Assessing the potential of para-donor and para-acceptor substituted 5-benzylidenebarbituric acid derivatives as push–pull electronic systems: Experimental and quantum chemical study

Article abstract of DOI:10.1016/j.saa.2021.119576

Electronic interactions in donor-π-linker-acceptor systems with barbituric acid as an electron acceptor and possible electron donor were investigated to screen promising candidates with a push–pull character based on experimental and quantum chemical studies. The tautomeric properties of 5-benzylidenebarbituric acid derivatives were studied with NMR spectra, spectrophotometric determination of the pKa values, and quantum chemical calculations. Linear solvation energy relationships (LSER) and linear free energy relationships (LFER) were applied to the spectral data - UV frequencies and ¹³C NMR chemical shifts. The experimental studies of the nature of the ground and excited state of investigated compounds were successfully interpreted using a computational chemistry approach including ab initio MP2 geometry optimization and time-dependent DFT calculations of excited states. Quantification of the push–pull character of barbituric acid derivatives was performed by the ¹³CNMR chemical shift differences, Mayer π bond order analysis, hole-electron distribution analysis, and calculations of intramolecular charge transfer (ICT) indices. The results obtained show, that when coupled with a strong electron-donor, barbituric acid can act as the electron-acceptor in push–pull systems, and when coupled with a strong electron-acceptor, barbituric acid can act as the weak electron-donor.

Full text of DOI:10.1016/j.saa.2021.119576

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Assessing the potential of para-donor and para-acceptor substituted
5-benzylidenebarbituric acid derivatives as push–pull electronic
systems: Experimental and quantum chemical study
a
b
c
´
ˇ ´ a
´
´
ˇ ´ c,
⇑
Ivana N. Stojiljkovic , Milica P. Rancic , Aleksandar D. Marinkovic , Ilija N. Cvijetic , Miloš K. Milcic
a Faculty of Forestry, University of Belgrade, Kneza Višeslava 1, 11030 Belgrade, Serbia
b Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia
c Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
5-benyzlidenebarbiturates were
investigated spectroscopically and
theoretically.
ꢀ
LFER analysis of spectral data and
optimized geometry parameters are
performed.
ꢀ
ꢀ
ICT analysis for quantification of the
efficiency of charge transfer is used.
The best candidate for the push–pull
system is para–N(CH3)2 substituted
derivative.
ꢀ
The weak electron-donating
properties of barbituric acid are
reported.
a r t i c l e i n f o
a b s t r a c t
Article history:
Electronic interactions in donor-p-linker-acceptor systems with barbituric acid as an electron acceptor
Received 29 September 2020
Received in revised form 19 January 2021
Accepted 29 January 2021
and possible electron donor were investigated to screen promising candidates with a push–pull character
based on experimental and quantum chemical studies. The tautomeric properties of
5-benzylidenebarbituric acid derivatives were studied with NMR spectra, spectrophotometric determina-
tion of the pKa values, and quantum chemical calculations. Linear solvation energy relationships (LSER)
and linear free energy relationships (LFER) were applied to the spectral data - UV frequencies and 13C
NMR chemical shifts. The experimental studies of the nature of the ground and excited state of investi-
gated compounds were successfully interpreted using a computational chemistry approach including
ab initio MP2 geometry optimization and time-dependent DFT calculations of excited states.
Quantification of the push–pull character of barbituric acid derivatives was performed by the 13CNMR
Available online 13 February 2021
Keywords:
Barbituric acid derivatives
Push–pull systems
LFER analysis
ICT process
chemical shift differences, Mayer
p bond order analysis, hole-electron distribution analysis, and calcula-
Hole-electron distribution analysis
tions of intramolecular charge transfer (ICT) indices. The results obtained show, that when coupled with a
strong electron-donor, barbituric acid can act as the electron-acceptor in push–pull systems, and when
coupled with a strong electron-acceptor, barbituric acid can act as the weak electron-donor.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
Barbituric acid and its synthetic derivatives are getting great
attention from researchers due to their unique structural proper-
⇑
Corresponding author.
E-mail address: mmilcic@chem.bg.ac.rs (M.K. Milcic).
ˇ ´
https://doi.org/10.1016/j.saa.2021.119576
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

ˇ
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I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
ties and important technological and pharmaceutical applications.
Various derivatives of barbituric acid were obtained by introducing
the substituents in the C5 and N1/N3 positions of the barbituric
acid ring, yielding compounds with a broad spectrum of biological
activities such as antibacterial, antioxidant, as inhibitors of enzyme
tyrosinase [1–3], and as selective oxidizing agents [4] for the
The nature of the ground and excited state of synthesized com-
pounds were characterized and investigated using spectrochemi-
cal, solvatochromic (including LFER and LSER analysis), and
computational chemistry approaches including optimization the
molecular geometry, time-dependent DFT calculations of excited
states, and the difference in charge distribution between the
ground and excited state. Besides, the push–pull character of inves-
tigated molecules was quantified through parameters such as
chemical shift differences, absorption spectra, HOMO-LUMO gap,
hole-electron distribution analysis, and intramolecular charge
transfer (ICT) analysis of excitations.
synthesis of
unsymmetrical
disulfide [5,6]. Moreover,
5-ethylidenepyrimidine-2,4,6(1H,3H,5H) trione moiety has a broad
range of applications in the design of dyes [7] and pigments [8]
with multifunctional properties and as organic nonlinear optical
materials [9,10].
Electron-donating substituent (D - push) covalently bonded to
electron-accepting substituent (A - pull) via a
(linker) represent a class of molecules known as push–pull elec-
tronic systems (D- -A). These compounds have attracted a great
p
-conjugated bridge
2. Experimental method
p
2.1. General method for synthesis of 5-benzylidenebarbituric acid
derivatives
attention as organic materials with promising electronic and opti-
cal properties applied in non-linear optics (NLO) [11–14], dye-
sensitized solar cells (DSSCs) [15,16], organic light-emitting diodes
(OLEDs) [17], colorimetric pH sensors, and ions detection [18,19].
The structural motifs, important for the potential application of
push–pull barbituric acid derivatives as NLO chromophores, are
summarized in a brief article of Ikeda et al [20]. Some of the main
features of push–pull systems are low energy intramolecular
charge-transfer (ICT) band in absorption spectra, low HOMO-
LUMO gap, and high value for first molecular hyperpolarizability.
In this study, a series of substituted 5-benzylidenebarbituric
acid derivatives were synthesized by Knoevenagel condensation
[35,36] of barbituric acid with the corresponding benzaldehyde.
3 mmol of barbituric acid was dissolved in 20 ml hot distilled
water and aromatic aldehyde (3 mmol) and 10 ml of 95% ethanol
were added. The solution was stirred at room temperature
between 0.5 h and 2 h, depending on the added aldehyde. The
crude product was filtered off, purified by recrystallization from
cold 95% ethanol, and dried. After the screening of different
reaction conditions of Knoevenagel condensation, we found out
that this was the most convenient way for the synthesis of
p-substituted 5-benzylidene barbiturates.
By varying three key elements (donor,
p-linker, and acceptor)
one can finely tune the HOMO-LUMO gap of the compound and
modulate corresponding charge transfer interactions, which can
influence its optoelectronic properties [21]. The computational
chemistry methods of today can predict and explain the push–pull
properties of a molecule. High level ground state quantum-
chemical calculations can accurately predict the HOMO-LUMO
gap, while time-dependent density functional theory (TDDFT) cal-
culations of the excited state allow us to estimate the potential for
2.2. Materials and methods
The chemical structure and purity of the synthesized com-
pounds were confirmed by melting point, IR, 1H and 13C NMR,
and elemental analysis. The melting points were determined on
Stuart SMP10 digital melting point apparatus in an open capillary
tube and are uncorrected. 1H and 13C NMR spectra were obtained
using Varian 2000 spectrometer (200 MHz for 1H and 50 MHz for
13C) at room temperature in deuterated dimethyl sulfoxide
(DMSO d6) with TMS as the internal standard. Chemical shifts are
reported in ppm (d). IR spectra were recorded in Perkin Elmer
FTIR-Spectrometer 1725X using the KBr pellet method. Microanal-
ysis of carbon, hydrogen, and nitrogen was carried on an Elemen-
tarVario EL III. UV/Vis spectra were recorded in Shimadzu UV-
1700 spectrophotometer using spectro-quality solvents at a fixed
concentration sample of 1ꢁ10ꢂ5 mol/dm3. The UV/Vis spectra for
pKa values determination were recorded on Cintra 6 spectropho-
tometer (GBC Dandenong, Australia) with a 1 cm quartz cuvette
in 220–700 nm range against the phosphate buffer as a blank.
The pH values were measured with CRISON 50 29 micro-
combined pH electrode (CRISON INSTRUMENTS, S.A. Spain). The
electrode was calibrated by standard buffer solutions (pH 4.01,
7.00, and 9.21). The pKa values for different tautomers of com-
pound 1 were predicted using MarvinSketch software (https://che-
maxon.com/products/marvin).
the ICT transition in the various organic p-systems, as well as elec-
tronic properties of push–pull chromophores [22–25]. Also, based
on the calculated electron density difference between ground
and excited state charge-transfer distance (DCT), which represents
the distance between the location of the departing electron from
the ground state and the locations of the arriving electron in the
excited state, and the amount of charge transferred upon excitation
(QCT) can be calculated [22].
The structure and properties of the barbituric acid moiety indi-
cate that pseudoaromatic pyrimidine-2,4,6-trione ring can act as
electron-donating or electron-accepting substituent in push–pull
systems [26]. Electron-accepting abilities of barbituric acid moiety
are well documented in the literature [7,27–30]. However, barbi-
turic acid as electron-donor moiety in the molecules of the push–
pull type is not yet confirmed [19,31–34].
In this study, a series of eleven 5-benzylidenebarbituric acid
derivatives with different substituents were synthesized (Fig. 1)
with the purpose of experimental and theoretical analysis on the
nature of compounds and intermolecular interactions. Based on
electron-donating/accepting properties the substituents can be
roughly divided into four groups: group I is composed of strong
electron-donors i.e. substituents with strong positive resonant
effect (R) and weak negative inductive effect (I); group II contains
weak electron-donors (alkyl substituents) with weak positive R
and negligible I; in group III – (halogen group) are weak electron-
acceptors, with large negative I and weak positive R and in group
IV are strong electron-acceptor substituents with negative R and
negative I. Also, an unsubstituted compound (compound 7) was
added for comparison purposes. A list of all substituents, together
with their Hammett rp+ parameters, as the good indicator of
electron-donating/accepting ability, is shown in Fig. 1.
Qualitative estimation of the influence of the solvent on UV
spectra was performed with a linear solvation energy relationship
(LSER) analysis using the Kamlet-Taft solvatochromic equation
[37]. This model separates non-specific (electrostatic effect - dipo-
larity/polarizability) and specific (hydrogen bond) solute-solvent
interactions as given by Eg. (1):
vmax
¼
vo þ s
p
ꢃ þbb þ a
a
ð1Þ
where mmax is the absorption maxima shifts; mo is the regression
value in cyclohexane as reference solvent; * is an index of the
p
2

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´
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Fig. 1. Chemical structures of 5-benzylidenebarbituric acid derivatives including atom numbering and Hammett rp+ values.
solvent dipolarity/polarizability;
hydrogen-bond donor (HBD) acidity and b is a measure of the sol-
vent hydrogen-bond acceptor (HBA) basicity. The Kamlet-Taft sol-
a is a measure of the solvent
ties with charge-transfer indices (DCT and QCT) [22]. The CT indices
were calculated in acetonitrile solution (simulated with PCM
method) with the method proposed by Jacquemin et al [23]. All
the theoretical calculations were done in the quantum chemistry
vent parameters [37,38]
p*, b, and a are given in Table S1. The
regression coefficients s, b, and a in Eq. (1) represent the individual
contribution of solvatochromic effects on absorption frequencies.
The influence of substituent on the electron density distribution
was studied by using a variety of linear free energy relationship
(LFER) models [39,40]. For the correlation of NMR spectral data,
the general form of one-parameter Hammett equation was initially
used (S4) and then analyzed along with UV frequencies using two-
parameter Hammett-Taft (extended Hammett Equation) and
Swain-Lupton type correlations of substituent effects in their gen-
eral form given by Eq. (S5) and (S6). Correlation analysis was per-
formed using Microsoft Excel computer software at a confidence
level of 95%. The correlation coefficient (r), the standard error of
the estimate (sd), Fisher’s significance test (F), and t-test (t) for
individual regression coefficients were used as a measure of good-
ness of fit in regression analysis. All dependent variables were
autoscaled between 0 and 1.
program Gaussian09 [44]. Mayer
p bond order analysis of the nat-
ure of chemical bond [45], hole and electron distribution, concep-
tual DFT global reactivity descriptors and interfragment charge
transfer analysis [46] was performed using Multiwfn 3.6 wave-
function analyzer [25] together with the Gaussian quantum chem-
ical code.
4. Results and discussion
4.1. Isomerism of 5-benzylidenebarbituric acid derivatives
4.1.1. Experimental 1H/13C NMR study
5-Benzylidenebarbituric acid derivatives are structurally versa-
tile compounds that might exist in several isomeric/tautomeric
forms in solution (Fig. S1). The 1H NMR spectra of compounds 1–
12 found triketo form as the dominant tautomeric species in
DMSO, as concluded by the presence of two –NH signals at
~11.3 ppm and the absence of signals of enolic –OH protons. Also,
three distinct signals from carbonyl groups (165–163 ppm,
162.7–161.5 ppm, and 150 ppm) are found in 13C NMR. These
results corroborate previous studies on the isomerism of barbituric
acid [47–49] as well as with crystal structures of benzylidenebar-
bituric acid compounds [50,51] deposited in the Cambridge
Structural Database [52].
2.3. Characterization of 5-benzylidenebarbituric acid derivatives
1H and 13C NMR, IR, melting points, and elemental analysis data
of compounds 1–12 are given in Supplementary material.
3. Computational methods
Optimal geometries of all investigated molecules were obtained
by using the second-order Møller-Plesset (MP2) method [41] with
the 6-311G(d,p) basis set. Absorption spectra were calculated
using the TDDFT method, more specifically B3LYP functional which
includes the Becke three-parameter exchange [42] and the Lee,
Yang, and Parr correlation functionals [43] with a 6-311G(d,p)
basis seton MP2/6-311G(d,p) optimized geometries. For absorption
spectra calculations PCM (polarizable continuum model) implicit
model of solvation effect was used for solvent simulation. The
amount of charge transfer between the donor and acceptor was
estimated as a difference between ground and excited state densi-
4.1.2. Spectrophotometric determination of the pKa values
To
confirm
the
tautomeric
preferences
of
5-benzylidenebarbituric acid derivatives, we determined the pKa
values in aqueous solution for compound 1 bearing the strongest
electron-donating substituent within the series. The macroscopic
pKa values are expected to depend on the tautomeric form of the
compound and the pKa for different tautomers predicted with
MarvinSketch software for compound 1 are shown in Fig. S2. The
changes in the ionization state of compound 1 were monitored at
the absorption maxima around 255 nm (Figs. S3–S5).
3

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I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
Compound 1 is expected to show three ionization steps in the
measurable pH range, two from barbituric acid moiety and one
from protonated N,N-dimethylamino group at low pH values.
Deprotonation of protonated N,N-dimethylamino group increases
the electron density at the benzene ring chromophore. This reflects
in the increased intensity of absorption peak at 255 nm (Fig. S3).
Such a trend is continued at higher pH values where ionization
of barbituric acid moiety occurs (Fig. S4). At even higher pH values,
deprotonation of the third ionization center of compound 1occurs
and results in the decreased absorbance of the benzene ring chro-
mophore (Fig. S5).
Optimized geometries of all compounds are in nonplanar con-
formation and this is in agreement with the geometries of crystal
structures of 3 keto tautomers of 5-benzylidenebarbituric acid
derivatives found in the Cambridge Structural Database (CSD)
(Fig. S10) [52]. The values of the dihedral angle h, defined as the
angle between planes containing benzene ring and barbituric acid
moiety, are shown in Table 2.The large value for angle h in unsub-
stituted compound 7 (42.4°) indicates a strong steric repulsion
between carbonyl group from barbituric acid and ortho-hydrogen
from the benzene ring, and a low amount of
p-electron delocaliza-
tion between two - systems. Since the introduction of the sub-
p
The absorption spectra of pure H2A and HAꢂ were detected in
the limited pH range where no further shift of the absorption spec-
tra was observed as pH increased. The ionization constants of com-
pound 1, as well as the absorbances of H3A+ and A2– forms, are
obtained through the linearization of spectral data according to
equations S1-S3 (Figs. S6–S8). Table 1 summarizes the pKa values
for compound 1. According to pKa predictions for three tautomeric
forms (Fig. S2), a high pKa3 value (12.27 0.02) indicates that tri-
keto form is the main tautomeric form in solution (Fig. S2, left).
It is reasonable to hypothesize that the other members of the con-
generic series also exist as triketo tautomer in aqueous solution.
stituent at C11 position will have no effect on the steric
interactions, the changes in the dihedral angle h can be a good indi-
cator of the influence of the electronic effects of the substituent on
the
The geometry of compound 1 is close to planar (h = 4.4°) indi-
cating a stronger delocalization of - electrons across benzylidene
p-electron delocalization in the system.
p
moiety. Other compounds from the group I also have substantially
lower values for angle h (around 20°) than unsubstituted com-
pound 7, but the deviation from planarity is higher than for com-
pound 1. On the other side, compounds from group IV have a
higher value of angle h than unsubstituted compound due to the
negative resonance effect of strong electron-accepting –CN and –
NO2 substituent.
4.1.3. Quantum chemical study
Additionally, the MP2/6-311G(d,p) method was used for opti-
mizing the geometry of all possible tautomers and determining
their relative stability in the gas phase. The calculations confirm
that the triketo forms are the most stable (Table S4). The diketo
and monoketo tautomers are by 15.4–20.6 kcal/mol and 32.5–35.
4.2.1. Correlation between dihedral angles and Hammett and Swain-
Lupton substituent constants
In order to differentiate between contributions of inductive
(field) and resonance effects of the substituent to the angle h,
regression analysis according to the dual substituent parameter
(DSP) Swain-Lupton model was done (equation S7). A good corre-
lation is observed (r = 0.98). A small value for f (ꢂ0.02) indicates
that the inductive (field) effect will not influence the value of a
dihedral angle. On the other side, large and positive value for cor-
relation coefficient r shows that the resonance effect of the sub-
stituent will be a dominant effect in determining the planarity of
the molecule. Substituents with large electron-donating resonance
effect (group I) will increase the delocalization of the electrons
between the benzene ring and the double bond in the benzylidene
fragment, resulting in the more planar geometry and lower values
of angle h. The opposite is true for compounds with –R effect (group
8
kcal/mol, respectively, less stable than triketo tautomer
(Table S4). The optimized structures of the most stable isomer of
5-benzylidenebarbituric acid derivatives from ab-initio calcula-
tions are depicted in Fig. S9.
4.2. Geometry optimization of 5-benzylidenebarbituric acid
derivatives
A previous ab initio theoretical study examining the molecular
structure and torsional profile of styrene found that the geometry
of styrene is principally determined by the
vinyl and phenyl groups and steric interactions [53]. The overesti-
mation of the -conjugation in DFT calculations leads to predic-
p – p interactions of the
IV), delocalization of
p-electrons between the benzene ring and a
p
double bond is decreased and the value for angle h is higher than
in unsubstituted compound. The same qualitative results are
obtained with extended Hammett equation (S8) (results of the cor-
relation are shown in SI).
tions of planar global minimum energy structure [54]. Other
theoretical studies for this system [55,56] also confirmed that the
results of the density functional theory calculations are in dis-
agreement with the results from the Møller-Plesset second-order
approximation. They concluded that the more conventional MP2
theory predicts much larger torsion angles, twisted styrene, and
reasonably good agreement with the experimental results [57].
In addition to the above approach, geometries of all investigated
compounds were fully optimized by minimizing the energies using
the MP2 method with a 6-311G(d,p) basis set, and the nature of
minima was confirmed by subsequent frequency calculations –
no imaginary frequencies were found. We have shown that this
method produced results that are in good agreement with experi-
mental findings [58,59]. The results of these calculations are pre-
sented in Fig. S9 and Table 2.
4.3. LFER analysis of NMR data and Mayer
p bond order analysis
In order to analyze the transfer of electronic effect of sub-
stituents through the molecules in their ground state, 13C NMR
´
data were fitted with Hammetts mono substituent parameter
´
´
(MSP) and extended Hammetts and Swain-Luptons dual sub-
stituent parameter (DSP) models. The results of 1H and 13CNMR
spectroscopy showed that the chemical shifts are very sensitive
to variable electron-donor and electron-acceptor substituents in
benzylidene moiety. The values of substituent-induced chemical
shifts (SCS) are shown in Table S5 as the difference between the
chemical shift of unsubstituted compound 7 and the corresponding
substituted derivative. As can be seen from Table S5, the high sen-
sitivity of the 13C NMR chemical shift to the local environment is
observed for carbon atoms in the phenyl moiety of the molecules
[60].
Table 1
An overview of experimentally determined pKas of compound 1.
Compound 1
Ionization center
pKa1
pKa2
pKa3
4.20 0.02
7.88 0.12
12.27 0.02
N,N-dimethylamino group
Barbituric acid amido NH
Barbituric acid amido NH
The substituents from groups I, II, and III increases
density around the C8 and C10 atoms, (average SCS values of
ꢂ7.4 ppm and ꢂ14.8 ppm, respectively) and decrease -electron
p-electron
p
4

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Table 2
The values of the dihedral angles for compounds 1–12 (in degrees) calculated at MP2 level.
Compound
1
2
3
4
5
6
7
8
9
10
11
12
Dihedral angle, h(°)
4.4
19.6
20.7
20.0
39.1
39.0
42.4
38.3
39.6
40.2
44.8
44.4
density on C9 atom (average chemical shift values of +4.6 ppm) due
to positive resonance effect. In the case of substituents with nega-
tive resonance effect (compounds from group IV) the electron den-
sities on C8 atom are reduced causing an upfield shift of 4.8 and
7.0 ppm for compounds 11 and 12. The chemical shift of C9 atom
is unaffected by electron-accepting substituent and there is a large
upfield shift for C10 atom in compound 12.
results of the calculated Mayer
are shown in Table 3.
Evident from the presented results is that the
the C7-C8 bond is decreasing (from 0.254 to 0.110) and the C5-C7
bond order is increasing (from 0.545 to 0.704) with increasing
electron-accepting capability of the substituent (Table 3). This is
in line with the increasing deviation from planarity found in opti-
p bond order for selected bonds
p bond order for
p
Much more interesting are SCS in C7 and C5 carbon atoms or C
mized geometries of investigated compounds, i.e. the lower the p
a
and Cb as often referred to in the literature [57,58,61,62]. The
chemical shift of C7 carbon atom are almost unaffected by sub-
stituents from groups I and II and shifted upfield for compounds
from groups III and IV. The opposite behavior is observed for C5,
strong upfield shift for compounds from groups I and II, and down-
field shift for compounds from groups III and IV.
bond order of the C7-C8 bond is, the higher is the angle between
two planes. This practically means that the increase in electron-
accepting capability of the substituent at the benzene ring will
decrease the delocalization of
the compounds from the group I (especially in compound 1) there
is a substantial amount of -electron delocalization between ben-
p-electrons across the molecule. In
p
In order to elucidate this, we investigated the distribution of
electronic densities from B3LYP wavefunctions for ground states of
compounds 1–12. This distribution is shown in Fig. 2. Also, the
p
-
zene moiety and exocyclic C5-C7 bond (Fig. 2). This delocalization
provides a resonant structure with the increased electron density
at the C5 carbon atom, causing a strong upfield SCS in 13C NMR
Fig. 2. Isosurface map (isosurface value = 0.027 au) of the distribution of p-electronic densities of compounds 1–12.
5

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I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
Table 3
Mayer
p
bond orders for compounds 1-12.
Bond
Bond order
1
2
3
4
5
6
7
8
9
10
11
12
C4-C5
C5-C7
C7-C8
0.130
0.545
0.254
0.118
0.597
0.209
0.118
0.597
0.209
0.118
0.596
0.210
0.109
0.649
0.158
0.109
0.650
0.158
0.105
0.669
0.140
0.107
0.656
0.154
0.105
0.667
0.145
0.104
0.669
0.143
0.097
0.700
0.113
0.096
0.704
0.110
spectra. For the compounds from group IV there is much less delo-
calization of electronic density between benzene moiety and
exocyclic C5-C7 bond (Mayer bond order less than 50% of the
From the DSP models (Tables 5 and 6), the positive sign of cor-
relation coefficients for the C8 and C5 atoms and k > 1 (resonance
effect predominates over inductive/field effects) reflect normal
p
p
bond order for a group I, Table 3), so the resonant effect have little
substituent electronic effect while the negative values of qI, qR, f,
influence on the SCS of C5. The strong downfield shift for C5 and
and r for C7, and k < 1 indicate the reverse effect. The obtained
results confirm the existence of a different mechanism of transmis-
sion of electronic substituent effects which includes localized and
upfield shift for C7 can be attributed to the localized
between the two -systems ( 1 and 2, Fig. 3). In a similar manner,
the weak and downfield shift of the C7 in the compound from the
p-polarisation
p
p
p
extended polarisation followed by extended
p-resonance interac-
group I can be attributed to extended p-polarisation in the p1-sys-
tion, resulting in a change in molecular geometry.
tem of these compounds (Fig. 3).
Also, results of the Mayer
there is very little delocalization of
acid moiety and the rest of the molecule, which is decreased when
going from the group I to group IV (from 0.130 in compound 1 to
0.096 in compound 12, Table 3). Based on the results of SCS from
p
bond order analysis indicate that
-electrons between barbituric
4.3.1. The difference of chemical shift values,
push–pull character
Dd, as a measure of the
p
Important properties of push–pull molecules are connected
with differences between the chemical shifts (
dC=C) of two sp2
carbon atoms which form an exocyclic double bond [63]. The indi-
cators for the push–pull effect are high values of dC=C and C@C
bond elongation. In case that the resonance effect is dominating,
high values of dC5=C7 are suited as direct measures of the push–
pull effect. As can be seen in Table 7, the highest value of dC5=C7
D
13C NMR spectra and the Mayer
stronger delocalization of
and vinyl group in compounds from the group I, we considered that
these compounds will have only two -systems ( 1 and 2, Fig. 3a).
On the other hand, compounds from group II, III, and IV will have
three -system (p1 2, and 3, Fig. 3b). This is further confirmed
p
bond order analysis indicating
D
p
-electrons between the benzene ring
D
p
p
p
D
(44.34 ppm) for the strongest electron-donor substituent (com-
pound 1) indicated a higher degree of resonance. On the contrary,
p
,
p
p
by very weak SCS on C4 (an only compound from the group I have
C4 SCS larger than 1 ppm) and negligible SCS on C2, Table S5.
To further analyze the substituent effect on 13C chemical shifts
of relevant carbon atoms (C5, C7, and C8), linear models of Hammett
and Swain-Lupton type were applied. The correlation results
obtained by applying regression (LFER) models with literature sub-
electron-accepting groups cause a decrease in
lower degree of -electron delocalization, which is confirmed by
the small value of dC5=C7 (28.81 ppm) for compound 12. Based
on the obtained results, compound 1 is the best candidate for the
push–pull system and further study in unraveling the existence
of the intramolecular charge transfer (ICT) process.
DdC5=C7 and the
p
D
stituent constants (rp, , rI, rR+, F, and R+) are given in Tables 4–6.
rp+
Better correlation with r+p electrophilic substituent constants
for C5 and C8 (Table 4) atoms indicates a considerable contribution
of extended resonance interaction in the overall electronic effect
4.4. Absorption spectra of 5-benzylidene barbiturates in different
solvents
transmitted through
also noticed for C5 and C8. On the other hand, a negative sign for
values and better correlations results for C7 atom in the case with
rp substituent constants indicates a reverse trend.
p-units. Normal substituent effect (q > 1) is
It is well known that the position of absorption maximum
depends on the chemical and physical properties of certain solvent
as a result of appropriate electronic transition which directly
depends on chromophores and type of electron transition. The
q
Fig. 3. Compounds from the groups I, II, III, and IV with the contribution of (a) localized p-polarisation and (b) extended p-polarisation.
6

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Table 4
Correlation results of 13C SCS values with the Hammett substituent constants rp and rp+
.
Atom
C5
q
h
r
sd
t
F
rp
rp+
rp
rp+
rp
rp+
2.18 0.19
1.42 0.03
ꢂ2.12 0.24
ꢂ1.24 0.22
2.16 0.21
1.39 0.09
0.01 0.08
0.37 0.02
ꢂ0.01 0.10
ꢂ0.33 0.16
0.01 0.09
0.37 0.07
0.97
1.00
0.94
0.87
0.96
0.98
0.27
0.08
0.36
0.51
0.31
0.22
11.72
42.71
ꢂ8.69
ꢂ5.64
10.37
14.64
137.38
1824.16
75.59
31.80
107.60
214.44
C7
C8
tkr(
= 2.26; Fkr( = 4.96.
a=0.05, m=1, m=9)
a
=0.05,
m
=9)
Table 5
Atom
Correlation results of 13C SCS values withrI and rR+
.
qI
qR
r
sd
It
Rt
F
qR/qF
C5
C7
C8
1.17 0.14
ꢂ2.75 0.54
0.83 0.36
2.19 0.09
ꢂ1.23 0.32
2.23 0.22
1.00
0.93
0.97
0.11
0.40
0.27
8.07
25.47
ꢂ3.83
10.34
479.42
30.44
71.93
1.87
0.45
2.69
ꢂ5.12
ꢂ2.27
I,R,f,r
d
t
kr(a
= 2.26; Fkr(
= 4.31.
m=9)
=0.05,
m=9)
a
=0.05,
m=2,
Table 6
Correlation results of 13C SCS values with F and R+.
Atom
f
r
r
sd
ft
rt
F
k(r/f)
C5
C7
C8
1.36 0.12
ꢂ3.12 0.45
1.10 0.33
1.43 0.04
ꢂ0.89 0.15
1.44 0.11
1.00
0.96
0.98
0.08
0.31
0.22
11.47
ꢂ6.89
3.34
35.25
ꢂ5.73
12.88
845.19
53.70
106.36
1.05
0.29
1.31
I,R,f,r
d
tkr(
= 2.26; Fkr(
= 4.31.
m=9)
a
=0.05,
m
=9)
a=0.05,
m=2,
Table 7
Selected chemical shift values of the carbon atoms (d, ppm) and chemical shift differences (
D
dC5=C7).
Compound
1
2
3
4
5
6
7
8
9
10
11
12
dC5
dC7
111.46
155.80
44.34
115.84
155.90
40.06
115.78
155.29
39.51
115.64
155.31
39.67
118.19
155.26
37.07
118.12
155.30
37.18
119.38
155.01
35.63
118.98
153.78
34.8
119.97
153.32
33.35
120.07
153.39
33.32
122.16
152.02
29.86
122.64
151.45
28.81
D
dC5=C7
term solvatochromism was used to describe the change of the posi-
tion of UV/Vis absorption band depending on dipolarity/polariz-
ability and proton or electron transfer between solvent and
solution, enabling assessment of basic properties of molecules
[64]. Generally, the planarity of molecules and the size of delocal-
ized p-electron systems have a decisive influence on the shape and
wavelength position of the UV/Vis spectra of a certain molecule.
Positive solvatochromism, appropriate bathochromic (red) shift,
ues of the maximum absorption frequencies, mmax, show that the
substituents on the phenyl ring of 5-benzylidenebarbituric acid
derivatives cause a bathochromic shift in relation to the unsubsti-
tuted compound 7 in all solvents for compounds from groups I, II,
and III. Compounds from group IV are hypsochromically shifted rel-
ative to the unsubstituted compound. The different behavior of
these compounds may be explained by the negative resonance
effect of the strong electron-acceptors (–CN and –NO2) in contrast
to the positive resonance effect of substituents in compounds from
other groups. Large positive resonance effect of substituents from a
group I induces the largest bathochromic shift. This shift is the
most pronounced in compound 1 because -NMe2 substituent has
the strongest resonance effect of all substituents (Table S6). Sub-
stituents in compounds from group II and III have similar, weakly
positive resonance effect, but compounds from group III are less
bathochromiaclly shifted because of large negative inductive effect
from their substituents. The influence of negative inductive effect
on the absorption band shift is the most visible in compounds from
group III; compound 10 is more bathochromcally shifted than com-
pounds 8 and 9 because -Br substituent has a weaker negative
inductive effect than -F and -Cl substituents. Compound 8 with flu-
orine substituent with strongest negative I is less bathochromically
shifted then compounds 9 and 10.
was connected with increased planarity which facilitates
electron delocalization.
p-
In order to analyze the effect of solvents, the absorption spectra
of the investigated compounds were recorded in 21 solvents with
different properties (polarity, HBA, and HBD ability), in the wave-
length range from 200 to 800 nm. The spectra show two absorption
maxima in the range of 230–480 nm. The lowest-energy maximum
at 310–480 nm and the second high-energy maximum at 230–
260 nm can be observed in the considered solvents. Since every
solvent has a UV absorbance cut-off wavelength, the second
absorption maximum, due to the absorption of some solvent in
that spectrum region, could not be registered in all the solvents,
so only the first absorption band was taken for further analysis.
The part of recorded UV spectra (280–600 nm) of all investigated
compounds in methanol, acetonitrile, THF, and chloroform are pre-
sented in Fig. 4. Table S6 contains absorption frequencies for the
first absorption band for all solvents.
4.5. Solvent effects on the UV/Vis absorption spectra - LSER analysis
The results clearly indicate that the values of the maximum
absorption frequencies,
substituents than on the nature of the solvents. Generally, the val-
mmax, depends more on the nature of the
From Fig. 4 it is obvious that the position of the first absorption
band of investigated compounds is very weakly influenced by the
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I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
Fig. 4. Absorption spectra of compounds 1–12 in MeOH (a), MeCN (b), THF (c), and TCM (d).
solvent, i.e. that the investigated compounds are weakly solva-
tochromic. Nevertheless, the effect of the solvent on the absorption
spectra of 5-benzylidenebarbituric acid derivatives was tested by
LSER analysis using the Kamlet-Taft solvent parameter sets. In
our research, 10 alcoholic solvents (fluorinated, mono- and polyhy-
droxy) and 11 aprotic solvents are selected for investigating the
position of the visible UV/Vis absorption band. In the case of alco-
holic solvents the Kamlet-Taft model including KAT solva-
tochromic parameters is the most suitable solvatochromic
method for the interpretation of the solvent polarity effects on
the absorption spectra [65]. The values of solvatochromic coeffi-
cients s, b, and a are listed in Tables S7. The presented results con-
firm that the solvatochromic effects observed in UV/Vis spectra of
1–12 are complex due to the interplay of both solvent and sub-
stituent effects.
Results of Kamlet-Taft analysis show that compounds with sub-
stituents with positive R (compounds from groups I and II) have a
negative sign of coefficient s indicating that increase of solvent
dipolarity/polarizability will increase the bathochromic shift of
the first absorption band. This effect is most pronounced for com-
pounds from the group I especially for compound 1 with strong
electron-donating -NMe2 substituent. This might be explained by
better solvation stabilization of the excited state relative to the
ground state. These results indicate that the excited state is more
polar and polarizabile compared with the ground state [21]. The
effect of solvent dipolarity/polarizability on compounds from group
III and IV is much weaker which is confirmed by negligible values
of regression coefficient s with high standard errors. A negative
sign of the coefficient a for all compounds (Table S7), except for
compound 11, indicates a bathochromic (red) shift with an
increase in the hydrogen-bond donor (HBD) capabilities of the sol-
vent. This indicates better stabilization of the excited state relative
to the ground state with increasing solvent acidity. Compound 11
is excluded from this analysis due to statistically insignificant
results. The highest absolute values of coefficient a was found for
compound 12.
The positive sign of the coefficient b for most compounds,
except for compound 12, reflects hypsochromic (blue) shift with
an increase of hydrogen-bond accepting (HBA) ability of the sol-
vent (Table S7). High positive values of the coefficient b indicate
higher contribution of solvent acidity to the ground state stabiliza-
tion. In this case, compounds 1 and 2 are excluded from the anal-
ysis (statistically insignificant results). Compound 12 is the
exception from this trend, negative and larger absolute value of
coefficient b indicates bathochromic (red) shift with increasing sol-
vent basicity (HBA). It can be noticed, from the results given in
Table S7, that in most cases kmax are more sensitive to the effects
of hydrogen bond acceptor ability of solvents. The obtained results
for coefficient b indicate that the nitro substituted derivative is
more susceptible to the solvent HBA stabilization in the excited
state. It could be postulated that the different behavior of com-
pound 12 is a consequence of the existence of
p3-electronic system
and the different conjugational ability of the
p-electron densities
through localized p-unit (p1-, p2-, and p3-) in the presence of the
NO2 group. This suggested that the introduction of the strong
electron-accepting substituents can lead to an increased proton-
donating ability of the NH group of barbituric acid.
On the other hand, the solvatochromic behavior of the enol
form of a nitro substituted barbiturates in solvents of different
polarity are well documented in the literature [19,31,33]. The enol
form of a nitro substituted derivatives of barbituric acid shows a
bathochromic shift in the UV/Vis absorption bands with increasing
8

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
I.N. Stojiljkovic, M.P. Rancic, A.D. Marinkovic et al.
solvent hydrogen-bond accepting capability. [19,31] The findings
indicate that the enol tautomer is stabilized by hydrogen bonds
between the enol OH group and solvent [19,31,33]. With this in
mind, it is possible that the presence of some amount of mono-
enol tautomeric form in solution of a nitro substituted derivative
12 will cause a bathochromic shift (and a negative sign of coeffi-
cient b) with increasing the solvent basicity. Based on this, broad-
ening and splitting of the first absorption band for compound 12
found in some of the considered solvents (Fig. S11) can originate
from keto-enol tautomerization of barbituric acid part of molecule
and specific solvent–solute interaction realized through hydrogen
bonding. In general, the effect of the solvent on the UV absorption
spectra of the investigated compounds is very complex and the
results suggest that there is a correlation between the potential
candidates for the push–pull or pull–push structure and the posi-
tion of absorption maxima in the solvents of different polarity.
0.90 eV to 1.47 eV) (Fig. S13). The same orbital transitions are
found in the second excitations with
a small admixture of
HOMO-2 ? LUMO transition with contributions from 14% to
23%. The lowest energy absorption maxima for compounds from
group IV is composed of three excitations with the second excita-
tion having the highest oscillator strength (Table S9, Fig. S12-d).
The dominant transition in this excitation is HOMO-1 ? LUMO
(49.03% and 40.20%), followed by HOMO ? LUMO (34.16% and
31.64%) and HOMO-2 ? LUMO (12.10% and 12.14%) transitions.
From MO energies shown in Fig. S13, it is obvious that the
energy of the LUMO orbital remains almost unaltered in com-
pounds from groups I, II, and III when compared to compound 7.
As expected, when strong electron-acceptors are introduced in
compounds from group IV the energy of the LUMO is significantly
lowered by 0.33 eV for compound 11 and 0.57 eV for compound
12. On the other side, the strongest electron-donor –NMe2 sub-
stituent in compound 1 will raise the energy of HOMO by 1.1 eV.
Other (weaker) electron-donating substituents from the group I
will also raise the energy of HOMO but by a lesser amount (from
0.44 eV to 0.51 eV). As a consequence, the HOMO-LUMO gap in
compounds from the group I will be significantly lower, resulting
in bathochromically shifted adsorption spectra [28,66] and higher
4.6. Substituent effects on the UV spectra- LFER analysis
To investigate the transmission of substituents effects in com-
pounds 1–12, extended Hammett’s equation (S5) was applied for
correlating light absorption properties. Based on the correlation
between the rI and rR substituent constants and the UV/Vis spec-
tral data (Table S8), a significantly stronger contribution of reso-
nance substituent effect relative to inductive/field effect is
value of electronic chemical potential (l) (Table S10). As expected,
compounds from group IV will have lowest value of electronic
chemical potential (highest electronegativity) and highest value
observed (ratio of
q
R/qI = 2.50–5.00 and r value between 0.95
of global electrophilicity index (x) (Table S10) since they have a
and 0.98) in all solvents. Generally, the electronic effects are trans-
mitted through the molecule in the same manner in all solvents, as
confirmed by the positive value of the proportionality constant, q.
strong electron-accepting groups in their structure. On the other
hand, compound 1, with strong electron-donating substituent will
be the strongest nucleophile, with highest nucleophilicity index
(N) among investigated compounds (Table S10) [67,68].
4.7. ICT process and evaluation of push–pull chromophores
The intramolecular charge transfer (ICT) process occurs during
the absorption of a photon by the excitation of molecules when
electron density is shifted from one moiety of a molecule to
another one. The electron density difference between the ground
and the excited state has been used to quantify the efficiency of
charge transfer in potential push–pull chromophores. ICT indices
i.e. charge-transfer distance (DCT) which represents the distance
between two barycenters and the amount of transferred charge
(QCT) for all investigated compounds were calculated and results
are presented in the last two columns of Tables S9 and in
Fig. S14. Based on TDDFT results obtained, it can be seen that the
strongest ICT occurs in compounds from the group I (DCT from
4.20 to 3.05 Å) with the strongest electron-donating substituents.
For example, in the compound 1, upon excitation,electronic
density is shifted across the longest distances (4.20 Å), from the
electron-donating -NMe2 substituent to the C7 atom in the vinyl
group and carbon atoms from the pyrimidine ring of barbituric
acid. This was confirmed by the hole-electron distribution analysis
of the first excitation of compound 1 (Fig. S15-a). In this excitation,
28% of the hole is located on the nitrogen atom from -NMe2 sub-
stituent and 40% of the electron is located on barbituric acid moi-
ety. The rest of the electron is located on the C7 atom (30%). Also,
carbon atoms from the benzene ring contribute substantially to
the hole (42%). Generally, as in the typical push–pull system, exci-
tation in compound 1 can be described as electron transfer from -
NMe2 substituent and benzene ring as electron-donors to the C7
vinyl atom and barbituric acid as electron-acceptors. Similar con-
clusions can be drawn for other compounds from group I. Com-
pounds from groups II and III have two excitations and none of
them can be described as ICT excitation (Fig. S14). In these com-
pounds, some amount of electron density is transferred from ben-
zene ring carbon atoms to the C7 atom from the vinyl group. Also,
upon excitation there is a noticeable rearrangement of electron
density in barbituric acid; electron density is transferred from oxy-
gen atoms to the carbon atoms from the same carbonyl group
(Fig. S14).
In order to further elucidate the nature of absorption spectra,
TDDFT calculations for all investigated compounds in solvent ace-
tonitrile were done and the results are shown in Table S9. Since our
goal was to investigate the nature of lowest energy absorption
maxima found in experimental spectra (between 310 and
480 nm, i.e. 4.00–2.58 eV) results are shown only for excitations
with wavelengths higher than 290 nm (4.28 eV) and oscillator
strength larger than 0.05. The results of TDDFT calculations
(Fig. S12) are in good qualitative agreement with experimental
spectra; compounds from a group I have the strongest bathochro-
mic shift compared to unsubstituted compound, the compounds
from groups II and III are slightly bathochromically shifted and
absorption maxima for compounds from group IV are at the same
wavelength as the unsubstituted compound (slightly hipsochromi-
cally shifted in experimental spectra). TDDFT calculations were
also able to correctly predict the influence of negative inductive
effect on absorption band in compounds from group III (Fig. S12-
c); compound 10 with -Br substituent (weaker –I effect) is more
bathochromically shifted than compounds with -Cl and -F sub-
stituent (stronger –I effect).
For compounds from a group I there is only one excitation in the
selected energy range and it can be characterized as the simple
HOMO ? LUMO transition with more than 95% contribution from
this orbital transition (Table S9). Compounds from groups II and III
and unsubstituted compound 7 have two excitations in the
selected energy range with similar oscillator strengths and energy
difference from 21 to 26 nm so the lowest energy absorption max-
ima observed in experimental spectra will be the superposition of
these two excitations (Fig. S12, b and c). The first excitation in
these compounds is dominated by HOMO ? LUMO transition
(from 55% to 75% contribution) with an admixture of HOMO-
1 ? LUMO (from 13% to 34%). This can be explained with smaller
HOMO to HOMO-1 gap in these compounds (from 0.30 eV to
0.61 eV) when compared to compounds from the group I (from
9

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119576
A reverse direction of ICT is observed in the first two excitations
of compound 12. Here a positive barycenter (representing a center
of the charge loss) is located close to barbituric acid and a negative
barycenter (representing center of the charge gain) is at benzene
ring with –NO2 substituent. This implies that, upon excitation,
there is some charge transfer from barbituric acid as an electron-
donor to –NO2 substituent as an electron-acceptor. This was fur-
ther confirmed with hole-electron distribution analysis (Fig. S15-
b). In the first excitation, the hole is mostly located at barbituric
acid oxygen atoms (60%) and the electron is mainly located at C7
atom (25%) and a smaller amount at –NO2 group (14%). ICT from
barbituric acid to NO2 is also noticed in the second excitation;
45% of the hole is located at barbituric acid moiety and 30% of
the electron is located at –NO2 group. Interfragment charge trans-
fer (IFCT) analysis [25] can be a valuable tool to quantitatively
describe electron redistribution between fragments during the
excitation [69]. The results of IFCT analysis show that a total of
0.11 electrons are transferred from barbituric acid moiety to –
NO2 substituent in the first excitation and 0.08 electrons are trans-
ferred from barbituric acid moiety to –NO2 substituent in the sec-
ond excitation. These results clearly point out that, when coupled
with the strong electron-accepting chromophore barbituric acid
can act as a very weak electron-donating group. This observation
shows that in all chromophores investigated, except compound
1, some extent degree of charge transfer occurs, but they are not
promising candidates for push–pull systems and cannot be used
as typical CT compounds.
of charge transfer in potential push–pull chromophores indicates
that the strongest ICT occurs in compounds with the strongest
electron-donating substituents (DCT from 4.20 to 3.05 Å).
The results obtained from the analysis of excitations show, that
when coupled with a strong electron-donor, barbituric acid can act
as the electron-acceptor and when coupled with a strong electron-
acceptor, barbituric acid can act as the weak electron-donor.
These studies clearly show that compound 1 can be considered
as the best candidate for a push–pull system. It means that excita-
tion in compound 1 can be described as electron transfer from -
NMe2 substituent and benzene ring as electron-donors to the C7
vinyl atom and barbituric acid as electron-acceptors. Other chro-
mophores investigated do not possess push–pull character and
cannot be used as typical CT compounds.
CRediT authorship contribution statement
Ivana N. Stojiljkovic: Investigation, Writing - original draft,
Writing - review & editing, Formal analysis, Project administration.
ˇ ´
Milica P. Rancic: Conceptualization, Validation. Aleksandar D.
´
Marinkovic: Supervision, Resources, Funding acquisition. Ilija N.
´
ˇ ´
Cvijetic: Investigation, Formal analysis. Miloš K. Milcic: Conceptu-
alization, Methodology, Writing - original draft, Writing - review &
editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
5. Conclusion
A systematic experimental and theoretical investigation of elec-
tron interactions in different substituted 5-benzylidenebarbituric
acid derivatives has been done. The tautomeric preferences of
investigated compounds have been studied by NMR spectra, spec-
trophotometric determination of the pKa, and quantum chemical
calculations, and our results confirm that the most stable tautomer
of the barbituric acid moiety is the triketo form.
Acknowledgement
This work was supported by the Ministry of Education, Science
and Technological Development of Republic of Serbia (Contract
number: 451-03-9/2021-14/200168).
MP2 optimizations suggest that the most stable keto isomers of
5-benzylidenebarbituric acid derivatives are generally nonplanar
structures except derivatives bearing strong electron-donating
substituents. The geometry of compound 1 containing strong
electron-donor substituent is close to planar (h = 4.4°) indicating
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.saa.2021.119576.
a stronger delocalization of p-electrons across benzylidene moiety.
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dependent than solvent-dependent. The Hammett SSP and Swain
Lupton DSP models of 13C NMR chemical shift indicate that polar
and resonance effects of substituent operate at the C8 and C5 atoms
and a reverse effect at C7.
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