The synthesis, spectroscopic characterization, DFT/TD-DFT/PCM calculations of the molecular structure and NBO of the novel charge-transfer complexes of pyrazine Schiff base derivatives with aromatic nitro compounds

Article abstract of DOI:10.1039/d0nj05397j

The novel charge-transfer (CT) solid complexes of pyrazine Schiff bases, derived from 2-aminopyrazine and substituted benzaldehydes (N-benzylidenepyrazin-2-amine, (NBPA)) and N-(((4-dimethylamino)benzylidene)pyrazin-2-amine) (NDMABPA) with some aromatic n

Full text of DOI:10.1039/d0nj05397j

View Article Online
View Journal
NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry
This article can be cited before page numbers have been issued, to do this please use: S. A. E. Abdel-
Latif, Y. Issa, A. L. Elansary and H. B. Hassib, New J. Chem., 2020, DOI: 10.1039/D0NJ05397J.
Volume 42
Number
March 2018
Pages 3963-4776
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
6
21
NJC
New Journal of Chemistry
rsc.li/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
rsc.li/njc

Page 1 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
Synthesis, spectroscopic characterization, DFT/TD-DFT/PCM calcu
D
la
O
t
I:
i
1
o
0.1
n
03
s
9/D0NJ05397J
o
f
molecular structure and NBO of novel charge-transfer complexes of pyrazine
Schiff base derivatives with aromatic nitro compounds
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
b,*
a
a
Yousry M. Issa , Samir A. Abdel-Latif Aida L. El-Ansary and Hekmat B. Hassib
a
Chemistry Department, Faculty of Science, Cairo University, Giza, 12613, Egypt.
bChemistry Department, Faculty of Science, Helwan University, Cairo, 11795, Egypt.
Novel Charge-transfer (CT) solid complexes of pyrazine Schiff bases, derived from
2-aminopyrazine and substituted benzaldehydes (N-benzylidenepyrazin-2-amine,
(
NBPA) and N-(4-dimethylamino)benzylidene)pyrazin-2-amine) (NDMABPA) with
some aromatic nitro compounds have been synthesized and characterized
experimentally using ultraviolet-visible (UV-Vis.) absorption, infrared spectra and
1
proton nuclear magnetic resonance ( HNMR) spectroscopy, the complexes were
formed in a mole ratio of 1:1, with good indications for existing charge-transfer in its
molecular structure. Theoretical studies were done on donors and acceptors
elucidating their structures and the active sites where the charge-transfer occurs.
The experimental work was done in ethanol. The solution characterizations included
the determination of the molecular structure of the formed CT complexes where it
verified 1:1 (donor:acceptor) in ethanol. Quantum mechanical calculations of
geometries, energies were attained using the density functional theory with Becke's
three parameter exchange functional method, the Lee-Yang-Parr correlation
functional approach (B3LYP/DFT) combined with 6.31G(d,p) basis set has been
consecutively out in solution using ethanol as solvent to compliment the measured
results and to justify the CT within the donors and the acceptors. The optimized
energy, complexation energy, geometrical parameters, Natural atomic charges as
well as 3D-plots of the molecular electrostatic potential maps (MEP) were computed
and elucidated; they resided with the experimental results where the complexes
stability are attributed to the presence of charge-transfer. The electronic spectra
_
*
______________
Corresponding author. Chemistry Department, Faculty of Science, Helwan University,
Cairo, Egypt
Email: SAMIR_ABDO@science.helwan.edu.eg (S. A. Abdel-Latif)

New Journal of Chemistry
Page 2 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
were computed and executed using time dependent-density functional theory (TD-
DFT) via adding polarizable continuum solvation method PCM, PCM-TD-DFT. The
allowed singlet transitions are positioned and their highest occupied molecular orbital
(
HOMO) and lowest unoccupied molecular orbital (LUMO) involvements are
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
represented. The describes of the frontier HOMO and LUMO molecular orbitals,
contributing in the first four singlet transitions were showed. For all the formed solid
complexes, the main relations between the donors and the acceptor
molecules take place through the π-π* interaction. Secondary n-π* transition
was noticed in some complexes. The vibrational wavenumbers have also been
performed using B3LYP/6-31G(d,p) and the results matches the experimental one.
The small energy gap between HOMO and LUMO energies expressions that the CT
occurs within the donors and the acceptors. Hyper conjugative interactions,
molecular stability, bond strength and intramolecular CT have been investigated by
applying natural bond orbital (NBO) analysis. Mean polarizability, total static dipole
moment, anisotropy of polarizability and mean first-order hyperpolarizability have
been also attained. The obtained values show that the CT complexes are
accomplished candidate to non-linear optical (NLO) materials.
Keywords: Aminopyrazine, Schiff bases, Charge-transfer complexes, Spectroscopic studies,
DFT/TD-DFT calculations, non-linear optical properties.
Introduction
Numerous investigations have been created concerning the charge-transfer complexes of
aromatic nitro materials with several aromatic donor compounds.1-5 Such studies were
devoted to evaluating the ionization potential of donors, or electron empathies of acceptors as
well as the consequence of π-π* bonding in complex construction. Limited studies are
involved with CT complexes of Schiff bases.6,7 Hindawey et al. studied the solid complexes
8
of p-nitrophenol, some dinitro and trinitrobenzenes with p-substituted benzylidenaniline. The
formed complexes of some hydroxyl Schiff bases with poly nitrobenzenes were studied.9
Complexes of the donor-acceptor type formed between aromatic amines and π-acceptors
were the subject of widespread studies.10-12 N-heterocyclic compounds were used as effective
donors in the formulation of CT complexes with different p-benzoquinone products.13,14 The
proton transfer complex of 2-amino-4-hydroxy-6-methylpyrimidine with salicylic acid has

Page 3 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
been separated and their crystal developed by slow evaporation technique.15 A new CT
View Article Online
DOI: 10.1039/D0NJ05397J
complex incorporating hydrogen bonding between the e-donor 3-amino-1,5-dimethylpyrazole
with the e-acceptor chloranilic acid has been created and characterized experimentally and
theoretically.16 A novel CT complex between the n- and π-donor of 5-amino-1,3-
dimethylpyrazole with the π-acceptor chloranilic acid was produced and illustrated
experimentally and theoretically. The experimental work was performed in solution and solid
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
7
state. Charge-transfer interactions between the electron donor gliclazide and the p-acceptors
,3-dichloro-5,6-dicyano-1,4-benzoquinone and tetracyanoethylene were investigated in
2
1
8
chloroform as solvent and in the solid state, New CT complex formed between the n- and p-
donor 1-Benzoylpiperazine with p- acceptor p-chloranil were carried out in polar solvent
(
acetonitrile) at several temperatures. The formed 1:1 chemical composition of the CT
complex is verified.19 The crystal and molecular structures of two novel separated CT
complexes created from the reaction of tris(hydroxymethyl)aminomethane and 2,5-dichloro-
3
,6-dihydroxy-1,4-benzoquinone (chloranilic acid) in the mole ratios of 2:1 and 1:1, have
2
0
been studied and characterized. The chemistry of an organic CT complex between pyrazole
donor and chloranilic acid acceptor has been studied in ethanol at room temperature. The
created complex has been described by different techniques.21 CT complexes of 4-(2-
thiazolylazo) resorcinol with 3,5-DNSA, PA, and CLA were made and depicted
spectroscopically and theoretically. Molecular structure of the formed complexes was
2
2
established in methanol. A molecule (2E)-3-(biphenyl-4-yl)-1-(4-bromophenyl) prop-2-en-
-one was separated, and the structure has been distinguished by using spectroscopic
1
techniques. DFT method were used within B3LYP/6–311++G(d,p) basis set to optimize the
molecular structure of the formed compound. Vibrational wavenumbers, geometrical
parameters and electronic properties have also been achieved.23 The CT complex of 1-
benzoylpiperazine as a donor with the π-acceptor 2,3-dichloro-5,6-dicyano-p-benzoquinone
has been examined spectrophotometrically in acetonitrile at various temperatures. The
computational analysis of the CT complex using density functional theory adopts the
experimental work. The charge transfer in the CT complex and its extreme stability are
showed through both experimental and theoretical studies.24
The present article deals with the preparation of CT complexes of some pyrazine
1
Schiff bases with di-and tri-nitrobenzene products and studied using IR, H-NMR and
UV visible spectroscopy to characterize the type of bonding between the donor and
acceptor molecules in the formed CT complexes. Theoretical computations by means of

New Journal of Chemistry
Page 4 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
the density functional theory (DFT) at the basis set B3LYP/6-311 G(d,p) will be included to
study the ground state properties as geometrical parameters, optimization structure, reactivity
parameters and 3D-plots of the molecular electrostatic potential maps (MEP). The derivation
of electronic spectra and the composition of the frontier molecular orbitals will be examined
using TD-DFT through the polarizable continuum solvation model (PCM). The natural
population analysis will be employed to compute the natural atomic charge of each site of the
complexes and natural bond order analysis to study the inter- and intra-molecular CT in the
constructed complexes. The reliability between measured and computed results is the
significant purpose of this work.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Experimental
Materials and apparatus
All chemicals used in this study were pure grade BDH and Fluka chemicals. They include 2-
aminopyrazine, benzaldehyde, p-N-dimethylbenzaldehyde, picric, 3,5-dinitrosalicylic, 3,5-
dinitrobenzoic acids and Nujol. The organic solvents used were, dimethylsulfoxide
(DMSO) and ethanol. The solvents were purified by recommended methods.25
Preparation of Schiff bases (Donors)
The aminopyrazine Schiff bases used in this investigation were synthesized by condensation
of equivalent amounts of 2-aminopyrazine and the corresponding aldehydes; benzaldehyde or
p-N-dimethylbenzaldehyde. 0.1 mole of 2-aminopyrazine was mixed thoroughly with 0.1
mole of aldehyde in 250 mL ground flask. About 100 mL of ethyl alcohol were added and the
mixture was refluxed for four hours. The product separated after cooling, collected and
recrystallized from hot ethanol till constant m.p.26 The obtained Schiff bases have the
resulting structural formulae (Scheme 1), their colors, melting points and elemental analysis
are listed in Table 1:
X
CH
N
N
N
Where X = H, N-benzylidenepyrazin-2-amine, (NBPA)
=
p-N(CH ) , N-(4-dimethylamino)benzylidene)pyrazin-2-amine, (NDMABPA)
3 2
Scheme 1. Structure of the prepared donors

Page 5 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
Table 1 Color, melting points and elemental analyses of 2-aminopyrazine Schiff bases
C%
Calc.
(Found)
H%
Calc.
(Found)
4.91
(5.10)
6.19
N%
Calc.
(Found)
22.92
(22.65)
24.75
(24.35)
X
Color
M. P. C
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
2.04
71.83|)
8.94
(69.70)
Light-brown
Green-yellow
H
116
60
(
6
p-N(CH3)
2
(6.60)
Preparation of charge-transfer complexes
The donors used in the preparation of the CT complexes were the prepared Schiff bases. The
acceptors used are picric (PA), 3,5-dinitrosalicylic (3,5-DNSA) and 3,5-dinitrobenzoic (3,5-
DNBA) acids. The 1:1 CT complexes were prepared by mixing a hot alcoholic saturated
solution of the donor (0.01 mole dissolved in the least amount of hot ethanol) with an
27
equimolecular amount of the acceptor as described previously. The solid complexes were
separated immediately (picric acid) or on standing (p-N-dimethylbenzalhedyde), they
recrystallized from ethanol. The complexes obtained displayed various colors depending on
the donor and acceptor used. The products then filtered off, dried and their melting points
determined, Table 1.
Physical measurements
The IR spectra were attained by using a Shimadzu FT-IR spectrometer in the range 4000-
-1
1
4
00 cm applying the KBr disc technique. The H-NMR spectra of the Schiff bases and their
CT complexes were recorded using Varian 300 MHz NMR spectrometer at room temperature
using TMS (tetramethylsilan) as an internal standard and d dimethylsulphoxide (DMSO)
6
provided from Merck used as solvent. The electronic absorption spectra were examined
within the visible and ultraviolet ranges (800-200 nm) using the same solvent as blank and
-3
determined using Perkin-Elmer lambda 4B UV-Vis spectrophotometer. 1x10 mol/L stock
solutions of the donors and their CT complexes were prepared via dissolving the accurate
weight in 25 mL of ethanol as solvent. The measured concentrations were prepared by
diluting the exact volume in 10 mL of ethanol. Measurements covered the ranges from 210-
400 nm and 230-700 nm for the Schiff bases and their CT complexes, respectively. Nujol
Mull method was used in which sample is combined with Nujol in a mortar to make a mull
which is then positioned in the spectrophotometer.

New Journal of Chemistry
Page 6 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
Computational Details
Owing to the absence of single crystal X-ray structure analysis and to obtain the molecular
conformation of the donors and their CT complexes, energy minimization analyses were done
2
8
by means of Gaussian-09W software package. The ground state geometrical structures of
the donors and their CT complexes were optimized by means of the density functional theory
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
9
with Becke's three parameter exchange functional method, the Lee-Yang-Parr correlation
functional (B3LYP) and the split-valence double zeta basis set with two polarized basis
functions (d and p), (DFT/B3LYP) at the 6-31G(d,p) with the B3LYP exchange correlation
3
0
31-33
functional approach. The basis set 6-31G(d,p) was applied for C, H, N and O atoms,
respectively. Through geometry optimizations, every bond length, bond angle and dihedral
angle could relax free of constraints, the geometry of the considered systems was totally
optimized in gas-phase. Several properties can be analysed using the DFT theory such as
optimization energy, geometrical parameters, 3D-plots of the molecular electrostatic potential
3
4
maps (MEP) and reactivity parameters. Gauss-View 5 software, Avogadro and Chemcraft
programs have been used to extract the calculated results, and visualize the optimized forms,
the frontier molecular orbitals and 3D-plots of the molecular electrostatic potential (MEP)
maps. The quantum chemical parameters of the donors and their CT complexes are gained
from calculations as energies of the lowest unoccupied molecular orbital (ELUMO), the highest
occupied molecular orbital (EHOMO), HOMO-LUMO energy gap, E , absolute
g
electronegativities, χ, chemical potentials, , absolute hardness, η, absolute softness, σ, global
electrophilicity, ω, global softness, S, and additional electronic charge, ΔNmax. These
parameters are computed using these equations;35,36 Eg = ELUMO – EHOMO, χ = - EHOMO
+
2
ELUMO/2, η = ELUMO – EHOMO/2, σ =1/η,  = -χ, S = 1/2η, ω =  /2η and ΔNmax. = -/η. The
spin density difference map computations were also performed to explain their optical
3
7
properties. Natural bond orbital (NBO) calculations were performed with the NBO code
contained in Gaussian 09 to understand different second order interaction between the filled
orbital of one subsystem and vacant orbital of another subsystem which is the calculate of the
molecular delocalization or hyperconjugation. The mean polarizability (<α>), the anisotropy
of the polarizability (Δα), the mean first order hyperpolarizability (<β>) and the total static
dipole-moment (μ) via the x, y, z components were analyzed.38-40 TD-DFT computations
were brought out at the same level of theory (B3LYP/6-31G(d,p)) to elucidate the origin of
electronic spectra, using polarizable continuum solvation method PCM, PCM-TD-DFT. In
PCM the solute part lying inside cavity, whereas the solvent part (ethanol) denoted as a

Page 7 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
structureless matter. In PCM method, the solvent is also considered by its dielectric constant
and other macroscopic parameters. The vibrational frequency calculations were achieved
utilizing the same level of theory (B3LYP/6-31G(d,p)) to confirm that the found optimized
geometries stand for local minima.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Results and Discussion
CT complexes are formed through primary interaction invoking the transfer of one electron
from the highest occupied molecular orbital on the donor molecule to the lowest unoccupied
molecular orbital on the acceptor molecule (-* HOMO  LUMO transition). In addition,
secondary interactions are liable to occur depending on the nature of acceptor and donor
molecules. These include n-* interactions. HOMO  LUMO investigation was undertaken
to elucidate the origin of both the intramolecular interaction of the free Schiff bases and the
intermolecular CT interaction with electron acceptors. The IR bands of the CT complexes
41
compared with those of the respective free donor and acceptors give a further prove of the
theoretical calculations.
FT-IR spectra of Schiff bases and their charge-transfer complexes
The infrared spectra of some Schiff bases prepared from 2-aminopyrazine were obtained as
KBr discs. The most important absorption bands are registered in Table 2 and Fig. 1. The
Schiff bases under investigation display the C=N band within the wavenumber range 1665-
-1
-1
1
590 cm . The bands display within the wavenumber region 3360-3200 cm are attributed to
the stretching frequency of CH bond in the heterocyclic moiety (pyrazine) which is in
accordance with previous investigations.42-45 The CH bands are observed as two bands with
-1
varying intensities within the region 1458-1426 cm . The CH band of the CH=N group is in
-1
the wavenumber range 1002-994 cm . The CH bands of the aromatic rings are observed
-1
within the wavenumber range 832-748 cm . The IR spectra of the formed complexes exhibit
some drastic changes which are useful for elucidation of their molecular structures. These
changes may be summarized as follows, the appearance of OH band in the spectra of 3,5-
-1
dinitrosalicylic acid molecular complex with wavenumber 3570 and 3460 cm may be
designated to the carboxylic OH group. The C=O bands of 3,5-dinitrosalicylic and 3,5-
dinitrobenzoic acids shift to lower wavenumber on complex formation because of the -*
-1
electronic interaction. The two bands at 1700 and 1675 cm assigned to C=O of the free 3,5-

New Journal of Chemistry
Page 8 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Vi-e1w Article Online
DNSA and 3,5-DNBA appear as a single band within the wavenumber 1688 and1680 cm in
DOI: 10.1039/D0NJ05397J
the spectra of the molecular complexes. Schiff bases are characterized by a weak band within
-1
46
the wavenumber range 1002-994 cm which was assigned to the CH (CH=N). This band
shifts to lower wavenumber on complex formation with PA, 3,5-DNSA and 3,5-DNBA,
except for donor (p-N(CH ) where it is shifted to higher values. 2-aminopyrazine Schiff
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
2
bases display the CH asymmetric and CH symmetric bands within the wavenumber 3215-
-1
-1
3020 cm and 3060-3020 cm , respectively, Tables 3-6. On complex formation these bands
are generally shifted to lower wavenumbers while the CH bands of the Schiff bases are
shifted to higher wavenumbers. This behaviour is ascribed to the decreased electron density
on the donor molecule through -* electronic interaction resulting from the transfer of one
electron for the HOMO of the donor to the LUMO on the acceptor molecules. This
conclusion is strengthened by the general shift of the CH bands of the heterocyclic ring of
donors (2 adjacent H-atoms for pyrazine Schiff bases) to higher wavenumbers in contrast to
those of the acceptor part which are shifted to lower wavenumbers. These shifts result from
the decrease of electron density on the heterocyclic ring of the Schiff base and its increase on
the acceptor molecule. One may deduce that the -* electron transfer originates from the
heterocyclic ring to the acceptor molecule. In case of p-N(CH ) donor the benzal ring was
3
2
the origin of CT.47 The C=O bands of 3,5-DNSA and 3,5-DNBA are shifted to lower
wavenumbers as a result of the raised electron density on the acceptor molecule. It is of
interest to notice that in the case of 3,5-DNSA CT complex the two C=O bands of the free
acceptor mostly appears as a single band. This is attributed to the rupture of the hydrogen
bond existing in the free acceptor leading to the disappearance of rotational isomerism that
4
8
take place in the free acceptor. The CT complexes of pyrazine Schiff bases with 3,5-DNSA
-1
acceptor exhibit the OH band within the wavenumber range 3250-3180 cm which is
assigned to the carboxylic OH group, Fig. S1. The NO bands of the three acceptors used
2
display some interesting behaviours which may be of great help for elucidation of the types
of bonding in the CT complexes. In the case of complexes of picric acid, Fig. 2, the three 
asymmetric bands appear mostly as two bands due to the decreased diversity of the energy
states of the NO group. This results from the rupture of the intra-molecular hydrogen bond
2
between the OH group and the neighboring NO group. One of these two asymmetric NO
2
2
bands is shifted to higher wavenumber, while the second displays a counteract shift.

Page 9 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
Table 2 Experimental, calculated and band assignments of infrared bands for 2-aminopyrazine Schiff bases.
Wavenumber cm-1
X
C-H
asym.
C=N
C-H
C-H
C-H
arom.
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
het.
Exp.)
sym.
CH=N
(Exp.)
Calc.
het.
(
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
Calc.
H
(3200)
(3058b)
3050
(2980)
2988
(1590)
1592
(1458w, 1426)
1458, 1430
(1192s)
1190
(994)
988
(748)
755
(917)
931
3
203
(3360)
271
p-N(CH3)2
(3210)
3208
(2940)
3020
(1665)
1666
(1444, 1438)
1458, 1430
(1237)
1241
(1002)
1020
(832, 813)
813
(828)
860
3

New Journal of Chemistry
Page 10 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 1. FT-IR spectra of 2-aminopyrazine Schiff bases donors NBPA (a) and
NDMABPA (b)
This may be justified by using the participation of the NO group in position 4 in electronic
2
interaction with the one nitrogen atom of the pyrazine moiety. The symmetric NO bands are
2
shifted to lower values and appear mostly as two peaks. In the case of 3,5-DNSA the
orientation of the molecule permits the participation of only one nitro group in the n-*
electronic transition namely that in position 5 in the case of pyrazine Schiff base derivative.
The  symmetric bands exhibit a shift to lower wavenumbers in most cases. The

Page 11 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 2. FT-IR spectra of charge transfer complexes of 2-aminopyrazine Schiff bases
donors NBPA (a) and NDMABPA (b) with Picric acid
asymmetric NO bands of 3,5-DNBA, Fig. S2, display shifts to lower wavenumbers. This
2
may be assigned to an intensified electron density on the acceptor molecule as a result of the
-* electronic transition. The  symmetric NO bands exhibit similar behaviour.
2
Electronic absorption spectra of 2-aminopyrazine Schiff bases
The absorption spectra of Schiff bases derived from 2-aminopyrzine were scanned in ethanol
and the values of max and max are listed in Table 7 and Fig. 3. Generally, all compounds
exhibit an intense band within the wavelength 246-242 nm range, this may be assigned to the
medium energy -* electronic transition within the benzene ring. For the unsubstituted
compound (H), a shoulder is observed at 289 nm which may be attributed to -* electronic
transition of the pyrazine ring and shifted to higher wavelength as a result of extended
conjugation. The bands observed within the wavelength 348-326 nm in the CT complexes
may be attributed to the -* electronic transition within the C=N linkage influenced by
intramolecular charge-transfer.

New Journal of Chemistry
Page 12 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
Table 3 Main IR bands of CT complexes of 2-aminopyrazine with PA, 3,5-DNSA and 3,5-DNBA
Donor
Color
M.P. C
OH
Exp.)
Calc.
C=O
(Exp.)
Calc.
NO2
(Exp.)
Calc.
CH
(Exp.)
Calc.
(
asym.
sym.
donor
acceptor
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Complex of picric acid
Free acceptor bands
(3110)
3275
(1555, 1540, 1530) (1350)
(784)
808
1535
1350
(1340) (812)
Brownish white
225
210
154
(3430)
-
(1555, 1550)
1550
(783)
783
3
425
1346
840
Complex of 3,5-dinitrosalicylic acid
Free acceptor bands
(3570-3460)
3721
(1700, 1675) (1540, 1530)
(1349)
1331
-
(925, 825)
915
1708
1515
Light yellow
(3380)
(1688)
1672
(1538, 1520)
1537
(1345) (822)
(915, 810)
915
3
330
1342
827
Complex of 3,5-dinitrobenzoic acid
Free acceptor bands
(3100)
(1707)
1708
(1555, 1450)
1502
(1350)
1387
(923, 808)
931
3268
Yellowish white
-
-
(1680)
1666
(1545, 1535)
1543
(1350) (828)
1347 850
(918, 800)
918

Page 13 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
Table 4 Main IR bands of CT complexes of 2-aminopyrazine Schiff bases with picric acid
Wavenumber cm-1
Donor part
Acceptor part
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
X
Color
M.P. C
C-H
(Exp.)
Calc.
C=N
(Exp.)
Calc.
C-H
((Exp.)
Calc.
NO2
(Exp.)
Calc.
C-H
(Exp.)
Calc.
het.
asym.
sym.
CH=N arom.
het.
asym.
(1555, 1540, 1530)
535
(825) (1560, 1550)
840 1542
(813) (1559, 1550)
803 1565
sym.
Bands of free picric acid*
(1350)
1350
(784)
771
1
H
Yellow
>240
231
(3378)
3425
(3199)
3200
(3060) (1648) (980sh) (772)
(1350, 1342)
1349, 1346
(750)
756
3034 1646
(3040) (1640) (923)
3042 1638 925
987
779
p-N(CH3)2 Brown yellow
(3358)
(3178)
3178
(850)
856
(1339, 1330)
1339, 1331
(738)
732
3
383
OH = 3110 cm-1
*

New Journal of Chemistry
Page 14 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
Table 5 Main IR bands of CT complexes of 2-aminopyrazine Schiff bases with 3,5-dinitrosalicylic acid
Wavenumber cm-1
Donor part
Acceptor part
NO2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
X
Color
M.P. C
C-H
C=O
C-H
C-H
het.
asym.
sym.
CH=N
arom.
het.
asym.
sym.
(Exp.) (Exp.) (Exp.)
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.) (Exp.)
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
Calc.
Calc.
Calc.
Calc.
Calc.
Bands of free 3,5-dinitrosalicylic acid*
(1700)
(1540, 1530)
1516
(1349)
1331
(925)
915
1
710
H
Light yellow
210
129
(3340)
3330
(3120) (3020) (1690) (998sh) (785)
(825)
837
1545, 1530, 1525) (1350)
(917)
915
3215
(3086) (3020) (1687) (899)
3081 3023 1699 906
3021
1700
997
784
1537
1351
p-N(CH3)2
Brownish yellow
(3384)
(820)
809
(820)
809
(1544s, 1534sh)
1542
(1350)
1348
(920)
9018
3
358
-1
*
OH = 3570, 3460 cm ,

Page 15 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
Table 6 Main IR bands of CT complexes of 2-aminopyrazine Schiff bases with 3,5-dinitrobenzoic acid
Wavenumber cm-1
Donor part
C=O
Acceptor part
NO2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
X
Color
M.P. C
C-H
C-H
C-H
het.
Asym. sym.
CH=N
arom.
het.
asym.
sym.
(Exp.)
Calc.
(Exp.) (Exp.) (Exp.)
Calc. Calc. Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
(Exp.)
Calc.
Bands of free 3,5-dinitrobenzoic acid*
(1707)
(725)
744
(811) (1555, 1540)
811 1502
(830) (1550, 1545, 1540)
850 1543
(815) (1550, 1540)
810 1544
(1350)
1387
(923)
931
1
708
(3350) (3100) (3040) (1675) (982sh)
3303 3187 3036 1666 984
(3280) (3100) (3050) (1690) (982sh)
283 3093 3038 1697 996
H
Light brown
151
107
(799)
810
(1370)
1378
(925)
934
p-N(CH3)2 reddish orange
(860)
850
(1345)
1354
(920)
919
3
*
OH = 3100 cm-1

New Journal of Chemistry
Page 16 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 3. Electronic absorption spectra of 2-aminopyrazine Schiff bases NBPA (a) and
NDMABPA (b) in ethanol
Ionization potential of 2-aminopyrazine Schiff bases
The amounts of the ionization potential (Ip) of the free donors are determined from their
electronic spectra applying the relation: Ip = a + b  , where  is the energy of the HOMO-
o
o
LUMO -* transition of the donor in the gas phase taken, a and b are constants amounting
4
9
to 5.11 and 0.701, Table 8. The value of experimental E can be calculated from max of
CT
5
0
electronic spectra applying the following equation :
E = 1239.9/ nm eV
CT
CT
The results are recorded in Table 8. It is apparent that the Schiff base with electron donating
group has lower ionization potential value than that with electron withdrawing group.
Charge-transfer complexes with PA, 3,5-DNSA and 3,5-DNBA acceptors
This group comprises complexes formed with PA, 3,5-DNSA and 3,5-DNBA when mixed
with the donor in the molecular ratio 1:1. The fact that such acceptors have strong acidic
character is supported by their low ionization constant (pK) values accounting to 1.60 and
.8251 for picric and 3,5-dinitrobenzoic acids and pK = 2.94, pK = 9.94 for 3,5-
52
2
1
2
dinitrosalicylic acid, respectively. The donors under investigation possess more than one
basic center in the molecule. Hence, an acid-base interaction is expected to take place. These
complexes are characterized by their relatively high melting points (Tables 3-6).

Page 17 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
Electronic absorption spectra of charge-transfer complexes
The electronic absorption spectra of CT complexes formed between picric acid and the
prepared Schiff bases were investigated applying the nujol mull technique. The spectra
display new very broad bands located at the longer wavelength side which were not observed
in the absorption spectra of either the donors (Schiff bases) or the acceptor (picric acid).
These bands may be due to the intermolecular CT interaction between the donor and acceptor
molecules, Fig. 4. For the verification of this postulation, the ECT values were analyzed from
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
7
the absorption spectra utilizing the above equation. The theoretical E values were also
CT
calculated using Briegleb equation:53
E = Ip – (E + C)
CT
A
Where Ip is the ionization potential of the donor as calculated in Table 8, E is the electron
A
4
8
affinity of the acceptor = 0.7 eV and C is the coulomb force taken as 4.7 eV. The  nm
max
and ECT values (observed and calculated) are depicted in Table 9. The values of ECT obtained
from the absorption spectra are nearly concordant with those calculated which is in favor for
Fig. 4. Electronic absorption spectra of CT complex of 2-aminopyrazine Schiff base with
picric acid

New Journal of Chemistry
Page 18 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
Table 7 Absorption bands of 2-aminopyrazine Schiff bases in ethanol and its CT complex with picric acid
X
-*
max x10-4
-* pyrazine
max x10-4
CT
ECT eV
Calc.
max nm
242
max nm
289 sh
-
max nm
326
max x10-4
1.08
max nm
390
Obs.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
H
2.90
1.60
0.55
-
3.18
2.95
2.38
2.21
p-N(CH3)2
246
348
2.67
420
Table 8 Calculated values of ionization potential of 2-aminopyrazine Schiff bases
X
CT
Ip
max nm
326
E
CT
a=5.11, b=0.701
H
3.81
3.57
7.78
7.61
p-N(CH3)2
348

Page 19 of 54
New Journal of Chemistry
1
2
3
View Article Online
DOI: 10.1039/D0NJ05397J
their assignment as CT bands. The CT band observed have a composite nature, this may be
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
considered as a further support for the existence of another interaction, i.e. n-*, besides the

-* electronic interaction. However, no clear band is observed which can be assigned to the
n-* interaction.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 9 Electronic absorption bands of CT complexes of 2-aminopyrazine Schiff bases with
picric acid
X
max nm
ECT eV
Calc.
Obs.
3.18
2.95
H
390
2.38
2.21
p-N(CH3)2 420
1H-NMR spectrum of charge-transfer complex of 2-aminpyrazine Schiff
base with picric acid
The NMR spectrum of CT complex of 2-aminopyrazine Schiff bases with picric acid was
investigated and compared to that of the free component. The NMR spectrum of picric acid
displays the aromatic proton signals at 8.30 and 8.40 ppm and that of OH at 6.16 ppm as
shown in Scheme 2 and Table 10. A comparison between the NMR spectra of the CT
complex and that of the component reveals that the signals of the acceptors are shifted to
higher field values while that of the donors are displayed to lower fields, Table 10 and Fig. 5.
This results from the -* electronic interaction leading to the increased -electron density
on the acceptor and its decrease on the donor molecule.
OH
1
3
2
O N 6
2 NO2
2
1
4
CH
N
3
2
5
3
5
6
4
N
4
1
6
N5
NO2
Scheme 2. Numbering system of picric and 2-aminpyrazine Schiff base

New Journal of Chemistry
Page 20 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
View Article Online
DOI: 10.1039/D0NJ05397J
Table 10 H-NMR signals of picric, 2-aminpyrazine Schiff base and its CT complex with picric
acid
Compound
Acceptor part
Donor part
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H2
H3
H4
H5
H6
-
CH OH
H3
-
H4
-
H5 H6
Picric acid
-
-
8.30
-
-
8.40
-
-
6.16
-
-
*8.46
*8.46 -
*6.16
2
-aminopyrazine
Signals of benzal ring
Signals of hetero ring
Schiff base (H)
7.60 7.50
8.70 8.70
8.36 7.50 7.60 9.7
8.70 8.70 8.70 9.00
-
-
8.35 8.70 -
8.15 7.05
8.70
8.67
CT complex with
picric acid
*
After complex formation
1
Fig. 5. H NMR of spectrum of CT complex of 2-aminopyrazine Schiff base NBPA
with picric acid

Page 21 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
Computational DFT Analysis
Density functional theory examination was done according to Becke’s three parameter
gradient-corrected exchange potential and the Lee-Yang-Parr Gradient-corrected correlation
potential (B3LYP), and significant calculations were achieved by using 6-311G** basis set.
This model has been extensively used for geometry optimization and the purpose of
electronic properties. The optimized amounts of bond lengths, bond angles, molecular
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
4
electrostatic potential map values, characterization of the frontier molecular orbital surfaces
and natural atomic charges were produced. Gauss-View 5.0 and Chemcraft programs have
been exploited to achieve the calculated results, and visualize the optimized structures, the
frontier molecular orbitals and molecular electrostatic potential maps. The optimized
geometries of NBPA and NDMABPA Schiff bases (donors); picric (PA), 3,5-dinitrosalicylic
(
3,5-DNSA) and 3,5-dinitrobenzoic (3,5-DNBA) acids (acceptors) and their CT complexes
with numbering system are shown in Fig. 6.
Molecular orbital treatment
Geometry of the donors, acceptors and CT complexes
The optimized bond lengths, recorded in Table 11 and Fig.6, show that the carbon nitrogen
bond length of picric acid (PA) in CT complex C25-N34 of the picric acid moieties of the
complex increased to 1.480 Å compared with 1.467 Å for free picric acid (PA), in case of
3
,5-dinitrosalicylic acid (3,5-DNSA) acceptor, the bond length C28-N36 in the CT complex
increases from 1.464 Å to 1.472 Å associated to free 3,5-DNSA while for 3,5-DNBA, the
bond length C27-N34 in the CT complex also increases from 1.486 Å to 1.491 Å related to
free 3,5-DNBA. This result indicates that the bond length of the carbon-nitrogen bond in PA,
3
,5-DNSA and 3,5-DNBA increases, thus confirming the electron transfer from the labile
lone pair of electrons of the 2-aminopyrazine Schiff donor (-N=CH) nitrogen towards the
carbon-nitrogen bond of PA, 3,5-DNSA and 3,5-DNBA acceptors. It is important to notice
that the nitrogen of 2-aminopyrazine Schiff base donor was found to be oriented towards the
PA, 3,5-DNSA and 3,5-DNBA acceptors and this orientation produced the resonating
structure from the electron transfer to the PA, 3,5-DNSA and 3,5-DNBA acceptors of the CT
complex. The bond lengths of PA, 3,5-DNSA and 3,5-DNBA decrease in the CT complex as
compared to themselves. This indicates the transfer of π-electron from the HOMO of 2-
aminopyrazine Schiff base donors to the π* LUMO of PA, 3,5-DNSA and 3,5-DNBA

New Journal of Chemistry
Page 22 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
moieties. The C6-N7 and C8-N7 bond lengths of the CT complex decreased to 1.279 Å and
1.394 Å compared with 1.282 Å and 1.402 Å, from 1.402 Å and 1.282 Å to 1.398 Å and
.278 Å, from 1.402 Å and 1.282 Å to 1.395 Å and 1.279 Å of free PA, 3,5-DNSA and 3,5-
1
DNBA acceptors, respectively. These results in the expansion of bond lengths because of the
increase of electron density of acceptor in the complex compared to free acceptor. It indicates
that the electron density on the donor moiety of the complex decreases, which results in the
contraction of bond lengths compared to donor alone. The CT complex is further confirmed
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
from the changes in its bond angles as compared to the reactants as presented in Table 11.
The free acceptor can be further confirmed from the decreasing bond angles of C2-N13-O18
and C2-N13-O19 of the CT complex (from 117.728° to 116.749° and from 118.825° to
1
1
16.811°), C28-N36-O38 from 119.091° to 118.014° and C25-C27-N34 from 118.873 to
18.813 of free PA, 3,5-DNSA and 3,5-DNBA acceptors, respectively. Moreover, the bond
angle of C24-C25-N34 decreases to 119.022° compared to 120.403° of picric acid acceptor.
In the case of 2-aminopyrazine Schiff base donor, the bond angle N4-C6-N7 decreases from
1
21.811° to 117.111°, 117.216°, 117.118° in the CT complexes as compared to the donor
alone which confirms the π-π* transition from HOMO to LUMO molecular orbitals of the
characterized CT complex. In case of p-N-dimethyl-2-aminopyrazine Schiff base donor, the
optimized bond lengths, recorded in Table S1 and Fig. 6, show that the carbon nitrogen bond
length of picric acid (PA) in CT complex C33-N44 of the picric acid moieties of the complex
increased to 1.473 Å compared with 1.467 Å for free picric acid, in case of 3,5-DNSA
acceptor, the bond length C26-N38 in the CT complex increases from 1.464 Å to 1.471 Å
associated to free 3,5-DNSA while for 3,5-DNBA, the bond length C26-N33 in the CT
complex also increases from 1.486 Å to 1.491 Å related to free 3,5-DNBA. These results
indicate that the bond length of the carbon-nitrogen bond in PA, 3,5-DNSA and 3,5-DNBA
increases thus confirming the electron transfer from the labile lone pair of electrons of the p-
N-dimethyl-2-aminopyrazine Schiff donor (-N=CH) nitrogen towards the carbon-nitrogen
bond of PA, 3,5-DNSA and 3,5-DNBA acceptors. It is important to note that the nitrogen of
p-N-dimethyl-2-aminopyrazine Schiff donor was found to be oriented towards the PA, 3,5-
DNSA and 3,5-DNBA acceptors and this orientation produced the resonating structure from
the electron transfer to the PA, 3,5-DNSA and 3,5-DNBA acceptors of the CT complex. The
bond lengths of PA, 3,5-DNSA and 3,5-DNBA decrease in the CT complex as compared to
themselves.
This indicates the π-electron transfer from the HOMO of

Page 23 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Donor (NBPA)
Acceptor (PA)
CT complex
Donor (NDMABPA)
Acceptor (PA)
CT complex
Donor (NBPA)
Acceptor (3,5-DNSA)
CT complex

New Journal of Chemistry
Page 24 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Donor (NDMABPA)
Acceptor (3,5-DNSA)
CT complex
CT complex
Acceptor (3,5-DNBA)
Donor (NBPA)
Donor (NDMABPA)
Acceptor (3,5-DNBA)
CT complex
Fig. 6 Optimized geometry, numbering system, and vector of dipole moment for the studied 2-aminopyrazine, (p-N(CH ) -2-
3
2
aminopyrazine Schiff bases (donors); Picric, 3,5-dinitrosalicylic and 3,5-dinitrobenzoic acids (acceptors) and their charge transfer
complexes using B3LYP/6-311G**

Page 25 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/D0NJ05397J
p-N-dimethyl-2-aminopyrazine Schiff donor to the π* LUMO of PA, 3,5-DNSA and 3,5-
DNBA moieties. The C6-N7 and C8-N7 bond lengths of the CT complex of PA, 3,5-DNSA
and 3,5-DNBA acceptors, respectively decreased to 1.272 Å and 1.381 Å compared with
1
1
.435 Å and 1.396 Å, from 1.396 Å and 1.288 Å to 1.393 Å and 1.285Å, from 1.396 Å and
.288 Å to 1.391 Å and 1.285 Å of free donor. These results in the expansion of bond lengths
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
because of the rise of electron density of acceptor in the complex compared to free acceptor.
It indicates that the electron density on the donor moiety of the complex decreases, which
results in the contraction of bond lengths compared to donor alone. The CT complex is
further confirmed from the changes in its bond angles as compared to the reactants as
presented in Table S1. The free acceptor can be further confirmed from the decreasing bond
angles of C2-N13-O18 and C2-N13-O19 of the CT complex (from 117.728° to 116.752° and
from 118.825° to 118.411°), C26-N38-O39 from 117.593° to 117.568° and C26-N38-O40
from 119.091 to 117.500 of free PA, 3,5-DNSA and 3,5-DNBA acceptors, respectively.
Moreover, the bond angle of C24-C27-N35 decreases to 120.855° compared to 122.269° of
3
,5-DNSA acceptor. In the case of p-N-dimethyl-2-aminopyrazine Schiff base donor, the
bond angle N4-C6-N7 decreases from 122.385° to 116.574°, 117.375° and 117.061° in the
CT complexes as compared to the donor alone which confirms the π-π* transition from
HOMO to LUMO molecular orbitals of the characterized CT complexes.
Global reactivity descriptors
Frontier molecular orbital energies calculation for CT complexes
Molecular orbital analysis shows that the frontier molecular orbitals are mainly composed of
N atomic orbitals. HOMO-LUMO calculation of 2-aminopyrazine and p-N-dimethyl-2-
aminopyrazine Schiff bases donors and PA, 3,5-DNSA and 3,5-DNBA acceptors, the
complexes in the ground state are obtained by the DFT method with basis sets B3LYP/6-
3
11G** and presented in Figs. 7-9. It is clear from the figure that the HOMOs are mainly
delocalized on the donor moiety (2-aminopyrazine and p-N-dimethyl-2-aminopyrazine Schiff
bases), while the LUMOs are localized on acceptors PA, 3,5-DNSA and 3,5-DNBA. The
molecular orbital HOMO is localized on the 2-aminopyrazine and p-N-dimethyl-2-
aminopyrazine Schiff bases part of the complex and particularly on the N atomic orbital.
Thus, one concludes that the n-electrons are localized in HOMO molecular orbital. The other
molecular orbitals are located on the p orbitals of the benzene moieties of 2-aminopyrazine
and p-N-dimethyl-2-aminopyrazine Schiff bases in the CT complexes, the HOMO can be

New Journal of Chemistry
Page 26 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
View Article Online
DOI: 10.1039/D0NJ05397J
considered as π molecular orbitals and the LUMO as the π* molecular orbital. Consequently,
the noticed transitions can be ascribed to n–π* and π–π* transitions. The energy values for
HOMO’s and LUMO’s of donors, acceptors and donor-acceptor complexes in the ground
state are provided in Table 12 and it is interesting to note from Table 12, that in case of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
-aminopyrazine Schiff base, the LUMO energy level of the donor-acceptor CT complexes
(-0.1381, -0.1337 and -0.1230 a. u.) compares well with the LUMO energy level of acceptors
-0.1506, -0.1610 and -0.1352 a. u.) while the HOMO energy (-0.2506, -0.2566 and -0.2567
(
a. u.) level of the CT complex are close to HOMO energy level of 2-aminopyrazine Schiff
base donor. In Figs. 2-4, some frontier orbitals are exhibited. The reason for localized frontier
molecular orbitals of donor-acceptor complex system is like other electron donor-acceptor
aggregate systems. Hence, the orbital interaction energy arises mainly due to the CT
concerning occupied and unoccupied orbitals. The energy difference between the HOMO and
LUMO, E , of the studied CT complexes occurs in the range 3.35-3.30 eV. The energy gap
g
for 3,5-DNSA CT complex is the maximum (3.35 eV) while 3,5-DNBA CT complex has the
minimum (3.30 eV) value. As a result, charge-transfer and polarization can easily occur
within the 3,5-DNBA CT complex than other complexes with more reactivity. The chemical
hardness, η, electronegativity, χ, chemical potential, , and global softness, S, were calculated
using HOMO and LUMO energies and listed in Table 12. PA complex has the lowest η and
maximum S values which means that the CT occurs easily in this complex and has a smaller
chemical hardness. It can be decided that the large E gap indicates a hardness of the
g
molecule, while smaller E gap is a characteristic for a soft and reactive molecule. The
g
electron affinity values of the CT complexes have the order: < 3,5-DNSA < 3,5-DNBA < PA.
Therefore, CT and polarization can easily occur within the PA complex than the other
complexes due to its higher reactivity. The computed reactivity parameters shown in Table 12
reveal that PA complex has the lowest η and maximum S values which means that the CT
occurs in this CT complex and has softer chemical hardness. It is interesting to note from
Table S2, that in case of p-N-dimethyl-2-aminopyrazine Schiff base, the LUMO energy level
of the donor-acceptor CT complexes (-0.1182, -0.1249 and -0.1217 a. u.) compares well with
the LUMO energy level of acceptors (-0.1506, -0.1610 and -0.1352 a. u.) while the HOMO
energy (-0.1652, -0.2135 and -0.2107 a. u.) level of the CT complex are close to HOMO
energy level of p-N-dimethyl-2-aminopyrazine Schiff base donor. The energy difference
between the HOMO and LUMO, E , of the studied CT complexes occurs in the range 2.42-
g

Page 27 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
Table 11 Selected geometric bond lengths, bond angles and dihedral angles of the optimized 2-aminopyrazine Schiff base (donor), Picric (PA),
3,5-dinitrosalicylic (3,5-DNSA) and 3,5-dinitrobenzoic (3,5-DNBA) acids (acceptors) and their CT complexes using B3LYP/6-311***
Ligand/CT
Bond lengths (Å)
Bond angles
Dihedral angles
complexes
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Donor (NBPA)
C5-C6
C6-N4
C6-N7
N7-C8
C8-C9
C9-C14
C9-C10
1.404
1.371
1.435
1.296
1.503
1.398
1.407
N1-C5-C6
C5-C6-N4
C5-C6-N7
N4-C6-N7
N7-C8-C9
C8-C9-C10
C8-C9-C14
C10-C9-C14
120.111
C5-C6- N7-C8
180.000
180.000
0.000
180.000
0.000
117.937
120.252
121.811
124.074
124.719
117.667
117.615
C6- N7-C8-C9
N7-C8-C9-C10
N7-C8-C9-C14
N4-C6-N7-C8
`
Acceptor (PA)
CT complex
C1-C2
C2-C5
C2-N13
N13-O18
N13-O19
1.428
1.387
1.467
1.244
1.212
O9-C1-C2
122.861
120.403
117.728
118.825
30.603
121.521
117.111
122.368
28.813
123.493
119.022
116.749
116.811
O9-C1-C2-C5
180.000
0.000
180.000
0.000
C1-C2-N13
C2-N13-O18
C2-N13-O19
N13-C2-C5
O9-C1-C3
N4-C6-N7
C5-C6-N7
C6-N7-C8
N7-C8-C9
C1-C2-N13-O18
C1-C2-N13-O19
O9-C1-C3-N11
C1-C3-C4-C6
N13-C2-C5-C6
N4-C6-N7-C8
C6-N7-C8-C9
N1-C5-C6-N7
N4-C6-N7-N34
N34-C25-C24-O30
N34-C25-C28-C29
0.000
-180.000
136.781
174.955
-179.496
-35.783
2.271
C8-N7
C6-N7
C6-N4
C8-C9
C25-N34
C24-C25
C24-O30
C25-C28
1.279
1.394
1.339
1.464
1.480
1.413
1.319
1.379
C24-C25-N34
C25- N34-O39
C25- N34-O40
178.952

New Journal of Chemistry
Page 28 of 54
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
Acceptor
(3,5-DNSA)
C2-C5
C5-C6
C5-N15
C2-O13
N15-O17
N15-O18
1.417
1.389
1.464
1.337
1.247
1.211
C2-C5-C6
121.715
120.693
122.269
117.593
119.091
C1-C2-C5-N15
C2-C5-N15-O17
C2-C5-N15-O18
O13-C2-C5-N15
C6-C5-N15-O17
C6-C5-N15-O18
-179.987
0.011
-179.980
0.023
179.983
0.026
C2-C5-N15
C5-C2-O13
C5-N15-O17
C5-N15-O18
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
CT complex
C6-N7
C8-N7
C8-C9
C6-N4
1.398
1.278
1.466
1.342
1.413
1.406
1.408
1.221
1.255
1.472
1.429
1.391
1.324
N4-C6-N7
C6-N7-C8
C5-C6-N4
C5-C6-N7
117.216
119.169
120.722
121.947
123.554
118.851
121.836
105.766
122.752
119.231
118.014
121.087
122.604
122.137
C3-N4-C6-N7
N4-C6-N7-C8
C6-N7-C8-C9
-178.675
-136.770
-174.022
29.943
N4-C6-N7-N36
C9-C8-N7-N36
C8-N7-N36O37
C8-N7-N36O38
C25-C28-N36-O38
O34- C25-C28-N36
C24-C25-C28-N36
C27-C29-C28-N36
C29-C28-C25-O34
C5-C6
N7-C8-C9
23.662
C9-C10
C9-C14
N36-O37
N36-O38
C28-N36
C25-C28
C28-C29
C25-O34
C8-C9-C10
C8-C9-C14
C6-N7-N36
O37-N36-O38
C28-N36-O37
C28-N36-O38
C25-C28-N36
C28-C25-O34
C25-C28-C20
-149.644
-26.922
-3.414
0.568
-179.213
179.607
-178.799
Acceptor
3,5-DNBA)
(
C3-C5
C5-C6
C5-N14
N14-O16
1.389
1.387
1.486
1.221
C3-C5-C6
122.546
118.873
118.581
117.168
C1-C3-C5-N14
C4-C6-C5-N14
C3-C5-N15-O16
C3-C5-N15-O17
180.000
180.000
0.000
C3-C5-N14
C6-C5-N14
C5-N14-O16
180.000

Page 29 of 54
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
N14-O17
1.2205
C5-N14-O16
117.229
CT complex
C3-N4
N4-C6
C5-C6
C6-N7
N7-C8
C8-C9
N7-N34
N34-O36
N34-O37
C27-N34
C25-C27
C27-C29
1.333
1.338
1.410
1.395
1.279
1.464
3.506
3.506
1.224
1.491
1.388
1.386
N4-C6-N7
C6-N7-C8
C5-C6-N4
C5-C6-N7
C8-C9-C10
C8-C9-C14
O36-N34-O37
C25-C27-N34
C27-N34-O37
C25-C27-C29
117.118
119.416
120.609
122.188
118.845
121.781
125.273
118.813
117.514
122.529
C3-N4-C6-N7
N4-C6-N7-C8
C6-N7-C8-C9
N4-C6-N7-N35
C8-N7-N34O36
C8-N7-N34O37
C29-C27-N34-O36
C29-C27-N34-O37
C24-C25-C27-N34
C28-C29-C27-N34
-179.228
-135.375
-175.142
37.539
-29.370
-154.720
-4.017
175.210
179.595
-179.633
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6