Electronic structure and absorption spectra of 6-picoline Schiff base: A DFT and XRD based approach

Article abstract of DOI:10.1016/j.molstruc.2013.07.008

The spectroscopic and molecular structure of 2-{(E)-[(6-methylpyridin-2-yl) imino]methyl}phenol (MPMP) has been characterized by FT-IR, UV-Vis and X-ray diffraction. The ground state geometry of MPMP has been optimized by using density functional theory a

Full text of DOI:10.1016/j.molstruc.2013.07.008

Journal of Molecular Structure 1050 (2013) 10–14
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Electronic structure and absorption spectra of 6-picoline Schiff base:
A DFT and XRD based approach
b
a
c
a
Ghulam Abbas a, , Ahmad Irfan , Mariam Mir , Mariya-al-Rashida , Ather Farooq Khan
⇑
a Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan
b Department of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia
c Department of Chemistry, Forman Christian College University, Lahore, Pakistan
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
A novel 6-picoline Schiff base has been synthesized and characterized.
The compound has been optimized by using density functional theory.
Role of frontier molecular orbitals in charge transfer properties is highlighted.
The solvent polarity effect was studied on the absorption wavelengths.
a r t i c l e i n f o
a b s t r a c t
Article history:
The spectroscopic and molecular structure of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol
(MPMP) has been characterized by FT-IR, UV–Vis and X-ray diffraction. The ground state geometry of
MPMP has been optimized by using density functional theory at DFT/B3LYP/6-31G** and DFT/B3LYP/6-
31 + G* level of theories. The DFT/B3LYP/6-31 + G* level of theory is good to reproduce the geometrical
parameters. We shed light on the frontier molecular orbitals; highest occupied molecular orbitals
(HOMOs) and lowest unoccupied molecular orbitals (LUMOs) which play important role in charge trans-
fer properties. The absorption spectrum was calculated by using time dependent density functional the-
ory at TD-B3LYP/6-31 + G* level of theory. The absorption wavelengths were computed in different
solvents by using polarizable continuum model (PCM). The solvent polarity effect was studied on the
absorption wavelengths. The structure–property relationship has been discussed intensively.
Ó 2013 Elsevier B.V. All rights reserved.
Received 14 June 2013
Accepted 9 July 2013
Available online 16 July 2013
Keywords:
Characterization
Density functional theory
Time dependent density functional theory
Frontier molecular orbitals
Solvent effect
Charge transport
1. Introduction
containing phenolic moieties in dyes, indicators, metallochromic
reagents, histological stains and liquid crystals has been studied
Schiff base compounds have attained considerable interest due
to their theoretical and experimental perspectives. Salen type
Schiff bases have wide applications in the synthesis of coordination
complexes [1], optical materials [2], ion selective electrodes [3],
metallo-enzymes [4] and medicinal compounds [5]. Structural
and spectroscopic investigations of pyridine–salicylaldehyde Schiff
bases have been compared with theoretical DFT calculations [6].
Alkyl derivatives of pyridine including methyl pyridines are used
as raw material for the synthesis of other heterocyclic compounds.
[8]. Schiff bases of salicylaldehyde and aminopyridines have been
known for their usefulness as metal extracting agents in solution
due to their excellent metal chelating ability [9]. Metal complexes
of chiral amino and imino pyridine ligands have found many appli-
cations in asymmetric catalytic processes [10]. Density Functional
Theory (DFT) has been employed extensively to calculate variety of
molecular properties e.g. equilibrium structure, charge distribu-
tion, UV–Vis and FTIR spectra. It has provided reliable results
which are in accordance with experimental data [11]. In this re-
search, the ground state (S0) geometry has been optimized by using
DFT at the Becke’s B3 exchange functional [12] combined with
Lee–Yang–Parr correlation functional LYP [13] expressed as
B3LYP [14] with 6-31G** and 6-31 + G* basis sets. The absorption
spectra was computed by time-dependent density functional the-
ory (TD-DFT) [15–18] by the hybrid functional TD-B3LYP [19–21]
and basis set 6-31 + G*. The effect of different solvents
(DMF, DMSO, methanol and ethanol) has been studied on the
Among methyl pyridines, three structural isomers a-, b- and c-pic-
oline (2-methyl, 3-methyl and 4-methyl pyridine) are particularly
well known and are extensively used as agricultural chemicals
such as nitrapyrin which causes slow release of ammonia from
the fertilizers [7]. The absorption spectra of azo compounds
⇑
Corresponding author. Tel.: +92 42 111 001 007x829; fax: +92 42 35321090.
E-mail address: abbas191@gmail.com (G. Abbas).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.07.008

G. Abbas et al. / Journal of Molecular Structure 1050 (2013) 10–14
11
Table 1
absorption spectra using polarizable continuum model (PCM) [22].
Crystal data of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol C13H12N2O.
All these calculations were carried out using GAUSSIAN 09 package
[23].
Mr = 212.25
Dx = 1.329 mg mꢁ3
Mo K radiation, k = 0.71073 Å
Cell parameters from 740 reflections
Orthorhombic, P212121
a = 4.4777 (6) Å
b = 9.6763 (9) Å
c = 24.4810 (17) Å
V = 1060.70 (19) Å3
Z = 4
a
In present work, we have synthesised, characterized a new
Schiff base 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol
and crystallographic data have been reported. We shed light on
the electronic and charge transfer properties. The computed
results were compared with experimental evidences where
available. The effect of solvents has been discussed on the absorp-
tion spectra. The structure–property relationship has also been
discussed.
h = 3.3–28.5°
l
= 0.09 mmꢁ1
T = 100 K
Polyhedra, yellow
0.15 ꢂ 0.08 ꢂ 0.03 mm
F(000) = 448
2.2. Synthesis
2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol was pre-
pared according to the following procedure:
2. Experimental
To a well stirred solution of salicyldehyde 1 mmol in 10 ml
absolute ethanol was added a solution of 1 mmol 2-amino-6-pico-
line in 10 ml ethanol and refluxed for 1 h. The resulting mixture
was cooled and left undisturbed. After 2 days yellow needle like
crystal of MPMP separated out. Yield 48%: m.p. 64.4 °C. Anal. Calc.
for C14H12 N2O: C, 73.56 H, 5.69 N, 13.19, found: C, 73.41 H, 5.61 N,
13.15. FT-IR (KBr cmꢁ1) 3294 (AOH), 1610 (C@N), 1580 (HOC),
1553 (HCC), 1499 (HCC), 1453 (HCH)
2.1. Material and measurement
All the chemicals and reagents were purchased from Across,
Alfa Aesar and Merck and were used without further purification.
Melting point was taken on a Gallenkamp apparatus and is uncor-
rected. The elemental analyses (C, H, N) were performed by using
an Elementar Vario EL analyzer. FT-IR (450–4000 cmꢁ1) spectrum
was taken with Thermo-nicolet 6700, USA FT-IR Spectrometer.
The electronic spectra were recorded on Labomed UVD2300 UV/
Vis spectrophotometer. Single crystal XRD data was collected on
an Oxford-Diffraction diffractometer, equipped with a CCD area
2.3. X-ray diffraction data of 2-{(E)-[(6-methylpyridin-2-
yl)imino]methyl}phenol Schiff base compound
detector and a graphite monochromator utilizing Mo Ka radiation
(k = 0.71073 Å).
The yellow single crystals of MPMP were obtained by slow
evaporation of solvent from ethanolic solution of the compound
at room temperature. All the measurements are performed by
Mo Ka radiation at 100 K: C13H12N2O, Mr = 212.25, Orthorhombic,
space group P212121, a = 4.4777 (6) Å, b = 9.6763 (9) Å,
c = 24.4810 (17) Å, V = 1060.70 (19) Å3, Z = 4, dcal = 1.329 gmꢁ3
l
2539 reflections were collected (hmax = 25.0°), (Rint = 0.027) with
,
= 0.09 mmꢁ1, crystal size = 0.15 ꢂ 0.08 ꢂ 0.03 mm. A total of
1448 reflections with I > 2r(I). The structure was solved by direct
methods using SIR92, [24] and refined on F2 using full-matrix least
squares using SHELXL97 [25] The final R indices of R1 = 0.059 and
wR2 = 0.179 were obtained. The ORTEP view and unit cell packing
diagram for MPMP are shown in Figs. 1 and 2. Tables 1–3 show
some X-ray crystallographic data of (1) compound. The XRD anal-
ysis reveals that the two aromatic rings (one of benzene and other
of pyridine) are almost planar and the planes of their rings make an
angle of 7.71°.
3. Results and discussion
In continuation of our research in search of new N,N-donor li-
gands, we have synthesised a new Schiff base (1) 6-picoline (2-
amino-6-methyl pyridine) with salicylaldehyde (Scheme 1). The
structure of (1) was confirmed using single crystal X-ray diffraction
Fig. 1. ORTEP plot of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol ellipsoids
are drawn at 50% probability level.
Fig. 2. Unit cell packing diagram for MPMP, intramolecular hydrogen bonds are indicated by dotted line.

12
G. Abbas et al. / Journal of Molecular Structure 1050 (2013) 10–14
Table 2
Table 4
Data collection of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol.
Selected optimized bond lengths in Angstrom (Å), bond angles and dihedral angles
(degrees) at the B3LYP/6-31G** and 6-31 + G* level of theories.
Radiation source: SuperNova (Mo) X-ray source
Detector resolution: 10.4223 pixels mmꢁ1
1731 independent
reflections
1448 reflections with
Bond lengths
Bond angles
Dihedral angles
B3LYP/6-31G** level of theory
I > 2r(I)
N2AC4
C4AC5
N1AC4
N1AC26
1.343 N2AC4AC5
1.400 N2AC4AN1
1.408 C4AN1AC26
122.73 N2AC4AN1AC26
119.81 C4AN1AC26AC16
120.13 O3AC17AC16AC26 ꢁ0.00
ꢁ0.04
x
scans
Rint = 0.027
180.00
Absorption correction: multi-scan
Empirical absorption correction (CrysAlis RED,
Oxford Diffraction)
hmax = 25.0°, hmin = 3.3°
h = ꢁ53
1.298 N1AC26AC16 121.67
C16AC26 1.444 C16AC17AO3 121.76
O3AC17 1.338
Tmin = 0.563, Tmax = 1.000
2539 measured reflections
k = ꢁ1110
l = ꢁ1629
6-31 + G* level of theory
N2AC4
C4AC5
N1AC4
N1AC26
1.343 N2AC4AC5
1.401 N2AC4AN1
1.409 C4AN1AC26
122.64 N2AC4AN1AC26
120.01 C4AN1AC26AC16
120.42 O3AC17AC16AC26 ꢁ0.007
0.23
ꢁ179.98
Table 3
Refinement Data of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol.
1.297 N1AC26AC16 122.04
C16AC26 1.448 C16AC17AO3 121.86
O3AC17 1.344
Refinement on F2
Hydrogen site location: inferred from
neighboring sites
Experimental data
Least-squares matrix: full
H atoms treated by a mixture of
independent and constrained
refinement
N2AC4
C4AC5
N1AC4
N1AC26
1.338 N2AC4AC5
1.390 N2AC4AN1
1.417 C4AN1AC26
123.34 N2AC4AN1AC26
119.43 C4AN1AC26AC16
119.13 O3AC17AC16AC26 ꢁ1.26
4.25
179.27
R[F2 > 2
r
(F2)] = 0.059
w = 1/[r
2(Fo2) + (0.1264P)2 + 0.6527P]
wR(F2) = 0.179
1.300 N1AC26AC16 121.27
where P ¼ ðF2o þ 2Fc2Þ=3
C16AC26 1.441 C16AC17AO3 120.90
O3AC17 1.360
S = 0.88
1731 reflections
153 parameters
0 restraints
Primary atom site location:
structure-invariant direct
methods
(D/r)max < 0.001
D
D
q
q
max = 0.20 e Åꢁ3
min = ꢁ0.26 e Åꢁ3
Extinction correction: none
Absolute structure: Flack H D (1983),
Acta Cryst. A39, 876–881
Secondary atom site location:
difference Fourier map
Flack parameter: 3 (4)
Scheme 1. Schematic representation of the synthesis of (1) with chemical
structure.
Fig. 3. Atomic numbering scheme for MPMP used for computational analysis.
studies. Thermal analysis and electronic spectra were also deter-
mined. The results from TGA/DSC indicate that the compound
decomposes in a single step between the temperature ranges
170–234 °C. The heat flow indicates two prominent endothermal
changes, one at 64.69 °C which is due to the melting of compound
and other at 238.09 °C.
Thus we selected the B3LYP/6-31 + G* level of theory for further
investigations.
3.2. Electronic properties
We were interested in exploring the electronic structure and
HOMO/LUMO make up of (1). For this purpose, first the geometry
was optimized using DFT (B3LYP with 6-31G** and 6-31 + G* basis
sets), the initial structure coordinates were taken from XRD data.
Electronic spectrum was computed using time-dependent DFT
(TD-B3LYP with basis set 6-31 + G*). The effect of different solvents
such as DMF, DMSO, MeOH and EtOH on the absorption spectra
was evaluated using PCM.
In Fig. 4 the distribution patterns of frontier molecular orbitals,
i.e., highest occupied molecular orbitals (HOMOs), first highest
occupied molecular orbitals (HOMOs-1), lowest unoccupied
molecular orbitals (LUMOs) and first lowest unoccupied molecular
orbitals (LUMOs + 1) at the ground state have been shown. The
HOMO and HOMO-1 are delocalized throughout the backbone.
The electron activating group (methyl) has no any contribution
in the formation of HOMO and HOMO-1. The LUMO is distributed
on whole of the compound while LUMO + 1 is localized on pyridine
moiety and methyl group. The nitrogen of pyridine has small con-
tribution in the formation of HOMO and HOMO-1. Contrary, more
charge is localized on nitrogen in LUMO and LUMO + 1. The oxygen
of hydroxyl group is taking part in the formation of HOMO and
HOMO-1 while little participation in the formation of LUMO and
no contribution in LUMO + 1. The HOMO, HOMO-1, LUMO,
LUMO + 1 energies, HOMO–LUMO energy gap (Eg) in eV for S0 state
at the B3LYP/6-31G** and B3LYP/6-31 + G* level of theories have
been presented in Table 5. The HOMO, HOMO-1, LUMO and
LUMO + 1 energies are smaller at B3LYP/6-31 + G* level of theory
3.1. Geometries
The geometrical parameters i.e., bond lengths, bond angles and
dihedral angles have been presented in Table 4, atomic numbering
scheme is given in Fig. 3. The bond lengths and bond angles at
B3LYP/6-31G** and B3LYP/6-31 + G* level of theories are in good
agreement with the experimental parameters. The torsion angles
at B3LYP/6-31 + G* level of theory are in better agreement with
experimental data. From Table 4, it can be seen that B3LYP/6-
31 + G* level of theory is better than B3LYP/6-31G** level of theory.

G. Abbas et al. / Journal of Molecular Structure 1050 (2013) 10–14
13
HOMO
HOMO-1
LUMO
LUMO+1
Fig. 4. Distribution patterns of frontier molecular orbitals at ground state.
THF are being 45 and 7 nm blue shifted compared to the absorp-
tion wavelengths in gas phase. The first and second absorption
peaks in CH3CN, DMF, acetone and methanol are almost same.
The second absorption peak in CH3CN, DMF, acetone and methanol
is being 6 nm red shifted compared to the gas phase. Generally, in
solvent the oscillator strengths improved revealing that the
absorption coefficient would be enhanced. The assignments for
first peak of absorption spectrum are from HOMO to LUMO while
second peak from HOMO-1 to LUMO. The computed absorption
wavelengths in different solvents are in better agreement with
the experimental data, see supporting information.
Table 5
The HOMO, HOMO-1, LUMO, LUMO + 1 energies and HOMO–LUMO energy gaps (Eg)
in eV for ground states at the B3LYP/6-31G** and B3LYP/6-31 + G* level of theories.
EHOMO
EHOMO-1
ꢁ6.38
ꢁ6.67
ELUMO
ꢁ1.87
ꢁ2.21
ELUMO+1
ꢁ0.50
ꢁ0.88
Eg
B3LYP/6-31G**
ꢁ5.78
B3LYP/6-31 + G*
3.91
3.90
ꢁ6.11
Table 6
Calculated absorption wavelengths (kabs) in nm, oscillator strengths (f), transitions
and % contribution at the TD-B3LYP/6-31 + G* level of theory.
4. Conclusions
Parameters
Gas phase
akabs
kabs
Transitions
Contributions (%)
f
A Schiff base 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phe-
nol has been synthesised and characterized by elemental analysis
(CHN), FTIR, UV–Vis spectroscopy and X-ray single crystal determi-
nation. Quantum chemical methods like DFT and TDDFT were used
to obtain geometrical parameters and the computed spectra were
compared with the experimental observations. The B3LYP/6-
31 + G* level of theory is better to reproduce the experimental
geometrical parameters of Schiff base compound. The HOMO,
HOMO-1 and LUMO are distributed all over the backbone while
LUMO + 1 is localized on pyridine moiety and methyl group. The
nitrogen of pyridine has small contribution in the formation of
HOMO and HOMO-1 while reverting affect has been observed for
oxygen. The HOMO–LUMO energy gaps at both the level of theories
are same. Generally, in solvent the oscillator strengths improved
revealing that the absorption coefficient would be enhanced. The
assignments for first peak of kabs are from HOMO to LUMO while
second peak from HOMO-1 to LUMO. The computed absorption
wavelengths in different solvents are in better agreement with
the experimental data. No solvatochromic effect has been observed
on the absorption wavelengths except the THF.
–
–
357
306
356
312
312
299
358
312
358
312
358
312
H–L
H-1–L
H–L
H-1–L
H–L
H-1–L
H–L
H-1–L
H–L
H-1–L
H–L
93
90
95
93
96
93
96
93
95
93
95
93
0.3248
0.4924
0.5043
0.4330
0.5105
0.4472
0.5286
0.4322
0.5064
0.4359
0.4994
0.4335
CH3CN
THF
DMF
Acetone
Methanol
–
H-1–L
a
Experimental data.
than that of B3LYP/6-31G** level of theory. The HOMO–LUMO en-
ergy gaps (Eg) at both the level of theories are same.
3.3. Absorption spectra
Absorption wavelengths (kabs), oscillator strengths (f), contribu-
tions and assignments at TD-B3LYP/6-31 + G* level of theory with
and without solvents have been listed in Table 6. In gas phase,
the computed maximum absorption wavelength is 306 nm (second
peak) while the first peak has been observed at 357 nm. In THF the
first and second absorption peaks have been observed at 312 and
299 nm, respectively. The first and second absorption peaks in
Supplementary material
CCDC 936163 contains the supplementary crystallographic data
of complex (1) of this article. The data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or

14
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from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 IEZ UK; fax +44 1223 336 033; or email: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found in the online version.
Acknowledgement
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