Synthesis, structures, spectral properties and DFT quantum chemical calculations of (E)-4-(((4-propylphenyl)imino)methyl)phenol and (E)-4-((2-tolylimino)methyl)phenol; their corrosion inhibition studies of mild steel in aqueous HCl

Article abstract of DOI:10.1016/j.molstruc.2017.03.071

Two Schiff base compounds with similar backbone, but different positions and lengths of alkyl groups, (E)-4-(((4-propylphenyl)imino)methyl)phenol (PMP) and (E)-4-((2-tolylimino)methyl)phenol (TMP) have been synthesized. Their structures were established by spectroscopic techniques which include FTIR, ¹H and ¹³C NMR, relative molecular mass (mass spect.), and single crystal X-ray crystallography. The spectroscopic methods revealed the characteristic imine functional groups and the exact molecular masses of the compounds. Single crystal analysis revealed an orthorhombic crystal system with Pca2₁ space group for the two compounds; and the structure of PMP reveal that the propyl group is disordered over two positions in 0.79:0.21 ratio. The corrosion inhibition behaviour of the two Schiff bases were studied by electrochemical measurements. The results suggest that the two derivatives adsorbed onto the mild steel surface and formed hydrophobic films, which to an appreciable extent, protected the surface of the mild steel. Potentiodynamic polarization results strongly suggested that the investigated Schiff bases behave as mixed-type inhibitors with a more pronounced cathodic nature, and the adsorption isotherm basically obeys the Langmuir isotherm. Quantum chemical computations were also employed to provide further explanations on the adsorption mode, and the theoretical predictions conformed to the experimental results.

Full text of DOI:10.1016/j.molstruc.2017.03.071

Accepted Manuscript
Synthesis, structures, spectral properties and DFT quantum chemical calculations of
(E)-4-(((4-propylphenyl)imino)methyl)phenol and (E)-4-((2-tolylimino)methyl)phenol;
their corrosion inhibition studies of mild steel in aqueous HCl
Elias E. Elemike, Henry U. Nwankwo, Damian C. Onwudiwe, Eric C. Hosten
PII:
S0022-2860(17)30360-5
10.1016/j.molstruc.2017.03.071
MOLSTR 23567
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 January 2017
Revised Date: 17 March 2017
Accepted Date: 19 March 2017
Please cite this article as: E.E. Elemike, H.U. Nwankwo, D.C. Onwudiwe, E.C. Hosten,
Synthesis, structures, spectral properties and DFT quantum chemical calculations of (E)-4-
(((4-propylphenyl)imino)methyl)phenol and (E)-4-((2-tolylimino)methyl)phenol; their corrosion
inhibition studies of mild steel in aqueous HCl, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.03.071.
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GRAPHICAL ABSTRACT

ACCEPTED MANUSCRIPT
Synthesis, structures, spectral properties and DFT quantum chemical calculations of
(E)-4-(((4-propylphenyl)imino)methyl)phenol and (E)-4-((2-tolylimino)methyl)phenol;
their corrosion inhibition studies of mild steel in aqueous HCl
Elias E. Elemike1,2,3, Henry U. Nwankwo1,2, Damian C. Onwudiwe*1,2, , Eric C. Hosten4
1.
Material Science Innovation and Modelling (MaSIM) Research Focus Area, Faculty of
Agriculture, Science and Technology, North-West University, Mafikeng Campus, Private Bag X2046,
Mmabatho 2735, South Africa.
2.
Department of Chemistry, School of Mathematics and Physical Sciences, Faculty of
Agriculture, Science and Technology, North-West University, Mafikeng Campus, Private Bag X2046,
Mmabatho 2735, South Africa.
3.
Department of Chemistry, College of Science, Federal University of Petroleum Resources,
P.M.B 1221, Effurun, Delta State, Nigeria.
4.
Department of Chemistry, Nelson Mandela Metropolitan University, P.O. Box 77000, Port
Elizabeth 6031, South Africa
* Corresponding author
Email: Damian.Onwudiwe@nwu.ac.za
Tel: +27 18 - 389 -2545; Fax: +2718 – 389 -2420
Abstract
Two Schiff base compounds with similar backbone, but different positions and lengths of
alkyl groups, (E)-4-(((4-propylphenyl)imino)methyl)phenol
(PMP) and (E)-4-((2-
tolylimino)methyl)phenol (TMP) have been synthesized. Their structures were established
1
13
by spectroscopic techniques which include FTIR, H and C-NMR, relative molecular mass
(mass spect.), and X-ray crystallography. The spectroscopic methods revealed the
characteristic imine functional groups and the exact molecular masses of the compounds.
Single crystal analysis revealed an orthorhombic crystal system with Pca21 space group for
the two compounds; and the structure of PMP reveal that the propyl group is disordered over
two positions in 0.79:0.21 ratio. The corrosion inhibition behaviour of the two Schiff bases
were studied by electrochemical measurements. The results suggest that the two derivatives
1

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adsorbed onto the mild steel surface and formed hydrophobic films, which to an appreciable
extent, protected the surface of the mild steel. Potentiodynamic polarization results strongly
suggested that the investigated Schiff bases behave as mixed-type inhibitors with a more
pronounced cathodic nature, and the adsorption isotherm basically obeys the Langmuir
isotherm. Quantum chemical computations were also employed to provide further
explanations on the adsorption mode, and the theoretical predictions conformed to the
experimental results.
Keywords: Crystal structure; schiff base compounds; quantum chemical calculations;
corrosion inhibition
1.0 Introduction
The rich chemistry of Schiff bases which centers on the azomethine functional group play
critical role in the understanding and fine tuning of their applications [1].The growing interest
in this class of compounds is, to a large extent, due to their structural resemblance with
naturally occurring biological molecules, relatively easy synthetic routes and synthetic
flexibility that give room for the design of required structural properties. Schiff base
compounds are very common intermediates for the preparation of arylacetamide, azetidinone,
formazone, thiazolidinone, including metal complexes and other derivatives [2, 3].
Synthesis of this type of carbon-nitrogen double bond requires careful procedure and has
involved methods such as solvent based, mechano-chemical or solvent-free and microwave
assisted methods but there is low yield and pH specifications associated with solvent based
methods. pH scale for such reactions normally span between 5 to 8 depending on the basic
nature of the amines [4]. In mechano-chemical synthetic method, Tella et al have applied
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ball-milling technique in the absence of solvent, for 20 min, to synthesize trimethoprim
complexes of Cu(II) and Ni(II) [5]. The microwave assisted synthetic method has proved to
be rapid and efficient involving solventless medium with high yield of products [6].
The increasing interest in imine compounds is associated with their presence in many
biological systems. They have found applications in organic synthesis, chemical catalysis,
medicine, pharmacy, chemical analysis, cation carriers in potentiometric sensors and as
corrosion inhibition agents [7-18]. Some of the applications in medicine and pharmacy
include use as antitumor, anti-inflammatory, antiviral, antifungal and antibacterial agents, and
in the treatment of cancer, HIV and diabetes, and also immobilization of enzymes [8-18].
Investigations focusing on the catalytic properties of Schiff bases have recorded success in
the hydrogenation of olefins. Moreover, due to their photo, thermochromic and optical non-
linearity properties, they have immense applications in modern technology ranging from
optical computers, detection and monitoring of the intensity of radiation, imaging systems,
optical sound recording technology, molecular memory storing systems, as organic materials
in reversible optical memories and optical detectors in biological systems, photostabilizers,
dyes for solar collectors and solar filters [19, 20]. Schiff bases can create the platform for
application as efficient molecular conductors using electrical properties to proton transfer [21,
22]. Because of their high catalytic properties, they are employed in photoelectrochemical
processes, serving great modification to electrode materials, micro-electronic devices, organic
batteries or graphical output devices [23]. Another interesting application of these
compounds, especially in this present study, is their use as efficient corrosion inhibitors. The
mode of action of an excellent corrosion inhibitor involves the spontaneous formation of
impervious system on the surface of interest [24]. The adsorption of Schiff base molecules
onto metal components is made possible by the presence of the imine group, the electron
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cloud present in the aromatic ring and other electronegative atoms which might be present
[25].
Most recently, density functional theory (DFT) has been broadly used in theoretical
modelling to provide deep understanding of molecular properties of compounds through the
development of exchange–correlation functionals in addition to its great accuracy in
replicating the experimental parameters such as in geometry, dipole moment, and vibrational
frequency [26-29].
Schiff base ligands derived from the condensation of 4-propylaniline and 2-methylaniline
with 4-hydroxybenzaldehye are presented in this study. Spectral properties of the compound
1
would be investigated using H-NMR, 13C-NMR, FTIR and mass spectroscopies. Single
crystal x-ray analysis of the structures are carried out, and the optimized molecular
geometries obtained by using theoretical models. Their electrochemical activity will be
investigated using potentiodynamic polarization (PDP) and electrochemical impedance
spectroscopy (EIS). The potentials of the compounds as corrosion inhibitors on mild steel are
investigated by determining the degree of protection offered by the molecules of the
compounds on mild steel in HCl medium. The interest on Schiff base among other corrosion
inhibitors is due to their inherent advantages such as green techniques, reduced toxicity,
enhanced safe level, ease of adherence onto steel surface, inexpensiveness, simple synthetic
route and excellent inhibition efficiency [30]. Furthermore, the effect of the different alkyl
groups on the ortho and para positions of the aromatic ring would provide insight into their
atomic and molecular behaviour; thereby shed light on their potential inhibition efficacies.
The crystal structures of these molecules are reported for the first time in this research and
evaluation of their corrosion inhibition properties has not been reported elsewhere.
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2.0 Materials and methods
2.1 Materials and physical measurements
The reagents, 4-propylaniline, 4-hydroxybenzaldehyde, o-toluidine and solvents used in this
work were of analytical grade and purchased from Sigma–Aldrich and Merck companies.
They were used without further purification. FTIR spectra of the synthesized ligands and the
adsorbed film of the ligands on mild steel surface, after immersion in acidic solution
containing 100 ppm Schiff base for 3 h, were recorded on a Bruker alpha-P FTIR
1
spectrophotometer in the wavenumber range 400 – 4000 cm−1. H and 13C NMR
measurements were performed on 600 MHz Bruker Avance III NMR spectrometers at room
temperature using DMSO as solvent. The molecular masses of the ligands were determined
using micrOTOF-Q 11 10390 Bruker compass Mass spectrometer. All electrochemical
measurements were performed using AUTOLAB Potentiostat PGSTAT 302 (Eco Chemie,
Utrecht, Netherlands) driven by the NOVA software version 1.10 in an electrochemical
workstation which consists of conventional three-electrode cell assembly system with a mild
steel as the working electrode, a silver-silver chloride with 3 M KCl as the reference
electrode, and a platinum (Pt) rod as the counter electrode.
2.2 Synthesis of (E)-4-(((4-propylphenyl)imino)methyl)phenol (PMP) and (E)-4-((2-
tolylimino)methyl)phenol (TMP)
In a general synthesis procedure, to a mixture of 1.20 g (10 mmol) p-hydroxybenzaldehyde in
20 mL ethanol, 1.50 mL (10mmol) 4-propylaniline in 20 mL ethanol was added with drops of
glacial acetic acid to give PMP. Similarly, in another reaction for TMP, 1.10 mL (10 mmol)
o-toluidine in 20 mL ethanol was mixed with 1.20 g (10 mmol) p-hydroxybenzaldehyde in 20
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mL ethanol with added drops of glacial acetic acid. The solutions were separately refluxed
with stirring. After 2 h, the resulting clear solutions were filtered and kept at room
temperature for slow evaporation. Fine yellow crystals suitable for single crystal X-ray
analysis were obtained after 5 days.
PMP: yield 47.5%, M.pt. 148-150 °C. Selected IR (cm−1): 3366(OH stretch), 3024 ν(C-H
aromatics), 2954, 2922, 2857 ν(sp3C-H), 2584 ν(HC=N), 1497ν(C=C), 1264, 1205 ν( C-O),
1
977 σ(O-H), 887, 836, 793 σ(p-OH). H NMR (DMSO, ppm, 600 MHz): δ = 0.84 -0.88 (m,
CH3), 1.49 - 2.05 (m, 6H), 2.31 - 2.54 (m, 3H), 6.45 - 7.78 (m, C6H4), 8.46 (s, N=CH), 9.82
(s, OH). 13C NMR (DMSO, δ ppm): δ = 11.89(CH3), 16.54(CH2), 39.25(CH2), 114.05,
115.20, 120.65, 129.05, 130.35, 132.30, 140.01, 145.75, 149.67, 159.33, 160.60 (C6H4),
161.73 (HC=N), 191.16 (C-OH). MS (ESI) m/z = C16H17NO Calcd. 239.29. Found [M+H]+ =
240.14 (100%). Anal. Calc. for C16H17NO: C, 80.30; H, 7.16; N, 5.85; Found: C, 81.09; H,
6.98; N, 6.15%.
TMP: yield 50.3%. M.pt. 172-175°C. Selected IR (cm−1): 3051, 3019 ν(C-H aromatics),
2865, 2822 ν(sp3 C-H), 2722 ν(HC=N), 1485 ν(C=C), 1269, 1215 ν(C-O), 997 σ(OH), 863,
782, 754 σ(p-OH).1HNMR (DMSO, ppm, 600 MHz): δ = 2.24 (s, CH3), 6.44 (s, 1H), 6.65 (s,
13
1H), 6.81-7.09 (m, C6H4), 7.19 (s, 2 H),7.78 (d, 2H),8.31 (s,1H), 9.77 (s, OH ); C NMR
(DMSO, ppm, 600 MHz): δ 17.84 (CH3, C-7), 116.19 (C-6), 117.97 (C-14), 126.59 (C-3),
130.69(C-3, C-11, C-15), 146.64(C-10), 151.22(C-4), 159.55(C-5), 160.63(-C=N), 191.19(C-
OH). MS (ESI) m/z = C14H13NO Calcd. 211.26. Found [M+H]+ = 212.11 (100%). Anal. Calc.
for C14H13NO: C, 79.59; H, 6.20; N, 6.63; Found: C, 80.05; H, 6.44; N, 6.54%.
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2.3 X-ray crystallography
X-ray diffraction studies were carried out at 200 K using a Bruker Kappa Apex II
diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). APEXII and
SAINT were used for data collection, cell refinement and data reduction [31]. Structural
elucidation were carried out using SHELXT-2014[32], and improved by least-squares
methods using SHELXL-2014 with SHELXLE as a graphical interface [32, 33]. Carbon-
bound H atoms were placed in calculated positions and were incorporated in the refinement
in the riding model approximation, with Uiso(H) set to 1.2Ueq(C) while all non-hydrogen
atoms were refined anisotropically. There was rotation of the H atoms of the methyl group
with a fixed angle around the C—C bond in order to give excellent fit to the experimental
electron density (HFIX 137 in the SHELX program suite,32with Uiso(H) set to 1.5Ueq(C).
Similar rotation was equally allowed for the hydroxyl H around the C—O bond to best fit the
experimental electron density (HFIX 147 in the SHELX program suite [32], with Uiso(H) set
to 1.5Ueq(O). The propyl group is disordered and necessitates the utilization of various
constraints. The data were corrected for absorption effects by the numerical method using
SADABS [31].
2.4 Electrochemical techniques
The complete electrochemical setup was allowed to reach the steady state, followed by
potential scan conducted for 1800 s in the aggressive solutions which contained various
concentrations of inhibitors and also a control (without inhibitor). Subsequently,
potentiodynamic polarization (PDP) measurements were carried out after 1800 s of mild steel
immersion in the aggressive solutions by sweeping the potential in the range of -0.25 to +0.25
V at the scan rate of 0.5 mVs-1. The corrosion current density (icorr), anodic and cathodic
Tafel slopes (βa and βc respectively) were obtained from the polarization curves by
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extrapolating the linear Tafel segments to the corrosion potential (Ecorr). The inhibition
efficiency (η%) was, thereafter, obtained. Electrochemical impedance spectroscopic (EIS)
techniques were carried out at the OCP by analysing the frequency response of the
electrochemical system in the range 10 mHz to 100 kHz at 10 mV root-mean-square (rms)
amplitude. The impedance spectra were fitted into appropriate equivalent circuit to obtain
required electrochemical data such as the charge transfer resistance (Rct).
2.5 Quantum chemical calculations details
Gaussian 09 software was used to calculate the quantum chemical parameters necessary to
explain the atomic and molecular interactions involved in the compounds. Density functional
theory (DFT) approach, which involves the Becke’s three-parameter hybrid functional and
Lee-Yang-Paar correlation functional (B3LYP) combined with the 6-31 G+(d) basis function
was used to obtain the optimized structures [34]. The DFT, among other functions, has shown
excellent promise in revealing the changes in electronic structure responsible for inhibitory
action of compounds on metal surfaces. GaussView 5.0 software was further applied in
visualizing the electron density graphical isosurfaces and the quantum chemical parameters
such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the
lowest unoccupied molecular orbital (ELUMO), the energy gap (ꢀ E = EHOMO− ELUMO),
chemical potential (ꢁ), absolute hardness (χ), absolute softness, nucleophilicity index (N) and
fraction of electron transfer from the inhibitor to the metal surface (ꢀN), were quantified
using equations 1-7 [35, 36].
lp (Ionization potential) = -EHOMO and EA (electron affinity) = -ELUMO
Chemical potential, µ =- ꢀꢁ(lp+EA)
(1)
(2)
Absolute hardness, χ =- ꢀꢁ(EHOMO – ELUMO
Energy gap ꢀE = ELUMO- EHOMO
)
(3)
(4)
(5)
ꢀ
Nucleophilicity index (N) = ꢂ
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Global softness (σ) =ꢃꢀ
(6)
(7)
ꢄ
ꢇꢄ
ꢈꢉꢊ
Amount of e- which migrates inhibitor to metal surface (ꢀN) = ꢁ(ꢃ
ꢅꢆ
ꢋꢃ
)
ꢅꢆ
ꢈꢉꢊ
3.0 Results and discussion
3.1 Synthesis
A condensation reaction between 4-propylaniline and 4-hydroxybenzaldehyde (1:1 molar
ratio) in ethanol afforded (E)-4-(((4-propylphenyl)imino)methyl)phenol (PMP), while similar
reaction
between
o-toluidine
and
4-hydroxybenzaldehyde
gave
(E)-4-((2-
tolylimino)methyl)phenol (TMP) as presented in scheme 1. Both compounds were obtained
at moderately high yield, are brownish yellow in colour and stable at ambient conditions. The
structures were elucidated using spectroscopic techniques and confirmed by X-ray
crystallography. The mass spectra results gave molecular ion peaks at 240.14 and 212.11 for
PMP and TMP respectively. Experimental and theoretical results were in agreement with
each other and confirmed the structures of the compounds.
Scheme 1: Synthesis route for the compounds, PMP and TMP.
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3.2 FTIR measurements
The spectra of both compounds, presented in Figures 1a and 1b, showed the absence of bands
due to NH2 and CO groups of the starting materials and the presence of bands due to the
azomethine group. The band which appeared around 3366 cm-1 could be assigned to the
vibrational frequency due to the phenolic OH [37]. The stretching vibrations around 3024 cm-
1 (for PMP) and 3019 cm−1 (for TMP) are due to C-H of the aromatic rings. The three weak
bands observed at 2954, 2922, 2857 for PMP are sp3 CH vibrations attached to the aromatic
ring. They were similarly observed in the range 2822 and 2865 cm-1 for TMP. Two strong
absorption bands which occurred around1265 and 1205 for PMP; and around 1269 and 1215
for TMP could be attributed to the O-H deformation modes and C-O stretching vibration
respectively. Sharp and intense bands characteristic of the vibrations of ν(C=C) and ν(CH=N)
were noticed around1898 and 1604 cm-1 for PMP, while for TMP they occurred around 1681
and 1625 cm−1 [38]. The O-H bending vibration in PMP and TMP were observed around 977
and 997 cm-1 respectively. The sharp bands around 887, 836, 782, and 754 in the spectra of
both compounds are diagnostic peaks attributed to p-substituted OH derivative of aromatic
ring [39].
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(a)
PMP
(b)
TMP
3000
2500
2000
1500
1000
Figure 1: FTIR spectra of the synthesized Schiff base ligands
3.3 NMR spectroscopy
In the 1H NMR spectra, the chemical shifts due to the aromatic protons, the hydroxyl protons
and protons due to alkyl substituents were clearly observed. The proton due to the HC=N
group appeared as singlet at 8.46 and 8.31ppm for PMP and TMP, respectively. The hydroxyl
group bounded to the aromatic ring showed signals at 9.85 for PMP and 9.77 ppm for TMP.
The positions of these peaks reflect the electronegative property of the aromatic ring, and
possible presence of hydrogen bonding leading to downfield resonance [39]. PMP showed
peaks in the range 0.81 - 0.88, 1.49 - 2.05, and 2.31 - 2.54 ppm, which are due to the terminal
CH3, CH2 at position 9, and the protons of the CH2 at position 8 respectively. In the spectrum
of TMP, the signals observed at 2.24 ppm could be due to protons of CH3 at position 7. The
aromatic protons appeared as multiplets and were observed between 6.44-7.78 ppm in both
compounds. The protons at positions 13 and 17 for PMP are chemically equivalent and
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resonated at 7.78 ppm, and similar situations were observed for the protons at positions 11
and 15 of TMP. Other protons with equivalent environments are found at positions 1 and 5, 2
and 4, 14 and 16 for PMP and 12 and 14 for TMP.
In the 13C-NMR spectra, the compounds showed signals in the region 11.00-191.00 ppm with
varied intensities which signify the type of carbon atom. The signals due to the phenyl group
were observed in the aromatic region between 110 and 160 ppm, and the position is a
function of the proximity of the carbon atoms to different electron donating or withdrawing
groups [39]. Signals at 161.73 and 160. 63 ppm in the spectra of PMP and TMP respectively
are ascribed to the C=N functional group. PMP showed CH3 signals at 11.89, and the signals
due to the CH2 at position 9 and 8 at 16.54 and 39.25 ppm respectively. In the spectrum of
TMP, the signal found at 17.84 ppm could be attributed to the CH3 attached to the aromatic
ring. A low intensity signal observed at 191.19 ppm in the spectra of both compounds is
ascribed to the carbon atoms bearing the OH group, and the enhanced downfield shift is due
to its para-position. The signals due to carbons at position 6 in PMP and the carbon at
position 5 in TMP were observed at 160.60 ppm and 159.55 ppm respectively. The proximity
of these carbons to the electronegative nitrogen atom enhances their deshielding effect and
caused them to resonate at higher δ-values [39]. Other aromatic carbons appeared in the range
115 to 151.22 ppm. Among the aromatic carbons, C-14, C-16 and C-1, C-5 for PMP showed
signals at 114 and 120 ppm respectively, while for TMP the C-6, C-12 and C-14 showed their
signals in relatively up-field regions: 116 to 117.97 ppm. This is because in TMP these
carbons are in β-positions to the electron withdrawing groups: N and OH, thereby
experiencing some deshielding effect. The highest observed peaks for PMP are at 114.05 and
129.05 ppm, and are assigned to C-16 and C-1 respectively.
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3.4 X-ray crystallography
An ORTEP structures and the crystal packing of PMP and TMP with the atom-numbering
scheme is presented in Figures 2 and 3 respectively. Table 1 presents the crystallographic
data of the compounds. Selected bond distances and angles are summarized in Table 2. Both
compounds crystallized in an orthorhombic crystal system with space group Pca21. In the
asymmetric unit there are four molecules of the compounds. The structure of PMP is non-
planar, with the least square plane through the phenol ring making a dihedral angle of
51.61(9)˚ with the least square plane through the propylbenzene ring. The propyl group is
disordered over two positions in a 0.79:0.21 ratio. The least square plane through both propyl
groups makes a dihedral angle of 84.6 (8)˚ with the propylbenzene. There is one
intermolecular O1—H1A…N1i (i: -x,1-y,1/2+z) interaction of length 1.88 Å which links
adjacent molecules and can be described with a C11(8) graph-set descriptor on the unary
level [40, 41]. There is no short π…π ring interactions, but there are two longish C—H…π
ring interactions with hydrogen to centroid length of 2.82 and 2.87 Å with the phenol and
propylbenzene rings respectively.
The structure of TMP shows that the N1-C2 bond length (1.2740) conforms to the double
bond for the imine group, while the N1-C11 (1.436) bond length agrees with the single bonds
for the amine groups [42]. Similar to the PMP, the structure of TMP is non-planar with the
least square plane through the phenol ring, making a dihedral angle of 77.44(8)˚ with the least
square plane through the toluene ring. There is an intermolecular O1—H1…N1i (i: 1/2+x,1-
y,z) interaction of length 1.92 Å which links adjacent molecules and can be described with a
C11(8) graph-set descriptor on the unary level [40, 41]. There is also a long C26—
H26…O1ii (ii: 2-x,1-y,1/2+z) interaction of length 2.53 Å. There are no short π…π ring or
X—H…π ring interactions.
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Table 1: Crystallographic data, data collection and structure refinement for the ligands
compound
(E)-4-(((4-propylphenyl)imino)
methyl)phenol
1527670
(E)-4-((2-tolylimino)
methyl)phenol
1491358
CCDC
Empirical formula
Colour/shape
Formular weight
Temperature (K)
Crystal system
Space group
a/Â
C16H17NO
yellow
C14H13NO
yellow
239.31 g/mol
200
211.25g/mol
200
orthorhombic
Pca21
orthorhombic
Pca21
10.8946(4)
13.0307(5)
9.3686(3)
1330.01(8)
4
10.7087(5)
12.0254(5)
9.0344(4)
b/Â
c/Â
V/Å3
1163.42(9)
4
Z
Densitycalc/g/cm3
1.195
1.206
Absorption coefficient (µ)/mm-1 0.074
0.076
F(000)
512
448
Crystal size/mm3
Θmin, θmax (º )
Dataset
0.06 x 0.25 x 0.51
2.4, 28.3
0.18 x 0.29 x 0.50
3.4. 28.3
-14:14, -17:17, -12:12
18252,3320,0.027
2918
-14.14, -16.15, -11.12
36154, 2831, 0.018
2673
Total uniq data, R(int)
a
R1 (I > 2σ(I))
R, wR2 ,S
0.0349, 0.0808, 1.04
0.0310, 0.0806, 1.03
-0.19, 0.15
Min and Max resd.dens.(e/ Å‒3) -0.16,0.17
Table 2: Bond lengths and bond angles of the synthesized Schiff bases
PMP
TMP
Bond lengths
Bond angles
Bond lengths
O(1)-(24) 1.3548(18)
N(1)-C(2) 1.2740(18)
N(1)-(11) 1.436(2)
Bond angles
O(1)-C(14)
1.350(2)
1.281(2)
1.421(2)
0.8400
C(1)-N(1)-C(21)
118.58(15)
109.00
C(2)-N(1)-(11)
C(24)-O(1)-(1)
N(1)-C(2)-(21)
116.20(12)
109.00
N(1)-C(1)
C(14)-O(1)-(1A)
N(1)-C(1)-C(11)
O(1)-C(14)-(15)
N(1)-C(21)
O(1)-H(1A)
C(1)-H(1A)
123.93(17)
117.79(16)
125.01(13)
O(1)-H(1) 0.8400
C(1)-(1A) 0.9800
C(25)-26) 1.381(2)
C(26)-26) 0.9500
O(1)-C(24)-(23) 117.86(13)
C(11)-C(12)-13) 116.56(16)
0.9500
C(11)-C(12)-(13) 121.25(16)
N(1)-C(1)-H(1) 118.00
C(12)-C(13)-(13) 120.00
C(25)-C(26) 1.384(3)
C(24)-H(25) 1.391
N(1)-C(2)-H(2)
117.00
C(12)-C(13)-13) 119.00
14

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C4
(a)
C22
C23
C13
C12
N1
C6
C7
C21
C3
C24
C11
C1
O1
C14
C2
C26
C25
C15
C16
(b)
Figure 2: (a) Molecular structure of (E)-4-(((4-propylphenyl)imino)methyl)phenol with
displacement ellipsoids drawn at 50% probability level, (b) Crystal packing drawn for the
clearest view.
15

ACCEPTED MANUSCRIPT
C1
(a)
C12
C13
N1
C23
C22
C11
C24
C2
C14
C21
O1
C16
C25
C26
C15
(b)
Figure 3: (a) The molecular structure of (E)-4-((2-tolylimino)methyl)phenol (TMP) with
displacement ellipsoids drawn at 50% probability level, (b)Crystal packing drawn for the
clearest view.
16

ACCEPTED MANUSCRIPT
3.5 Electrochemistry of the studied compounds on mild steel
3.5.1 Potentiodynamic polarization studies
Polarization tests were conducted to probe the kinetics of the anodic and cathodic reactions
[43]. Figure 4 presents the results of the effect of different concentrations of (PMP) and
(TMP) on the cathodic and the anodic polarization curves of mild steel in 1 M HCl. The
electrochemical data acquired from the polarization curves are presented in Table 3. Upon the
addition of the Schiff bases, both the cathodic and anodic partial reactions were affected.
Consequently, this caused a further reduction in the cathodic (βc) and anodic current densities
(βa), and the resulting corrosion current density (icorr). Therefore, the addition of PMP and
TMP diminishes the anodic dissolution and also lessens the cathodic hydrogen evolution
reaction [43]. As a result of recorded retardation of the cathodic and anodic current densities
upon the addition of the compounds in the aggressive solution, they can be classified as a
mixed-type inhibitor for the corrosion of MS in 1 M HCl [44, 45]. A close inspection of
Figure 4 and Ecorr values given in Table 3 reveals that the addition of the Schiff bases plays
more active role on the cathodic arm of the Tafel plots in addition to the shifting of Ecorr
values towards more negative region, respectively. In addition, study conducted by Yadav et
al. reported that the change in Ecorr values above 85 mV suggests that a compound is an
anodic or cathodic inhibitor [46]. In this present study, the highest displacement of Ecorrwas
about 46.91 mV towards cathodic direction. These findings support the idea that the studied
ligands function as a mixed-type inhibitor but with much more cathodic effect [44-47].
17

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(PMP)
0.001
1E-4
1E-5
1E-6
1E-7
Blank
5 ppm
10 ppm
20 ppm
60 ppm
80 ppm
100 ppm
-0.7
-0.6
-0.5
-0.3
-0.2
E (V vs Ag/AgCl, 3 -M0.4KCl)
(TMP)
0.001
1E-4
1E-5
1E-6
1E-7
Blank
5 ppm
10 ppm
20 ppm
60 ppm
80 ppm
100 ppm
-0.6
-0.5
-0.4
-0.3
-0.2
-0.7E (V vs Ag/AgCl, 3 M KCl)
Figure 4: Polarization curves for mild steel in the absence (blank) and presence of different
concentrations (in ppm) of PMP and TMP at 303 K.
18

ACCEPTED MANUSCRIPT
Table 3: Tafel polarization parameters for MS in 1 M HCl solution in absence and at different
concentration of the inhibitor ligands
βa(mV/dec) βc(mV/dec)
Inhibitor Conc
(ppm)
-Ecorr
(mV)
452.72
icorr (ꢀA cm-2) η%
Blank
-
98.93
72.08
236.22
-
PMP
5
151.53
84.33
66.04
48.43
45.07
38.86
78.43
123.62
141.84
62.99
61.11
54.82
471.4
119.42
39.24
14.64
14.62
13.57
8.74
49
83
94
94
94
96
10
20
60
80
100
499.63
488.76
464.33
464.06
459.34
TMP
5
127.72
133.61
54.44
61.25
58.98
72.62
80.90
74.14
122.1
42.67
31.88
35.33
481.76
483.91
486.59
493.4
486.98
485.62
164.74
132.94
54.53
23.87
20.22
12.0
30
44
77
90
91
95
10
20
60
80
100
The inhibition efficiency (%IEPDP) presented in Table 3 was computed from the icorr using the
formula:
ꢑ
ꢕ
%ꢌꢍꢎꢏꢎ = 100 ꢐ1 − ꢑ ꢒꢓꢔꢔ ꢖ
(13)
ꢒꢓꢔꢔ
Where iocorr and icorr are corrosion current densities in the absence and presence of inhibitors
respectively.
Table 3 shows that as the inhibitor concentration increased, there was improved tendency of
corrosion inhibition with corresponding decrease in corrosion current density. This trend can
be attributed to the adsorption of inhibitor molecules on mild steel-acid interface which
consequently shielded the metal surface from direct acid attack [46]. The lowest icorr values
which resulted to maximum inhibition efficiency obtained for PMP and TMP were 8.74 and
12.0 ꢀA cm-2 at 100 ppm inhibitor concentration, respectively.
19

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3.5.2 Electrochemical impedance spectroscopy
Figure 5 shows typical Nyquist plots for mild steel (MS) in 1 M HCl in the absence and
presence of different concentrations of PMP and TMP. All of the impedance spectra can be
seen to display depressed semicircles with centres below the real axis. This characteristic
unique property of Nyquist plot is caused by unavoidable roughness and inhomogeneity of
metal electrodes that result during MS polishing [45, 48]. The inhibition efficiencies, %IEEIS,
were calculated from Nyquist plots by the equation:
ꢕ
ꢇꢚ
%ꢌꢍꢗꢘꢙ = 100 ꢐꢚ
ꢒꢛꢖ
(14)
ꢒꢛ
ꢚ
ꢒꢛ
where Rct and R0 are the charge transfer resistance in the presence and absence of inhibitor
ct
respectively. Other derivable relevant impedance data are reported in Table 4.The impedance
response of mild steel in 1 M HCl solution was greatly affected after the addition of all the
ligands, and the impedance of the inhibited system improved significantly with increase in
inhibitor concentration.
In addition, at 100 ppm concentration of ligands, larger diameter semicircles were significant
compared to the other five lower concentrations of ligands (Fig. 5). In the presence of the
Schiff base ligands, the entire Nyquist diagrams showed a diminished capacitive loop in the
high frequency (HF) range and an inductive loop in the lower frequency (LF) range. The
often observed HF capacitive loop has been ascribed to the charge transfer process and time
constant of the electric double layer as well as the inhomogeneity of the surface which
characterises numerous adsorption activities on metal [46]. Moreover, the LF inductive loop
may be a result of the relaxation process obtained by adsorption of certain species, in this
instance N and O on working electrode surface [49].
20

ACCEPTED MANUSCRIPT
To ensure a proper interpretation of depressed semicircles with their foci below the real axis,
a constant phase element (CPE) was introduced which is given by [46]:


1
−1
(15)




ZCPE
=
[(
j
ω
) ]
n
Y0


where, Y0 is the CPE constant; ω is the angular frequency; j is the imaginary number and n is
the phase shift (exponent). Figure 6 represents the equivalent circuit employed for the fitting
of impedance spectra. Subsequently, the double layer capacitances, Cdl, for the circuit
including a CPE were computed by using the following equation:
1/ n
1−n
ct
Cdl =
(
Y0R
)
(16)
Other useful impedance parameters which includes Rs, Rct, Y0, n and %η obtained from fitting
the recorded EIS data using the equivalent circuit of Figure 6, are presented in Table 4. The
data in Table 4 indicates that Rct value improved significantly with the concentration of the
Schiff bases. Consequently, the value of proportional factor Y0 and Cdl obtained was
decreased significantly upon the addition of inhibitor. However, no regular trend could be
established across concentrations. Such anomaly may suggest that possible interaction of
chemical species on MS surface cannot be ruled out, whereas the decrease in Cdl values
connotes successful displacement of water molecules that have been found to have high
dielectric constant [46]. The change in the values of Rct and Y0 can be related to the moderate
displacement of water molecules by Schiff base molecules on the working electrode surface
which eventually results to the reduction in the number of active sites exposed to corrosion
[50]. The recorded increase in Rct threshold values is usually ascribed to the formation of
protective film on the metal/solution interface [43, 47]. The observed stability of n values
within the range of 0.78-0.86 is an indication that the MS dissolution in the present report is
hugely dependent on the charge transfer process [46]. In general, the results recorded for the
inhibition efficiency of the two Schiff bases against MS corrosion in 1 M HCl using the EIS
21

ACCEPTED MANUSCRIPT
technique are in tandem with those recorded using the PDP technique.
(PMP)
300
Blank
5 ppm
10 ppm
20 ppm
200
60 ppm
80 ppm
100 ppm
100
0
0
100
200
300
2
Z
(Ω cm )
re
250
200
150
100
50
(TMP)
Blank
5 ppm
10 ppm
20 ppm
60 ppm
80 ppm
100 ppm
0
0
50
100
150
(Ω cm )
200
250
2
Z
re
Figure 5: Nyquist plots for mild steel in theabsence and presence of different concentrations
of PMP and TMP at 303 K.
Figure 6: Equivalent circuit employed for the fitting of impedance spectra.
22

ACCEPTED MANUSCRIPT
Table 4: Electrochemical impedance parameters obtained for MS in 1 M HCl in the absence
and presence of different concentration of Schiff base ligands.
Conc
Rs
Rct
Y0
Cdl
n
η%
ꢁSsncm−2
)
(
ꢁSsncm−2
)
Inhibitor
Blank
(ppm)
(ꢂ cm-2)
(ꢂ cm-2)
(
-
2.50
16.6
677
4000.8
0.84
-
PMP
5
2.02
3.35
2.11
4.13
3.55
2.01
32.9
92.8
236
291
300
342
457
241
169
116
136
221
2187.3
2525.1
1271.2
981.6
0.86
0.81
0.84
0.83
0.83
0.86
50
82
93
94
94
95
10
20
60
80
100
1196.4
1375.8
TMP
5
3.59
2.88
3.38
2.50
3.35
4.22
22.9
28.7
67.6
199
211
267
496
483
371
175
143
129
3851.9
3918.6
2954.2
2390.6
2623.1
2456.3
0.82
0.82
0.83
0.80
0.78
0.78
28
42
75
92
92
94
10
20
60
80
100
Figure 7 shows the Bode impedance modulus and phase angle plots for mild steel in 1 M HCl
in the absence and presence of different concentration of the Schiff bases. The Bode plots
depict a single peak as can be observed for phase angle against frequency. This observation
entails that the dissolution of MS is determined by a single charge transfer process [51].
Furthermore, Daoud et al. alluded that this phenomenon is due to a single relaxation time
constant [51]. At higher frequencies, it can be observed that the phase angle increased with
resultant increase ascribed to the induction process involving the connecting cables [52]. In
order to unravel the capacitive behaviour of the entire system under study, the relationship
between log |Z| vs. log f at intermediate frequencies was explored, and presented in Table 5.
It is obvious from the results presented in Figure 7 that at intermediate frequencies, there is a
slight broadening of one time constant in the presence of inhibitor which could be due to the
appearance of a surface film [46, 52]. Yadav et al. prescribed that an ideal capacitive
behaviour would give a slope (Ѕ) of -1 and maximum phase angle (α) of -90 [46]. For the
23

ACCEPTED MANUSCRIPT
present system at intermediate frequencies, the slope and maximum phase angle values in the
range of -0.34 to -0.59; -52.17 to -69.30; and -0.49 to -0.62;-38.26 to -57.60 for PMP and
TMP respectively were obtained. These deviations can be regarded as departure from the
ideal capacitive behaviour at intermediate frequencies [46]. As a result of this, at increased
concentrations, both PMP and TMP tend to attain steady state (of -Ѕ and – α) faster than in
their absence. This behaviour reflects the catalytic activity of the two Schiff base ligands
during the MS dissolution process [53].
3.0
2.5
2.0
1.5
1.0
0.5
0.0
80
70
60
50
40
30
20
10
0
Blank
60 ppm
5 ppm
10 ppm
20 ppm
80 ppm
100 ppm
(PMP)
0
1
2
3
4
log f (Hz)
3.0
70
Blank
5 ppm
10 ppm
20 ppm
60 ppm
80 ppm
100 ppm
60
50
40
30
20
10
0
2.5
2.0
1.5
1.0
0.5
0.0
(TMP)
0
1
2
3
4
log f (Hz)
Figure 7: Bode impedance modulus and phase angle plots for mild steel in 1 M HCl in
absence and presence of different concentration of PMP and TMP.
24

ACCEPTED MANUSCRIPT
Table 5: The slopes of the Bode impedance magnitude plots at intermediate frequencies (S)
and the maximum phase angles (α) for MS in 1 M HCl solution in the absence and presence
of inhibitors
.
Inhibitor
Concentration
-S
-α
(ppm)
-
0.30
40.47
Blank
PMP
TMP
5
100
5
0.34
0.59
0.49
0.62
52.17
69.30
38.26
57.60
100
3.6 Adsorption study
A good number of previous corrosion inhibition studies support the idea that metal samples in
aqueous medium are usually covered with water dipoles adsorbedon the surface of the metal
[44, 45, 54]. As a result, inhibitor molecules that eventually adsorb onto metal surface do so
via a quasi-substitution mechanism. It is on this note that we subjected EIS data obtained in
this research to two adsorption isotherms which includes Temkin (Fig. 8a) and Langmuir
(Fig. 8b) models. Incidentally, the Langmuir isotherm produced the best fit from the
experimental data. The Langmuir equation used in fitting the data is given by:
Cinh
1
=
+ Cinh
θ
Kads
(17)
where, θ is the degree of surface coverage; Cinh is the inhibitor concentration and Kads is the
equilibrium constant of the adsorption/desorption process. The choice of Langmuir over
Temkin was based on the correlation coefficient (R2) values closer to unity that were obtained
for the Langmuir isotherm. In addition, an ideal Langmuir isotherm expects slope around
unity[54], which is in line with values obtained in this study.
25

ACCEPTED MANUSCRIPT
In order to attribute the adsorption process to either physisorption or chemisorption, the free
energy of adsorption (ꢀGads) was calculated for the Langmuir isotherm by making use of
equation 18:
∆G =−RTln
(
55.5Kads
)
ads
(18)
Where ꢀGads is the standard free energy of adsorption; R is the gas constant and T is the
absolute temperature. The value of 55.5 is the concentration of water in solution in molL−1.
The values of Kads and ꢀGads are presented in Table 6. The obtained ꢀGads values suggest that
both PMP and TMP adsorb on mild steel surface in 1 M HCl via competitive physical and
chemical adsorption mechanisms [45]. The negative sign mainly describes the spontaneity of
the adsorption process [45, 54].
26

ACCEPTED MANUSCRIPT
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
(a)
PMP: R2=0.65634: Slope=0.00291: Intercept=0.71319
TMP: R2=0.86742: Slope=0.00625: Intercept=0.41843
0
20
40 60
Cinh (ppm)
80
100
120
100
80
60
40
20
0
PMP: R2=0.9995: Slope=1.02421: Intercept=2.70874
TMP: R2=0.9954: Slope=0.93246: Intercept=11.74545
(b)
0
20
40 60
Cinh (ppm)
80
100
Figure 8: Representative adsorption isotherms for MS in 1 M HCl containing various
concentrations of the studied Schiff base ligands: (a) Temkin and (b) Langmuir.
Table 6: Langmuir parameters representing the adsorption mechanism on mild steel surface
obtained from EIS parameters at 303 K.
Inhibitor
Kads×103
-ꢀG°ads (kJ/mol)
PMP
TMP
88.340
17.987
38.81
34.80
27

ACCEPTED MANUSCRIPT
3.7 Molecular interactions
The frontier molecular orbital and Fukui functions were used to explain the reactivity of the
compounds and their various interactions with the surface of the metal. The optimized
structures, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital
(LUMO) and the Fukui indices of the Schiff base molecules are presented in Figure 9. Other
quantum chemical parameters as shown in Table 7 were basically calculated from the EHOMO
and ELUMO. The activity of the molecules is determined by the EHOMO and ELUMO. The higher
value of EHOMO shows its readiness to donate electrons and form coordinate covalent bonds
with the empty d-orbitals of the metal atoms thereby enhancing the corrosion inhibition
effect. From the studied molecules, PMP has higher EHOMO and reasonably low ELUMO values
that afford it the ability to donate and accept electrons under favourable conditions than TMP.
The energy gap ꢀE is smaller in PMP compared to TMP signifying that it is unstable, more
polarised, of higher reactivity and can easily be adsorbed on the metal surface than TMP [46].
The values of the dipole moment are supposed to support the other parameters in the sense
that molecules with higher dipoles reflect stronger dipole-dipole interactions with metal
surfaces. In otherwords, PMP should have a higher dipole moment than TMP, however, there
are some reports which show that experimental inhibition efficiencies sometimes do not
conform to the values of the dipole moments [55-58]. From the Figure 9, there is better
electron distribution in TMP compared to PMP which attest to the value of the dipole
moment that measures the polarity of the molecules. The propyl and methyl groups attached
to the para and ortho positions of the backbone molecule respectively have played a
significant role in affecting the surface layer of the molecules. Our result in this research
therefore shows that decrease in dipole moment increases the inhibition efficiency which is
adequately supported by the experimental results. Furthermore, other quantum chemical
28