Experimental and quantum chemical computational study of (E)-1-[5-(3,4-dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone

Article abstract of DOI:10.1016/j.saa.2012.02.092

In this work, the azo dye, (E)-1-[5-(3,4-dimethylphenyldiazenyl)-2- hydroxyphenyl]ethanone, has been synthesized and characterized by IR, and X-ray single-crystal determination. In the theoretical calculations, the stable structure geometry of the isolate

Full text of DOI:10.1016/j.saa.2012.02.092

Spectrochimica Acta Part A 93 (2012) 208–213
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Experimental and quantum chemical computational study of
(E)-1-[5-(3,4-dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone
b
c
a
a
Serap Yazıcıa,∗, C¸ ig˘dem Albayrak , Ismail Erdem Gümrükc¸ üog˘lu , Ismet S¸ enel , Orhan Büyükgüngör
˙
˙
a Department of Physics, Ondokuz Mayıs University, Samsun 55139, Turkey
b Department of Science Education, Sinop University, Sinop, Turkey
c Department of Chemistry, Ondokuz Mayıs University, Samsun, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the azo dye, (E)-1-[5-(3,4-dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone, has been
synthesized and characterized by IR, and X-ray single-crystal determination. In the theoretical calcu-
lations, the stable structure geometry of the isolated molecule in gas phase was investigated under
the framework of the density functional theory (B3LYP) with 6-31G (d, p). To designate lowest energy
molecular conformation of the title molecule, the selected torsion angle was varied every 10◦ and the
molecular energy profile was calculated from −180◦ to +180◦. Besides, molecular electrostatic potential
(MEP), frontier molecular orbitals (FMO) analysis, and thermodynamic properties were described from
the computational process. In addition to these calculations, we were investigated solvent effects on the
nonlinear optical properties (NLO) of the title compound.
Received 14 October 2011
Received in revised form 18 February 2012
Accepted 23 February 2012
Keywords:
Crystal structure
Diazenyl
Density functional theory (DFT)
Conformational analysis
Non-linear optical properties (NLO)
Solvent media
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental and computational methods
It is well known that azo dyes play an important role in dyes
and pigments industry. They have been most widely used in fields
such as dyeing textile fiber, biomedical studies, advanced applica-
tions in organic synthesis and high technology areas like lasers,
electro-optical devices, and ink-jet printers [1–3]. In addition,
azobenzene and its derivatives have attracted much attention for
their nonlinear optical properties [4–6]. Azo dyes contain at least
2.1. Instrumentation
The FT-IR spectrum of the title compound was recorded in the
4000–400 cm−1 region with a Bruker Vertex 80 V FT-IR spectrom-
eter using KBr pellets.
2.2. Synthesis
one chromophoric azo group (
N N
), links two sp2 hybridized C
atoms, attached to the substituents. The low-lying azo centered ␲*
molecular orbital is responsible for the intense long-wavelength
absorption (dye function) and the nonlinear optical properties
(information storage function) [7].
Here, we wish to report the synthesis crystal and molecular
structure of the dye derived from 2-hydroxyacetophenone: (E)-1-
[5-(3,4-dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone by
IR and X-ray determination. The conformational analysis of the title
molecule with respect to the selected torsion angle is achieved
by DFT. Besides these, the molecular structure and geometry,
FMOs, MEP, thermodynamic properties have also been studied and
investigated that the optical response of the title molecule in the
solvent media with the computational studies.
The compound was prepared by reflux
a mixture of
3,4-dimethylaniline (0.944 g, 7.8 mmol), water (20 ml) and
concentrated hydrochloric acid (1.97 ml, 23.4 mmol) was
stirred until a clear solution was obtained. This solution was
cooled down to 0–5 ◦C and a solution of sodium nitrite (0.75 g,
7.8 mmol) in water was added dropwise while the tempera-
ture was maintained below 5 ◦C. The resulting mixture was
stirred for 30 min in an ice bath. 2-Hydroxyacetophenone
(1.067 g, 7.8 mmol solution (pH 9) was gradually added to
a
cooled solution of 3,4-dimethylbenzenediazonium chlo-
ride, prepared as described above, and the resulting mixture
was stirred at 0–5 ◦C for 2 h in ice bath. The product was
recrystallized from ethyl alcohol to obtain solid (E)-1-[5-
(3,4-dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone.
Crystals of (E)-1-[5-(3,4-dimethylphenyldiazenyl)-2-hydroxy-
phenyl]ethanone were obtained after one day by slow evaporation
from acetic acid (yield 78%, m.p. = 405–407 K).
∗
Corresponding author. Tel.: +90 362 3121919x5285; fax: +90 362 4576092.
E-mail address: yserap@omu.edu.tr (S. Yazıcı).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.02.092

S. Yazıcı et al. / Spectrochimica Acta Part A 93 (2012) 208–213
209
Table 1
Crystal data and structure refinement parameters for the title compound.
C16
C3
C2
O1
C14
Chemical formula
C16H16N2O2
C13
C4
N2
Color/shape
Formula weight
Temperature
Brown/prism
268.31
150 K
H1
O2
C9
C1
Crystal system
Space group
Unit cell parameters
Triclinic
P−1
C15
N1
C12
C5
C6
C10
˚
a = 7.1429(4) A
C11
C7
˚
b = 9.9121(7) A
˚
c = 10.0125(6) A
˛ = 93.345(5)◦
ˇ = 102.297(5)◦
C8
ꢀ = 91.957(5)◦
3
Fig. 1. ORTEP III diagram of the title compound.
˚
Volume
690.67(8) A
Z
2
Density
Absorption coefficient
Tmin, Tmax
1.291 g/cm3
the effect of solvent on the energetic behavior and NLO properties
of the title molecule, the PCM (polarizable continuum model) [16]
was employed by B3LYP/6-31G (d, p).
0.086 mm−1
0.9698, 0.9893
STOE IPDS 2/ꢁ-scan
2.83◦ to 26.50◦
11,629
2857/1924
1924/0/209
Diffractometer/meas. meth.
ꢂ range for data collection
Unique reflections measured
Independent/observed reflections
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2ꢃ(I)]
R indices (all data)
3. Results and discussion
1.090
3.1. Description of the crystal structure
R1 = 0.0543, wR1 = 0.1256
R2 = 0.0857, wR2 = 0.1400
The
molecular
structure
of
(E)-1-[5-(3,4-
dimethylphenyldiazenyl)-2-hydroxyphenyl]ethanone with the
atom numbering scheme is shown in Fig. 1 and selected geometric
parameters are given in Table 2. The title compound crystallizes
in the triclinic space group P−1 with two molecules (Z = 2) in the
unit cell. The compound has two aromatic rings and an azo moiety.
In the molecule, the benzene rings which linked to azo bridge
adopt trans configuration with respect to the N N double bond.
The title molecule is almost coplanar, making a dihedral angle
between the aromatic rings of 1.83(1)◦. In the title compound,
the C1–N1–N2–C9 torsion angle is 180(2)◦ as a result of the
partial electron delocalization on the azo bridge. It is expected
that C6–C1–N1 and C14–C9–N2 angles are 120◦ since C1 and C2
atoms have sp2 hybridization. Due to the steric effect between
the azo moiety and substitute groups, these angles have been
deviated from 120◦ with dihedral angle of 116.7(2)◦, 115.7(2)◦,
respectively. The most important bonds of the azo compounds
2.3. Crystal data for the title compound
A
brown crystal of the compound with dimensions
0.53 mm × 0.32 mm × 0.14 mm was mounted on goniometer
and data collection was performed on a STOE IPDS II diffrac-
tometer by the ω scan technique using graphite-monochromatic
˚
Mo K␣ radiation (ꢄ = 0.71073 A) at room temperature (296 K).
Unit cell parameters were determined from least-squares refine-
ment of setting angels with ꢂ in the range 2.83 ≤ ꢂ ≥ 26.50. The
refinement was carried out by full-matrix least-squares method
on the positional and anisotropic temperature parameters of
the non-hydrogen atoms, or equivalently corresponding to 209
crystallographic parameters. Atom H1 was located in a difference
Fourier map and refined isotropically, and the other H atoms
were positioned geometrically and treated using a riding model,
˚
are N1 N2, C1 N1 and N2 C9 and values of them are 1.254(3) A,
˚
˚
fixing the bond lengths at 0.93 A and 0.96 A for aromatic CH and
CH3 atoms, respectively. The displacement parameters of the
H atoms were fixed at Uiso(H) = 1.2Ueq(C) (1.5Ueq for methyl)
of their parent atoms. For the title compound data collection
and cell refinement: X-AREA [8]; data reduction: X-RED32 [8];
program used to solve structure: SHELXS 97 [9]; program used to
refine structure: SHELXL 97 [9]; molecular figure: ORTEP III [10]
publication software: WinGX [11]. Details of the data collection
conditions and the parameters of the refinement process are given
in Table 1.
˚
˚
1.428(3) A and 1.424(3) A, respectively and these bond lengths are
in a good agreement with found of those in the literature [17,18].
Furthermore, C7 O2 and C4 O1 bonds which have distance of
˚
˚
1.233(3) A, 1.349(3) A, respectively, are also compared well with
the values reported previously [19].
In the title azo dye, there is a strong intramolecular O H· · ·O
hydrogen bond which generates S(6) ring motif [20]. The sum of
˚
the van der Waals radii of the two O atoms (3.04 A) is significantly
longer than the intramolecular O· · ·O hydrogen bond length [21].
The crystal packing is stabilized by van der Waals interactions and
the details of the intramolecular hydrogen bond are summarized
in Table 2.
2.4. Computational procedure
The molecular structure of the title molecule is optimized
by the DFT calculations with a hybrid functional B3LYP (Becke’s
Three parameter Hybrid Functional using the LYP Correlation
Functional) at 6-31G (d, p) basis set [12,13] and all of the calcu-
lations were carried out using Gaussian 03 program package [14].
The vibrational frequencies of the optimized molecule have also
been calculated at the same level of the theory and achieved fre-
quencies were scaled by 0.9627 [15]. To elucidate conformational
features of the molecule, the selected degree of torsional freedom, T
(C6–C1–N1–N2), was varied from −180◦ to +180◦ in every 10◦ and
the molecular energy profile was obtained with the B3LYP/6-31G
(d, p) method. In addition, FMO’s, MEP and thermodynamic prop-
erties were performed with the same level of theory. To investigate
3.2. Optimized geometry and conformational analysis
Selected structural parameters obtained experimentally and
calculated theoretically using B3LYP with the 6-31G (d, p) basis
set are tabulated in Table 3 as comparative. The optimized param-
eters by DFT show a small difference from those obtained by
Table 2
Hydrogen-bond geometry (Å,◦).
D
H· · ·A
D
H
H· · ·A
D· · ·A
D
H· · ·A
O1–H1· · ·O2
0.91(3)
1.70 (3)
2.544(2)
152(3)

210
S. Yazıcı et al. / Spectrochimica Acta Part A 93 (2012) 208–213
Table 3
Selected molecular structure parameters for the title compound.
X-ray
DFT
C7–O2
C4–O1
N1–C1
N1–N2
N2–C9
C7–C8
C12–C15
C13–C16
1.233(3)
1.349(3)
1.428(3)
1.254(3)
1.424(3)
1.488(4)
1.508(3)
1.515(3)
1.24196
1.33313
1.41173
1.26311
1.41539
1.51346
1.50852
1.50992
O1–C4–C3
O1–C4–C5
C5–C7–C8
C6–C1–N1
C14–C9–N2
O1–C4–C5–C7
C4–C5–C7–C8
C15–C12–C13–C16
C2–C1–N1–N2
N1–N2–C9–C10
C6–C1–N1–N2
117.9(2)
118.11439
122.21741
120.26315
115.98414
115.43609
0.00224
−180.00000
−0.00255
−0.01630
−0.00526
179.98452
121.7(2)
120.6(2)
116.7(2)
115.7(2)
−0.2(4)
178.0(2)
0.8(4)
1.1(3)
0.0(4)
−178.45(1)
Fig. 3. Molecular energy profile using DFT against the selected torsional degree of
freedom.
3.3. Vibrational spectra
X-ray diffraction. It is not suprising, because the DFT calculations
ignore molecular interactions. The largest difference between the
experimental observations and those obtained from the theoret-
The harmonic vibrational frequencies for the title azo dye have
been calculated by using DFT method at 6-31G (d, p) level. It is
known that there is a systematic error in the calculated harmonic
vibrational frequency when it is compared to the experimental
fundamental vibrational frequency [22]. The reason for this is the
incorrect description of the electron–electron interaction and the
neglect of anharmonicity in the theoretical calculations. In order to
compare the theoretical results with experimental values of those,
scaling factor which is 0.9627 for B3LYP/6-31G (d, p) is applied to
all of the calculated frequencies. In Fig. 4, the theoretical spectrum
is not scaled for the qualitative comparison.
◦
˚
ical calculations is 0.02546 A for bond lengths and 0.71586 for
bond angles. On the other hand, the root mean square error (RMSE)
is detected about 1.3 × 10−2 A for bond lengths. When the exper-
˚
imentally observed and theoretically calculated geometries are
superimposed (Fig. 2), RMSE is found to be about 4.7 × 10−2 A.
˚
This RMSE value support that there is the conformational harmony
between the observed and calculated geometries. In addition to
these, in the optimized molecule, the dihedral angle between the
aromatic rings found to be 0◦.
In order to determine the most favourable conformation as the-
oretically, the energy profile of the title compound as a function
of C6–C1–N1–N2 torsion angle was obtained from B3LYP/6-31G
(d, p) method (Fig. 3). Selected torsion angle, T (C6–C1–N1–N2), is
−178.45(1)◦ for X-ray and 179.98452◦ for DFT-optimized geometry.
The energy profile shows two maxima at 90◦ and three minima
at 0◦ and 180◦. The hydroxyl attached ring perpendicular to the
plane of the other aromatic ring at two maxima values of selected
torsion angle. The two energy barriers arise from the steric inter-
actions between the two aromatic rings. In the gas phase, it is
clear that the most stable conformers must have a minima value of
selected torsion angle and these minima are 0◦ and 180◦. The opti-
mized crystal structure has a planar conformation at these values
of the torsion angle.
Some of the characteristic frequencies for the title compound
have given in Table 4 and the correlation graphic which described
harmony between the calculated and experimental frequencies
are plotted (Fig. 5). As can be seen from Fig. 5, experimental
fundamentals have a better correlation with B3LYP. The experi-
mental frequencies are slightly lower than the calculated values
for the title molecule. Two factors may be responsible for the
discrepancies between the experimental and theoretical spectra.
The first is caused by the environment and the second reason
for these discrepancies is the fact that the theoretical calcula-
tions neglect the anharmonicity effects [23]. In the experimental IR
results, O H stretching band was observed at 2000–3000 cm−1 as
a broad peak due to the strong intramolecular O H· · ·O hydrogen
bond. Its value obtained by DFT calculations is 3101 cm−1. While
the N N and C O stretching vibrations were seen at 1420 cm−1
and 1643 cm−1, respectively, the same bands were calculated as
1495 cm−1 and 1648 cm−1,respectively. These stretching modes are
consistent with those reported in the literature [24–26].
Table 4
Characteristic IR absorption bands of the title molecule.
Assignments
Experimental (cm−1
)
Calculated (cm−1
)
ꢅO–H
2000–3000
1643
1420
1242
1327
1606–1574
626–780
829
3101
1648
1495
1293
1391
1605–1558
862
ꢅC
ꢅN
O
N
ꢅC–O
ıO–H
ꢅC–C
ꢀO–H
Fig. 2. Atom-by-atom superimposition of the calculated structure (red) over the X-
ray structure (black) for the title molecule. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
ꢀC–H(ring)
831
ꢅ; stretching, ı; in-plane bending, ꢀ; out-of-plane bending

S. Yazıcı et al. / Spectrochimica Acta Part A 93 (2012) 208–213
211
Fig. 4. Experimental (red) and calculated (black) IR spectra of the title molecule. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of the article.)
3.4. Molecular electrostatic potential
diffraction methods, as well as computationally [27,28]. Its sign in a
given region depends upon whether the positive contribution of the
nuclei or the negative one of the electrons is dominant there [29].
Thus possible sites for electrophilic or nucleophilic attack may eas-
ily be identified, and a measure of the interaction of the molecule
with its environment obtained.
To predict reactive sites for electrophilic attack, molecular
electrostatic potential (MEP) was calculated at the B3LYP/6-31G
(d, p) optimized geometry and demonstrated in Fig. 6. It is
known that the negative (red) and the positive (blue) regions
of the MEP represent to regions which prone to electrophilic
and nucleophilic attack, respectively. As can be seen from Fig. 6,
there are two negative regions localized on the O1 and O2
The electrostatic potential at any point, V(r), is the energy
required to bring a single positive charge from infinity to that point.
As each pseudo atom in the refined model consists of the nucleus
and the electron density distribution described by the multipole
expansion parameters, the electrostatic potential may be calculated
by the evaluation of
ꢂ
ꢀ
ZA
(rꢀ)drꢀ
|rꢀ − r|
ꢁ
ꢁ
ꢁ
ꢁ
V(r) =
−
(1)
RA − r
A
where ZA is the charge on nucleus A located at RA. V(r) is a
physical observable, which can be determined experimentally, by
Fig. 5. Correlation graphic of calculated and experimental frequencies of the title
compound.
Fig. 6. Molecular electrostatic potential map calculated at B3LYP/6-31G (d, p) level.

212
S. Yazıcı et al. / Spectrochimica Acta Part A 93 (2012) 208–213
Fig. 7. Molecular orbital surfaces and energies for the HOMO and LUMO of the title compound.
Table 5
Theoretical energies (eV), dipole moments (Debye), molecular polarizabilities (Å3) and hyperpolarizabilities (×10−30 esu) in different media of the title compound.
Media
Gas (ε = 1)
Chloroform (ε = 4.711)
Ethanol (ε = 24.852)
Water (ε = 78.355)
E
ꢆ
−23,927.315
3.5913
−23,927.549
4.6350
−23,927.640
4.8891
−23,927.674
4.9400
˛
ˇ
ꢇEa
34.853062
7.5389
3.6942
43.031982
13.4813
3.6634
46.221860
17.1427
3.6484
47.043659
18.5744
3.6408
a
ꢇE = EHOMO − ELUMO
.
atoms with values around −0.0348 and −0.0374 a.u., respec-
tively. These are the most preferred regions for any electrophilic
attack on the whole molecule. So, these regions indicate that the
intramolecular hydrogen bond formed between the O1 and O2
atoms.
In order to inspect the effects of solvent on (hyper) polarizabil-
ities, the NLO properties of the title molecule were calculated at
B3LYP/6-31G (d, p) level by adding the self-consistent reaction field
(SCRF) approach with polarizable continuum model (PCM) [41,42].
In different solvents (chloroform, ethanol, water) energy, dipole
moment, (hyper) polarizability and the HOMO–LUMO energy gap
of the title compound are tabulated in Table 5. As can be seen
from the table, with increasing polarity of the solvent, a decline
was observed for the HOMO–LUMO energy gap of the title azo
dye. On the other hand, dipole moment, polarizability and hyper-
polarizability of the title molecule were raised with polarity of the
solvent. As the HOMO–LUMO energy range decreases, the electrons
are more easily excited from the HOMO to the LUMO and the elec-
tronic charge asymmetry of the molecule can be easily changed
when the external electric field is applied. So, with polarity of the
solvent, the increment of polarizability and hyperpolarizability val-
ues of the title compound is not surprising for this study. In addition
to these, together with the increasing polarity of the solvent, the
stability of the title compound decreases in going to the solution
phase.
3.5. Frontier molecular orbitals
The highest occupied molecular orbitals (HOMOs) and the
lowest-lying unoccupied molecular orbitals (LUMOs) are named as
frontier molecular orbitals (FMOs). The energy gap between the
HOMO and LUMO is very important in determining the chem-
ical activity of the molecule. A small HOMO–LUMO energy gap
implies low kinetic stability, because it is energetically favourable
to add electrons to a low-lying LUMO and to receive electrons
from a high-lying HOMO [30–32]. The distributions of the HOMO
and LUMO orbitals computed at the B3LYP/6-31G (d, p) level for
the title molecule and illustrated in Fig. 7. As can be clearly seen
from the figure, HOMO and LUMO orbitals are extended over the
entire molecule. The energy difference between the HOMO and
LUMO was obtained as 3.6942 eV in the gas phase calculations.
With this energy gap, it can be said that the title molecule has
high kinetic stability and low chemical reactivity as in our previous
study [26].
3.7. Thermodynamic properties
Based on the vibrational analysis at B3LYP/6-31G (d, p) level and
statistical thermodynamics, the standard thermodynamic func-
tions: heat capacity (Cp0,m), entropy (Sm0 ), and enthalpy (Hm0 ) were
obtained at constant pressure (Table 6) and at constant tempera-
ture (Table 7).
3.6. The solvent effect on the nonlinear optical properties
Nonlinear optical (NLO) effects arise from the interactions of
electromagnetic fields in various media to produce new fields
altered in phase, frequency, amplitude or other propagation charac-
teristics from the incident fields. The molecules, exhibiting efficient
nonlinear optical properties, have been investigated intensively
because of their potential applications in telecommunication, data
storage and optical signal processing [33–39]. The theoretical and
experimental works clearly shows that the environment plays a
remarkable role in the considerations of the first- and the second-
order hyperpolarizabilities of the molecules. On the other hand, the
linear polarizability is less affected by the solvent than the higher
order polarizabilities [40].
Table 6
Thermodynamic properties at different temperatures at B3LYP/6-31G (d, p) level.
T (K)
Hm0 (kcal mol−1
)
Cp0,m(cal mol−1 K−1
)
Sm0 (cal mol−1 K−1
)
100
200
298.15
300
400
1.910
6.049
12.133
12.267
20.551
30.709
28.955
49.733
70.293
70.680
90.688
107.984
88.527
116.395
140.870
141.319
165.013
187.609
500

S. Yazıcı et al. / Spectrochimica Acta Part A 93 (2012) 208–213
213
Table 7
Thermodynamic properties at different pressures at B3LYP/6-31G (d, p) level.
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increase with increasing temperature, due to the intensities of
molecular vibration increase as temperature increase. While the
pressure is increased at 298.15 K, it was observed that the entropy
increases but the enthalpy and the heat capacity remain constant
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solvent media, while the HOMO–LUMO energy gap of the molecule
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in the gas phase more than in polar solvents. The thermodynamic
properties of the title compound were also obtained at constant
temperature and at constant pressure. We hope that our paper
will be helpful for new researchers are working on the organic
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The authors wish to acknowledge the Faculty of Arts and Sci-
ences, Ondokuz Mayis University, Turkey, for the use of STOE IPDS
2 diffractometer (purchased under grant F.279 of the University
Research Fund).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.02.092.