International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 10, October 2013
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Theoretical study of substituent effects on
electronic and structural properties and IRspectrum of 1,3,6,8-tetrahalogens pyrene
compounds via density functional theory.
Ali Taher Mohi*
* Department of Physics, College of Education, Al-Mustansiriyah University, Baghdad, Iraq.
ABSTRACT
Density functional theory (DFT) was used to systematically examine the affects of halogens substituted (fluorine (F), chlorine
(Cl) and bromine (Br)) on electronic and structure properties of Pyrene. The B3LYP/6-31G (d,p) protocol has been used for
structure optimization and all other properties. The optimized structures, total energies, electronic states, energy gaps, ionization
potentials, electron affinities, chemical potential, global hardness, softness, global electrophilictity, dipole moment and dipole
polarizability were calculated. The harmonic vibrational frequencies calculated and compared with available experimental data.
The results showed a decrease in gap energies and the presence of the electron-accepting groups leads to easy oxidation.
Keywords: DFT, Ionization potential, electron affinity, energy gap, and IR spectrum.
1. INTRODUCTION
Aromatic compounds are a large class of conjugated π-electron systems of great importance in many research areas, such
as materials science, astrochemistry and molecular electronics [1]. In the last case molecules with electron-accepting
(high electron affinity) and electron-donating (low ionization potential) properties strongly effecting charge-transporting
properties of thin films based on these materials are often used. In contrast to inorganic materials that consist of covalent
or ionic bonds of atoms over the entire solids, organic materials are based on independent molecules and are
characterized by weak intermolecular interactions [2].As a result, characteristics of energy levels and energy
transformation for single molecules and condensed media differ a little, while the difference among these characteristics
for inorganic compounds is much stronger. Thus, the properties of individual molecules can be used to describe molecular
solids in the first approximation. Molecular electronics is one of the most important developments in modern-day
technology [3].
Organic semiconductors have been profitably used since the time of their discovery due to their flexibility, low cost [4],
and intrinsic properties such as light emission. Some applications of organic semiconductors include: organic light
emitting diodes (OLEDs), photovoltaic cells, thin film transistors, and biosensors. It is important to mention that most of
the materials used for these applications contain conjugated π systems [3].
Pyrene molecule and its derivatives are used commercially to make dyes and dye precursors, for example pyranine and
naphthalene-1,4,5,8-tetracarboxylic acid. Its derivatives are also valuable molecular probes via fluorescence spectroscopy,
having a high quantum yield and lifetime (0.65 and 410 nanoseconds, respectively, in ethanol at 293 K) [5].
The goal of this work is the theoretical investigation of the effect of substitution groups (tetra- halogens) on electronic and
structural properties and the reactivity of pyrene molecular and the evaluation of their dipole polarizability by DFT
method and comparison with experimental results.
2. Computational details
All the computational studies were carried out using the density functional theory (DFT) methods implemented in the
Gaussian 09 suite of programs [6]. The model molecular structures tetrahalogens pyrene in different positions is given in
figure 1. The molecular properties of the compounds have been computed by DFT using the standard 6-31G(d,p) basis
set. In the DFT calculations the Lee, Yang and Parr correlation functional [7] is used together with Becke’s three
parameters [8] exchange functional B3LYP. Conformational analysis of the molecules has been performed to have an
idea about the lowest energy structures of the species. Atoms in the molecule are numbered according to their order in the
molecule specification section of the input.
The geometry optimization was performed at the B3LYP density functional theory with the same basis set. Harmonic
vibrational frequencies were computed at the same level of theory. The hybrid functional B3LYP had been shown to be
highly successful for calculation the electronic properties such as ionization potentials, electronic states and energy gaps
[9],[10].
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Volume 2, Issue 10, October 2013
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The geometry optimized structures are obtained by restricted closed-shell formalism and without any symmetry
restriction, and vibrational analysis for each structure does not yield any imaginary frequencies, which indicates that the
structure of each molecule corresponds to at least a local minimum on the potential energy surface [11]. The vibrational
wave number assignments were made with a high degree of accuracy has been carried out by combining the results of the
Gaussview 5 program with symmetry consideration.
In this investigation, the more relevant ionization potential (IP), electron affinities (EA), chemical potential (K) (it is
the negative of electronegativity (χ)), hardness (η), softness (S), electrophilic index (ω) and the electric dipole
polarizability (α) were calculated. The ionization potential is calculated as the energy difference between the energy of the
molecule derived from electron-transfer (radical cation) and the respective neutral molecule; IPv = Ecation - En. The EA
was computed as the energy difference between the neutral molecule and the anion molecule: EA = En- Eanion [12]. The
HOMO and LUMO energy was used to estimate the IP and EA in the frame work of Koopmans’ theorem:
and
[7], [12].
The other parameters are calculated as follows [13]:
Electronegativity (χ):
Hardness
(2)
Softness
(3)
Electrophilic index
(4)
Density functional theory has also been used to calculate the dipole moment µ, mean polarizability <α>, the total dipole
moment and the mean polarizability in a Cartesian frame is defined by [13],[14]:
µ = (µx2 + µy2 +µz2)1/2
(5)
(6)
Where
are the eigenvalues of the polarizability tensor.
Figure 1: The optimized structures of pyrene and tetrahalogens pyrene molecules using B3LYP/6-31G(d,p)
3
Results and discussion
3.1 Molecular geometry
Geometry optimization is one of the most important steps in the theoretical calculations. The optimized structure
parameters of molecules calculated by DFT-B3LYP levels with the 6-31G(d,p) basis set are listed in the table1 in
accordance with the atom numbering scheme given in figure1. Table 1 compares the calculated bond lengths and angles
for pyrene molecule with those experimentally available from X-ray diffraction data.
From the theoretical values, we can find that most of the optimized bond angles are slightly larger than the
experimental values, due to the theoretical calculations belong to isolated molecules in gaseous phase and the
experimental results belong to molecules in solid state.
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Table 1: Optimized geometrical parameters of molecules, bond length R (A°), and bond angles (°).
Molecules
1
2
3
4
bond angles
Our data
1.394
Expt.
[15]
1.395
A(C1 – C6 – C5)
120.755
Expt.
[16]
120.600
R(C4 – C8)
1.426
1.425
A(C5 – C4 – C8)
120.057
119.900
bond length
Our data
R(C1 – C6)
R(C5 – C6)
1.404
1.406
A(C16–C15–C8)
118.559
117.960
R(C16 – C17)
1.361
1.367
A(C9 – C8 – C15)
119.866
120.000
R(C2 – H22)
1.087
1.091
A(H22 – C2 – C3)
119.141
-
R(C13 – H25)
1.086
1.089
A(H25–C13–C14)
119.707
-
R(C1 – C2)
1.388
-
A(C2 – C1 – C6)
118.116
-
R(C4 – C8)
1.427
-
A(C2 – C3– C7)
122.956
-
R(C10 –H21)
1.084
-
A(H18–C15–C14)
118.467
-
R(C12–H22)
1.083
-
A(H20–C1–C2)
120.942
R(C2 –F23)
1.349
-
A(C3-C2- F23)
118.555
-
R(C13 –F26)
1.349
-
A(C12-C13-F26)
118.296
-
R(C1 – C2)
1.389
-
A(C2 – C1 – C6)
119.535
-
R(C4 – C8)
1.435
-
A(C2 – C3– C7)
123.193
-
R(C10 –H21)
1.083
-
A(H18–C15–C14)
118.642
-
R(C12–H22)
1.083
-
A(H20–C1–C2)
120.233
-
R(C2 –Cl23)
1.756
-
A(C3-C2- Cl23)
120.620
-
R(C13–Cl26)
1.756
-
A(C12-C13-Cl26)
117.306
-
R(C1 – C2)
1.388
-
A(C2 – C1 – C6)
119.580
-
R(C4 – C8)
1.435
-
A(C2 – C3– C7)
117.779
-
R(C10 –H21)
1.083
-
A(H18–C15–C14)
118.423
-
R(C12–H22)
1.082
-
A(H20–C1–C2)
120.210
-
R(C2 –Br23)
1.910
-
A(C3-C2- Br23)
120.916
-
R(C13–Br26)
1.910
-
A(C12-C13-Br26)
117.016
-
3.2 Energies
Table 2 shows the values of the total energy and electronic states for the analyzed structures and the energy gap
(
of the studied molecules. The total energies for all studied molecules as a linear function of halogens
substituted side group number adding to the molecule.
It is clear that from table 2, the total energy for all tetrahalogens pyrene molecules is increase and depending on the
atomic number of halogens atoms, and it is observed that substitution of tetrahalogens groups (electron-accepting) causes
decreasing the HOMO and LUMO energy, it is known that the electron accepting substituents decreasing the LUMO and
HOMO energies [17], and energy gap decreased. Therefore, the presence of substituent decreases the energy gaps
improves the conductivities and also enhances the solubilities of these molecules.
The LUMO-HOMO energy gaps of tetrahalogens pyrene molecules is small than that of the original molecules, with
decreasing energy gap, electrons can be easily excited from the ground state. This effect of the side group was the largest
in molecule 4 which have energy gap of (3.467eV). The energy gap of pyrene (3.84eV) is agreements with experiment
value (3.78eV ± 0.05) [18]. The table 2 shows also the symmetry of studied molecules, the all molecules have D2h
symmetry (high symmetry) and are the planar with inversion center.
Table 2: Total energy, electronic states and energy gap for molecules.
Structure
Energy
(a.u)
Symmetry
1
-615.789
D2h
Volume 2, Issue 10, October 2013
Electronic States(eV)
HOMO
LUMO
-5.336
-1. 496
Energy
Gaps (eV)
Our data
3.840
Energy Gaps (eV)
Expt.
3.78±0.05
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2
-1012.714
D2h
-5.502
-1.739
3.763
-
3
-2454.157
D2h
-5.856
-2.343
3.513
-
4
-10900.198
D2h
-5.807
-2.340
3.467
-
3.3 Some important variables.
B3LYP functional used in this study has a high efficient to calculate the electronic properties for the organic studied
molecules, such as ionization potentials (IP), electron affinities (EA), chemical potential (K), absolute hardness (η),
absolute softness (S), electrophilic index (ω). The properties that are displayed in figure (2–7) for each variable are
computed by two different ways: The first one being energy-vertical is based on the differences of total electronic energies
when an electron is added or removed in accordance with the neutral molecule.The second one is based on the differences
between the HOMO and the LUMO energies of the neutral molecule and is known as orbital-vertical (Koopmans’
theorem).
The calculated properties for each variable as shown in figure (2-7) clearly reveal that these tetrahalogens pyrene
compounds have a tendency to capture electrons instead of donating them. The ionization potential for the tetrahalogens
pyrene molecules is higher than that for the original molecule, but the 1,3,6,8-chloropyrene molecule has the largest
value of ionization potential, this indicates that the tetrahalogens pyrene molecule needs high energy to become cation
comparing with the others.
The strength of an acceptor is measured by its electron affinity (EA) which the energy released when adding one electron
to LUMO. An acceptor must have a high EA, adding the halogens atoms to the pyrene molecule leads to increasing the
ability of the electron affinity for the molecule; EA for molecule 1,3,6,8-chloropyrene is the largest, as we see in figure 3.
The calculation value of IP, EA and energy gaps for pyrene molecule is a good agreement with experimental value 7.439
± 0.006 eV, 0.430 ± 0.005 eV and 3.43 ± 0.05 eV [18], respectively.
Few interesting observations have been made from the results that are shown in figure (2-7) obtained through the energyvertical and orbital-vertical (KT) methods. The electron affinities (EA) computed from the energy of the lowest
unoccupied molecular orbital (LUMO) are higher for all study molecules than that of the energy-vertical method, the
negative value of (EA) indicates that the total energy of the system is increasing with acceptation an electron. The
ionization potential (IP) that results from the highest occupied molecular orbital is smaller for all study molecules than
that of the energy-vertical method. The figure 4 shows the values of electronegativity that calculate by two ways are
identified. From the previous investigations, it has been found that for almost all the commonly used exchangecorrelation functional such as B3LYP, B3PW91, Koopman’s theorem is not satisfied accurately [12].
The two results obtained by the calculation of electronegativity and electrophilicities also agreed very well with the
difference in the result. This could be the reason for the low hardness values obtained from the orbital-vertical method
than from the method of energy-vertical. Koopman’s theorem neglects the relaxation effect by using the frozen-orbital
approximation. However, this error is frequently compensated by the oppositely directed error due to the electron
correlation effect, neglected in the Hartree-Fock (HF) method. Therefore, the Koopmans’ theorem is a crude but useful
and fast approach [17,19]. The behavior of electronegativity, softness and electrophilic index for the studied molecules
shows the magnitude large than these for the original ring, adding the tetrahalogens atoms gives the molecules more
softness.
Figure 2: Shows the calculated Ionization Potential for molecules under study.
Figure 3: Shows the calculated Electron Affinity for molecules under study.
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Volume 2, Issue 10, October 2013
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Figure 4: Shows the calculated electronegativity for molecules under study.
Figure 5: Shows the calculated absolute hardness for molecules under study.
Figure 6: Shows the calculated absolute softness for molecules under study.
Figure 7: Shows the calculated electrophilic index for molecules under study.
The molecule dipole moment represents a generalized measure of bond properties and charge densities in a molecule
[20]. The calculated total dipole moment for all molecules in present work is zero, and this value is symmetric due to the
molecules and the inherent tendency of π-electrons for strong delocalization. Theoretically this is true, yet it is pointed out
that π -electrons in tetrahalogens pyrene are distortive in nature, hence, each end of the molecule will have the same
properties. Accordingly, the tetrahalogens pyrene molecules are non-polar molecules, leading to have permanent dipole
moment.
The results of the calculated polarizability for (1 – 4) molecules in table 3 showed that all halogens substitution leads to
increase the average polarizability and cause more reactive then the original molecules (1). The molecules 3 and 4 have
average dipole polarizability equal 414.011 and 469.612 a.u, they have the highest polarizability and have highest
reactivity. This due to the ring delocalizing electron resonance from the phenyl groups [13].
Table 3: calculated dipole moment μ (debye), components of αi (i = xx,yy,zz) an average of the dipole polarizability
in atomic units for molecules.
molecules
μ
αxx
αyy
αzz
1
0.000
537.171
356.062
81.602
324.945
2
0.000
544.404
377.363
87.519
336.429
3
0.000
650.768
480.144
111.122
414.011
4
0.000
547.658
547.658
151.017
469.612
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3.4 IR Spectra
The IR spectra of tetrahalogens pyrene compounds are provided in figure 8. The harmonic vibrational frequencies
calculated for study molecules at B3LYP level using the 6-31G(d,p) basis sets. The calculated vibrational spectra of
tetrahalogens pyrene have been divided into two regions; a low wave number fingerprint region (<2000 cm−1) and a high
wave number (low intensity) functional group region (3500 – 2000 cm−1) [21]. The number of vibrational frequency
modes depending on the number of atoms (N) in molecules (3N-6), the neutral pyrene and tetrahalogens pyrene are
characterized by 72 normal vibrational modes. Absence of imaginary frequency represents the stable nature and the
validity of the molecules.
These characteristic of normal vibrational modes are used for the identification of materials and for the determination of
structure in an unknown pure compound. This can be analysis as:
(i) C – H vibrations
The (C – H) stretching vibrations of aromatic molecules in the region (3200 – 2900) cm-1 which is characteristic region
for ready identification of (C – H) stretching vibrations and particularly the region (3200 – 3100) cm-1 for asymmetric
stretching and (3100 – 2900) cm-1 for symmetric stretching modes of vibration [22].
For tetrahalogens pyrene, the (C – H) stretching stayed in the region (3242– 3213) cm-1, with slightly increased campare
with natural pyrene. For most cases, the aromatic compound (C-H) vibration absorption bands are usually weak; in many
cases it is too weak for detection. In this region, the bands are not affected, appreciably by the nature of substituents.
The aromatic molecules frequency have both in-plane (1700 – 1100) cm-1 and out-of-plane (below 1000 cm-1 ) (C – H)
bending vibrations, for tetrahalogens pyrene the in-plane vibration calculated at (1510 - 1150) cm-1, and for out-of-plane
(C – H) bending vibrations are found in the region (1002 - 810) cm-1.
(ii) Phenyl ring vibrations: (C - C) stretching vibrations
The aromatic (C - C) stretching vibrations occurs in the region (1600 - 1500) cm -1, for tetrahalogens pyrene (C - C)
stretching located at (1650-1420) cm-1.As for the previous band, (C - C) stretching and in-plane (C – H) bending
vibrations have been identified. The ring in plane vibrations has given rise to weak bands across the medial frequency
region, that is to say, below 1600 cm-1
(iii) (C –X) vibrations (X=F, Cl and Br)
The heavier mass of halogens obviously makes the C-X stretching mode to appear in longer wavelength region.
Krishnakumar et al. [23] assigned vibrations of C−X group (X=F, Cl and Br) in the frequency range (1292−485) cm -1.
Based on the above literature, the stretching of (C‫ــــ‬F) bond has been observed at (1151.09) cm-1, the stretching of
(C‫ــــ‬Cl) bond has been observed at (1084.83) cm-1 and the stretching of (C‫ــــ‬Br) bond has been observed at
(1058.56) cm-1, which highly coupled with (C−C−C) in-plane deformation. The (C−X) stretching occurs in strong to
medium intensity [24].
While the (C‫ــــ‬F) in-plane bending vibration appeared at (325.43) cm-1, the (C‫ــــ‬Cl) in-plane bending vibration appeared
at (215.87) cm-1 and the (C‫ــــ‬Br) in-plane bending vibration appeared at (280.45) cm-1. The (C-F) out-of-plane bending
vibration is assigned to (217.96) cm-1 , The (C-Cl) out-of-plane bending vibration appeared at (153.94) cm-1 and the (CBr) out-of-plane bending vibration appeared at (145.92) cm-1.
Figure 8: Calculated IR spectra of study molecules
4. Conclusions:
We have use DFT in this study to compute geometry optimization and electronic properties of pyrene and tatrahalogens
pyrene by using B3LYP function. The calculated electronic properties such as ionization potential, electron affinity,
electronegativity, hardness, softness and electrophilic index by using two different ways: energy-vertical method and
orbital-vertical method (KT), the important conclusions are:
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1- Geometry optimization for molecule 1 has been found in a good agreement with experimental data, while for other
studied molecules (2 – 4) it has not been found a reference data.
2- The presence of the substituents decreases the energy gap of the molecules study, this is one of the important properties
obtained in this work, and a small energy gap means small excitation energies of manifold of the exited states.
3- The electronic properties (IP, EA, K, η, S, ω) was calculated by using energy-vertical method are a good agreement
with experimental result (especially for pyrene) and better than was calculated by using orbital-vertical method, thus
Koopman’s theorem is not satisfied accurately.
4- The results showed that all substitution groups leads to increase the average polarizability and cause to more reactive
than original molecules.
5- In IR spectra calculation shows a good agreement with experimental data for pyrene, adding the halogens groups leads
to increasing the vibrational mode, and highest stretching vibrational wave numbers.
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AUTHOR
Ali Taher Mohi received the B.S., M.S. and Ph.D. degrees in physics in 1991, 1996 and 2013, respectively.
During 2000-2005, I'm lecture in Department of Physics, College of Education 7th April University, Al-Zawia,
Libya. I'm now lecture in Department of Physics, College of Education, Al-Mustansiriyah University,
Baghdad, Iraq.
Volume 2, Issue 10, October 2013
Page 235