An experimental and theoretical study on the electrostatic effect of an appended cationic group on electronic properties of aromatic systems

Article abstract of DOI:10.1039/b709751d

Tetraalkylammonium salts, characterized by an aromatic system pending from one of the alkyl chains, are taken as model systems to study the spectroscopic and redox properties of the aromatic centre under the field effects exerted by the charged group through alkyl bridges of varying length. The changes in the aromatic's redox properties, due to the net field effect and its different components, are interpreted on theoretical bases. the Owner Societies.

Full text of DOI:10.1039/b709751d

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
An experimental and theoretical study on the electrostatic effect of an
appended cationic group on electronic properties of aromatic systemsw
Viviana Grosso,a Carlos Previtali,a Carlos A. Chesta,*a D. Mariano A. Vera*b and
Adriana B. Pierini*b
Received 27th June 2007, Accepted 10th September 2007
First published as an Advance Article on the web 28th September 2007
DOI: 10.1039/b709751d
Tetraalkylammonium salts, characterized by an aromatic system pending from one of the alkyl
chains, are taken as model systems to study the spectroscopic and redox properties of the
aromatic centre under the field effects exerted by the charged group through alkyl bridges of
varying length. The changes in the aromatic’s redox properties, due to the net field effect and its
different components, are interpreted on theoretical bases.
We report here a study on the field effects exerted by a
Introduction
tetraalkylammonium (TAA) group on the properties of a
Through-space Coulombic interactions between a remote
series of aromatic compounds. This study involves the spectro-
unipole (or dipole) and a molecule can produce quantifiable
scopical and electrochemical characterization of the cnAr salts
changes in its reactivity.
in Chart 1 (Ar = aromatic chromophore, cn = carbon chain
In principle, the magnitude of the Coulombic effects de-
of n methylene units) and a comparison with the correspond-
pends on the unipolar charge–dipole moment, orientation of
ing neutral derivatives, in which the –(CH2)n–TAA group has
the dipole, distance between the reactive centre and the uni-
been replaced by H (or CH3). All experiments were carried out
pole (or dipole), and on the effective dielectric constant.1 The
in a polar solvent (acetonitrile) and at low concentrations of
importance of these electrostatic effects (or field effects) was
first recognized by Bjerrum2 and Kirkwood3 and further
investigated in more recent years in connection with the
electrostatic catalysis in enzymes4–10 and with the significant
changes observed in the reactivity of organic molecules when
exposed to the electric field exerted by remote ions. Remark-
able examples of these effects are the enhancement of the rate
of enolization of cationic ketones,11 the changes on the
regioselectivity and kinetics of the reductive cleavage of alkyl
aryl ethers,12 the modification of the photophysical and
photochemical properties of diverse substrates absorbed in
zeolites13,14 and the changes observed in the photophysical and
redox properties of a chromophore/fluorophore after metal
complexation in crown-ether-based sensors.15,16 However,
despite this apparent interest, only a few studies involving
experimental and theoretical analyses of these Coulombic
effects have been reported.8,13
a
´
Departamento de Quımica, Universidad Nacional de Rıo Cuarto,
5800–Rıo Cuarto, Argentina. E-mail: chesta@exa.unrc.edu.ar
´
´
b
´
´
INFIQC, Departamento de Quımica Organica, Facultad de Ciencias
´
´
´
Quımicas, Universidad Nacional de Cordoba, 5000–Cordoba,
Argentina. E-mail: mariano@fcq.unc.edu.ar. E-mail:
adriana@fcq.unc.edu.ar
w Electronic supplementary information (ESI) available: Supplemen-
tary Figures to the experimental and theoretical subsections: KT
calculations on the electron affinities and ionization potential of
structurally related compounds. Additional models and whole sum-
mary of orbital energies for the whole species and models under study
in gas phase and in PCM acetonitrile. DFT calculations details for the
thermochemistries on Chart 2 for naphthalene, anthracene and pyrene
series, including total energies in atomic units for each substrate and
oxidized/reduced product both in gas phase and in acetonitrile. Details
on the NBO analysis of the orbital interactions. Cartesian coordinates
of main species and models under study. See DOI: 10.1039/b709751d
Chart 1
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the salts (o10ꢁ2 M). Under these experimental conditions the
salts are expected to be completely dissociated, leaving the
TAA group free to exert a strong Coulombic effect on the
aromatic rings. Calculations using first-principles methods
were carried out on several model systems to evaluate the
nature and the magnitude of the effects produced by the
charged group due to its electrostatic potential and to its
electronic coupling through the alkyl bridge. We analyzed
here the effects of the distance/orientation of the charge with
respect to the chromophore, the role of the bridge itself, and
the effect exerted by a polar solvent on the electrostatic effects.
c1DMeOB. 1H NMR (Cl3DC, TMS) d/ppm: 1.44 (t, 9H,
–CH3), 3.26 (q, 6H, –CH2–), 3.88 (s, 6H, –OCH3), 4.30 (s, 2H,
–CH2–, benzylic), 6.7 (m, 3H, aromatic). Elem. anal. calcu-
lated for C15H26ClNO6: C: 51.21%, H: 7.5%, Cl: 10.08%, N:
3.98%. Found: C: 51.20%, H: 7.6%, N: 3.95%. IR (KBr):
1090 and 620 cmꢁ1
.
1
c1Ant. H NMR (Cl3DC, TMS) d/ppm: 1.45 (t, 9H), 3.28
(q, 6H), 4.87 (s, 2H), 7.3–7.7 (m, 9H). Elem. anal. calculated
for C21H26ClNO4: C: 64.36%, H: 6.69%, Cl: 9.05%, N:
3.57%. Found: C: 64.22%, H: 6.62%, N: 3.52%. IR (KBr):
1090 and 620 cmꢁ1
.
Instrumental
Material and methods
Absorption spectra were recorded using a HP 8453 UV-visible
spectrophotometer. IR spectra were obtained using a Nicolet
Impact 400 spectrophotometer. 1H NMR spectra were re-
corded on a Brucker 200 MHz nuclear magnetic resonance
spectrometer.
Reagents
Acetonitrile (HPLC grade) was from Sintorgan. 1-(Chloro-
methyl)benzene (Aldrich, 97%), 1,3-dichloro-5-(chloromethyl)-
benzene (Aldrich, 97%), 1-(chloromethyl)-4-methoxybenzene
(Aldrich,
98%),
1-(chloromethyl)-3,5-dimethoxybenzene
9-(chloromethyl)anthracene (Aldrich,
Stationary fluorescence experiments were carried out using a
Spex Fluoromax spectrofluorometer. All measurements were
performed in deoxygenated solutions at (298 ꢂ 1) K. Lifetime
measurements were performed by using the time-correlated
single photon counting technique (TCSPC) with an Edinburgh
Instruments OB900 apparatus.
(Aldrich, 99%),
98+%), and 1-(chloromethyl)naphthalene (Aldrich, 90%)
were used as received. Triethylamine (Aldrich, 98%) was
distilled prior to use.
Synthesis
Cyclic voltammetry experiments were done using an EG &
G Princeton Applied Research PAR-273 potentiostat–gal-
vanostat. The current and potentials were registered either
on an EG & G PAR RE 0150 XY recorder or with a Keithley
194A high speed voltmeter connected to a computer. A
conventional three-compartment Pyrex cell was used. A Pt
microelectrode (d = 25 mm) or a standard Pt electrode (A =
0.23 cm2) were used as working electrodes. All the potentials
were measured with reference to the saturated calomel elec-
trode (SCE). All redox potentials were determined in acetoni-
trile using tetrabutylammonium perchlorate (0.1 M) as the
supporting electrolyte. Depending on the experiment, the
concentration of the salts was varied between 10ꢁ2–10ꢁ3 M.
c1Py and c4Py were provided by Molecular Probes. c1Naph
was prepared as previously reported.17 The other salts in Chart
1 were prepared from the corresponding 1-(chloromethyl)
aromatics and triethylamine according to an adaptation of
the procedure reported by Arnold et al.18 Typically, 2 g of the
halide and 25 mL of freshly distilled triethylamine were
refluxed with stirring for 8 hours. The salt was filtered out,
dissolved in water, and was then precipitated out as the
perchlorate salt by addition of a saturated aqueous solution
of NaClO4. Finally, the salts were recrystallized from ethyl
acetate and vacuum dried at 45 1C for 24 hours. Yields varied
from 60–70%.
c1ClB. 1H NMR (Cl3DC, TMS) d/ppm: 1.44 (t, 9H, –CH3),
3.26 (q, 6H, –CH2–), 4.47 (s, 2H, –CH2–, benzylic), 7.26 and
7.43 (s, aromatics, 4H). Elem. anal. calculated for
C13H21Cl2NO4: C: 47.86%, H: 6.49%, Cl: 21.74%, N:
4.29%. Found: C: 47.95%, H: 6.43%, N: 4.36%. The IR
(KBr) showed two pronounced absorpꢁtions at 1090 and
620 cmꢁ1, both characteristic of the ClO4 ion.
Computational details
The conformational space of the compounds shown in Chart 1
was first explored at the semiempirical AM1 level and char-
acterized by means of normal analysis. Next, they were
optimized at the HF/6-31G* level. A series of cnNaph salts
(n = 1–16) was also calculated. Apart from the structures
having a totally gauche chain (cnAr series, where the distance
varies with n), other conformers (see below) were also opti-
mized for exploring the effect of changing the orientation of
the charged group with respect to the aromatic ring.
c1DClB. 1H NMR (Cl3DC, TMS) d/ppm: 1.45 (t, 9H,
–CH3), 3.29 (q, 6H, –CH2–), 4.50 (s, 2H, –CH2–, benzylic),
7.4–7.5 (m, 3H, aromatic). Elem. anal. calculated for
C13H20Cl3NO4: C: 43.29%, H: 5.6%, Cl: 29.49%, N: 3.88%.
Found: C: 43.26%, H: 5.7%, N: 3.85%. IR (KBr): 1090 and
The difference in energy between the HOMOs of the
cnAr–MeAr pair and the energy difference between their
LUMOs were evaluated. These energy differences are referred
to as the Koopman’s ionization potential (DI) and electron
affinity shifts (DEA), respectively, and are expected to give a
first estimate of the modifications in the donor and acceptor
capabilities due to the presence of the charged group in the
absence of a solvent.
620 cmꢁ1
.
c1MeOB. 1H NMR (Cl3DC, TMS) d/ppm: 1.45 (t, 9H,
–CH3), 3.25 (q, 6H, –CH2–), 3.85 (s, 3H, –OCH3), 4.38 (s, 2H,
–CH2–, benzylic), 7.4–7.5 (m, 4H, aromatic). Elem. anal.
calculated for C14H24ClNO5: C: 52.25%, H: 7.5%, Cl:
11.02%, N: 4.35%. Found: C: 52.22%, H: 7.4%, N: 4.41%.
The Koopman’s theorem (KT)19 estimation of these shifts is
IR (KBr): 1090 and 620 cmꢁ1
.
expected to give a linear correlation with the actual change of
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the gas-phase donor/acceptor properties of the systems20–23
and it was indeed reasonable as confirmed by two independent
criteria: (i) linear correlations of KT values with experimental
I or EA values for structurally related compounds20 (the details
are included as ESIw) and (ii) highly correlated calculations of
thermodynamic values of these properties in selected systems
(i.e. by computing the total energy of the oxidized and reduced
forms of the cnAr–MeAr pair, see below).
For the naphthalene, anthracene and pyrene derivatives on
Chart 1, the shifts in the standard oxidation or reduction
potentials were evaluated through the thermodynamic cycle on
Chart 2 at the B3LYP/6-31+G** // B3LYP/6-31G* level of
theory.24,25 Further corrections were obtained at the B3LYP/
6-311++G(2d,p) // B3LYP/6-31G* level.26,27 The potentials
were evaluated in acetonitrile (the solvent used in the electro-
chemical and spectroscopic studies). This solvent was modeled
by means of Tomasi’s polarized continuum model (PCM), as
implemented in Gaussian 98;28,29 the solvation energies were
obtained using the gas-phase equilibrium geometries.
Fig. 1 Absorption (left) and fluorescence spectra (right) of 1.2 ꢃ
10ꢁ5 M c1Naph in acetonitrile at 298 K.
Fig. 2 shows the voltagrams obtained for the reduction of
c1Py using either a Pt microelectrode (d = 25 mm) (a) or a
standard Pt electrode (A = 0.23 cm2) (b). As it can be
concluded from voltagram (a) the reduction of the salt takes
place at Bꢁ1.4 V. Although a similar potential can be guessed
from voltagram (b), the lack of a reverse wave shows the
irreversibility of the reductive process. It can be concluded that
the anion radical of c1Py undergoes fast secondary reactions
and, therefore, that the potential measured from the experi-
ments in Fig. 2 does not represent a formal reduction potential
(E1) value. Similar behaviours were observed for all the salts
studied: for both types of processes, oxidations and reduc-
The charge localization and the electrostatic potential cal-
culations (ESP) were done by using the Merz–Singh–Kollman
(MK) scheme.30 Models to evaluate the effect of an external
(non-bonded) electric field on the unsubstituted aromatics
were considered, by combining the MeAr molecule with an
+
array of point charges. Under this approach, the –NMe3
group was modeled by a set of atomic point charges positioned
at the TAA atomic coordinates and chosen to fit its electro-
static potential (ESP) in the real salt.31 For some species under
study, the interaction between the aromatic p system and the
substituent was analyzed using localized natural bonding
orbitals, with the NBO 3.1 standard program from the
Gaussian 98 package.28,32
tions. Thus, the oxidation (Eox) and the reduction (Ered
)
potentials reported in Table 1 could depart from the standard
values by B0.06–0.07 V.38
The oxidation potentials of salts c1Py, c1Naph, c1Ant and
c1DMeOB are more anodic that those observed for the
unsubstituted (or methylsubstituted) compounds. The relative
Results and discussion
Table 1 Redox potentials in V vs. SCE, singlet–singlet excited state
energies (Eoo) and emission lifetimes (to) of the compounds studied in
acetonitrile at 298 K
Characterization of the salts
The shape and energy (wavelength) of the maxima of the
absorption (and emission, when detected) spectra of the salts
do not differ significantly from those of the MeAr aromatics.
The absorption and emission spectra of c1Naph in acetonitrile
are shown in Fig. 1. Approximated values of the salt’s
singlet–singlet excited state energies (E00) were estimated from
the wavelength of the 0–0 absorption/emission vibronic bands
as shown in Fig. 1 and collected in Table 1.
ꢄ
ꢄ
/
t0/
ns
EAr+
eV
/
/Ar
E
Ar/Arꢁ
eV
E00/
eV
d
Compound
Pyrenea
c4Py
c1Py
1.20
1.25
1.52
ꢁ2.10
3.34 350
3.34 210
ꢁ2.10
ꢁ1.40
B3.30
62
Naphthalenea
c1Naph
1.43
1.85
ꢁ2.62
ꢁ2.03
3.95
B3.90
96
3.4
3.9
In agreement with earlier studies on the photochemistry of
structurally related compounds,33,34 some of the salts studied
here photodecompose upon irradiation. The decomposition
rates depend on the nature of the aromatic chromophore.
While salts c4Py and c1Py are photostable, c1Naph and c1Ant
quickly decompose to produce 1-methylnaphthalene and
9-methylanthracene, respectively. Both products were detected
by GC-mass spectroscopy upon photolysis of the salts in
acetonitrile–2-propanol (9 : 1) (Fig. S1 in ESIw). The lack of
emission from c1Ant indicates the particular importance of
this photodecomposition process for the anthracene deriva-
tive. The fluorescence lifetimes (t0) measured in acetonitrile for
the pyrene, naphthalene and anthracene compounds are
collected in Table 1.
Anthracenea
c1Ant
1.09
1.35
ꢁ1.98
ꢁ1.27
3.31
B3.27
—
Methoxybenzeneb
c1MeOB
1.76
42.20
1.45
—
—
—
—
—
—
—
—
—
—
—
—
1,3-Dimethoxybenzeneb
c1DMeOB
1.90
Chlorobenzenec
c1ClB
—
—
—
—
ꢁ2.74
—
—
—
—
—
—
—
—
oꢁ2.30
ꢁ2.57
1,3-Dichlorobenzenec
c1DClB
ꢁ2.02
a
b
Redox potentials were obtained from ref. 35. Values taken from
c
ref. 36. Reduction potential in DMSO vs. SCE, ref. 37. Expressed
d
according to IUPAC convention.
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Fig. 2 Cyclic voltagrams for c1Py in acetonitrile obtained using a Pt
microelectrode (a) and a standard Pt electrode (b).
changes in Eox are in the order of 0.30–0.50 V. The opposite
behavior is observed in the reduction potentials. The salts
c1Py, c1Naph, c1Ant and c1DClB are easier to reduce than the
corresponding aromatics. The relative changes in Ered are
remarkably larger: ca. B0.50–0.70 V. Interestingly, the differ-
ences observed in the Eox and Ered for the pyrene derivatives
c4Py and c1Py reveal the significance of the aromatic/ammo-
nium group separation in determining the magnitude of the
electrostatic effect. The value Eox for c1MeOB and Ered for
c1ClB could not be determined because their values are out-
side the working potential window of the system.
Fig. 3 summarizes the shifts in the redox potentials in Table 1:
Fig. 3 Summary of the shifts in the electrochemical potentials, DEred
and DEox, in acetonitrile. The actual DEred and DEox for c1ClBe and
c1MeOBe should exceed the values shown in the Figure.
results were found for the naphthalene derivatives as shown in
Fig. 6.
ꢄ
ꢄ
ꢄ/Ar
DEred = EAr/Arꢁ – ESalt/Saltꢁ and DEox = ꢁEAr+
+
ꢄ
ꢄ
.
ESalt+
In order to estimate the magnitude of the effect exerted by
an external (non-bonded) electric field, cnPy models were
calculated. These models were built as a combination of MePy
plus an array of atomic charges, placed as the TAA group in
the cnPy molecule and chosen to fit the electrostatic potential
(ESP) of the group. The results, presented in Fig. 4, show that
for the c1Py and c4Py distances, the HOMO and LUMO
shifts are 1.8 and 1.1 eV respectively (compare white vs. black
circles in Fig. 4). Based on this model, roughly 60% of the
overall shift calculated for c1Py (B3 eV) and c4Py (B1.7 eV)
is due to the net electric field. The remaining shift is attributed
to the polarization exerted by TAA through the alkyl bridge
and to the through-bond orbital coupling between the p
system of the chromophore, mainly the s*(C–C) of the
methylene(s) of the bridge, and the s* orbitals of the TAA
group. The contribution of the latter so-called ‘‘orbital effect’’
was estimated from the differences in shifts between the C1 and
Cs conformers of the salts. In the Cs conformers the s* of the
methylene and the aromatic p systems remains orthogonal, as
shown in Fig. 5 for c4Py and so the Py-bridge–TAA orbital
coupling is negligible due to symmetry [the –(CH2)n– chain is
in the same plane as the pyrene]. In contrast, for the C1
conformers discussed so far the chain axes lie 83o above the
pyrene plane and orbital coupling is optimal (see Fig. 5). The
total energy of the Cs conformer lies 9.4 kcal molꢁ1 above of
the C1 conformer in the case of c1Py and 2.2 kcal molꢁ1 in the
case of the c4Py species.39
/Salt
In principle, the changes observed in the redox properties of
the salts, when compared to that of the unsubstituted com-
pounds, could be taken as evidence of an internal field effect.
The presence of the charged ammonium group (TAA) exerts a
remarkable influence on the development of a second charge in
the molecule and a straightforward reasoning can be used to
anticipate the direction of observed effects. The generation of
the aromatic cation radicals seems to be less favourable when
the TAA group is attached to the aromatic ring, while the
same group apparently assists the formation of the anion
radical. As it is shown in the next section, the magnitude of
the changes observed along each direction depends on a
number of factors acting simultaneously.
Computational results
The HOMO and LUMOs energies evaluated for MePy and
the most stable conformations (hereafter referred as confor-
mers of C1 symmetry) of c1Py and c4Py are shown in Fig. 4 as
solid circles. As expected, by comparing the HOMOs and
LUMOs energies of MePy and the salts, a remarkable change
in the gas phase donor/acceptor capabilities of the salts is
observed. The magnitude of the effect increases as the
ring–TAA separation decreases. For instance, the DI and
DEA for c4Py are B1.7 eV and those for c1Py are B3 eV.
On the other hand, for a given separation, the TAA group
increases both the ionization potentials and the electron
The calculated shifts for the Cs conformers are shown as
gray circles on Fig. 4. The slight change observed between C1
and Cs in c4Py (o0.05 eV) indicates that orbital effects are
negligible for this compound and that polarization is respon-
sible for the major part of the remaining effect (almost 0.6 eV
affinities to
a
similar extent. As
a
consequence,
the HOMO–LUMO gap of the salts is nearly independent of
the ring–TAA distance. This result is in agreement with the
slight changes observed in the E00 along the series. Similar
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Fig. 4 HOMO–LUMO energies for different separations between the nitrogen of the TAA group and the ipso-carbon of the pyrenyl system, r.
Frontier orbital energies for the C1 conformers of MePy, c4Py and c1Py species (K); conformers of Cs symmetry of the salts ( ); MePy exposed to
an external (non-bonded) array of charges that reproduces the electrostatic potential of the TAA group (J).
of the whole 1.7 eV shift). For c1Py the orbital effect is
B0.3 eV, but even in this case it is fairly small when compared
to the 3.1 eV whole shift.
on the polarization and the role of the alkyl bridge
was performed by analyzing the folded conformers of c7
and c8Naph. In these two U-shaped conformers the
TAA–naphthyl distance is comparable to that of the
linear c2Naph, although the TAA and the ring are not close
enough to overlap, as checked by NBO analysis.41 The smaller
shift (B0.5 eV) of the folded conformers with respect
to c2Naph can be mainly attributed to the polarization of
the bridge. This is due to the fact that in the linear c2Naph
the field propagates both through the space and through
the bonds of the polarized chain; while in the folded
conformers, the chromophore senses the field essentially
through the space.
Despite the charge–chromophore orientation changes be-
tween the Cs and C1 conformers (Fig. 5), we have checked that
for a given distance, the effect is practically the same whether the
TAA group approaches the aromatic system either on the plane
(as in the Cs conformers) or above the plane (as in the C1
conformers). This has been tested with an external (non-bonded)
charge model by varying the ring–TAA separation with different
axis orientation (details are available as ESIw).
Similar results were obtained for the naphthalene
derivatives with a wider variety of chain lengths (see Fig. 6).
As can be observed in the figure, the field effect holds even at
very long distances (i.e. 23 A for c16Naph). Again, the
ring–TAA distance is the dominant effect in the MOs energy
shifts. The difference in shifts between the C1 and Cs
conformers is 0.26 eV for c1Naph, B0.1 eV for c2Naph,
and practically negligible for c4Naph.40 A further investigation
A
polar solvent screens the net field sensed by the
chromophore and favors charge localization on the TAA and
aromatic moieties as illustrated in Fig. 7 for c4Naph. While
the bridge is smoothly polarized by the TAA charge in the
gas phase (Fig. 7a), in a continuum polar solvent a higher
charge separation is favored and the aromatic system appears
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Fig. 5 Structures of the (a) C1 or totally gauche and (b) Cs con-
formers of c4Py, optimized at the HF/6-31G* level.
Fig. 7 Calculated electrostatic potentials (ESP) for c4Naph and
MeNaph in the gas phase (a) and in acetonitrile (b).
at a more negative potential, similar to that observed for
MeNaph (Fig. 7b).
black triangles on Fig. 8) and thus, they acquire relative
importance in solution.
Fig. 8 shows the calculated LUMOs energies for a cnNaph
(n = 1–4) series in acetonitrile. As can be seen, the major
part of the electrostatic effects is screened by the polar solvent
and the shifts calculated for c1Naph decreases from B3 eV
(gas phase) to B0.8 eV in acetonitrile. The effect practically
vanishes for n 4 4, while it still holds for more than 16
methylenes in the gas phase; the shifts for c4Py and
c4Naph were reduced from B1.7 eV to less than 0.15 eV
in acetonitrile. The HOMOs follow a similar trend. Interest-
ingly, the orbital coupling effects for the shortest chain
derivatives do not change appreciably (compare gray vs.
The above results illustrate the different contributions that
modify the donor/acceptor capabilities of the aromatics. How-
ever, in order to estimate the actual changes in the thermo-
dynamic redox potentials of the salts with respect to the
reference methylaromatics, the solvation free energies of all
oxidized and reduced species (reactants and products) should
also be taken into account.
The thermodynamic cycles in Chart 2 can be used to analyze
the factors responsible for the electrochemical shifts deter-
mined in acetonitrile (Fig. 3). The shift in the electrochemical
Fig. 6 LUMOs and HOMOs of cnNaph , n = 1, 2, 3, 4, 7, 8, 10, 16 (C1 conformers) and MeNaph (solid triangles). c7Naph and c8Naph folded
conformers (crossed diamonds). In order to compare the linear and folded species, r is taken between the TAA group and the center of the
naphthyl ring.
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where
DDG0solvðreactantsÞ ¼ DGs0olvðArÞ ꢁ DGs0olvðsaltÞ
ð2aÞ
ð2bÞ
DEA ¼ EAðArÞ ꢁ EAðsaltÞ
DDG0solvðred; productsÞ ¼ DGs0olvðArꢄꢁÞ ꢁ DGs0olvðsaltꢄꢁÞ ð2cÞ
Similarly, the following expressions apply to the oxidations:
DDG0ox ¼ ꢁ nFDEo0x ¼ ꢁDDGs0olvðreactantsÞ
ð3Þ
þ DI þ DDG0solvðox; productsÞ
Fig. 8 LUMOs energies for cnNaph series, C1 conformers (m) n = 1
to 4; Cs conformers ( ) n = 1, 2 and 4, in acetonitrile; shifts with
respect to the MeNaph (K).
where
DDG0solvðreactantsÞ ¼ DGs0olvðArÞ ꢁ DGs0olvðsaltÞ
DI ¼ IðArÞ ꢁ IðsaltÞ
ð4aÞ
ð4bÞ
standard reduction potential of c1Ar (for which the electro-
static effects are maxima) with respect to the methyl aromatic:
DEr0ed ¼ EA0 r=Arꢁꢄ ꢁ Es0alt=salt ꢁꢄ
;
DDG0solvðox; productsÞ ¼ DGs0olvðArꢄþÞ ꢁ DGs0olvðsaltꢄþÞ ð4cÞ
can be computed by taking the paths shown in Chart 2 and
then subtracting the two expressions obtained from cycles in
Chart 2a and Chart 2b for the MeAr and the c1Ar, respec-
tively. This leads to
The results are summarized on Table 2.42 The change in the EA
values and I values (gas phase values) are in fact as big as B3
eV, indicating that the KT values based on the orbital shifts
were in the right order of magnitude. The DE0red and DEo0x
,
DDG0red ¼ ꢁ nF DEr0ed ¼ ꢁDDGs0olvðreactantsÞ ꢁ DEA
calculated from eqn (1) and (3), reproduced considerably well
(despite the limitations of the model) the direction and mag-
nitude of the experimental redox shifts. An inspection of the
data in Table 2 shows that these shifts mainly arise from
changes in EAs and Is which are not fully compensated by
solvation terms even in a polar medium such as acetonitrile.
The magnitude of the estimated shifts increases in the series
Py, An and Naph, which seems reasonable considering that in
the smaller aromatics the chromophore is more exposed to the
TAA field. However, this trend is not evident in the experi-
mental potentials. The greater shift of the experimental reduc-
tion with respect to the oxidation potentials (Fig. 3) has also
been found in the calculated values, even though they have
shown smaller asymmetries.
ð1Þ
þ DDG0solvðred; productsÞxx
Remarks
In summary, the theoretical studies confirm the tendency
observed in the experimental redox potentials (Fig. 3). The
changes in the donor/acceptor capability of the aromatics
linked to the TAA group are the result of a combination of
electrostatic, bridge polarization and orbital coupling effects
acting together. Both in the gas phase and in a polar medium,
the field due to the positively charged TAA enhances the
acceptor capabilities of the aromatic ring practically to the
same extent to which it precludes its donor strength. These
results give further support to the electrostatic catalysis hy-
pothesis in enzymes and could also help to understand the
changes in reactivity observed in other systems where the
substrate is directly exposed to the field exerted by an ion or
dipole; such as in crown ethers, zeolites, and other charged
interfaces.
Chart 2
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Table 2 Summary of DFT calculations.a Calculated changes in the redox properties of the salts with respect to the MeAr and their contributions
(Chart 2, eqn (1) and (3))
DG0solv
(Ar or salt)
DG0solv (Arꢄ+) or
DG0solv (Arꢄꢁ) or
+
ꢁ
(saltꢄ
)
(saltꢄ
)
I
EA
0.31
3.47
0.38
3.54
0.44
DG0red
DE0ox
MePyb
c1Pyb
MePyc
c1Pyc
0.12
ꢁ1.38
0.12
ꢁ1.30
ꢁ5.23
ꢁ1.30
ꢁ5.23
ꢁ1.32
ꢁ5.02
ꢁ1.48
ꢁ5.75
ꢁ1.51
ꢁ0.35
ꢁ1.51
ꢁ0.35
ꢁ1.50
ꢁ0.08
ꢁ1.63
ꢁ0.23
7.00
9.92
7.04
9.96
6.94
9.98
7.72
—
ꢁ0.50
—
0.49
—
0.48
—
0.51
—
—
ꢁ0.50
—
ꢁ1.38
MeAnb
c1Anb
MeNaphb
c1Naphb
a
0.14
ꢁ1.04
0.12
3.66
ꢁ0.35
ꢁ0.61
—
ꢁ0.66
ꢁ1.46
11.05
3.29
0.64
All energies in eV and DE0 in V. At the B3LYP/6-31+G** // B3LYP/6-31G* level. Changes in the ion/molecule properties improved at the
b
c
B3LYP/6-311++G(2d,p) // B3LYP/6-31+G* level.
18 B. Arnold, L. Donald, A. Jurgens and J. Pincock, Can. J. Chem.,
1985, 63, 3140.
19 T. A. Koopmans, Physica A, 1933, 1, 104.
Acknowledgements
Authors thank Consejo Nacional de Investigaciones Cientı
cas y Tecnicas (CONICET-Argentina) and Agencia Nacional
de Promocion Cientıfica (FONCYT-Argentina) for financial
support. C.A.C. thanks the Secretarıa de Ciencia y Tecnica de
la Universidad Nacional de Rıo Cuarto. D.M.A.V. and A.B.P.
thank the Secretarıa de Ciencia y Tecnica de la Universidad
Nacional de Cordoba (SECYT). The INFIQC is jointly
sponsored by CONICET and the Universidad Nacional de
Cordoba.
´
fi-
20 Although the I and EA obtained by directly applying the KT are
not expected to reproduce actual values, the linear relationships of
type I = a e(HOMO) + b [where e(HOMO) is the corresponding
orbital energy and a and b can be fitted for a given method and
basis set; an analogous expression holds for the EAs] are justified
and extensively used for correlating experimentalvalues (PES, ETS
spectroscopies) and orbital energies, even in the most conflicting
cases of compounds having negative electron affinities (see ref. 21,
22 and 23). Nevertheless, in this study we are not concerned with
the absolute values of I and EA but with their differences between
the values for the salt cnAr and the reference methylaromatic
MeAr.
´
´
´
´
´
´
´
´
´
´
21 K. D. Jordan and M. N. Paddon-Row, Chem. Rev., 1992, 92, 395.
22 R. Stowasser and R. Hoffmann, J. Am. Chem. Soc., 1999, 121,
3414.
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26 We have already demonstrated that the DFT formalism is reliable
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31 (a) Within the Gaussian program either the ‘‘Pop=MK’’ or
‘‘Pop=ESP’’ keywords start the MK analysis for obtaining
the MK charges (calculations on the salt structure). The ‘‘Char-
ge=Angstroms’’ keyword allows for computing the effect of
external charges at given coordinates in Angstroms (applied to
the neutral methylaromatics); see also; (b) G. G. Hall and C. M.
Smith, Int. J. Quantum Chem., 1984, 25, 881; (c) C. M. Smith and
G. G. Hall, Theor. Chim. Acta, 1986, 69, 63.
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F. Weinhold, Chem. Rev., 988, 88, 899; (d) Program NBO 3.1: E. D.
Glendering, A. E. Reed, J. E. Carpenter and F. Weinhold,
University of Wisconsin.
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34 B. Arnold, L. Donald, A. Jurgens and J. Pincock, Can. J. Chem.,
1985, 63, 3140.
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and c4Naph respectively. A complete scan of the torsion around
the relevant dihedral, monitoring both the total energies and the
shifts in the HOMO and LUMO energies, is given for the c1Naph
in the ESI, Fig. S5w.
40 Complete Table S2 with the gas-phase shifts of the different models
available as ESIw.
41 Inspection of the second-order perturbation analysis reveals
the absence of interactions (delocalizations) between the
p(aromatic)/p*(aromatic) and the s*(CH)/s(CH) greater
than 0.05 eV. On the other hand, by setting to zero all non-
diagonal elements of the Fock matrix involving NBOs of
the naphthyl and of the TAA group, the changes in the energy
of the HOMO and LUMO are negligible. In NBO analysis
this means to ‘‘delete’’ the ‘‘through-space’’ orbital interaction
between the TAA and the aromatic. Thus, when comparing
the shifts in these particular U-shaped conformers with respect
to the linear chain species with similar TAA–aromatic distances,
we are viewing different polarization patterns in the bonds of the
chain, but we are not simultaneously introducing an important
through-space orbital interaction due to the closeness of the
groups.
42 The shifts in the standard potentials in acetonitrile (eqn (1) and (3))
as well as their components (eqn (2) and (4)) were calculated at the
B3LYP/6-31+G(d) and B3LYP/6-311++G(2d,p) levels of theory
for the pyrene derivatives couple MePy and c1Py. Since the results
were essentially equivalent, especially for the most sensitive case of
the electron affinity changes, the results for the anthracene and
naphthalene derivatives were obtained just at the B3LYP/6-
31+G(d) level.
37 J. R. Wiley, E. C. M. Chen, E. S. D. Chen, P. Richardson, W. R.
Reed and W. E. Wentworth, J. Electroanal. Chem., 1991, 307, 169.
38 A. J. Bard and L. R. Faulkner, in Electrochemical Methods:
Fundamental and Applications, Wiley & Sons, New York, 2nd
edn, 2001.
39 Similar conformational energy differences have been obtained for
the naphthyl derivatives; these being 8.3, 2.8 and 2.0 kcal for c1, c2
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