Effect of hydroxyl group position on adsorption behavior and corrosion inhibition of hydroxybenzaldehyde Schiff bases: Electrochemical and quantum calculations

Article abstract of DOI:10.1016/j.molstruc.2012.11.013

The corrosion inhibition and adsorption of N,N′-bis(n- hydroxybenzaldehyde)-1,3-propandiimine (n-HBP) Schiff bases has been investigated on steel electrode in 1 M HCl by using electrochemical techniques. The experimental results suggest that the highest inhibition efficiency was obtained for 3-HBP. Polarization curves reveal that all studied inhibitors are mixed type. Density functional theory (DFT) at the B3LYP/6-31G(d,p) and B3LYP/3-21G basis set levels and ab initio calculations using HF/6-31G(d,p) and HF/3-21G methods were performed on three Schiff bases. By studying the effects of hydroxyl groups in ortho-, meta-, para- positions, the best one as inhibitor was found to be meta-position of OH in Schiff base (i.e., 3-HBP). The order of inhibition efficiency obtained was corresponded with the order of most of the calculated quantum chemical parameters. Quantitative structure activity relationship (QSAR) approach has been used and a correlation of the composite index of some of the quantum chemical parameters was performed to characterize the inhibition performance of the Schiff bases studied. The results showed that %IE of the Schiff bases was closely related to some of the quantum chemical parameters but with varying degrees/order. The calculated %IE of the Schiff base studied was found to be close to their experimental corrosion inhibition efficiencies.

Full text of DOI:10.1016/j.molstruc.2012.11.013

Journal of Molecular Structure 1035 (2013) 247–259
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Effect of hydroxyl group position on adsorption behavior and corrosion inhibition
of hydroxybenzaldehyde Schiff bases: Electrochemical and quantum calculations
a
a
b
a
I. Danaee a, , O. Ghasemi , G.R. Rashed , M. Rashvand Avei , M.H. Maddahy
⇑
a Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
b Department of Chemistry, K.N. Toosi University of Technology, Tehran, Iran
h i g h l i g h t s
"
"
"
"
The inhibition ability of Schiff bases against the corrosion of steel was evaluated.
Schiff bases retard both the cathodic and anodic reactions through chemical adsorption.
The correlation of inhibition effect and molecular structure was discussed.
Frontier orbital theory which was applied to the results of quantum chemistry.
a r t i c l e i n f o
a b s t r a c t
The corrosion inhibition and adsorption of N,N0-bis(n-hydroxybenzaldehyde)-1,3-propandiimine (n-HBP)
Schiff bases has been investigated on steel electrode in 1 M HCl by using electrochemical techniques. The
experimental results suggest that the highest inhibition efficiency was obtained for 3-HBP. Polarization
curves reveal that all studied inhibitors are mixed type. Density functional theory (DFT) at the B3LYP/
6-31G(d,p) and B3LYP/3-21G basis set levels and ab initio calculations using HF/6-31G(d,p) and
HF/3-21G methods were performed on three Schiff bases. By studying the effects of hydroxyl groups
in ortho-, meta-, para- positions, the best one as inhibitor was found to be meta-position of OH in Schiff
base (i.e., 3-HBP). The order of inhibition efficiency obtained was corresponded with the order of most of
the calculated quantum chemical parameters. Quantitative structure activity relationship (QSAR)
approach has been used and a correlation of the composite index of some of the quantum chemical
parameters was performed to characterize the inhibition performance of the Schiff bases studied. The
results showed that %IE of the Schiff bases was closely related to some of the quantum chemical param-
eters but with varying degrees/order. The calculated %IE of the Schiff base studied was found to be close
to their experimental corrosion inhibition efficiencies.
Article history:
Received 16 May 2012
Received in revised form 5 November 2012
Accepted 7 November 2012
Available online 16 November 2012
Keywords:
Inhibition
Schiff base
AFM
Density functional theory (DFT)
Ab initio calculation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
diffusion of ions to the metallic surface and increasing the electri-
cal resistance of the metallic surface [6].
Acid solutions with pH values below one are generally used for
industrial acid cleaning, acid descaling, oil well acidizing, the pick-
ling and removal of undesirable rust [1–3]. Mild steel which are
extensively used in a lot of industrial processes, could corrode dur-
ing these acidic applications particularly with the use of HCl [4,5].
Corrosion prevention systems favor the use of corrosion inhibi-
tors with low or zero environmental impacts. Inhibitors are chem-
icals that react with a metallic surface or the environment.
Inhibitors decrease corrosion processes by, increasing the anodic
or cathodic polarization behavior, reducing the movement or
The use of organic molecules as corrosion inhibitor is one of the
most practical methods for protecting against the corrosion and it
is becoming increasingly popular. These compounds in general are
adsorbed on the metal surface, blocking the active corrosion sites.
Four types of adsorption may take place by organic molecules at
metal/solution interface:
(a) electrostatic attraction between the charged molecules and
the charged metal
(b) interaction of unshared electron pairs in the molecule with
the metal
(c) interaction of
(d) combination of a and c
p-electrons with the metal
⇑
Corresponding author. Tel.: +98 631 4429937.
E-mail address: danaee@put.ac.ir (I. Danaee).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.11.013

248
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
Nomenclature
ba,bc
l
2-HBP
3-HBP
4-HBP
A
anodic and cathodic Tafel slopes
dipole moment, electronic chemical potential
N,N0-bis(2-hydroxybenzaldehyde)-1,3-propandiimine
N,N0-bis(3-hydroxybenzaldehyde)-1,3-propandiimine
N,N0-bis(4-hydroxybenzaldehyde)-1,3-propandiimine
electron affinity
Icorr
IE
corrosion current
inhibition efficiency
Experimental inhibition efficiency
Theoretical inhibition efficiency
lowset unoccupied molecular orbital
molecular orbital
IEexp
IETheor
LUMO
MO
NLM
n
AFM
C
atomic Force Microscopy
capacitor
nonlinear model
Qdl exponent
Cdl
double layer capacitance
constant Phase Element (Q)
density functional theory
corrosion potential
electrochemical impedance spectroscopy
frequency
Qdl
QSAR
Rct
Rp
SCE
double layer constant phase element
quantitative structure activity relationship
charge transfer resistance
polarization resistance
saturated Calomel Electrode
energy gap
fraction of charge transferred
absolute hardness
absolute electronegativity
CPE
DFT
Ecorr
EIS
f
D
D
g
E
N
HF
Hartree–Fock
HOMO highest occupied molecular orbital
ionization potential
I
v
2. Experimental and computational details
The adsorption ability of inhibitors onto metal surface depends
on the nature and surface charge of metal, the chemical composi-
tion of electrolytes, and the molecular structure and electronic
characteristics of inhibitor molecules. It was shown that organic
compounds containing heteroatoms such as nitrogen, sulfur, oxy-
gen and phosphor are capable of forming coordinate covalent bond
with metal owing to their free electron pairs and thus acting as
2.1. Material preparation
All chemicals used in present work were of reagent-grade
Merck product and used as received without further purification.
The three Schiff bases were prepared according to the described
procedure [17]. The compounds were synthesized from 1:2 mol ra-
tios of 1,3-diaminopropane and n-Hydroxybezaldehyde (2-, 3- and
4-hydroxybenzaldehyde) through a condensation reaction in etha-
nolic media then recrystallized in ethanol. The physical and analyt-
ical data for the Schiff bases is given in Table 1. Identification of
structure of synthesized Schiff base was performed by IR, 1HNMR
and 13C-NMR spectroscopic techniques. The Schiff base derivatives
used in this study are presented in Fig. 1.
inhibitor. The conjugated double bonds or
p electrons in triple
bonds exhibit good inhibitive properties [7–9]. Schiff bases (with
RC = NR0 as general formula) have the both features combined with
their structure which may then give rise to particularly potential
inhibitors [10]. Schiff bases are condensation products of an amine
and a ketone or aldehyde and the first Schiff base metal complexes
were prepared and described in the literature by Schiff in (1864).
An interesting phenomenon is that Schiff bases systematically dis-
play considerably stronger corrosion inhibition efficiencies than
the corresponding amines. Schiff bases have been recently re-
ported as effective corrosion inhibitors for steel, aluminum and
copper in acidic media. The greatest advantages of Schiff bases
are [11,12]:
2.2. Computational techniques
All the quantum chemical calculations were performed with
complete geometry optimizations using standard Gaussian-03
software package [18]. Geometry optimization were carried out
by two different methods: ab initio methods at the Hartree–Fock
(HF) level with the 3-21G and 6-31G(d,p) basis sets and at the den-
sity functional theory (DFT) level with the non-local hybrid density
functional B3LYP [19], combining Becke’s three-parameter hybrid
exchange functional with the correlation functional of Lee et al.
[20] at basis sets 3-21G and 6-31G(d,p) [21,22].
(a) they can be synthesized conveniently from inexpensive raw
materials
(b) they contain the electron cloud on the aromatic ring or, the
electronegative atoms such as nitrogen and oxygen in the
relatively long chain compounds
(c) harmless for environment
Statistical analyses were performed using SPSS program version
15.0 for windows. Non-linear regression analyses were performed
by unconstrained sum of squared residuals for loss function and
estimation methods of Levenberg–Marquardt using SPSS program
version 15.0 for windows.
Theoretical chemistry has been used recently to explain the
mechanism of corrosion inhibition, such as quantum chemical cal-
culations [13,14]. Quantum chemical calculations have been pro-
ven to be a very powerful tool for studying the mechanism
[15,16]. The objective of this work is to present the relationships
between the Schiff bases reactivity and their ability to inhibit the
corrosion of mild steel in hydrochloric acid to understand if any
structural differences induced by different positions of the hydro-
xyl group may be related to the experimentally observed differ-
ences of corrosion efficiency. The structural parameters, such as
the frontier molecular orbital energy HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied molecular orbi-
tal), the charge distribution of the studied inhibitors, the absolute
2.3. Electrochemical measurements
Working electrodes were prepared from steel specimens of the
chemical composition (wt.%) C = 0.16%, Si = 0.32%, Mn = 0.35%,
P = 0.03%, S = 0.02%, and the remainder Fe. Samples were cut from
a cylindrical rod with a cutter machine. The exposed surface area of
each electrode is equal to 0.81 cm2. These specimens were used as
working electrode in electrochemical measurements and the ex-
posed areas of the electrodes were mechanically abraded with
220, 400, 600, 800, 1000 and 1200 grades of emery paper,
electronegativity (v) values, and the fraction of electrons (DN)
transfer from inhibitors to iron were calculated and correlated with
inhibition efficiencies.

I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
249
Table 1
Physical and analytical data of the n-HBP Schiff bases.
Compound
Compound formula
Formula weight
Yield (%)
Color
Found (calculated)
%C
%H
%N
2-HBP
3-HBP
4-HBP
C
17H18N2O2
282.3
282.3
282.3
97
98
98
Bright yellow
72.45 (72.32)
72.55 (72.32)
72.75 (72.32)
6.56 (6.42)
6.43 (6.42)
6.01 (6.42)
9.89 (9.92)
9.80 (9.92)
9.99 (9.92)
C17H18N2O2
C17H18N2O2
Yellow
Light-yellow to light-brown
Fig. 1. The chemical structure of the Schiff bases.
degreased with acetone and rinsed by distilled water before each
electrochemical experiment.
written least square software based on the Marquardt method
for the optimization of functions and Macdonald weighting for
the real and imaginary parts of the impedance [23,24].
A very high-resolution type of atomic force microscopy (Nano-
surf easyScan2 AFM, Switzerland), with demonstrated resolution
on the order of fractions of a nanometer, was used as a surface
scanning probe. Before AFM analyses, the steel specimens were im-
mersed in solutions (with and without inhibitors separately) for
1 day at 25 °C.
Electrochemical measurements were carried out in a conven-
tional three-electrode system.
A saturated calomel electrode
(SCE) was used as the reference electrode and a platinum sheet
was used as the counter electrode. Before each experiment, the
working electrode was immersed in the test cell for 30 min until
to reach steady state condition. The electrochemical measure-
ments were carried out using computer controlled Auto Lab poten-
tiostat/galvanostat (PGSTAT 302N). Polarization curves were
recorded at constant sweep rate of 1 mV sꢀ1 in 1 M HCl solution.
All tests were carried out at constant temperatures by controlling
the cell temperature using a water bath. All the experiments were
performed in quiescent conditions and solutions were open to the
atmosphere under unstirred conditions.
3. Results and discussion
3.1. Potentiodynamic polarization measurements
Corrosion current Icorr were calculated from Tafel extrapolation
methods. The measurements were repeated three times for each
condition and the average values were presented. The polarization
resistance was calculated from the slope of the potential vs. cur-
rent plots.
Electrochemical impedance spectroscopy (EIS) measurements
were carried out in frequency range of 100 kHz to 10 mHz with
amplitude of 10 mV peak-to-peak using AC signals at open circuit
potential. Fitting of experimental impedance spectroscopy data
to the proposed equivalent circuit was done by means of home
Figs. 2 and 3 show representative Tafel polarization curve of
steel in 1 M HCl solution in the presence of 5 ꢁ 10ꢀ5 and
2 ꢁ 10ꢀ4 M of 2-, 3- and 4-HBP. According to Figures, lower corro-
sion current was obtained for 3-HBP and therefore 3-HBP is more
beneficial rather than 2-HBP and 4-HBP. Table 2 shows the electro-
chemical corrosion kinetic parameters such as corrosion potential
(Ecorr vs. SCE), corrosion current density (Icorr), cathodic and anodic
Tafel slopes (ba bc]), the degree of surface coverage (h) and inhibi-
tion efficiency (IE%) obtained by extrapolation of the Tafel lines for
inhibitors. The degree of surface coverage and inhibition efficiency

250
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
show that the hydrogen evolution is activation-controlled and
the reduction mechanism is not affected by the presence of the
inhibitors. No definite trend was observed in the shift of Ecorr values
in the presence of different concentrations of the inhibitors, sug-
gesting that these compounds behave as mixed-type inhibitors
[27]. However, the influence is more pronounced in the cathodic
polarization plots compared to that in the anodic polarization
plots. It is clear that the addition of the inhibitor shifts the cathodic
curves to a greater extent toward the lower current density when
compared to the anodic curves. The Ecorr value is also shifted to the
more negative side with an increase in the inhibitor concentration
[28]. It can be seen that the corrosion rate decreased and inhibition
efficiency (IE%) increased by increasing inhibitor concentration.
Polarization resistance (Rp) values were determined from the slope
of the polarization curve and calculated using Stern–Geary equa-
tion which is given below [29,30]:
ba ꢂ bc
1
Rp ¼
ꢁ
ð3Þ
2:303ðba þ bcÞ Icorr
Fig. 2. Tafel polarization curves for steel in 1 M HCl (1), in the presence of
5 ꢁ 10ꢀ5 M of 2-HBP (2), 4-HBP (3) and 3-HBP (4).
By increasing the inhibitors concentration the polarization
resistance increases in the presence of compound, indicating
adsorption of the inhibitor on the metal surface to block the active
sites efficiently and inhibit corrosion [31].
3.2. Electrochemical impedance spectroscopy
Figs. 4 and 5 show Nyquist plots for steel in 1 M HCl solution in
the presence of 5 ꢁ 10ꢀ5 and 2 ꢁ 10ꢀ4 M of 2-, 3- and 4-HBP. The
plots show a depressed capacitive loop which arises from the time
constant of the electrical double layer and charge transfer resis-
tance. As can be seen, higher charge transfer resistance was ob-
tained in presence of 3-HBP. The impedance of the inhibited steel
increases with increasing the inhibitor’s concentration and conse-
quently the inhibition efficiency increases. The equivalent circuit
compatible with the Nyquist diagram recorded in the presence of
inhibitors was depicted in Fig. 6.
The simplest approach requires the theoretical transfer function
Z(x) to be represented by a parallel combination of a resistance Rct
and a capacitance C, both in series with another resistance Rs [32]:
Fig. 3. Tafel polarization curves for steel in 1 M HCl (1), in the presence of
2 ꢁ 10ꢀ4 M of 2-HBP (2), 4-HBP (3) and 3-HBP (4).
1
Zð
x
Þ ¼ Rs þ
ð4Þ
x = 2pf and f is frequency in Hz.
1=Rct þ i
is the frequency in rad/s,
To obtain a satisfactory impedance simulation of steel, it is nec-
xC
x
for different concentrations of inhibitor is calculated using the fol-
lowing equations [7,25]:
essary to replace the capacitor (C) with a constant phase element
(CPE) Q in the equivalent circuit. The most widely accepted expla-
nation for the presence of CPE behavior and depressed semicircles
on solid electrodes is microscopic roughness, causing an inhomo-
geneous distribution in the solution resistance as well as in the
double-layer capacitance [33]. Constant phase element CPEdl, Rs
and Rct can be corresponded to double layer capacitance, solution
resistance, and charge transfer resistance respectively. To corrobo-
rate the equivalent circuit, the experimental data are fitted to
equivalent circuit and the circuit elements are obtained. Table 3
illustrates the equivalent circuit parameters for the impedance
spectra of corrosion of steel in 1 M HCl solution. The results dem-
onstrate that the presence of inhibitors enhance the value of Rct ob-
tained in the pure medium while that of Qdl is reduced. The
decrease in Qdl values was caused by adsorption of inhibitor indi-
cating that the exposed area decreased. On the other hand, a de-
crease in Qdl, which can result from a decrease in local dielectric
constant and/or an increase in the thickness of the electrical double
layer, suggests that Schiff base inhibitors act by adsorption at the
metal–solution interface.
!
_
Icorr ꢀ Icorr
h ¼
ð1Þ
ð2Þ
_
Icorr
!
_
Icorr ꢀ Icorr
IE% ¼
ꢁ 100
_
Icorr
_
where Icorr and Icorr are the corrosion current densities determined
by the intersection of the extrapolated Tafel lines and the corrosion
potential for steel in uninhibited and inhibited acid solution,
respectively.
As was expected both anodic and cathodic reactions of steel
electrode corrosion were inhibited with the increase of the inhibi-
tors concentration. This result suggests that the addition of the
inhibitors reduces anodic dissolution and also retards the hydro-
gen evolution reaction [26]. The electrochemical processes on the
metal surface are related to the adsorption of the inhibitors and
the adsorption is known to depend on the chemical structure of
the inhibitors. The parallel cathodic Tafel curves in Figs. 2 and 3

I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
251
Table 2
Potentiodynamic polarization parameters for the corrosion of mild steel in 1 M HCl solution in absence and presence of different concentrations of n-HBP.
Inhibitor
Concen. (M)
ꢀEcorr (mV)
Icorr
(
l
A cmꢀ2
)
Corrosion rate (mpy)
ba (mV decꢀ1
)
ꢀbc (mV decꢀ1
)
Rp
(X
cm2)
h
IE%
Blank
442
485
495
710
294
270
324.5
134.3
123.4
84
80
95.7
111
146
133
29
76
90
–
0.58
0.62
–
58
62
2-HBP
5 ꢁ 10ꢀ5
2 ꢁ 10ꢀ4
3-HBP
4-HBP
5 ꢁ 10ꢀ5
2 ꢁ 10ꢀ4
496
499
173
114
79.1
52.1
91.2
83
126
124
133
189
0.76
0.84
76
84
5 ꢁ 10ꢀ5
2 ꢁ 10ꢀ4
481
499
242
128
110.6
58.5
87.5
87.3
138
123
96
173
0.66
0.82
66
82
Fig. 6. Equivalent circuits compatible with the experimental impedance data in
Figs. 4 and 5.
Table 3
Impedance data for mild steel in 1 M HCl solution without and with different
concentrations of n-HBP.
Inhibitor
2-HBP
3-HBP
4-HBP
Concen (M)
Rs (O)
Rct (O)
Qdl ꢁ 103 (F)
2.5
1.8
1.5
n
Blank
1.5
1.5
1.4
42
44
47
0.86
0.88
0.88
5 ꢁ 10ꢀ5
Fig. 4. Nyquist plots for steel in 1 M HCl (1) in the presence of 5 ꢁ 10ꢀ5 M of 2-HBP
2 ꢁ 10ꢀ4
(2), 4-HBP (3) and 3-HBP (4).
5 ꢁ 10ꢀ5
2 ꢁ 10ꢀ4
1.5
1.4
65
81
0.7
0.5
0.9
0.92
5 ꢁ 10ꢀ5
1.5
1.4
63
81
1.3
0.7
0.9
0.89
2 ꢁ 10ꢀ4
potentiostatic current–time transients were recorded. Fig. 7 shows
the current transients of steel electrode at ꢀ0.4 V vs. SCE applied
anodic potential. Initially the current decreases monotonically
with time. The decrease in the current density is due to the forma-
tion of corrosion products layer on the anode surface. However, in
later times the current increases tracing approximately a straight
line to reach a steady state value depending on applied potential
(Fig. 7). The increase in current is related to the dissolution of
the steel and pit nucleation and pit growth. In presence of inhibi-
tor, increasing current was not observed and electrode was inhib-
ited from corrosion due to inhibitor adsorption.
Fig. 5. Nyquist plots for steel in 1 M HCl (1) in the presence of 2 ꢁ 10ꢀ4 M of 2-HBP
3.4. Molecular structure and quantum chemical calculation
(2), 4-HBP (3) and 3-HBP (4).
As the Qdl exponent (n) is a measure of the surface heterogene-
ity, values of n indicates that the steel surface becomes more and
more homogeneous as the concentration of inhibitor increases as
a result of its adsorption on the steel surface and corrosion inhibi-
tion. The increase in values of Rct and the decrease in values of Qdl
with increasing the concentration also indicate that Schiff bases act
as primary interface inhibitors and the charge transfer controls the
corrosion of steel under the open circuit conditions.
The effectiveness of an inhibitor is related to its spatial molec-
ular structure as well as with its molecular electronic structure
[34]. In this regard, quantum chemical calculations have proved
to be a powerful tool for studying corrosion inhibition mechanism
and recently, corrosion publications have contained substantial
quantum chemical calculations [35,36]. In this study, quantum
chemical calculations were conducted at two different methods:
ab initio methods at the Hartree–Fock (HF) level and the density
functional theory (DFT) level with the 3-21G and 6-31G(d,p) basis
sets by geometry optimization of the studied compounds in order
to support experimental data and to investigate the relationship
between molecular structure of the Schiff bases and their inhibi-
tion effects.
3.3. Chronoamperometry
In order to gain more insight about the effect of inhibitors
on the electrochemical behavior of steel in 1 M HCl solution,

252
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
Table 4
Selected bond lengths (in angstroms) and bond angles (in degrees) of the n-HBP Schiff
bases with B3LYP/6-31G(d,p).
2-HBP
3-HBP
4-HBP
Bond lengths
C1AN2
N2AC3
C3AC4
1.29
1.47
1.54
1.54
1.47
1.28
1.40
1.39
1.29
1.29
1.47
1.54
1.54
1.47
1.29
1.40
1.39
1.47
1.54
1.54
1.47
1.29
1.40
1.39
C4AC5
C5AN6
N6AC7
(CAC)Ar
CAO
Bond angles
C1AN2AC3
N2AC3AC4
C3AC4AC5
C4AC5AN6
119.14
110.51
112.09
109.34
123.16
120.46
174.26
143.57
175.66
124.43
110.22
114.24
109.76
124.71
ꢀ144.95
ꢀ61.56
133.44
ꢀ63.44
119.44
110.41
111.51
118.74
122.24
121.65
174.44
0.15
C5AN6AC7
C1AN2AC3AC4
N2AC3AC4AC5
C4AC5AN6AC7
C3AC4AC5AN6
Fig. 7. Current transients of steel electrode at ꢀ0.4 V vs. SCE: (1) Blank, (2)
2 ꢁ 10ꢀ4 M of 2-HBP, (3) 4-HBP and (4) 3-HBP.
179.53
bases are shown in Fig. 9. It can be seen in Fig. 9 that, the frontier
molecular orbital distribution obtained from four different calcula-
tion methods have given very close results. Three Schiff bases
investigated in the present study consist of symmetrical two parts
and contain two benzene rings and two imine groups. Difference
between the structures is related with the position of the substitu-
tion of OH group at ortho, meta and para position. It can be seen in
Fig. 9 that, the HOMO location of 2-HBP Schiff base is relatively dis-
tributed throughout the molecule while the HOMO location of 3-
HBP and 4-HBP Schiff bases are distributed over the benzene ring
and a small part of the carbon chain that take place one side of
the molecule. According to HOMO distribution of Schiff bases, it
can be said that 3-HBP and 4-HBP molecules carry their rich
Fig. 8 represents the optimized structure of three Schiff bases.
The selected bond lengths and bond angles for the studied Schiff
bases are presented in Table 4. It could be easily seen that, mole-
cules are not fully planar, which may result in relatively weak
interaction between molecules and metal surface. However, differ-
ent factors need to be considered for elucidating the orientation of
organic molecules on the electrode surface. The atoms and groups
of the molecules may interact with the electrode surface depend
on the geometry of the inhibitor as well as the nature of their fron-
tier molecular orbitals. Frontier molecular orbital (HOMO and
LUMO) theory is useful in predicting the adsorption centers of
the inhibitor responsible for the interaction with metal surface
[35,37]. The HOMO and LUMO populations of the studied Schiff
Fig. 8. The optimized structure of three n-HBP Schiff bases obtained at the B3LYP level (right) ‘side on’ view of the optimized structure.

I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
253
Fig. 9. HOMO and LUMO populations of investigated Schiff bases obtained at (1) B3LYP/6-31G(d,p), (2) B3LYP/3-21G, (3) RHF/6-31G(d,p) and (4) RHF/3-21G.
Table 5
Quantum chemical parameters for the studied Schiff bases.
Inhibitor
2-HBP
Models
ELUMO (eV)
EHOMO (eV)
ELUMO–EHOMO (eV)
D
N
l
(Debye)
Molar volume (cm3 molꢀ1
)
%IE
DFT-B3LYP (6-31G(d,p))
DFT-B3LYP (3-21G)
HF (6-31G(d,p))
ꢀ0.98
ꢀ0.81
2.90
ꢀ5.97
ꢀ5.87
ꢀ8.40
ꢀ8.39
ꢀ5.91
ꢀ5.80
ꢀ8.28
ꢀ8.27
ꢀ5.95
ꢀ5.81
ꢀ8.54
ꢀ8.49
4.99
5.06
11.29
11.47
0.7063
0.7236
0.3763
0.3787
3.31
3.52
3.22
3.23
207.051
255.653
194.940
246.565
57
73
81
HF (3-21G)
3.08
4-HBP
3-HBP
DFT-B3LYP (6-31G(d,p))
DFT-B3LYP (3-21G)
HF (6-31G(d,p))
ꢀ1.07
ꢀ0.87
2.83
4.84
4.93
11.11
11.31
0.7252
0.7435
0.3852
0.3877
3.12
3.08
2.87
2.70
216.316
179.647
195.121
204.090
HF (3-21G)
3.04
DFT-B3LYP (6-31G(d,p))
DFT-B3LYP (3-21G)
HF (6-31G(d,p))
ꢀ1.28
ꢀ1.07
2.72
4.67
4.75
11.26
11.46
0.7253
0.7497
0.3634
0.3704
5.41
4.83
5.62
5.20
214.001
204.778
190.780
189.178
HF (3-21G)
2.97

254
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
Fig. 10. Variation of experimental inhibition efficiency (IE%) with (A) the LUMO energy, (B) the HOMO–LUMO gap, (C) the electronic chemical potential and (D)
DN of n-HBP
Schiff bases: (a) 2-HBP, (b) 4-HBP and (c) 3-HBP.
Fig. 11. Electrostatic properties of (A) 2-HBP (B) 3-HBP and (C) 4-HBP: side views of the dipole and the Mulliken charge populations are displayed on the left while the middle
and right panels show the contour and isosurface representation of electrostatic potential respectively (the electron rich region is red and the electron poor region is blue).
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
255
negative centers in a small region, in comparison with 2-HBP mol-
ecule. The parts of the molecules with low HOMO density probably
more oriented towards to cathodic sites of the steel surface and
afterwards adsorption occurs by sharing of electrons. Computed
EHOMO, ELUMO, ELUMO – EHOMO, DN and molar volume values of stud-
ied Schiff bases were listed in Table 5. In order to investigate the
agreement of the quantum chemical results with experimental
observation, the measured inhibition efficiencies (%) of these three
n-HBP Schiff bases in the concentration of 1.0 ꢁ 10ꢀ4 M, were also
listed in Table 5.
According to Fukui’s frontier molecular orbital theory [38], reac-
tive ability of the inhibitor is related with frontier molecular orbital
(MO), including highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO). Higher HOMO
energy (EHOMO) of the adsorbent leads to higher electron donating
ability [39–41]. Low LUMO energy (ELUMO) indicates that the accep-
tor accepts electrons easily. Among these Schiff bases, 3-HBP and
4-HBP have the highest HOMO energy, they donate electrons eas-
ily, and they have the best inhibitive property but the changing
tendency of 3-HBP and 4-HBP is not correlated to inhibition effi-
ciencies. So in these systems, the theoretical parameters EHOMO
cannot be used to predicate the inhibition efficiencies of the inhib-
itors. On the other hand, ELUMO values of three Schiff bases in-
creased in the order: 2-HBP < 4-HBP < 3-HBP. These results agree
with the experimental observations, which imply that 3-HBP com-
pound has better corrosion performance (Fig. 10A).
The energy gap between LUMO and HOMO (
D
E = ELUMO–EHOMO
)
is an important stability index [42]. The smaller is the value of
D
E,
the more probable it is that the molecule has inhibition efficien-
cies. The results obtained from quantum chemical calculation
using B3LYP/6-31G(d,p) are listed in Table 5 that are in a good
agreement with experimental results (Fig. 10B).
From the quantum chemical point, the fraction of electronic
charge (DN) transferred from the inhibitor to the metal is another
important factor [43]. When a bulk metal and an organic corrosion
inhibitor are brought together, electron flow will occur from the
atom of the lower
v value to the atom of the higher v value until
the chemical potentials become equal. Then,
D
N, the fraction of
charge transferred, may be written as:
vM
2ðgM
ꢀ
vI
gI
D
N ¼
ð5Þ
þ
Þ
where the subscripts M and I represent the metal and inhibitor,
respectively. According to Pearson, operational and approximate
definitions of the electronic chemical potential (
l
) and the absolute
hardness (
g
) of a chemical system are given by [44]:
ðI þ AÞ
ꢀ
l
¼
¼
v
ð6Þ
ð7Þ
2
ðI ꢀ AÞ
g
¼
2
where I is the ionization potential and A is the electron affinity.
Since (I + A)/2 is the Mulliken electronegativity for atoms, the value
of
v for any system is known as the absolute electronegativity.
According to Koopman’s theorem, the frontier orbital energies are
given by [45]:
I ¼ ꢀEHOMO
ð8Þ
A ¼ ꢀELUMO
ð9Þ
The percent inhibition of steel with n-HBP Schiff bases inhibi-
Fig. 12. Correlation of experimental and theoretical inhibition efficiencies of the
tors shows good correlation with the electronic chemical potential,
studied Schiff bases using the B3LYP/6-31G(d,p) method.

256
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
IEexp ¼ 2:66EHOMO ꢀ 0:23ELUMO þ 16:03
IEexp ¼ 0:66EHOMO ꢀ 1:73ELUMO þ 11:16
IEexp ¼ 0:77EHOMO ꢀ 2:7ELUMO þ 15:37
ð11Þ
ð12Þ
ð13Þ
as depicted in Fig. 10C. It is interesting to note that the percent
inhibition increases progressively with an increase the electronic
chemical potential (R2 = 0.66).
Using a theoretical vM value of 7 eV molꢀ1 and gM value of
0 eV molꢀ1 for iron atom [43],
D
N, the fraction of electrons trans-
ferred from inhibitor to the iron molecule, was calculated and
listed in Table 5. The values of N clearly revealed that the inhibi-
tion efficiency increased with the N increase (Fig. 10D). According
N showed inhibition effect re-
From the above equations, it can be seen that the coefficients of the
EHOMO are positive while that of ELUMO are negative indicating that
the formation of a feedback bond is favored by increasing value of
EHOMO but with decreasing value of ELUMO. Correlations between
the above equations and experimental inhibition efficiencies are
excellent (R2 = 1). However, when all the calculated quantum chem-
ical parameters are used for developing suitable models for the dif-
ferent computational methods (through the multiple regression), it
is not possible to obtain simple equations such as those given in
Eqs. (10)–(13). This indicates that corrosion inhibition process is a
composite function of some quantum chemical descriptors.
Though a number of satisfactory correlations have been re-
ported by other investigators [43,48–51] between the inhibition
efficiency of various inhibitors used and some quantum chemical
parameters, a composite index and a combination of more than
one parameter [52,53] has been used to perform QSAR which
might affect the inhibition efficiency of the studied molecules. Con-
sequently, a relation may exist between the composite index and
the average corrosion inhibition efficiency for a particular inhibitor
molecule. Therefore, for this study, parameters have been selected
relevant to the activity of the molecules under investigation. The
linear model approximates inhibition efficiency (IETheor%) as in
the equation below:
D
D
D
to other reports [43,46], values of
sulted from electrons donation.
Agreeing with Lukovits’s study [46], if
DN < 3.6, the inhibition
efficiency increases with increasing electron-donating ability at
the metal surface. In this study, the three Schiff bases are the
donators of electrons, and the iron surface is the acceptor. The
compounds are bound to the metal surface, and thus form an
inhibition adsorption layer against corrosion. The molecule
3-HBP has the highest inhibition efficiency because it has the low-
est LUMO energy, the electronic chemical potential,
DE and DN
values, and it has the greatest ability of offering electrons, and 2-
HBP has the lowest inhibition efficiency, for vice versa.
The Mulliken charge populations and the direction of dipole
moment calculated for the three Schiff bases are projected to the
molecular plane. The charge population and direction of the dipole
moment can be understood by considering the electrostatic poten-
tial (middle and right panels of Fig. 11), which discerns electron
density rich regions centered on O atoms in hydroxyl group, N
atoms in AC@NA group and C atoms of carbon bone chain and
some of carbon atoms of benzene rings. The regions of highest elec-
tron density are generally the sites to which electrophiles attacked.
So O, N, and C atoms are the active center, respectively, which have
the strongest ability of bonding to the metal surface. On the other
side, HOMO (Fig. 9) was mainly distributed over the mentioned
atoms. Thus, these areas are probably the primary sites of the
bonding.
IETheor ¼ AxiCi þ B
ð14Þ
where IETheor is the inhibition efficiency, A and B are the regression
coefficients determined by regression analysis, xi is a characteristic
quantum index for the inhibitor molecule (i), and Ci denotes the
experiment’s concentration of the inhibitor. Such linear approach
was not found to be satisfactory for correlating the present results.
Consequently, the nonlinear model (NLM) proposed by Lukovits
et al. [54] and also used by Khaled [53] for studying the interaction
of corrosion inhibitors with metal surfaces in acidic solutions de-
rived from the equation below (15) based on the Langmuir adsorp-
tion isotherm has been used:
3.5. Quantitative structure activity relation (QSAR) study
From the present study, it has been established that the mech-
anism of inhibition involves the donation of electron to Fe in mild
steel by the electron. However, it has also been found that the
inhibitor can not only donate electron to the metal but can also ac-
cept electron from the lone pair of Fe, leading to the formation of a
feedback bond [47]. The formation of a feedback bond can be ana-
lyzed by considering a quantitative relationship between the
EHOMO, ELUMO and the experimental inhibition efficiency. The con-
siderations led to the establishment of Eqs. (10)–(13) for B3LYP/
ðAxi þ BÞCi
IETheor
¼
ꢁ 100
ð15Þ
1 þ ðAxi þ BÞCi
where IETheor is the inhibition efficiency, A and B are the regression
coefficients determined by regression analysis, xi is a quantum
chemical index characteristic for the molecule (i) and Ci denotes
the experimental concentration i. Application of equation 15 for
3-HBP Schiff base yields Eqs. (16)–(19) for B3LYP/6-31G(d,p),
B3LYP/3-21G, HF/6-31G(d,p) and HF/3-21G methods, respectively.
6-31G(d,p),
B3LYP/3-21G,
HF/6-31G(d,p)
and
HF/3-21G,
respectively.
IEexp ¼ 2:48EHOMO ꢀ 0:57ELUMO þ 14:88
ð10Þ
ðꢀ9:11EHOMO ꢀ 1:17ELUMO þ 8:93
D
E þ 2:23
D
N þ 10:19
l
þ 364:47V þ 1ÞCi
þ 364:47V þ 1ÞCi
IETheor
IETheor
IETheor
IETheor
¼
¼
¼
¼
ꢁ 100
ð16Þ
ð17Þ
ð18Þ
ð19Þ
1 þ ðꢀ9:11EHOMO ꢀ 1:17ELUMO þ 8:93
D
E þ 2:23
D
N þ 10:19
l
ðꢀ9:57EHOMO ꢀ 1:26ELUMO þ 9:31
D
E þ 2:29
D
N þ 10:58 þ 380:88V þ 1:08ÞCi
l
ꢁ 100
1 þ ðꢀ9:57EHOMO ꢀ 1:26ELUMO þ 9:31
D
E þ 2:29
D
N þ 10:58
l
þ 380:88V þ 1:08ÞCi
ðꢀ11:06EHOMO ꢀ 0:55ELUMO þ 11:51
D
E þ 2:32
D
N þ 11:47
l
þ 408:13V þ 1:23ÞCi
ꢁ 100
1 þ ðꢀ11:06EHOMO ꢀ 0:55ELUMO þ 11:51
D
E þ 2:32
D
N þ 11:47 þ 408:13V þ 1:23ÞCi
l
ðꢀ11:22EHOMO ꢀ 0:43ELUMO þ 11:79 E þ 2:32
D
D
N þ 11:48
l
þ 411:57V þ 1:25ÞCi
þ 411:57V þ 1:25ÞCi
ꢁ 100
1 þ ðꢀ11:22EHOMO ꢀ 0:43ELUMO þ 11:79 E þ 2:32
D
D
N þ 11:48
l

I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
257
The same results are obtained for other Schiff bases. From the
results obtained, it can be seen that there is a good and acceptable
coefficient correlation between the experimental and calculated/
estimated inhibition efficiencies of the studied Schiff bases using
the B3LYP/6-31G(d,p) method as shown in Fig. 12. Also it is neces-
sary to mention that there is no general way of predicting com-
pounds usefulness to be a good corrosion inhibitor or find some
universal type of correlation. A number of excluded parameters
that should be involved such as effect of solvent molecules, surface
nature, adsorption sites of the metal atoms or oxides sites or
Fig. 13. 2D and 3D of AFM images of steel exposed to 1 M HCl solution (a), in the presence of 2 ꢁ 10ꢀ4 M of 2-HBP (b), 3-HBP (c), 4-HBP (d) (scan size: 49.5 ꢁ 50
l).

258
I. Danaee et al. / Journal of Molecular Structure 1035 (2013) 247–259
Fig. 13. (continued)
vacancies, competitive adsorption with other chemical species in
the fluid phase and solubility must also be given due consideration.
However, the quintessential component of the QSAR approach is
the determination of the correct molecular descriptors.
By studying the effects of hydroxyl groups in ortho-, meta-,
para- positions, the best one as inhibitor was found to be meta-po-
sition of OH in Schiff base (i.e., 3-HBP).
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