Linoleic acid and its potassium and sodium salts: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2017.08.065

Linoleic acid (cis, cis-9,12-octodecadienoic acid) is the main polyunsaturated -omega 6- essential fatty acid. The conformational behaviour of linoleic acid (LA) in the gas phase was investigated by means of density functional theory (DFT). The structures of conformers of LA were fully optimized by using the B3LYP/6-311++G(d,p) method. The theory showed that the tttttts′CssCs′tt conformation of LA (conformer I) is the more stable than the other conformations. Fourier Transform Infrared (FTIR) and micro-Raman spectra of pure LA in liquid form were recorded in the region 4000–450 and 3500–100 cm^?1, respectively. The DFT calculations on the molecular structure and vibrational spectra of the dimer form of most stable conformer of LA were also performed using the same method. The assignment of the vibrational modes was made based on calculated potential energy distributions (PEDs). The simulated spectra of dimer form of LA are in reasonably good agreement with the experimental spectra. The sodium and potassium salts of LA were synthesized and characterized by FTIR and Raman spectroscopy, X-ray diffraction and DFT calculations. Several molecular and electronic properties of LA and its salts such as HOMO-LUMO energies, chemical hardness and electronegativity were also calculated and interpreted.

Full text of DOI:10.1016/j.molstruc.2017.08.065

Journal of Molecular Structure 1150 (2017) 68e81
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Linoleic acid and its potassium and sodium salts: A combined
experimental and theoretical study
a
b,
Tugba Gocen , Sevgi Haman Bayarı *, Mehmet Haluk Guven a
ꢀ
a Bülent Ecevit University, Department of Physics, 67100, Zonguldak, Turkey
b Hacettepe University, Department of Physics Eng., 06800, Beytepe-Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Linoleic acid (cis, cis-9,12-octodecadienoic acid) is the main polyunsaturated -omega 6- essential fatty
acid. The conformational behaviour of linoleic acid (LA) in the gas phase was investigated by means of
density functional theory (DFT). The structures of conformers of LA were fully optimized by using the
B3LYP/6-311þþG(d,p) method. The theory showed that the tttttts0CssCs0tt conformation of LA
(conformer I) is the more stable than the other conformations.
Received 22 March 2017
Received in revised form
19 July 2017
Accepted 17 August 2017
Fourier Transform Infrared (FTIR) and micro-Raman spectra of pure LA in liquid form were recorded in
the region 4000e450 and 3500e100 cmꢀ1, respectively. The DFT calculations on the molecular structure
and vibrational spectra of the dimer form of most stable conformer of LA were also performed using the
same method. The assignment of the vibrational modes was made based on calculated potential energy
distributions (PEDs). The simulated spectra of dimer form of LA are in reasonably good agreement with
the experimental spectra.
Keywords:
Linoleic acid
Sodium and potassium salts
DFT
FTIR
Raman
The sodium and potassium salts of LA were synthesized and characterized by FTIR and Raman spec-
troscopy, X-ray diffraction and DFT calculations. Several molecular and electronic properties of LA and its
salts such as HOMO-LUMO energies, chemical hardness and electronegativity were also calculated and
interpreted.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
study on linoleic acid, alpha-linoleic acid and arachidonic acid
showed that polyunsaturated chains in crystals form highly or-
A rapidly growing literature describes the benefits of poly-
unsaturated fatty acids in alleviating cardiovascular disease,
atherosclerosis, autoimmune disorder, diabetes, and other diseases.
Omega 6 Fatty acids are a class of polyunsaturated fats which have
been growing in prevalence in modern diets [1].
Linoleic acid, an omega-6 fatty acid (cis, cis-9,12-
octodecadienoic acid) is considered essential fatty acid because it
cannot be synthesized by human body. It is used in the biosynthesis
of prostaglandins and cell membranes [2].
In all omega-6 fatty acids, the first double bond is located be-
tween the sixth and seventh carbon atom from the methyl end of
the fatty acid. Linoleic acid (LA) has two cis double bonds with an
18-carbon chain (Scheme 1). It is present mostly in plant oils [3].
In the body, essential fatty acids serve multiple functions. It is
important to elucidate the structural properties. Single crystal
dered, and The closely packed molecules are stretched at a specific
conformation [4].
Rich [5] used quenched molecular dynamics simulations in
vacuo and investigated conformational space of arachidonic (20:4)
and related fatty acids: linoleic (18:2), oleic (18:1), arachidic (20:0),
and stearic (18:0). Resent investigation using powder X-ray
diffraction (XRD) indicated that there are three kinds of poly-
morphic phases in linoleic acid, low-temperature (LT, ꢀ20ꢁ),
middle-temperature (MT, ꢀ40ꢁ) and high-temperature (HT, ꢀ60ꢁ)
[6].
Vibrational spectroscopy has been used extensively for getting
molecular structure information about biological compounds. FTIR
spectroscopy has been applied to the determination of standard
indices used in the classification of edible oils, such as the free fatty
acids content [7e9].
Temperature-dependent FTIR spectra of linoleic acid were
measured over a temperature range of ꢀ80 to ꢀ30 ꢁC to explore
solid-state phase transition of linoleic acid [10]. In a recent study,
* Corresponding author.
E-mail address: bayari@hacettepe.edu.tr (S. Haman Bayarı).
http://dx.doi.org/10.1016/j.molstruc.2017.08.065
0022-2860/© 2017 Elsevier B.V. All rights reserved.

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
69
patterns (10ꢁ ꢃ 2
q
ꢃ 60ꢁ) solid form of sodium and potassium salts
of LA at room temperature employing CuKa (1.5405 Å) radiation.
2.4. Calculation
All the calculations were performed using Gaussian 09 Revision
A.02 program [12] running on a unix system (Ubuntu 14.04.4 LTS)
and GaussView 5.0.8. was used for visualization of the structure
and simulated vibrational spectra.
The initial structure [4] of linoleic acid was obtained from
Cambridge Structure Database (CSD LINOLA, NBS515621) and
conformational analysis were carried out by using the density
functional theory DFT(B3LYP) method. The level of theory used was
Becke's three-parameter exchange functional [13] in combination
with the correlation functional of Lee, Yang, and Parr [14] and the 6-
311þþG(d,p) basis set.
Scheme 1. Chemical structure of linoleic acid (LA).
Raman spectroscopy was used for the screening the oxidation
process of linoleic acid [11].
Although these studies are helpful in elucidating the confor-
mational space of LA, they did not analyse the vibrational proper-
ties of LA, in detail. On the other hand, up till now no vibrational
study has been reported on the dimer form of LA. The present study
describes a thorough conformational analysis of linoleic acid (LA)
using density functional theory (DFT). A total of three energy
minima were identified and characterized by means of B3LYP/
6e311þþG(d,p) geometry optimization in the gas phase for LA.
We also synthesized sodium (Na) and potassium (K) salts
[sodium linoleate and potassium linoleate] of LA. These salts are
commonly called soap salts. Fatty acids are poorly soluble in water
in their undissociated (acidic) form, whereas they are relatively
hydrophilic as potassium or sodium salts. Some potassium salts of
fatty acids are also used as multi-purpose food additives. There-
fore, the studies presented here also aiming to understanding of
structural, electronic and vibrational properties for Na and K salts
of LA.
The relaxed potential energy scans were performed for rotation
around the C9-C10-C11-C12 and C10-C11-C12-C13 dihedral angles
(Fig. 1), starting from the initial optimized geometry of LA. The
relaxed potential energy scan was also performed for rotation
around the C1-C2-O25-H52 dihedral angle, starting from the
lowest energy minimum of LA in the first PES. Conformational
searches were performed by varying the respective dihedrals in
steps of 10ꢁ from ꢀ180ꢁ to 180ꢁ. Then all energy minima were re-
optimized again at the DFT/B3LYP/6-311 þþ G(d,p) and the three
lowest-energy conformers are predicted in gas phase.
Geometry optimizations were followed by harmonic vibrational
frequency calculations. We verified that all the vibrational fre-
quencies of each equilibrium structure were real (no imaginary
vibrational frequencies).
The experimental FTIR and Raman spectra of liquid LA, Na and K
salts of linoleic acid (solid form) were recorded. Density functional
theory (DFT) calculations have been used to predict the theoretical
spectra and to assist reliable assignments of the experimental
frequencies.
The harmonic vibrational frequencies were scaled by 0.96
(above 1800 cmꢀ1) and 0.98 (under 1800 cmꢀ1). The vibrational
normal modes were analysed by carrying out the Potential Energy
Distribution (PED) calculations using VEDA (Vibrational Energy
Distribution Analysis) program [15]. This program reads the input
data (molecular geometry in cartesian coordinates, force constant
matrix and frequencies) from the output file of Gaussian program.
In this program, the theoretical normal modes are represented by
internal coordinates to generate symmetry coordinates. The initial
coordinates (stretching and bending) is replaced by the set of in-
ternal coordinates, which was defined following the recommen-
dations of Pulay et al. [16] and PED is recalculated. The average max.
potential energy (EPM) value obtained for LA in this study was
equal to 71.67%. Vibrational assignments were made on the basis of
PED and with the help of GaussView graphical animation.
Calculation of LA in the dimer form was also carried out to
improve the assignment of the bands in the liquid state experi-
mental spectrum. The optimum geometries and harmonic vibra-
tional wavenumbers of Na and K salts of LA were calculated using
same method.
2. Materials and methods
2.1. Experimental
Linoleic acid (Sigma-Aldrich, >99% purity) was purchased and to
avoid oxidation stored at ꢀ20 ꢁC. All chemical compound used here
were grade and used without further purification.
Linoleic acid (7.0 mmol) was dissolved in absolute ethanol
(10 mL) and was reacted with 1 M sodium hydroxide or potassium
hydroxide stock solution (7.0 mL). The mixture was refluxed for
60 min. The precipitate was filtered and washed with ethanol (70%).
Sodium [sodium linoleate (C18H31NaO2) and potassium [potassium
linoleate (C18H31KO2)] salts of linoleic acid were dried at 35 ꢁC in a
vacuum oven for one day.
2.2. Vibrational spectra
The calculated Raman activities by GAUSSIAN-09 program were
subsequently converted to relative Raman intensities with Gauss-
Sum program [17] according to the following equation [18]:
ATR-FTIR spectra of LA and its salts were recorded using Perkin
Elmer Spectrum 100 FTIR spectrometer at a spectral resolution of
4 cmꢀ1 over 32 scans in the range of 4000e650 cmꢀ1. FTIR spectra
(4000e450 cmꢀ1) were also recorded from KBr pellets of salts.
Micro-Raman spectra were collected on a LabRAM (Horiba-
Jobin-Yvon) spectrometer using a 632.8 nm excitation line from a
HeeNe laser. A 100 ꢂ objective (Olympus, France) was used for
focusing and collecting scattered Raman light. The laser power was
approximately 10 mW on the sample surface.
f ð
n
ꢀ
niÞ4Si
h
ꢀ
ꢁi
Ii ¼
(1)
hcni
ni 1 ꢀ exp
ꢀ
kT
where no is the exciting frequency (in this study we have used
15,822.78 cmꢀ1), ni is the calculated vibrational frequency (cmꢀ1) of
the ith normal mode, Si is the Raman scattering activity of the ith
normal mode. h; c and k are fundamental constants, and f is
normalization factor (10ꢀ12) for all peak intensities. For the plots of
simulated Raman spectra, Lorentzian band shapes were used with a
2.3. XRD patterns of salts
The powder X-ray diffractometer (XRD, Rigaku D-Max B hori-
zontal diffractometer) was used in order to record the diffraction
bandwidth (FWHM) of 10 cmꢀ1
.

70
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
Fig. 1. Atom numbering of linoleic acid (LA).
Fig. 2. Relaxed potential energy scans of linoleic acid.
The energy values of the highest occupied molecular orbital
Minimum energy conformation I occurred at C9-C10-C11-C12 and
C10-C11-C12-C13 dihedral angles of carbon-carbon bonds between
double bonds of near 120ꢁ and of O]CeOeH angle of 180ꢁ (s-trans
arrangement) in the DFT calculations. This structure is remarkably
good agreement with the X-ray crystal structure of LA [4] as shown
Table 2. In the X-ray crystal structure, LA is stretched with confor-
mation tttttts0CssCs0tt (here dihedral angles are denotes as
cis ¼ 0ꢁ ¼ C, skew ¼ 120ꢁ ¼ s, s0 ¼ ꢀ120ꢁ, trans ¼ 180ꢁ ¼ t) [20].
The gas-phase conformer I with predicted population of 70.0% is
most stable conformer. Although I is the energetically most stable
conformer, conformer II (tttttts0Css0Cs0tt) is also energetically close
to I (0.5 kcal molꢀ1).
(EHOMO) and the lowest unoccupied molecular orbital (ELUMO) of the
studied compounds were calculated. Several other electronic
properties, such as ionization potential, electronic affinities, hard-
ness and electronegativity were also calculated [19].
3. Results and discussion
3.1. Conformational stability and geometric structure of
Three conformers were found as shown in Fig. 2. The calculated
relative energies, dihedral angles of optimized geometries, equi-
librium populations at 298.15 K and dipole moment values for the
three conformers of LA are summarized in Table 1.
According to the results of optimized structures, among these
three, conformer I is the most stable. The cis configuration in a
naturally unsaturated fatty acid has hydrogen atoms on the same
side of the carbons, resulting in a kink in the fatty acid chain.
The highest energy conformation III is about 5.35 kcalmol-1
higher in energy than the minimum. There is only difference be-
tween conformers I and III is the dipole moment of conformer III
which has different orientation of the eOH group (s-cis arrange-
ment; O]CeOeH angle of 0ꢁ). Only the trans arrangement of
COOH monomeric LA was considered in the present work, as the cis
Table 1
Relative total (
D
Etotal; electronic þ zero-point vibrational) energies, the equilibrium populations (%) of the LA conformers and. dipole moment values.
Conformer
E þ ZPE (Hartree)
D
Etotal (kcalmolꢀ1
)
Dihedrals (ꢁ)
P (%)
Dipole Moment (D)
C9-C10-C11-C12
C10-C11-C12-C13
C1-C2-O-H
Conformer I
ꢀ855.4194
ꢀ855.4186
ꢀ855.4109
ꢀ1710.8614
0.00
0.50
5.35
120.6
126.8
118.1
ꢀ119.2
117.8
ꢀ180.0
180.0
0.0
ꢀ180.0
70.0
30.0
0.0
1.80
1.86
4.64
0.08
Conformer II
Conformer III
Dimer form of I
ꢀ123.7
114.5
ꢀ120.5

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
71
Table 2
DFT/6-311þþ G (d,p) optimized geometric parameters of the three stable conformers of linoleic acid monomer and dimer form of conformer I.
Geometric parameters
Conformers (monomer)
Dimer
Experimentala
I
II
III
Bond Length (Å)
C(l) ¼ O(24)
C(l)-O(25)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
1.205
1.360
1.510
1.528
1.533
1.533
1.533
1.532
1.540
1.504
1.337
1.511
1.511
1.337
1.504
1.540
1.532
1.533
1.532
e
1.205
1.360
1.510
1.528
1.532
1.533
1.533
1.533
1.540
1.504
1.336
1.510
1.510
1.336
1.504
1.540
1.533
1.533
1.532
e
1.198
1.366
1.522
1.529
1.533
1.533
1.533
1.532
1.540
1.504
1.336
1.511
1.511
1.337
1.504
1.540
1.532
1.533
1.532
e
1.225
1.324
1.510
1.528
1.533
1.533
1.533
1.532
1.540
1.504
1.337
1.510
1.511
1.337
1.504
1.540
1.532
1.533
1.531
1.688
0.998
Overlayb
1.238
1.287
1.511
1.540
1.529
1.518
1.532
1.519
1.522
1.517
1.324
1.507
1.526
1.318
1.507
1.556
1.538
1.549
1.532
C(9)-C(8)
C(9) ¼ C(10)
C(10)-C(11)
C(11)-C(12)
C(12) ¼ C(13)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
OeH/O
OeH
0.969
0.969
0.965
Bond Angles (ꢁ)
O(24)-C(l)-O(25)
C(2)-C(l)-O(24)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(4)-C(5)-C(6)
C(5)-C(6)-C(7)
C(6)-C(7)-C(8)
122.2
126.5
113.8
112.5
113.3
113.5
113.5
113.2
112.8
128.5
128.1
111.7
128.0
128.4
112.9
113.3
113.6
113.3
122.2
126.5
113.8
112.5
113.2
113.5
113.5
113.3
112.7
128.4
128.0
112.0
128.0
128.3
112.9
113.3
113.6
113.3
119.6
125.3
113.6
112.7
113.1
113.6
113.4
113.4
112.6
128.5
128.1
111.8
128.0
128.4
112.6
113.4
113.5
113.3
123.5
123.6
114.5
112.4
113.3
113.5
113.5
113.2
112.8
128.4
128.0
111.8
128.0
128.4
112.9
113.3
113.6
113.3
124.0
122.3
115.7
109.7
112.5
111.7
114.1
112.9
112.8
127.0
127.6
108.0
128.1
125.4
109.9
110.9
110.9
112.4
C(7)-C(8)-C(9)
C(8)-C(9)-C(10)
C(9)-C(10)-C(11)
C(10)-C(11)-C(12)
C(11)-C(12)-C(13)
C(12)-C(13)-C(14)
C(13)-C(14)-C(15)
C(14)-C(15)-C(16)
C(15)-C(16)-C(17)
C(16)-C(17)-C(18)
Dihedral Angles (ꢁ)
O(24)-C(1)-O(25)-H(52)
C(2)-C(1)-O(25)-H(52)
C(3)-C(2)-C(1)-O(24)
C(3)-C(2)-C(1)-O(25)
C(1)-C(2)-C(3)-C(4)
C(2)-C(3)-C(4)-C(5)
C(3)-C(4)-C(5)-C(6)
C(4)-C(5)-C(6)-C(7)
C(5)-C(6)-C(7)-C(8)
C(6)-C(7)-C(8)-C(9)
C(7)-C(8)-C(9)-C(10)
C(8)-C(9)-C(10)-C(11)
C(9)-C(10)-C(11)-C(12)
C(10)-C(l1)-C(12)-C(13)
C(l1)-C(12)-C(13)-C(14)
C(12)-C(13)-C(14)-C(15)
C(13)-C(14)-C(15)-C(16)
C(14)-C(15)-C(16)-C(17)
C(15)-C(16)-C(17)-C(18)
0.0
0.0
180.0
ꢀ0.3
ꢀ180.0
0.0
0.9
0.0
ꢀ180.0
ꢀ180.0
0.0
0.0
20.3
180.0
ꢀ179.9
ꢀ179.8
179.8
ꢀ179.5
179.5
ꢀ176.2
ꢀ123.9
ꢀ0.1
179.7
179.7
179.7
179.6
179.6
179.4
ꢀ177.7
ꢀ121.7
ꢀ0.6
ꢀ179.2
ꢀ179.8
ꢀ180.0
179.4
ꢀ179.9
178.9
ꢀ176.7
ꢀ123.6
ꢀ0.4
ꢀ180.0
ꢀ180.0
ꢀ179.9
ꢀ179.9
ꢀ179.8
180.0
177.0
120.6
0.0
ꢀ119.2
ꢀ120.5
0.1
120.5
176.7
179.9
179.9
ꢀ159.6
ꢀ170.4
ꢀ177.8
ꢀ179.6
179.6
179.1
ꢀ177.7
ꢀ119.4
ꢀ2.3
120.5
117.8
0.1
ꢀ118.8
ꢀ177.2
ꢀ179.6
179.9
126.8
ꢀ123.7
ꢀ0.6
118.1
114.5
ꢀ0.4
122.7
123.5
ꢀ3.3
ꢀ116.4
ꢀ177.4
ꢀ179.9
179.8
ꢀ120.0
ꢀ177.4
ꢀ179.9
179.8
ꢀ120.9
ꢀ176.4
ꢀ179.6
ꢀ178.3
a
Ref. [4].
b
Overlay of X-ray structure (blue) and predicted structure of conformer I (green).

72
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
Fig. 3. Comparison of experimental ATR-FTIR spectrum linoleic acid (liquid) with simulated gas phase infrared spectra of monomer and dimer forms of linoleic acid (LA).

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
73
COOH exists in a very low concentration (in fact 0.01%) in the gas
phase. Note that the predicted infrared spectrum of conformer III in
the OeH provides a striking contrast to those of I and II.
Although the B3LYP/6-311þþG(d,p) calculations were per-
formed for an isolated molecule in the gas phase, the obtained
geometrical parameters (Table 2) revealed a very good agreement
with the experiment. The predicted distance from C1 atom (car-
boxylic-sided chain) to a carbon atom (C18) of the methyl sided
chain is 19.42 Å.
LA molecules form centrosymmetric OeH/O hydrogen bonded
dimers in the low temperature crystal structure [4]. Therefore, the
cyclic dimer having two OeH/O]C hydrogen bonds was consid-
ered in the present work. The energy of dimer form of conformer I
is lower than the total energy of the two monomer (two conformer
I) units, indicating that intermolecular hydrogen bonding plays an
important role in stabilization of the molecule. B3LYP/6-
311þþG(d,p) optimized structure of dimer form of LA is given in
the supplementary material (Fig. S1) and geometric parameters are
given in Table 2 along with monomer form of LA.
The predicted OeH, C]O and CeO bond length changes caused
by hydrogen bond formation calculated for dimer, showing inter-
action. In the crystal structure [4], C]O/HeO length is 2.65 Å. The
calculated length is 2.35 Å in the optimized dimer structure.
The minimum differences (0.001 Å) observed between experi-
mental and calculated geometric parameters (bond lengths and
angles) of dimer form of LA. Bond lengths and angles in LA resemble
closely values found in other unsaturated carboxylic acids [21].
We observed two the short spacing peaks at 0.43 nm and 0.33 nm
in the XRD pattern of KLA at room temperature (Fig. S2). We did not
observed strong peak at near 0.460 nm for
double in the CH2 rocking vibrational mode near 720 cmꢀ1 in the FTIR
spectra of salts indicated that
0 form exist and dominant in salts. The
a form. We also observed a
b
splitting of the band was explained by the perturbation of the CH2
rocking vibration by the interaction of nearest neighbor methylene
chains in the crystalline array. These results also reveal that the salts
present an orthorhombic symmetry in the subcell. The existence of
crystalline peaks in the XRD pattern of KLA can be considered that
molecules are arranged in ordered microscopic structure.
Molecular calculations for Na and K salts of monomer form of LA
(conformer I) were performed at B3LYP/6-311þþG(d,p). The geo-
metric parameters of optimized structures of salts are given in
Table S1 (Supporting Information).
The calculated bond length of C]O is higher whereas CeO bond
length is shorter in the salts of LA with respect to that in LA. The
calculated Naþ/O distances are shorter than the Kþ/O distances.
O/Kþ/O angles is lower than O/Naþ/O. These values match
closely to the results in the literature regarding alkali metals with
different molecules.
Our calculations reveal that the total energy of the salts becomes
less negative and decrease from LA to KLA indicates that the sta-
bility of salt molecule increase in comparison to free molecule.
3.3. Vibrational spectra
3.3.1. Monomer and dimer forms of linoleic acid
3.2. Structure of NaLA and KLA salts
The experimental FTIR and Raman spectra of the liquid linoleic
acid (LA) are presented in comparison with those of simulated gas
phase spectra of monomer and dimer forms in Fig. 3 and Fig. 4,
respectively.
The calculated wavenumbers and the potential energy distri-
butions of the vibrational modes of monomer and dimer forms of
LA for conformer I are given in Table 3, in comparison with the
experimental vibrational spectra of the investigated molecule.
LA molecule has twelve methylene (CH2) one methyl (CH3), one
carboxylic acid (COOH) groups.
A carboxylic acid is a functional consisting a carbonyl group (C]
O) with a hydroxyl group (OH) attached to the same carbon atom. In
the infrared spectra, we expected CO, COH and OH vibrations of this
group. It is generally known that carboxylic acid form strong
dimeric complexes which have different properties compared to
monomer form. The hydrogen bonds in carboxylic acids are so
strong that this group bound together in dimers when in the solid
and liquid phase [25].
The experimental single crystal structure of salts of LA is not
known. The XRD patterns of sodium (Na) and potassium (K) salts of
LA are given in Fig. S2. The XRD pattern of LA was not measured
since this compound is liquid at room temperature.
The polymorphism in the crystal structure of fatty acids is re-
flected in the lateral packing and lamellar stacking of the chains can
be measured by XRD and vibrational spectroscopy.
The long spacing in a crystal structure is related to the length of
the carbon chain and refers to the interplanar distance along the c
axis. This value increases with increasing size of the metal cation.
The short spacing values upper 20ꢁ
2q (wide angle region) calcu-
lated from XRD pattern are related to a lateral packing mode of the
long chain aliphatic chains of the fatty acids.
The short spacing peak near 0.42 nm (strong) and two peaks
between 0.37 and 0.40 nm and between 0.42 and 0.43 nm in the
XRD pattern refer to
) is characterized by a strong peak at near 0.46 nm. This form does
a form and b
0 form respectively. The other form
The OH stretching band is calculated at 3759 cmꢀ1 for monomer
form and 3245 cmꢀ1 and 3155 cmꢀ1 for dimer form of LA. The OH
stretching wavenumbers due to H-bonds in dimer form showed
negative shifts. The high (514 cmꢀ1 and 604 cmꢀ1) shift values
suggest that the presence of strong H-bond in dimer form. We
could not observe the OH stretching (intense) bands of COOH group
in FTIR and Raman spectra. The characteristic OH stretching band
(broad, weak intense) near 2700e2600 cmꢀ1 due to formation of
H-bonds and OH out-of-plane bending band at 935 cmꢀ1 in the
FTIR spectrum of LA indicates the presence of carboxyl group in
form of dimeric structure.
The in-plane OH bending mode appears in the 1440-
1395 cmꢀꢀ1 infrared spectral region. In this spectral region, apart
from OH bending vibration, also CH bending vibrations can be
observed. However, OH bending band is broader than CH bending
bands. The calculated bands at 1477 cmꢀ1 and 1455 cmꢀ1 were
assigned to the in plane OH bending modes in dimer form of LA.
These modes are not pure (Table 3).
(
b
not satisfy criteria
a
or b0 forms [22].
Among the three forms, the b0 polymorph reveals the most
optimal functional physical properties compared with the ther-
modynamically least stable
forms [23].
a forms as well as the more stable b
Three polymorphs of LA confirmed by means DSC and XRD by
Ueno et al. [6] XRD diffraction pattern of low temperature (ꢀ20 ꢁC)
LT form of LA between 18ꢁ and 26ꢁ 2
q showed short-spacing peaks
near 0.43 nm (strong) and 0.39 nm (medium) [6].
The long spacing (39e43 nm as strong) and short spacing values
[0.45 nm (strong) and 0.29 nm (medium)] of sodium salt of LA have
been reported by Jandacek and Broering 1989 [24]. We could not
observe the long spacing peaks due to measured 2q range. We
observed three short spacing peaks at 0.46 nm (strong), 0.43 nm
(strong) and 0.30 nm (medium) in the XRD pattern of NaLA. The
strong peaks are not well resolved. The existence of peak at 0.46 nm
may indicate that
b
form has occurred with b0 form.

74
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
calculated at 987 cmꢀ1 (and at 931 cmꢀ1) in dimer form of LA.
The C]O stretching absorption occurs near 1760 cmꢀ1 in the
case of monomeric form of carboxylic acid in the vapor phase. The
force constant of C]O bond in dimer form is lower than the
monomer form resulting lower frequency of C]O stretching in the
case of dimer form. In carboxylic acid dimer, antisymmetric C]O
stretching mode is infrared active whereas symmetric stretching is
Raman active. The intense band at 1708 cmꢀ1 and weak band at
1699 cmꢀ1 were assigned to the C]O stretching vibrations in the
FTIR and Raman spectra, respectively. This mode predicted at
1812 cmꢀ1 (intense) in monomer and as two bands at 1754 cmꢀ1
(high intensity) and 1707 cmꢀ1 (weak intensity) in dimer.
The CeO stretching mode appears in the spectra of carboxylic
acid dimer near 1210e1320 cmꢀ1. It usually occurs as a double in
the spectra of long chain fatty acids. The bands calculated at
1253 cmꢀ1 and 1249 cmꢀ1 in dimer form of LA were assigned to the
CeO stretching modes. The PED calculation reveals that this a
mixed mode. The 44% is from CeO stretching and 31% contribution
from in plane OH bending. The observed infrared band at
1223 cmꢀ1 was assigned to the CeO stretching vibration.
In the unsaturated acids a band near 3000 cmꢀ1 which corre-
spond to the CH stretching of the double bonds in chain. The band
at 3010 cmꢀ1 in the FTIR spectrum and at 3014 cmꢀ1 in Raman
spectrum of LA was assigned to CH stretching mode of cis-double
bonds [11].
The position of infrared and Raman bands varies depending on
conformation and number of carbon atoms and also number of
ethyl (CH2) groups. The observed bands in the 2978 to 2855 cmꢀ1
spectral region in the FTIR and Raman spectra of LA were assigned
to the CH stretching vibrations of CH3 and CH2 groups (Table 3). The
band at 1458 cmꢀ1 in FTIR spectrum and at 1453 cmꢀ1 in Raman
spectrum of LA was assigned to CH2 bending mode. The observed
infrared band at 1412 cmꢀ1 (1417 cmꢀ1 in Raman) was also assigned
to CH2 bending mode which is adjacent to the COOH group.
We observed very intense band at 1439 cmꢀ1 in Raman spec-
trum. This mode was calculated at 1489 cmꢀ1 (with higher Raman
activity) and was assigned to CH2 scissoring mode which is adjacent
to the C]C bond. The assignments of the other vibrational modes
of methylene groups are presented in Table 3. Raman band at
1304 cmꢀ1 was assigned to CH2 twisting mode.
The C]C stretching band was observed at 1656 cmꢀ1 on the
shoulder of the C]O stretching band in the FTIR whereas very
intense band was observed at 1659 cmꢀ1 in Raman spectrum of LA.
We observed a weak band at 1048 cmꢀ1 in FTIR spectrum and
assigned as CeC stretching mode.
3.3.2. Potassium and sodium salts of linoleic acid
Fig. 5 shows the experimental (a) and simulated gas phase (b)
infrared spectra and Fig. 6 shows the experimental (a) and simu-
lated gas phase (b) Raman spectra of potassium (K) and sodium
(Na) salts of linoleic acid in solid state together with the FTIR and
Raman spectra of linoleic acid in liquid state for the comparison
purpose.
Fig. 4. Comparison of experimental micro-Raman spectrum of linoleic acid (liquid)
with simulated gas phase Raman spectra of monomer and dimer forms of linoleic acid
(LA).
Theoretical spectra of salts were also calculated by DFT/B3LYP
method on 6-311þþG (d,p) basis. The all calculated wavenumbers
and the potential energy distributions of the vibrational modes of K
and Na salts of LA are given in Table S2 as supporting material. The
experimental vibrational bands of salts and their assignments are
presented in Table 4.
The observed bands in the FTIR and Raman spectra of free LA
and its salts show differences in the positions and intensities. The
changes of the bands of the COOH group in salts discussed by
comparison with the free LA as below:
The out-of plane OH bending mode is generally found in the
960e900 cmꢀ1 region (broad with medium intensity). We
observed two bands at 935 cmꢀ1 and 920 cmꢀ1 (broad and medium
intensity) in the FTIR spectrum of LA in this range. The observed
infrared band at 935 cmꢀ1 was assigned to the OH out-of plane-
bending mode of COOH group in dimer form of LA. This band is

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
75
Table 3
Calculated B3LYP/6-311þþG(d,p) harmonic wavenumbers (cmꢀ1), intensities of monomer and dimer forms for conformer I, experimental(IR and Raman) wavenumbers (cmꢀ1
)
and assignments.
Experimental
Monomera
Dimera
Assignments
PEDd (%)
Calculated Scaledb IIR
IR
Calculated
Scaledb
IIR
IR
c
c
c
c
IR
Raman
3759
3133
3106
3083
3078
3069
3057
3046
3037
3028
3018
3609
3008
2982
2960
2955
2946
2935
2924
2916
2907
2897
2891
2889
2883
2875
2872
1740
1683
1668
1484
1475
1470
1465
1459
66.8
25.4
12.9
45.5
76.7
46.2
73.1
49.5
42.5
4.6
51.8
31.1
62.1
0.8
22.9
0.9
16
19
8
16
0
4
7
7
1
16
25
14
17
4
87
7
3
100
0
0
0
4
3245; 3155 3115; 3053 4202.9; 0.0 0; 93
vðOHÞ (100)
3010w
3014m
3133; 3133 3008; 3008 25.6; 21.7
3106; 3106 2982; 2982 7.6; 2.4
3083; 3083 2960; 2960 89.0; 2.9
3077; 3077 2954; 2954 120.1; 0.1
3069; 3069 2946; 2946 75.5; 21.8
17; 19
4; 13
1; 27
0; 0
n
ðCHÞ (89)
ðCHÞ (90)
n
2978m
2959w
2937m
naðCH3Þ (95)
naðCH3Þ (94)
naðCH2Þ (89)
naðCH2Þ (80)
naðCH2Þ (87)
nsðCH2Þ (89)
nsðCH2Þð92Þ
nsðCH3Þ (98)
naðCH2Þð94Þ
naðCH2Þð87Þ
nsðCH2Þ (94)
nsðCH2Þ (89)
nsðCH2Þ (91)
2955w
2925m
4; 3
3056; 3056 2934; 2934 116.9; 48.4 8; 2
2928m
3045; 3045 2923; 2923 65.6; 21.0
3037; 3036 2916; 2916 79.6; 0.0
3027; 3027 2906; 2906 9.8; 0.0
3018; 3018 2897; 2897 75.0; 30.9
3011; 3011 2891; 2891 68.1; 3.6
3010; 3010 2890; 2890 116.1; 6.8
3003; 3003 2883; 2883 0.9; 0.0
2995; 2995 2875; 2875 24.7; 4.3
2992; 2992 2872; 2872 6.8; 7.0
1754; 1707 1719; 1639 715.3; 0.0
1717; 1717 1683; 1683 3.0; 1.9
1702; 1702 1668; 1668 3.8; 1.6
1515; 1515 1485; 1485 28.1; 0.0
1506; 1506 1476; 1476 1.73; 0.0
1500; 1500 1470; 1470 12.5; 3.8
1496; 1496 1466; 1466 10.6; 0.0
1490; 1490 1450; 1450 1.5; 0.1
4; 7
0; 1
0; 46
12; 29
18; 1
12; 17
3; 2
2; 98
5; 5
0; 8
75; 100
0; 0
0; 0
0; 0
2; 5
2895mw 3011
2855w
2855s
3009
3003
2995
2992
1812
1717
1702
1514
1505
1500
1495
1489
1708vs
1656sh
1699vw
1659vs
1636vw
299.8
2.5
2.7
13.5
0.4
7.9
n
n
n
ðC ¼ OÞð84Þ
ðC ¼ CÞð81Þ þ Þ þ ðCHÞð10Þ
d
ðC ¼ CÞð80Þ þ
d
ðCHÞð10Þ
d
d
d
d
ðCH2Þ (89)
ðCH2Þ (85)
ðCH3Þ (93)
ðCH2Þ (88)
1466vw
1458m
1431w
1453m
1439s
5.1
1.6
1
17
0; 0
1; 45
12; 0
0; 0
0; 6
1; 5
1; 0
0; 0
1; 0
1; 2
0; 1
14; 1
6; 2
3; 4
76; 59
Sciss.ðCH2Þ (88)
1477,1455
1447; 1426 0.0, 120.6
d
d
d
d
ðCOHÞð47Þ þ
ðCH2Þ (85)
ðCH2Þ (92)
ðCH2Þ (81)
n
ðC ꢀ OÞð25Þ
1473
1460
1434
1416
1413
1402
1377
1338
1330
1318
1444
1431
1405
1388
1385
1374
1349
1311
1303
1292
1285
1278
1273
1261
1253
1247
1233
1119
1216
1209
1206
1198
1141
1121
1110
1104
1077
1049
1043
1040
1027
996
0.3
15.8
0.4
38.4
2.2
2.1
1.2
3.3
0.1
2.7
1.6
1.9
3.7
0.5
0.6
11.5
2.6
251.5
0.6
2.4
5.1
0.5
1.5
1.6
2.2
1.5
61.4
20.3
8.4
3.5
1.7
4.3
2.7
2
3
4
0
0
0
2
1
5
8
2
64
20
1
0
1
1
3
1
1
1
1
0
2
1470; 1470 1441; 1441 1.1; 1.0
1460; 1456 1431; 1426 119.0; 0.0
1435; 1435 1406; 1406 0.2; 0.0
1408; 1406 1380; 1378 0.0; 0.5
1414; 1414 1386; 1386 3.0; 1.5
1403; 1402 1375; 1374 0.0; 1.4
1379; 1379 1351; 1351 2.1; 0.5
1341; 1341 1314; 1314 5.0; 0.2
1334; 1333 1307; 1306 68.6; 0.3
1320; 1320 1293; 1293 3,6; 2.4
1310; 1309 1284; 1283 4.2; 6.3
1304; 1304 1278; 1278 11.9; 0.1
1412mbr 1417w
u
ðCH2Þ (63)
1378w
1325m
d
ðCH3Þ (91)
u
ðCH2Þ (74)
ðCH2Þð74Þ
u
g
ðCH2Þ (83)
1304m
1294w
tðCH2Þ (75)
ðCH2Þ (72)
g
1284m
1279mw 1311
u
ðCH2Þ (77)
1271m
1304
1299
1287
d
d
d
ðCHÞð70Þ
ðCOHÞ (76)
ðCHÞ (75)
1264m
1287; 1287 1261; 1261 0.5; 0.1
1281; 1281 1255; 1255 1.6; 0.5
1; 2
0; 1
1246mw
1223mw
1256mw 1279
1272
g
ðCH2Þ (77)
d
ðCOHÞð73Þ þ
ðCH2Þ (79)
u
ðCH2Þð21Þ
1258
1260; 1260 1235; 1235 5.5; 2.1
1253; 1249 1228; 1224 146.6; 0.1
1240; 1240 1215; 1215 2.7; 0.1
1232; 1232 1207; 1207 6.0; 0.2
1224; 1223 1200; 1199 54.0; 0.0
1220; 1218 1196; 1194 47.7; 0.0
1164; 1164 1141; 1141 0.0; 0.0
1146; 1146 1123; 1123 0,9; 0.7
1135; 1135 1112; 1112 1.8; 0.3
1128; 1128 1105; 1105 1.7; 0.9
1107; 1107 1085; 1085 8.8; 0.4
1071; 1070 1049; 1048 1.0; 19.6
1064; 1064 1043; 1043 1.3; 0.1
1062; 1062 1041; 1041 2.7; 2.5
1049; 1049 1028; 1028 1.7; 0.1
2; 2
1; 1
0; 0
1; 0
1; 1
0; 1
0; 0
3; 0
2; 0
5; 12
1; 34
2; 0
0; 2
2; 2
1; 7
2; 3
4; 1
0; 0
0; 3
0; 2
5; 9
0; 1
0; 9
2; 6
0; 9
0; 1
0; 0
0; 1
1; 1
4; 7
u
1217w
1198w
1142
1241
1234
1231
1222
1164
1144
1133
1127
1099
1070
1064
1061
1048
1016
1006
n
ðC ꢀ OÞð44Þ þ
d
ðCOHÞð31Þ
g
g
g
g
g
g
g
ðCH2Þ (62)
ðCH2Þ (65)
ðCH2Þ (75)
ðCH2Þ (72)
ðCH2Þð64Þ þ vðC ꢀ CÞð10Þ
ðCH2Þ (62)
1101w
1048w
1
ðCH2Þ (74)
1078w
976w
10
21
1
1
1
6
3
3
r
ðCH3Þð46Þ þ
d
ðCCCÞð20Þ þ
n
ðC ꢀ CÞð18Þ
ðC ꢀ OÞ (12)
ðCHÞð10Þ
d
ðCCCÞð40Þ þ
n
ðC ꢀ CÞð15Þ þ
n
n
n
n
n
ðC ꢀ CÞð46Þþt(CH2) (17)þ
d
ðC ꢀ CÞð77Þ
ðC ꢀ CÞð65Þ
ðC ꢀ CÞð42Þ þ
d
ðCHÞð25Þ
ðCCCÞð17Þ
ðC ꢀ CÞð60Þ þ ðCHÞ (23)
ðCO::OHÞð94Þ
ðCH2Þ (58)
ðC ꢀ CÞð41Þ þ
ðC ꢀ CÞð48Þ þ ðCC ¼ CCÞ (25)
ðCH2Þð34Þ þ ðC ꢀ CÞð14Þ
ðCH2Þ (40) þ (CH) (17)
ðC ꢀ CÞð40Þ þ
1018; 1018 998; 998
1007; 1007 987; 987
4.0; 0.1
3.6; 1.4
188.4; 0.3
0.5; 0.5
16.0; 5.4
8.8; 4.5
9.3; 3.0
1.5; 1.4
0.4; 0.1
5.8; 0.0
1.7; 0.8
4.0; 2.3
3.1; 0.0
9.6; 6.5
34.9; 3.0
g
n
t
ðCHÞð74Þ þ
d
986
g
935mbr
920mbr
987; 931
977; 977
963; 963
946; 946
928; 928
907; 907
894; 894
911; 908
853; 853
834; 834
773; 773
740; 740
735; 735
967; 912
957; 957
944; 944
927; 927
909; 909
889; 889
876; 876
893; 889
836; 836
817; 817
758; 758
725; 725
720; 720
976
963
946
927
906
893
878
851
833
772
739
736
956
944
927
908
888
875
860
834
816
757
724
721
0.4
9.2
6.8
7.9
0.7
0.2
2.7
1.2
6.4
2.3
8.4
19.6
2
2
9
0
1
5
8
1
0
0
1
6
g
n
n
g
d
ðCCCÞð18Þ þ ðCHÞð13Þ
d
n
r
g
r
881vw
843w
771vw
n
n
ðCH3Þ (25)
847w
ðC ꢀ CÞð59Þ þ
n
ðC ꢀ OÞ (18)
g
g
g
ðCH2Þð57Þ þ
ðCH2Þ (67)
ðCH2Þ (70)
r
ðCH3Þð10Þ
723m
r
ðCH2Þ (72)
g
ðCHÞð67Þ þ
r
ðCH2Þð11Þ
(continued on next page)

76
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
Table 3 (continued )
Experimental
Monomera
Dimera
Assignments
PEDd (%)
Calculated Scaledb IIR
IR
Calculated
Scaledb
IIR
IR
c
c
c
c
IR
Raman
733
727
648
636
594
582
718
712
635
623
582
570
7.0
1
1
1
7
1
2
734; 734
729; 729
719; 719
714; 714
4.4; 1.3
58.9; 19.3
0; 0
0; 0
r
ðCH2Þ (79)
36.2
85.6
25.8
0.8
g
ðCHÞ(74)
t
ðCOOHÞð85Þ
664vw
600w
682; 667
593; 593
579; 579
577; 570
554; 552
542; 528
668; 654
581; 581
567; 567
565; 559
543; 541
531; 517
51.3; 0.1
0.5; 0.1
30,9; 18.7
0.0; 0.0
3,3; 0.0
11.8; 0.0
0; 5
1; 1
2; 2
1; 0
1; 6
0; 7
dðOCOÞð67Þ þ
dðCCCÞð29Þ þ
dðCCCÞð45Þ þ
n
n
t
ðC ꢀ OÞð16Þ
ðC ꢀ CÞð13Þ þ
ðCC ¼ CCÞð25Þ
g
ðCH2Þð11Þ
25.9
g
ðC ꢀ COOHÞð52Þ þ
r
ðCH2Þð24Þ
552
541
0.3
5
d
ðCCCÞð51Þ þ
ðCCOÞ (51)þ
g
u
ðCH2Þð11Þ
d
(CH2) (27)
512vvw
480mw
518
516
492
437
508
506
482
428
29.1
17.5
1.2
4
0
2
2
t
ðCCOHÞ (78)
d
d
d
d
d
ðCCOÞ (61)
ðCCCÞð34Þ þ
ðCCCÞ (59)
ðCCOÞ (56)
ðCCCÞð54Þ þ
494; 493
441; 440
425; 415
484; 483
432; 431
417; 407
12.1; 0.6
0.0; 14.1
6.9; 0.1
0; 1
3; 1
0; 2
t
ðCC ¼ CCÞ (19)
1.4
409
401
373
315
302
266
246
238
201
189
169
150
147
130
116
109
104
97
87
65
53
47
42
34
20
17
401
393
366
309
296
261
241
233
197
185
166
147
144
127
114
107
102
95
85
64
52
46
41
33
20
17
5.0
0.3
0.1
1.4
0.4
0.3
0.0
0.2
0.3
1.0
0.2
0.0
0.0
0.1
0.7
0.0
0.2
0.1
0.1
0.0
0.1
0.0
0.2
0.1
0.0
1.3
0.0
0.0
0
5
6
4
8
10
0
11
7
8
20
0
0
0
0
0
0
0
0
0
81
0
0
d
ðCCOÞ (18)
ðCCCÞð27Þ
404; 402
372; 372
327; 325
311; 305
275; 269
243; 243
247; 242
204; 204
156; 156
151; 154
135; 135
122; 114
112; 109
104; 104
102; 98
94; 78
68; 63
59; 51
49; 49
46; 39
74; 69
32; 30
24; 22
16; 10
396; 394
365; 365
320; 319
305; 299
270; 264
238; 238
242; 237
200; 200
153,151
148; 143
132; 132
120; 112
107,110
104; 104
100; 96
92; 77
67; 62
58; 51
48; 48
39,45
73; 68
31; 30
24; 22
16,10
0,1; 0.7
0,3; 0.1
0.0; 32.0
3.0; 0.4
10.1; 0.0
0.3; 0.0
7.8; 0.1
2.5; 0.0
0.1; 0.0
0.1; 0.2
0.5; 0.0
0.1; 0.3
0.1; 0.0
0.2; 0.1
0.4; 0.0
0.2; 0.1
0.8; 0.1
0.0; 1.1
0.1; 0.0
0.0; 0.4
0.0; 0.6
0.0; 0.0
0.0; 0.0
0.1; 0.0
0.0; 0.0
0.0; 0.0
0.0; 0.0
6; 3
0; 11
3; 1
1; 17
1; 14
0; 2
6; 25
2; 8
0; 2
3; 0
1; 3
3; 0
t
ðCCCCÞð32Þ þ
d
d
d
d
d
ðCCCÞð59Þ þ vðC ꢀ CÞð12Þ
ðCCCÞð52Þ þ
ðCCCÞ (57)
t
ðCC ¼ CHÞð11Þ
ðCCCÞð50Þ þ
t
ðCCCHÞð13Þ
t
ðCH3Þ
d
d
d
ðCCCÞð53Þ þ vðC ꢀ CÞð10Þ
ðCCCÞð43Þ þ
ðCCCÞð47Þ
t
ðCCCCÞð15Þ
t
t
t
t
ðCCCCÞð35Þ þ
d
ðCCCÞð25Þ þ vðC ꢀ CÞð10Þ
ðCCCCÞ (69)
ðCCCCÞ (57)
ðCCCCÞ (66)
5; 0
0; 2
6; 2
2; 7
d
ðCCCÞ (59)
t
ðCCCCÞ (68)
dðCCCÞ (62)
0; 15
17; 0
0; 88
36; 12
22; 21
18; 21
95; 113
491; 355
589; 478
177; 1955
9852; 15,536
t
t
t
t
t
t
t
t
t
t
t
ðCCCHÞð58Þ þ
ðHCCHÞ (56)
ðCCCCÞ (73)
ðCCCHÞð51Þ þ
ðCCCHÞð54Þ þ
ðCCCCÞð40Þ þ
ðCCCCÞð72Þ
t
ðCCCCÞð13Þ
t
t
ðOCCCÞð21Þ þ
ðCCCCÞð13Þ þ
t
ðCH3Þð10Þ
t
ðOCCCÞð10Þ
t
ðCCCHÞð25Þ
0
529
1454 20; 15
0
0
ðCCCCÞð62Þ
20,15
10; 6
4; 3
ðCCCOÞð57Þ þ
ðCCCCÞ (74)
ðCCCCÞ (77)
t
ðCCCCÞð21Þ þ ðCCCHÞð12Þ
7
4
7
4
10; 6
4; 3
n; stretching, s; symmetric a; anti-symmetric,
d; in-plane bending; g; out of plane bending, u; wagging, r; rocking, t; twist, t; torsion.
vs: very strong, s: strong, m:medium, w:weak, vw: very weak, br: broad, sh: shoulder.
a
The calculated modes having the same wavenumber and assignment are not listed in the Table.
b
c
Wavenumbers in the ranges from 4000 cmꢀ1 to 1800 cmꢀ1 and lower than 1800 cmꢀ1 are scaled with 0.960 and 0.980, respectively.
IR absorption intensities, IIR (km molꢀ1) and Relative Raman intensities (IR) calculated by Eq. (1) and normalized to 100.
PED; potential energy distribution for vibrational normal modes. PED's lower than 10% are not included.
d
1. The characteristic vibrational mode v(C]O) of the carboxyl
group in LA present at 1708 cmꢀ1 (high intensity) in the infrared
and 1699 cmꢀ1 (weak intensity) in Raman spectra were
disappeared.
(unidentade, bridging bidentate and bidentate chelating). Since the
degree of interaction between the cationic centre (metal ion) and
the coordinated carboxylate group affects electron delocalization
and consequently the stretching frequencies of the carboxylate ion.
The difference between the wavenumbers of the symmetric and
2. In the spectra of salts, new bands appeared.
3. Two new bands at 1560 cmꢀ1 and 1423 cmꢀ1 in the ATR-FTIR
spectrum of NaLA (1561 cmꢀ1, and 1410 cmꢀ1 in the FTIR
spectrum of KLA) which are typical bands for salts of carboxylic
acids (symmetric and asymmetric stretching vibrations of COOꢀ
anions, respectively). The appearance of two absorption bands
shows that NaLA and KLA salts possess an ionized structure.
4. Besides these bands, symmetric and asymmetric in plane and
out-of-plane deformations vibrations of COOꢀ anions were
observed in the FTIR spectra of salts (Table 4).
asymmetric stretching of carboxylate anion [
D
¼
nas(COOꢀ)e
ns(COOꢀ) ] could be used as spectroscopic indication to determine
the mode of the carboxylate binding in a carboxylate compound
[26,27].
There are no crystal structural data for sodium or potassium
salts of linoleic acid. The carboxylates of highly electropositive el-
ements such as sodium and potassium are ionic in nature. Linoleic
acid conformation can be stabilized by the ionic interaction be-
tween Na (or K) cation and carboxylate anion [24].
The symmetric and asymmetric stretching of carboxylate anion
¼ 137 cmꢀ1
There are three coordination modes in the carboxylate com-
plexes depending on the nature of the M-O2CR interaction
of sodium and potassium salts of linoleic acid give
D
and
D
¼ 151 cmꢀ1, respectively. The difference between the

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
77
Fig. 5. The experimental (solid phase) ATR-FTIR (A) and simulated gas phase (B) infrared spectra of potassium (K) and sodium (Na) salts of linoleic acid with experimental ATR-FTIR
spectrum of liquid linoleic acid (LA).
predicted wavenumbers of the symmetric and asymmetric
stretching of carboxylate anion were also calculated as 142 cmꢀ1 for
Na and 155 cmꢀ1 for K salts of LA (Table S2). According to spec-
troscopic criteria [27], the metal -to- oxygen bond in salts has an
(721 cmꢀ1 and near 700 cmꢀ1) infrared bands were observed salts
of LA. The doublet band indicates that existence of inter chain
interaction in solid phase salts.
Finally, it may be concluded, like the other salts of fatty acid,
sodium or potassium salts of LA consist of a nonpolar end (hydro-
phobic, the hydrocarbon chain of the fatty acid) and polar end
(hydrophobic, the ionic carboxylate).
ionic character. The higher value of
D means that the ionic inter-
action between alkali metal and carboxylate group in KLA is higher
than NaLA.
The bands at 924 cmꢀ1 and 916 cmꢀ1 in the FTIR spectra of NaLA
and KLA, respectively characterizes COOꢀ deformation mode of the
ionized carboxylate groups. The band resulting from eCH2 -groups
rocking vibration at 723 cmꢀ1 with strong intensity was observed
as a singlet in the infrared spectrum of liquid LA whereas a doublet
3.4. The Frontier orbital energies and related molecular properties
The highest occupied and lowest unoccupied molecular orbitals
(electron donating HOMO and electron accepting LUMO) are the

78
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
Fig. 6. The experimental (solid phase) (A) and simulated gas phase (B) Raman spectra of potassium (K) and sodium (Na) salts of linoleic acid with experimental micro-Raman
spectrum of liquid linoleic acid (LA).
two most important frontier molecular orbitals [28]. The difference
It is known that electronegativity decrease, reactivity increase
as atomic radius increase in alkali metals. As the energy gap value
decreases, the reactivity of the molecule increases. In our study,
the energy gap value (6.23 eV) of LA is decreasing in NaLA
(4.84 eV) and KLA (4.75 eV) which is consistent with the
increasing alkali metal atomic radius and also ionic radius from Na
to K. This can be explained that KLA would interact more strongly
with LA.
in energy between HOMO and LUMO (
bility index [29,30].
DEgap) is an important sta-
As seen in Fig. 7, the HOMO is mainly localized on double
bonds for LA, whereas LUMO is generally localized on carboxylic
acid group of LA. For the salts, the HOMO is mainly located
around the COOꢀ group and the LUMO is localized on the Na or K
ions.

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
79
Table 4
Experimental wavenumbers (cmꢀ1) and assignments of the absorption bands in the infrared (KBr, ATR) and Raman spectra of linoleic acid (liquid), sodium and potassium, salts
of linoleic acid (solid).
Linoleic acid
Sodium salt of linoleic acid
Potassium salt of linoleic acid
Assignment
ATR
Raman
ATR
KBr
Raman
ATR
KBr
Raman
3388mbr
3010w
3421mbr
3012mw
3371w
3009w
3421mbr
3011mw
n
ðOHÞ
3010w
2955w
2925m
2855w
3014m
2978m
2959w
2937m
2928m
2895mw
2855s
3010m
2957w
3009m
nðCHÞ
naðCH3Þ
2955w
2935w
2921s
2958w
2922s
2957w
2923ms
2853m
2958w
2925s
naðCH3Þ
nsðCH3Þ
naðCH2Þ
naðCH2Þ
naðCH2Þ
2935m
2919s
2883vs
2860ms
2852ms
2882s
2850m
2853m
2853m
2853m
1708vs
1656sh
1699vw
1659vs
1636vw
n
n
n
ðC ¼ OÞ
ðC ¼ CÞ
ðC ¼ CÞ
1657vs
1560w
1648w
1631w
1563vs
1465mw
1430m
1416ms
1659vs
1633w
1562w
1457w
1437s
1625w
1561vs
1465w
1426mw
1410vs
1560vs
1461w
1445m
1423ms
1562vs
1463mw
1446ms
1426m
naðCOOꢀÞ
1458m
1431w
1453m
1439m
d
ðCH2Þ
1438s
1404w
sciss:ðCH2Þ
1420w
nsðCOOꢀÞ
1412mbr
1378w
1417vw
d
d
ðCH2Þ þ
ðCH3Þ
d
ðCOHÞ
1379vw
1346vw
1320vw
1378w
1346w
1320w
1366vw
1378w
1346w
1318w
1347w
1320w
u
ðCH2Þ
ðCH2Þ
1325m
1304m
1294w
1279mw
1271m
1264m
1256mw
1217w
1198w
1326vw
1303mw
1326w
u
tðCH2Þ
1293m
1278w
g
ðCH2Þ
1284m
1279vw
1279w
1278m
1277w
1250w
1278w
u
ðCH2Þ
d
d
ðCHÞ
1264m
1256w
1234w
1265m
ðCHÞ þ
g
ðCH2Þ
1246mw
1223mw
g
ðCH2Þ
1234vw
1198vw
1234w
1197w
n
g
g
ðCOÞ þ
d
ðCOHÞ
1197w
1095w
1064w
ðCH2Þ
ðCH2Þ
1096ms
1064m
1092vw
1062w
1094ms
1078w
1078w
976w
1066wbr
983wbr
r
ðCH3Þ
1048w
n
g
ðCCÞ þ
ðCHÞ
ðCO::OHÞ
u
ðCH2Þ
985w
988w
985w
975mw
916ms
935mbr
920mbr
t
924m
822w
767w
721m
697ms
924
823m
e
925ms
916m
824w
760w
721mw
694ms
916ms
830m
e
n
ðCCÞ þ
d
ðCCCÞ
d
ðCOOꢀÞ
g
ðCOOꢀÞ þ
g
ðCH2Þ
723ms
722m
699m
580w
537m
720m
702ms
589w
535m
477w
r
r
ðCH2Þ
ðCH2Þ
g
ðCOOꢀÞ þ
d
ðCCCÞ
ðCOÞ
d
d
ðCOOꢀÞ þ
n
480mw
ðCCCÞ
n; stretching, s; symmetric a; anti-symmetric,
d
; in-plane bending;
g
; out of plane bending, u; wagging, r; rocking, t; twist sciss. scissoring.
vs: very strong, s: strong, m:medium, w:weak, vw: very weak, br: broad, sh: shoulder.
The hardness and softness are other important parameters and
commonly used as a criterion of chemical reactivity and stability.
4. Conclusions
The hardness (
h
) and softness (
s
) can be estimated from the
In this work the structural and vibrational properties of the most
stable conformer of linoleic acid (LA) as monomer and dimer forms
were investigated by DFT-B3LYP method with 6e311þþG(d,p)
basis sets. Sodium (Na) and potassium (K) salts of linoleic acid were
synthesized. The FTIR and Raman spectra of LA in liquid state and
its salts in solid state were recorded. The fundamental vibrations of
LA and its salts were assigned by means potential energy distri-
butions (PEDs) using VEDA program. In order to gain further insight
into structures HOMO, LUMO energies and other related molecular
properties were also calculated.
Vibrational results confirm that the LA in the liquid state exists
with dimeric structure through hydrogen bonding whereas the
metal-to-oxygen bond in salts in the solid state has an ionic character.
The possible polymorphic forms were determined using the
short spacing values of X-ray diffraction patterns of Na and K salts of
LA.
calculated HOMO and LUMO energies [19]. The smaller values of
hardness imply higher reactivity, which means that a molecule
with a small HOMO-LUMO gap is more reactive and is a softer. In
our study, the hardness value of KLA salt (2.38 eV) is lower than that
of NaLA salt (2.42 eV) and LA (3.12 eV). The calculated electroneg-
ativity of KLA (3.65eV) is also lower than that of the NaLA salt
(3.98 eV) and LA (3.49 eV).
The dipole moment (m) is also important parameter of the
electronic distribution in a molecule, which can be related to the
interaction strength of molecules and metal. Dipole moment value
of LA is 1.80 D is an agreement by comparing the available result in
literature [31].
The dipole moment values of salts (6.03 D for NaLA and 7.63 D
for KLA) are higher than LA. The high value of dipole moment in
salts can therefore form a strong ionic interaction.

80
T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
Fig. 7. Molecular orbital (HOMO and LUMO) diagram and energy gap for LA, NaLA and KLA.
spectroscopy as a utilitarian tool for the routine determination of acidity in
ester-based oils, J. Agric. Food Chem. 63 (2015) 8333e8338, http://dx.doi.org/
10.1021/acs.jafc.5b02738.
In summary, we have shown that complementary experimental
and theoretical methods provided structural insight into structural
and vibrational features of LA and its two alkali salts.
[10] F. Pi, F. Kaneko, M. Iwahashi, M. Suzuki, Y. Ozaki, Solid-state low temperature
middle temperature phase transition of linoleic acid studied by FTIR spec-
troscopy, J. Phys. Chem. B 115 (2011) 6289e6295, http://dx.doi.org/10.1021/
jp200760p.
Conflicts of Interest
[11] N.F.L. Machado, L.A.E.B. De Carvalho, J.C. Otero, M.P.M. Marques, The
autooxidation process in linoleic acid screened by Raman spectroscopy,
J. Raman Spectrosc. 43 (2012) 1991e2000, http://dx.doi.org/10.1002/jrs.4121.
[12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino,
G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,
K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam,
M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski,
R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador,
The authors declare no conflict of interest.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2017.08.065.
References
[1] E. Patterson, R. Wall, G.F. Fitzgerald, R.P. Ross, C. Staton, Health implications of
high dietary omega-6 polyunsaturated fatty acids, J. Nutr. Metabol. (2012)
1e17, http://dx.doi.org/10.1155/2012/539426.
[2] U.N. Das, Essential fatty acids: biochemistry, physiology and pathology, Bio-
technol. J. 1 (2006) 420e439, http://dx.doi.org/10.1002/biot.200600012.
[3] M.I. Gurr, Lipids and nutrition, in: F.B. Gunstone, Frank D. Padley (Eds.), Lipid
Technologies and Applications, Marcel Dekker, New York, 1997, pp. 79e93.
€
J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz,
J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, vol. 34, Gaussian Inc
Wallingford CT, Wallingford CT, 2009, http://dx.doi.org/10.1159/000348293.
[13] A.D. Becke, Density-functional exchange-energy approximation with correct
asymptotic behavior, Phys. Rev. A 38 (1988) 3098, http://dx.doi.org/10.1103/
PhysRevA.38.3098.
€
[4] J. Ernst, J.F. William, S. Sheldrick, Die Strukturen der essentiellen ungesattigten
[14] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-en-
ergy formula into a functional of the electron density, Phys. Rev. B 37 (1988)
785e789, http://dx.doi.org/10.1103/PhysRevB.37.785.
€
€
Fettsauren,Kristallstruktur der Linolsaure sowie Hinweise auf die Kristall-
€ €
strukturen der aLinolensaure und der Arachidonsaure, Zeitschrift Fur Natur-
forschung - Sect. B J. Chem. Sci. 34 (1979) 706e711, http://dx.doi.org/10.1515/
znb-1979-0512.
ꢁ
[15] M.H. Jamroz, Vibrational Energy Distribution Analysis VEDA 4, Warsaw, 2010.
[16] P. Pulay, G. Fogarasi, F. Pang, J.E.B. Ib, Systematic ab initio gradient calculation
of molecular geometries, force constants, and dipole moment derivatives,
J. Am. Chem. Soc. 101 (1979) 2550e2560, http://dx.doi.org/10.1021/
ja00504a009.
[17] N.M. O'Boyle, A.L. Tenderholt, K.M. Langner, cclib: a library for package-
independent computational chemistry algorithms, J. Comput. Chem. 29
(2008) 839e845, http://dx.doi.org/10.1002/jcc.20823.
[18] G. Keresztury, Handbook of Vibrational Spectroscopy, Wiley, 2002.
[19] C.G. Zhan, J.A. Nichols, D.A. Dixon, Ionization potential, electron affinity,
electronegativity, hardness, and electron excitation energy: molecular prop-
erties from density functional theory orbital energies, J. Phys. Chem. A 107
(2003) 4184e4195, http://dx.doi.org/10.1021/jp0225774.
[5] M.R. Rich, Conformational analysis of arachidonic and related fatty acids using
molecular dynamics simulations, Biochim. Biophys. Acta (BBA) - Mol. Cell Res.
1178 (1993) 87e96.
[6] S. Ueno, A. Miyazaki, J. Yano, Y. Furukawa, M. Suzuki, K. Sato, Polymorphism of
linoleic acid (cis-9, cis-12-Octadecadienoic acid) and
cis-12, cis-15-Octadecatrienoic acid), Chem. Phys. Lipids 107 (2000) 169e178,
http://dx.doi.org/10.1016/S0009-3084(00)00169-9.
a-linolenic acid (cis-9,
[7] J. McCarthy, D. Barr, A. Sinclair, Determination of trans fatty acid levels by FTIR
in processed foods in Australia, Asia Pac. J. Clin. Nutr. 17 (2008) 391e396.
[8] J. V Kadamne, C.L. Castrodale, A. Proctor, Measurement of conjugated linoleic
acid (CLA) in CLA-rich potato chips by ATR-FTIR spectroscopy, J. Agric. Food
Chem. 59 (2011) 2190e2196, http://dx.doi.org/10.1021/jf104204e.
[20] W.T.M. Schurink, S. De Jong, Ab initio molecular orbital calculations on the
[9] X. Meng, L. Li, Q. Ye, F. Van De Voort, Fourier transform infrared (FTIR)

T. Gocen et al. / Journal of Molecular Structure 1150 (2017) 68e81
81
conformations of some unsaturated fragments of lipid molecules: 1-Butene,
cis-2-pentene and 1,4-pentadiene, Chem. Phys. Lipids 19 (1977) 313e322,
http://dx.doi.org/10.1016/0009-3084(77)90062-7.
on the electronic system of biologically important ligands: spectroscopic
study of halogenobenzoic acids,halogenobenzoates and 5-halogenouracils,
Inorg. Chim. Acta 358 (2005) 2155e2166, http://dx.doi.org/10.1016/
j.ica.2004.10.035.
[21] J.N. Low, C. Scrimgeour, P. Horton, Elaidic acid ( trans -9-octadecenoic acid),
Acta Crystallogr. Sect. E Struct. Rep. Online 61 (2005) o3730eo3732, http://
dx.doi.org/10.1107/S1600536805033040.
[27] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, John Wiley & Sons, New York, 1997.
[22] C. Himawan, V.M. Starov, A.G.F. Stapley, Thermodynamic and kinetic aspects
of fat crystallization, Adv. Colloid Interface Sci. 122 (2006) 3e33, http://
dx.doi.org/10.1016/j.cis.2006.06.016.
[23] P. D, Fat crystal structure in cream and butter, in: K.S. Nissim Garti (Ed.),
Crystallization and Polymorphism of Fats and Fatty Acids, Marcel Dekker, New
York, 1988, pp. 305e361.
[24] R.J. Jandacek, W.B. Broering, X-Ray diffraction study of sodium soaps of
monounsaturated and polyunsaturated fatty acids, Lipids 24 (1989)
1008e1013, http://dx.doi.org/10.1007/BF02544070.
[25] B.C. Smith, Infrared Spectral Interpretation: a Systematic Approach, CRC Press,
1998.
[28] I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley and
Sons, New York, 1976.
[29] Z. Zhou, R.G. Parr, Activation hardness: new index for describing the orien-
tation of electrophilic aromatic substitution, J. Am. Chem. Soc. 112 (1990)
5720e5724, http://dx.doi.org/10.1021/ja00171a007.
[30] R.G. Pearson, Electronic spectra and chemical reactivity, J. Am. Chem. Soc. 110
(1988) 2092e2097, http://dx.doi.org/10.1021/ja00215a013.
[31] E.B. Ituen, J.E. Asuquo, O.R. Ogede, Computational (DFT) simulations for
comparative prediction of chemical reactivity and stability of linoleic and
stearic acid molecules, Int. J. Comput. Theor. Chem. 2 (2014) 14e19, http://
dx.doi.org/10.11648/j.ijctc.20140202.11.
[26] W. Lewandowski, M. Kalinowska, H. Lewandowska, The influence of halogens