript.
The authors can, in all honesty, state that the work presented therein is original and does not
copy from any other articles, written by any other authors, in full or part.
Declaration of interest statement
We, the authors of this manuscript, declare with absolute certainty that there is no conflict of
interest concerning the work in this manuscript. The work presented therein is solely for
academic purposes and was conducted by the authors listed on this manuscript.
Reference
[
1]
Gäbler, H. E.; Schneider, J., Assessment of heavy-metal contamination of floodplain
soils due to mining and mineral processing in the Harz Mountains, Germany. Environmental
Geology 2000, 39 (7), 774-782.
[
2]
Dhakate, R.; Singh, V. S.; Hodlur, G. K., Impact assessment of chromite mining on
groundwater through simulation modeling study in Sukinda chromite mining area, Orissa,
India. Journal of Hazardous Materials 2008, 160 (2), 535-547.
[
3]
Garcia-Sanchez, A.; Alvarez-Ayuso, E., Arsenic in soils and waters and its relation to
geology and mining activities (Salamanca Province, Spain). Journal of Geochemical
Exploration 2003, 80 (1), 69-79.
22

[
4]
Coynel, A.; Schäfer, J.; Dabrin, A.; Girardot, N.; Blanc, G., Groundwater
contributions to metal transport in a small river affected by mining and smelting waste. Water
Research 2007, 41 (15), 3420-3428.
[
5]
by spectrophotometry of its dithizone complex. Microchimica Acta 1994, 113 (1), 53-59.
6] Tsunogai, S.; Nishimura, M.; Nakaya, S., Complexometric titration of calcium in the
presence of larger amounts of magnesium. Talanta 1968, 15 (4), 385-390.
7] Zhai, J.; Xie, X.; Bakker, E., Ionophore-based ion-exchange emulsions as novel class
of complexometric titration reagents. Chemical Communications 2014, 50 (84), 12659-
2661.
8]
I: A comparison of direct methods. Talanta 1963, 10 (11), 1177-1182.
9] Leray, I.; Valeur, B., Calixarene-Based Fluorescent Molecular Sensors for Toxic
Metals. European Journal of Inorganic Chemistry 2009, 2009 (24), 3525-3535.
10] Adhikari, B. B.; To, C.-A.; Iwasawa, T.; Schramm, M. P., Pb, Sr and Ba calix[6]arene
Théraulaz, F.; Thomas, O. P., Complexometric determination of mercury(II) in waters
[
[
1
[
Lyle, S. J.; Rahman, M. M., Complexometric titration of yttrium and the lanthanons—
[
[
hexacarboxylic acid octahedral complexation: a dramatic effect of dealkylation. Supramol
Chem 2015, 27 (10), 724-730.
[
11] Talanova, G. G.; Elkarim, N. S. A.; Talanov, V. S.; Bartsch, R. A., A Calixarene-
Based Fluorogenic Reagent for Selective Mercury(II) Recognition. Analytical Chemistry
999, 71 (15), 3106-3109.
12] Métivier, R.; Leray, I.; Valeur, B., Lead and Mercury Sensing by Calixarene-Based
1
[
Fluoroionophores Bearing Two or Four Dansyl Fluorophores. Chemistry – A European
Journal 2004, 10 (18), 4480-4490.
[
13] Ghosh, P.; Bharadwaj, P. K.; Roy, J.; Ghosh, S., Transition Metal (II)/(III), Eu(III),
and Tb(III) Ions Induced Molecular Photonic OR Gates Using Trianthryl Cryptands of
Varying Cavity Dimension. Journal of the American Chemical Society 1997, 119 (49),
1
[
1903-11909.
14] Muthukumar, C.; Kesarkar, S. D.; Srivastava, D. N., Conductometric mercury [II]
sensor based on polyaniline–cryptand-222 hybrid. Journal of Electroanalytical Chemistry
007, 602 (2), 172-180.
15] Pouretedal, H. R.; Shamsipur, M., A PVC-based cryptand C2B22 membrane
potentiometric sensor for zinc(II). Fresenius' Journal of Analytical Chemistry 1998, 362 (4),
15-418.
16] Sadhu, K. K.; Sen, S.; Bharadwaj, P. K., Cryptand derived fluorescence signaling
systems for sensing Hg(ii) ion: A comparative study. Dalton Transactions 2011, 40 (3), 726-
34.
17] Gokel, G. W.; Leevy, W. M.; Weber, M. E., Crown Ethers:ꢀ Sensors for Ions and
Molecular Scaffolds for Materials and Biological Models. Chemical Reviews 2004, 104 (5),
723-2750.
18] Suzuki, Y.; Morozumi, T.; Nakamura, H.; Shimomura, M.; Hayashita, T.; Bartsh, R.
2
[
4
[
7
[
2
[
A., New Fluorimetric Alkali and Alkaline Earth Metal Cation Sensors Based on Noncyclic
Crown Ethers by Means of Intramolecular Excimer Formation of Pyrene. The Journal of
Physical Chemistry B 1998, 102 (40), 7910-7917.
[
19] Maeda, H.; Tierney, D. L.; Mariano, P. S.; Banerjee, M.; Cho, D. W.; Yoon, U. C.,
Lariat-crown ether based fluorescence sensors for heavy metal ions. Tetrahedron 2008, 64
(22), 5268-5278.
[
20] Hayashita, T.; Teramae, N.; Kuboyama, T.; Nakamura, S.; Yamamoto, H.; Nakamura,
H., Chromoionophores Based on Crown Ethers and Related Structures for Alkali Metal Ion
Sensing in Aqueous Media. Journal of inclusion phenomena and molecular recognition in
chemistry 1998, 32 (2), 251-265.
23

[
21] Xie, J.; Ménand, M.; Maisonneuve, S.; Métivier, R., Synthesis of Bispyrenyl Sugar-
Aza-Crown Ethers as New Fluorescent Molecular Sensors for Cu(II). The Journal of Organic
Chemistry 2007, 72 (16), 5980-5985.
[
22] Huang, M.; Ma, X.; Li, X., Macrocyclic compound as ionophores in lead(II) ion-
selective electrodes with excellent response characteristics. Chinese Science Bulletin 2008, 53
(21), 3255.
[
23] Shamsipur, M.; Ganjali, M. R.; Rouhollahi, A., Lead-selective membrane
potentiometric sensor based on an 18-membered thiacrown derivative. Anal Sci 2001, 17 (8),
35-938.
24] Bocheńska, M.; Guziński, M.; Kulesza, J., Lower Rim Substituted p-tert-Butyl-
9
[
Calix[4]arene. Part 15. Pb(II)-Ion-Selective Electrodes Based on p-tert-Butyl-calix[4]arene
Thioamides. Electroanalysis 2009, 21 (17‐18), 2054-2060.
[
25] (a) Kulesza, J.; Alves Júnior, S.; Guziński, M.; Bocheńska, M.; Hubscher-Bruder, V.;
Arnaud-Neu, F., Calix[4]Arenes Appended with Thioamide Moieties as Powerful Tool for
Heavy Metals Recognition. Advances in Science and Technology 2013, 77, 77-81; (b) Dai,
Y.; Liu, C. C., A Simple, Cost-Effective Sensor for Detecting Lead Ions in Water Using
Under-Potential Deposited Bismuth Sub-Layer with Differential Pulse Voltammetry (DPV).
Sensors (Basel) 2017, 17 (5), 950.
[
26] Jackson, D. T.; Nelson, P. N., Preparation and properties of some ion selective
membranes: A review. Journal of Molecular Structure 2019, 1182, 241-259.
27] Boda, A.; Ali, S. M.; Shenoi, M. R. K.; Rao, H.; Ghosh, S. K., DFT modeling on the
[
suitable crown ether architecture for complexation with Cs+ and Sr2+ metal ions. Journal of
Molecular Modeling 2011, 17 (5), 1091-1108.
[
28] More, M. B.; Ray, D.; Armentrout, P. B., Intrinsic Affinities of Alkali Cations for 15-
Crown-5 and 18-Crown-6:ꢀ Bond Dissociation Energies of Gas-Phase M+−Crown Ether
Complexes. Journal of the American Chemical Society 1999, 121 (2), 417-423.
[
29] Chu, I.-H.; Zhang, H.; Dearden, D. V., Macrocyclic chemistry in the gas phase:
intrinsic cation affinities and complexation rates for alkali metal cation complexes of crown
ethers and glymes. Journal of the American Chemical Society 1993, 115 (13), 5736-5744.
[
30] Anderson, J. D.; Paulsen, E. S.; Dearden, D. V., Alkali metal binding energies of
dibenzo-18-crown-6: experimental and computational results. International Journal of Mass
Spectrometry 2003, 227 (1), 63-76.
[
31] Maleknia, S.; Brodbelt, J., Gas-phase selectivities of crown ethers for alkali metal ion
complexation. Journal of the American Chemical Society 1992, 114 (11), 4295-4298.
32] Choi, C. M.; Heo, J.; Kim, N. J., Binding selectivity of dibenzo-18-crown-6 for alkali
[
metal cations in aqueous solution: A density functional theory study using a continuum
solvation model. Chem Cent J 2012, 6 (1), 84-84.
[
33] (a) Jones, R. L.; Homa, D. M.; Meyer, P. A.; Brody, D. J.; Caldwell, K. L.; Pirkle, J.
L.; Brown, M. J., Trends in Blood Lead Levels and Blood Lead Testing Among US Children
Aged 1 to 5 Years, 1988–2004. Pediatrics 2009, 123 (3), e376; (b) Pieper, K. J.; Tang, M.;
Edwards, M. A., Flint Water Crisis Caused By Interrupted Corrosion Control: Investigating
―Ground Zero‖ Home. Environmental Science & Technology 2017, 51 (4), 2007-2014.
[
34] Pieper, K. J.; Martin, R.; Tang, M.; Walters, L.; Parks, J.; Roy, S.; Devine, C.;
Edwards, M. A., Evaluating Water Lead Levels During the Flint Water Crisis. Environmental
Science & Technology 2018, 52 (15), 8124-8132.
[
35] Barn, P.; Kosatsky, T., Lead in School Drinking Water: Canada Can and Should
Address This Important Ongoing Exposure Source. Canadian Journal of Public Health 2011,
02 (2), 118-121.
1
[
(
36] Patel, H. A., Proton Conduction in Tröger's Base-Linked Poly(crown ether)s. 2018, 10
30), 25303-25310.
24

[
37] Rey Eliseo, C. T.; Grace, M. N.; Arnel, B. B.; Myoung Jun, P.; Basavaraj, R. P.;
Seong-Poong, L.; Jeong Gil, S.; Wook-Jin, C., Microwave-Assisted Synthesis of Dibenzo-
Crown Ethers. Letters in Organic Chemistry 2014, 11 (2), 109-115.
[
38] M. J. Frisch, G. W. T., H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich,
J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F.
Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings,
B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G.
Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J.
E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N.
Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J.
W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox,
Gaussian 16, Revision B.01. 2016.
[
39] Becke, A. D., Density-functional thermochemistry. III. The role of exact exchange.
Journal of Chemical Physics 1993, 98, 5648.
40] Lee, C.; Yang, W.; Parr, R. G., Development of the Colle-Salvetti correlation-energy
formula into a functional of the electron density. Physical Review B 1988, 37 (2), 785-789.
41] Yanai, T.; Tew, D. P.; Handy, N. C., A new hybrid exchange–correlation functional
using the Coulomb-attenuating method (CAM-B3LYP). Chemical Physics Letters 2004, 393
[
[
(1), 51-57.
[
42] Cossi, M.; Rega, N.; Scalmani, G.; Barone, V., Polarizable dielectric model of
solvation with inclusion of charge penetration effects. The Journal of Chemical Physics 2001,
14 (13), 5691-5701.
43] Cossi, M.; Rega, N.; Scalmani, G.; Barone, V., Energies, structures, and electronic
1
[
properties of molecules in solution with the C-PCM solvation model. Journal of
computational chemistry 2003, 24 (6), 669-81.
[
44] Hay, P. J.; Wadt, W. R., Ab initio effective core potentials for molecular calculations.
Potentials for the transition metal atoms Sc to Hg. The Journal of Chemical Physics 1985, 82
(1), 270-283.
[
45] Wadt, W. R.; Hay, P. J., Ab initio effective core potentials for molecular calculations.
Potentials for main group elements Na to Bi. The Journal of Chemical Physics 1985, 82 (1),
84-298.
46] Nelson, P. N.; Ellis, H. A., Structural, odd–even chain alternation and thermal
2
[
investigation of a homologous series of anhydrous silver(i) n-alkanoates. Dalton
Transactions 2012, 41 (9), 2632-2638.
[
47] Nelson, P. N.; Taylor, R. A., Theories and experimental investigations of the
structural and thermotropic mesomorphic phase behaviors of metal carboxylates. Applied
Petrochemical Research 2014, 4 (3), 253-285.
[
48] Nelson, P. N.; Ellis, H. A.; Taylor, R. A., Effects of molecular and lattice structure on
the thermal behaviours of some long chain length potassium(I) n-alkanoates. Journal of
Molecular Structure 2014, 1058, 234-243.
[
1
[
49] Bernstein, H. J., Bond distances in hydrocarbons. Transactions of the Faraday Society
961, 57 (0), 1649-1656.
50] A. C. Alves, J.; V. Barkley, J.; F. Brigas, A.; A. W. Johnstone, R., Metal-assisted
reactions. Part 26.1 Catalytic reactivity and ether bond lengths in aryloxytetrazoles and
aryloxypseudosaccharins. Journal of the Chemical Society, Perkin Transactions 2 1997, (4),
6
69-678.
25

[
51] Trotter, J., Bond lengths in benzene derivatives: Hybridization or resonance.
Tetrahedron 1960, 8 (1), 13-22.
52] Kusaka, R.; Inokuchi, Y.; Ebata, T., Structure of hydrated clusters of dibenzo-18-
[
crown-6-ether in a supersonic jet—encapsulation of water molecules in the crown cavity.
Physical Chemistry Chemical Physics 2008, 10 (41), 6238-6244.
[
53] Robert M. Silverstein, F. X. W., David J. Kiemle, David L. Bryce, Spectrometric
Identification of Organic Compounds. 8 ed.; Wiley: 2014; p 464.
54] Glendening, E. D.; Feller, D.; Thompson, M. A., An Ab Initio Investigation of the
[
Structure and Alkali Metal Cation Selectivity of 18-Crown-6. Journal of the American
Chemical Society 1994, 116 (23), 10657-10669.
[
55] Randles, J. E. B., A cathode ray polarograph. Part II.—The current-voltage curves.
Transactions of the Faraday Society 1948, 44 (0), 327-338.
56] Ševčík, A., Oscillographic polarography with periodical triangular voltage. Collect.
Czech. Chem. Commun. 1948, 13, 349-377.
57] Zanello, P.; Cinquantini, A.; Mangani, S.; Opromolla, G.; Pardi, L.; Janiak, C.;
[
[
Rausch, M. D., The redox behaviour of ferrocene derivatives: VI. Benzylferrocenes. The
crystal structure of decabenzylferrocenium tetrafluoroborate. Journal of Organometallic
Chemistry 1994, 471 (1), 171-177.
[
58] Chambers, J. Q.; Adams, R. N., Nitro-group elimination in the reduction of
dinitrobenzenes in dimethylformamide. Journal of Electroanalytical Chemistry (1959) 1965,
(5), 400-407.
59] Tanaka, K.; Shichiri, T.; Wang, S.; Yamabe, T., A study of the electropolymerization
of thiophene. Synthetic Metals 1988, 24 (3), 203-215.
60] Bard, A. J., Chemical modification of electrodes. Journal of Chemical Education
983, 60 (4), 302.
61] Untereker, D. F.; Lennox, J. C.; Wier, L. M.; Moses, P. R.; Murray, R. W.,
9
[
[
1
[
Chemically modified electrodes: Part IV. Evidence for formation of monolayers of bonded
organosilane reagents. Journal of Electroanalytical Chemistry and Interfacial
Electrochemistry 1977, 81 (2), 309-318.
26

Graphical Abstract
27