G Model
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H.R. Memarian et al. / C. R. Chimie xxx (2012) xxx–xxx
atom, on the phenyl ring at the C4-position, on the
electrochemical oxidation potentials are illustrated in
Fig. 5.
negative inductive effect of the phenyl group in 1-position
of the heterocyclic ring, the electronic effect of the
additional substituent on the phenyl ring at C-4 position
of the heterocyclic ring, and also the type of the substituent
on the 5-position. The nature of solvent, protic vs. aprotic,
has a drastic effect on the electrochemical oxidation
potentials.
Computational study at DFT-B3LYP/6-31++G** level of
theory is carried to calculate different structural and
bonding characteristics, and to investigate their correla-
tion with the measured electrochemical oxidation poten-
tials. These computational results approve the proposed
It can be seen from Fig. 5 that C4 and N1 do not carry
significant spin density in the radical cation and radical
species, respectively. Correlations between E and the spin
densities on the N1 and C4 atoms in the radical cations of
the 1a, 1f, 1g, 3a, 3f and 3g series of THPMs, illustrated in
Fig. 5a–d, show that the spin N1 density orders of
1f < 1a < 1g and 3f < 3a < 3g, corresponding to the reverse
order, namely 1f > 1a > 1g and 3f > 3a > 3g, for the spin
density on the C4 atom in the corresponding radical
species (Fig. 5d) indicates that the presence of the electron-
releasing 4-OCH3 group on the C4-phenyl ring (in 1f)
facilitates electron removal from neutral molecule, where-
as the presence of electron-withdrawing 4-NO2 group (1g)
retards this process. This can be attributed to the
fact that the balance of the negative inductive and the
positive resonance effect of the 4-OCH3 group shift the
electron density of the aromatic ring toward C11 attached
to C4.
mechanism in which
a protic solvent stabilizes the
intermediate radical cation resulting in easier electro-
chemical oxidation of THPMs.
4. Experimental
4.1. Cyclic voltammetry measurements
Further electron pushing through the sigma bonds
stabilizes the formation of the positive charge on N1 atom
during the electron removal step from 1f THPM, whereas
4-NO2 group in 1g THPM has an opposite effect as
illustrated in Scheme 4.
This suggestion is supported by the lower spin density
on C4, because the positive charge and the unpaired
electron is delocalized dominantly over the N1 and C5
atoms than on the C4 atom, as shown in Fig. 4b. It should be
noted that the electron-donating effect of the methoxy
group is also supported by the obtained dihedral angles of
the methoxy group on the 2-, 3- and 4-positions of the
phenyl ring with respect to the aromatic ring atoms
(between 176.18 and 179.88) and also the Caromatic–O–CH3
bond angle (between 118.58 and 118.98). This is shown in
Fig. S1 in the supplementary data.
Various substituted 2-oxo-1,2,3,4-tetrahydropyrimi-
dines (1a-q, 3a-q and 5a,k,g) were synthesized according
to the reported procedure [24]. Solvent used in this study,
acetonitrile, ethanol and propan-2-ol were purified and
dried by standard method. Solutions with concentrations
of 1 mM of THPM and 100 mM of tetrabutylammonium
perchlorate in acetonitrile, ethanol and propan-2-ol were
prepared. The cyclic voltammetry (CV) measurements
were performed using SAMA 500 Potentio/Galvanostat. All
electrochemical experiments were carried out in
a
conventional three electrode system at room temperature.
A silver electrode, a large area Pt plate (99.99%), and a glass
carbon electrode (GCE) were used as reference, counter
and working electrodes, respectively.
4.2. Computational methods
The interesting point is that after proton detachment
+
˙
˙
from THPM and formation of TrHPM , the radical center
formed on the C4 atom has simultaneously allylic and
benzylic characters. The higher weights of the benzylic
Geometries of THPMs and DHPMs were optimized
using the density functional theory (DFT) B3LYP method
with 6-31++G(d,p) basis set. All computations are carried
out using G98 program package [25]. Compounds 1a, 3a
and 5a (R1 = H, CH3 and Ph, respectively) all with X = H, are
considered as representative THPMs to investigate com-
paratively the effect of the substitution on the aryl group in
the C4-position and the substituent on the N1 atom on the
optimized structures of compounds. Charge density
radical over the allylic radiacl of the
compared with those of the f compounds (Table S8 of
the supplementary data) are possibly due to:
g compounds
ꢀ
ꢀ
the more effective SOMO-LUMO interaction over the
SOMO-HOMO interaction facilitated by the presence of
the 4-NO2 group;
closer orientation of the aryl ring to the coplanarity with
the heterocyclic ring in the NO2-substituted g THPMs
(e.g. 31.98 to 33.58 in 1f vs. 46.88 to 27.08 in 1g).
distributions for all THPMs, and the radical cation
+
˙
˙
(THPM ) and radical (TrHPM ) species derived from
them have also been studied using natural bond orbital
(NBO) analysis [26–29]. To elucidate the effect of solvent
on the structure and bonding characteristics, and the
consequent effects on the electrochemical behavior, two
sets of computations are carried out on these compounds;
one in the bulk of solvent using the SCRF = CPCM model,
and one in the presence of a solvent molecule (acetonitrile,
ethanol or propan-2-ol) coordinating the primary oxida-
tion site (i.e. the N1 atom of the heterocyclic ring). Note
that, in the geometry optimization of the explicit THPM–
ethanol interaction, another initial geometry in which the
ethanol molecule faces the N3 atom is also adopted.
3. Conclusions
The effect of the substituent on the electrochemical
oxidation of various in 1-, 4- and 5-positions substituted 2-
oxo-1,2,3,4-tetrahydropyrimidines (THPMs) in acetonitrile
is studied using cyclic voltammetry. The results clearly
indicate that several factors affect the electrochemical
behavior of these compounds. The important factor is the
positive inductive effect of the methyl group vs. the
Please cite this article in press as: Memarian HR, et al. Substituent effects on the voltammetric studies of 2-oxo-1,2,3,4-
Memarian, Hamid R.
