Luanny M. B. Cardosoa,
João G. de Oliveira Netob,
Gilberto D. Saraivac,
Fábio F. Leited,
Alejandro P. Ayalae,
Adenilson O. dos Santosb and
Francisco F. de Sousa*a
aInstitute of Exact and Natural Sciences, Federal University of Para – UFPA, Belém, Pará CEP 66075-110, Brazil. E-mail: ffs@ufpa.br
bCenter for Social Sciences, Health, and Technology, Federal University of Maranhao – UFMA, Imperatriz, Maranhão CEP 65900-410, Brazil
cPhysics Course, State University of Ceara – UECE, Campus FECLESC, Quixadá, Ceará CEP 63900-000, Brazil
dDepartment of Exact and Technological Sciences, Federal University of Amapá – UNIFAP, Macapá, Amapá CEP 68903-419, Brazil
eDepartment of Physics, Federal University of Ceara – UFC, Fortaleza, Ceará CEP 65455-900, Brazil
First published on 21st November 2023
Saturated monocarboxylic fatty acids with long carbon chains are organic compounds widely used in several applied fields, such as energy production, thermal energy storage, antibactericidal, antimicrobial, among others. In this research, a new polymorphic phase of arachidic acid (AA) crystal was synthesized and its structural and vibrational properties were studied by single-crystal X-ray diffraction (XRD) and polarized Raman scattering. The new structure of AA was solved at two different temperature conditions (100 and 300 K). XRD analysis indicated that this polymorph belongs to the monoclinic space group P21/c (C2h5), with four molecules per unit cell (Z = 4). All molecules in the crystal lattice adopt a gauche configuration, exhibiting a R22(8) hydrogen bond pattern. Consequently, this new polymorphic phase, labeled as B form, is a polytype belonging to the monoclinic symmetry, i.e., Bm form. Complementarily, Hirshfeld's surfaces were employed to analyze the intermolecular interactions within the crystal lattice of this polymorph at temperatures of 100 and 300 K. Additionally, density functional theory (DFT) calculations were performed to assign all intramolecular vibration modes related to experimental Raman-active bands, which were properly calculated using a dimer model, considering a pair of AA molecules in the gauche configuration, according to the solved-crystal structure.
Chemically, FAs are commonly described as organic substances duly consisting of a carbon chain (skeletal) attached to a methyl group (CH3) and a carboxylic group (COOH), also often found in the form of esters.12 In addition, in the crystalline state, these compounds are duly arranged in crystalline structures forming dimers through intermolecular interactions, mainly hydrogen bonds (H–O⋯H), which have an essential role in the formation of saturated FA crystals. The crystal structure of monocarboxylic FAs is formed by packed molecules via hydrogen bonds with dimers predominantly adopting an all-trans or a gauche configuration that generally allow three-dimensionally ordering from a typical R22(8) hydrogen bond pattern for monocarboxylic FAs.13,14 These molecular configurations are responsible for the polymorphism phenomenon; so, FAs can crystallize in more than one crystal structure.15–18
According to the literature, molecular systems formed by monocarboxylic acids with an even number of carbon atoms, such as capric acid (C10H20O2),19 lauric acid (C12H24O2),20 myristic acid (C14H28O2),21 palmitic acid (C16H32O2),22 stearic acid (C18H36O2),23 arachidic acid (C20H40O2),24 behenic acid (C22H44O2),25 and lignoceric acid (C24H48O2),26 can be crystallized in different polymorphic phases: A1, A2, A3, Asuper, Bo/m, C, and Eo/m, depending on the conditions of temperature, pressure, solvent, among others.27,28 Amidst these polymorphic phases, the C form is the most stable at high temperatures. Moreover, the A phases have a triclinic system, the Bo and Eo structures are found in the orthorhombic symmetry, and the Bm/Em and C forms belong to the monoclinic system. It is important to notice that the indices “o” and “m” are associated with two polytypes belonging to the orthorhombic and monoclinic structures, respectively.29 As mentioned above, the dimers formed through O–H⋯OC hydrogen bonds of each pair of molecules influence the polymorphism at saturated monocarboxylic FAs. Such a phenomenon is duly associated with the molecular configurations into the crystal lattice. Thereby, the polymorphic phases named by A and B exhibit a gauche configuration while the E and C forms present an all-trans configuration.15,17,30
Arachidic acid (AA), also known as eicosanoic acid, is a saturated fatty acid with the chemical formula C20H40O2. It is classified as a long-chain fatty acid due to its skeletal structure containing more than 13 carbon atoms; it can be found in the human organism from the plasma phospholipid; it is an almost hydrophobic compound, i.e., it is practically insoluble in water.24 Among various saturated fatty acids, AA is present in lower quantities in cupuaçu butter, cocoa butter, perilla oil, peanut oil, and corn oil. It also constitutes approximately 7.0% of the fats found in Durio graveolens fruit.31,32 It is also found naturally in cannabis, fish, and other plant oils. In the area of materials development, AA is used to produce detergents, lubricants, and photographic materials.33
Due to its high melting point, differently of unsaturated FAs, the AA is highly stable from a physicochemical point of view, since the carbon chain does not have double bonds.34 In a study reported by B. Rogers et al.,33 the influence of temperature on the composition and thermo-oxidative stability of AA in Arabica coffee oil was investigated. The analyzed samples showed that the thermal treatment does not significantly affect the grains.34 Besides that, this FA can be used in processes involving high temperatures, such as non-ambient conditions employed in the food industries.35 However, even in the face of these concepts and properties, AA was superficially investigated in the literature, with a lack of results associated with polymorphic variations, structural and spectroscopic characterizations.
In this paper, we report the crystal growth of a new AA polymorph (Bm form) by slow evaporation method in acetone and its structure determination through single-crystal X-ray diffraction (XRD) at 100 and 300 K. Complementarily, we performed a computational study using crystal voids and Hirshfeld's surfaces to describe the intermolecular interactions present in this new polymorphic phase. In addition, spectroscopic analyses using polarized Raman spectroscopy at two scattering geometries were performed to evaluate the crystalline system's inter- and intramolecular vibration modes. Furthermore, the correct assignment of intramolecular Raman modes was described through quantum chemical calculation based on density functional theory (DFT) using a dimer system as model, which is formed by two molecules of AA in the gauche configuration.
I = f(ν0 − νi)4·νi(−1)·Bi(−1)·Si |
(1) |
The AA crystals grown in this study have a lozenge-like shape, resembling a small transparent plate. They typically exhibit the B-form crystal structure with monoclinic symmetry, specifically the polytype Bm. Fig. 1 exhibits the simulated morphology from the CIF file (code 2177009 for 300 K) using the Mercury program, showing that the crystal has grown as platelets with a lozenge-like prismatic shape. Fig. 1 shows the crystal terminal planes corresponding to the main crystallographic planes. It is worth noting that the acute angles are visibly distinguished. According to the literature,59 the two internal acute angles are near 75° (smaller acute angle between the two angles) and 105° (larger acute angle between the two angles).
Fig. 1 Growth habit of AA crystal showing that the crystal has grown in the Bm form and it has a lozenge-like morphology. |
Table 1 presents the structural data associated with the final atomic parameters determined for this sample. From single-crystal XRD results' analysis, the polymorphic phase of AA crystal belongs to the polytype Bm form, corresponding to monoclinic symmetry with P21/c (C2h5) space group, with the AA molecules in the crystal lattice adopting a gauche configuration, as shown in Fig. 2. Additionally, it was verified that the crystalline structure remains stable down to 100 K. However, the unit cell parameters (a, b, and c) undergo a contraction effect with the cooling and, consequently, the volume of unit cell decreases from 2004.5(2) to 1912.0(6) Å3.
Temperature | 100 K | 300 K |
Empirical formula | C20H40O2 | C20H40O2 |
Formula weight | 304.47 g mol−1 | 304.47 g mol−1 |
Crystal system | Monoclinic | Monoclinic |
Space group | P21/c (C2h5) | P21/c (C2h5) |
a (Å) | 48.445(7) | 48.478(19) |
b (Å) | 7.1499(10) | 7.400(3) |
c (Å) | 5.5267(8) | 5.597(2) |
β (°) | 92.785(5) | 93.301(9) |
V (Å3) | 1912.0(6) | 2004.5(2) |
Z | 4 | 4 |
Radiation | MoKα (λ = 0.71073 Å) | MoKα (λ = 0.71073 Å) |
R-factor (%) | 6.84 | 9.50 |
Fig. 2a shows the AA molecule in the gauche configuration at 300 K, viewed along the c-axis, with all carbon and oxygen atoms duly numbered. Additionally, it illustrates the presence of a saturated carbon chain and a carboxylic group in the monomer, which is connected to carbon chain via the C1 atom. Fig. 2b shows the arrangement of AA molecular units in the Bm form, where the dimers form alternating monolayers composed of carboxylic and methyl terminal groups along the [001] direction.
In addition, in the crystal lattice, the monomers interact through the hydrogen bonds between the carboxylic groups (O–H⋯O/O⋯H–O) with a distance of 6.981 Å and an angle of 140.94° at 300 K, which change to 6.845 Å and 146.09°, respectively, when the temperature reaches 100 K. This structural phenomenon can be also interpreted as tri-axial contraction undergone by unit cell at 100 K, which leads to a compaction of the AA units within the crystal lattice. As expected, the cryogenic effect induces a shorting in the lengths of the intermolecular bonds, promoting modifications such that the two molecules belonging to a dimer approach each other.
In order to detail the atomic packing of AA molecules in the unit cell, crystal void surfaces were generated and analyzed at both temperatures of 100 and 300 K. Fig. 3a–c shows the unit cell projected in the ab plane without void surfaces and with void centers in the unit cell at 100 K (Fig. 3b) and 300 K (Fig. 3c). The void spaces are described through gray electronic density isosurfaces, where the free volume in the unit cells, the surface areas, the percentages of voids present in the lattice, and the globularity index were calculated.
Fig. 3 (a) Crystal voids viewed along the ab plane in the unit cell of AA in the Bm form without presence of isosurfaces. (b) At 100 K and (c) 300 K with presence of isosurfaces. |
According to our computational calculations, at 100 K the Bm form of AA has a void volume of 64.60 Å3, equivalent to 3.38% void centers, and a void surface area of 373.63 Å2, as pictured in Fig. 3b, while at 300 K (see Fig. 3c) higher values are observed for these parameters, such as a percentage of 6.89% of free space, corresponding to a volume of 138.17 Å3, and a surface area of 719.68 Å2. This change in unit cell volume can be clarified by different temperatures measured. In both cases, there are four molecules per unit cell, but the AA molecules are closer together at 100 K owing to volume compaction, so there are fewer void centers in the crystal lattice. This also justifies the disparity between the surface area values, as at 300 K there is a greater free volume; thus, the electronic isosurfaces occupy a greater area.
Another important descriptor was calculated from the crystal voids: the globularity (G).60 This index property describes the measurement of the degree to which the isosurface area differs from that of a perfect sphere with the same volume, being equivalent to 1.0 as it is thoroughly spherical, and progressively less than a unit as the isosurface becomes more structured. For the AA polymorph, values of G = 0.208 and G = 0.180 were obtained for temperatures of 100 and 300 K, respectively, suggesting that at lower temperatures, the isosurfaces have more rounded shapes than those noticed at 300 K.
Fig. 4 (a) Hirshfeld surface of AA crystal in the Bm form at 300 K given in terms of dnorm, (b) de, and (c) di; (d) shape index; (e) curvedness of crystal structure. |
Complementarily, the distances from the Hirshfeld surface to the nearest atoms outside (de) and inside (di) the surface, as displayed in Fig. 4b and c, are given in terms of de and di, respectively. On the de surface shown in Fig. 4b, the red region around O atom indicates the receptor site of intermolecular contacts, while on the di surface displayed in Fig. 4c, the red area presented under the OH group reveals a hydrogen donor zone. Furthermore, in the main chain that appears on the Hirshfeld surface mapped in terms of de and di, slightly yellowish portions are verified, corresponding to the recipient and donor contacts of the CH2 groups, respectively, which are established between the neighboring units. Both areas complement each other for the formation of hydrogen bonds and structural stabilization of the packing of AA units into the unit cells.
Fig. 4d and e shows the shape index and curvedness of the AA molecule, which are unique surfaces that provide information about the topology of the contacts. The shape index is particularly sensitive to even subtle changes in surface shape. In this representation, warm-colored regions indicate concave locations, while cool-colored regions are associated with convex areas. These two surfaces complement each other, identifying the points where two AA surfaces come into contact to facilitate intermolecular interactions. As seen in Fig. 4d, the concave regions of AA molecule are found between the C–C and CO units, while the convex areas predominate over the CH3, CH2, and C–OH groups; thus, these zones form the structural lattice through the hydrogen bonds performed between dimers.
The curvedness surface is a measurement that details the crystal shape. In Fig. 4e, the flat areas, depicted in green, correspond to the regions with low curvedness values. On the other hand, the areas delineated by blue edges are indicative of high curvedness values. Furthermore, these edges serve as dividers, demarcating the surface into distinct parts and indicating the locations where neighboring molecules interact with each other. This observation aligns with the findings of McKinnon et al.,45 who described the core of n-alkanes. The close approach of identical atoms (H⋯H) produces flat areas in the Hirshfeld surface clearly evidenced in the curvedness index mapping. In addition, the flat areas enhance the sensibility of the shape index to minor changes in the surface as the set of blue spots characteristic of the approach of CH2 groups from adjacent molecules.
A computational study based on Hirshfeld surfaces was also performed to calculate the percentage of contacts present in AA units. Fig. 5a–e shows the cumulative and stratified 2D-fingerprint plots from the intermolecular interactions between the chemical species in the crystalline system. The same pattern is obtained for both temperatures of 100 and 300 K, where four different types of contacts are revealed: H⋯H, H⋯O/O⋯H, O⋯O, and H⋯C/C⋯H, which correspond to the following intermolecular interactions: induced dipole, hydrogen bonds, induced dipole, and dipole–dipole, respectively. These interactions contribute for the structural stability of AA crystal. In addition, the fraction of colored dots (red and yellow) highlighted in Fig. 5b characterizes specific close contacts (H⋯H) and the blue and green dots represent distant contacts (O⋯O and H⋯C/C⋯H). However, as observed in Fig. 5f, slight variations are verified in relation to the percentage contributions of the contacts in the two temperatures measured herein. At 100 K, the H⋯O/O⋯H hydrogen bonds are favored, while the H⋯H induced-dipole interactions suffer a little reduction, owing to the contraction effect in the unit cell.
Furthermore, the long and thin peaks in the lower regions of de–di values in the fingerprint plots represent strong and greater contacts on the surfaces, as shown in Fig. 5c. Indeed, hydrogen bonds are predominant intermolecular forces in the crystalline lattice of FAs, even though they are not the main contact in terms of percentages. On the other hand, hydrogen bonds in terms of percentage exhibit a more significant amount in other compounds based on organic molecules such as amino acids and their complexes.61,62
Here, it is done a comparison between experimental Raman spectra obtained in the y(xx)y (red spectrum) and y(zz)y (blue spectrum) scattering geometries and calculated Raman spectra in both temperatures of 300 K (black spectrum) and 100 K (pink spectrum). The dimer model used in our DFT calculations is provided in Fig. S2.† For a better discussion, the Raman spectra are divided into four distinct wavenumber regions: 30–690 cm−1; 700–980 cm−1; 1100–1690 cm−1; and 2780–3000 cm−1, as shown in Fig. 6 and 7.
Fig. 6a exhibits the Raman spectra containing inter- and intramolecular vibration modes in the region of 160–690 cm−1, which includes an inset with experimental Raman spectra between 40 and 155 cm−1 in both y(xx)y and y(zz)y scattering geometries. All bands that appeared in this wavenumber region (40–155 cm−1) are associated with lattice modes, which can give important information to understand the intermolecular interactions, thermodynamic stability, and physicochemical properties of organic systems, as well as polymorphism in materials in the crystalline state.15,16,64–66 According to the literature,15 these vibration modes have coupling with hydrogen bonds belonging to dimers responsible for the crystalline lattice. The inset of Fig. 6a shows little differences in the Raman spectra profile and relative intensity in the lattice vibration bands due to the polarization effect. In particular, the bands centered at 112, 127 and 133 cm−1 were not observed at y(xx)y geometry.
According to literature,15,29 the set of bands that appears within the 200–1000 cm−1 spectral range (Fig. 6a and b) is intrinsically characteristic of the Bm form of saturated FAs. This spectral region has predominant contribution from deformations of CH2 units. These vibrations are influenced by monoclinic angle β of the unit cell, as well as molecular configuration strongly impacts in the positions and intensities of intramolecular Raman bands in the 200–1000 cm−1 range. Indeed, when we compared the number and profiles of bands from the Bm-Raman spectrum with the Raman spectrum of the C form of other saturated FAs, one can notice that such an C polymorphic phase has a spectrum with number of bands minor than the Bm form.15,29 This suggests that the molecular configuration of dimers has an essential role on alterations in the Raman bands related to the CH2 groups, consequently, the Raman spectra of both polymorphic phases are well distinct. Table 2 presents the Raman modes of the Bm form of AA, both experimental and calculated at T = 300 K.
ωy(zz)y | ωy(xx)y | ωcal. | Assignments (VMARD)a |
---|---|---|---|
a Nomenclature: τ-torsion; δ-bending; sc-scissoring; ν-stretching; γ-out-of-plane deformation. | |||
2953 | 2953 | 2945 | ν(C121H122) + ν(C121H123) + ν(C121H124) |
2940 | 2940 | 2939 | ν(C121H123) + ν(C121H124) |
2927 | 2927 | 2923 | ν(C64H72) |
2900 | 2900 | 2886 | ν(C121H124) + ν(C121H123) + ν(C121H122) |
2883 | 2882 | 2867 | ν(C92H98) + ν(C92H99) + ν(C92H100) |
2848 | 2848 | 2853 | ν(C77H85) |
1647 | 1647 | 1679 | ν(C2O10) + ν(C71O80) |
1503 | 1503 | 1490 | sc(H16C11H17) + sc(H6C15H25) + sc(H42C32H43) |
1479 | 1478 | 1476 | γ(H4C3H5C12) |
1466 | 1465 | 1463 | τ(C115C116C121H124) |
1440 | 1440 | 1429 | τ(C37C46C55H63) |
1416 | 1416 | 1391 | sc(C87C92H100) + sc(C87C92H98) + sc(C87C92H99) |
1370 | 1368 | 1368 | δ(C55C64H72) |
1294 | 1296 | 1293 | δ(H89C84C91) |
1235 | — | 1233 | δ(C109C112H114) |
1217 | — | 1214 | τ(C46C55C64C71) |
1200 | 1201 | 1196 | τ(H9O1C2C7) |
1184 | 1184 | 1181 | τ(C112C115C116H119) |
1129 | 1129 | 1118 | δ(C109C112C115) |
1105 | 1105 | 1093 | ν(C3C12) |
1068 | 1067 | 1049 | ν(C55C64) |
1063 | 1040 | 1040 | ν(C3C12) |
— | 993 | 996 | ν(C31C40) |
983 | 983 | 983 | ν(C37C46) |
959 | 959 | 953 | ν(C84C91) |
929 | 929 | 925 | ν(C64C71) |
898 | 898 | 892 | δ(C55C64H73) |
892 | 892 | 878 | ν(C77C87) + ν(C87C92) + δ(C87C92H100) |
— | 873 | 865 | τ(H9O1C2O10) |
864 | 864 | 855 | ν(C14C23) |
843 | 843 | 835 | ν(C64C71) |
815 | 815 | 808 | ν(C71O81) |
786 | — | 780 | τ(C32C41C50C59) |
766 | — | 758 | τ(C3C12C20C28) |
631 | 632 | 642 | δ(O1C2O10) + δ(C64C71O80) |
575 | 575 | 580 | δ(C46C55C64) |
557 | 557 | 557 | δ(C64C71O81) |
— | 511 | 515 | δ(C58C67C76) |
— | 480 | 480 | δ(C23C31C40) |
463 | — | 465 | δ(O1C2C7) |
402 | 402 | 401 | δ(C109C112C115) |
329 | 329 | 328 | δ(O1C2C7) |
— | 280 | 285 | δ(O1C2O10) |
229 | 229 | 226 | τ(O10C2C7H13) |
186 | 186 | 186 | δ(C55C64H73) |
— | 166 | 157 | Lattice |
132 | 134 | 135 | Lattice |
126 | — | 128 | Lattice |
111 | — | 113 | Lattice |
— | 108 | 105 | Lattice |
88 | 87 | 84 | Lattice |
76 | 71 | 60 | Lattice |
59 | 58 | 60 | Lattice |
52 | 52 | 53 | Lattice |
As mentioned above, the Raman vibrational calculations were also performed for the AA structure in the Bm form at 100 K; however, significant changes between the Raman spectra profile and peak positions were not observed when the calculated Raman bands were compared to the experimental bands, indicating that inter- and intramolecular vibration modes of the Bm form of the crystal are stable under lower-temperature conditions.
According to our DFT calculations, intramolecular vibration modes in the wavenumber region between 300 and 690 cm−1 (Fig. 6a) appear mainly related to motions from bending of skeletal chain. One can notice that the calculated modes' positions exhibit a good agreement with experimental modes in both y(xx)y and y(zz)y scattering geometries (Table 2).
Comparing the vibration bands' profile in the 708–860 cm−1 range (Fig. 6b) between experimental Raman spectra in both scattering geometries, three bands located at about 750, 766 and 785 cm−1 (marked by *) in the y(zz)y geometry have higher intensity due to polarization effect. Interestingly, a similar phenomenon also occurs in the spectral profile of the polarized-Raman spectra of the C form of stearic- and palmitic-acid crystals within the same wavenumber region.15 Furthermore, when we compare the two scattering geometries, it is possible to observe differences in the relative intensity of bands related to the vibration modes at about 843, 864, 892, 898, 929 and 959 cm−1 as shown in the inset of Fig. 6b.
The Raman vibration modes located within the 800–1100 cm−1 region are mostly related to the stretching vibrations of C–C units, which are very sensitive to carbon chain modifications. The literature has shown the relation between gauche C–C stretching peak position and chain length for saturated FAs.15,29 Indeed, the gauche configuration influences the C–C bonds around COOH groups, which directly connect with hydrogen bonds leading in the Raman spectrum. It is worth noting that in the 30–1000 cm−1 spectral range significant differences could not be observed between the calculated Raman spectra profile at temperatures of 100 K and 300 K, suggesting a good structural stability for the AA crystal under low-temperature conditions.
Fig. 7a shows a characteristic peak located nearly 1105 cm−1 corresponding to gauche C–C stretching for a C20 chain length, in agreement with literature.67 The spectral region of 1100–1400 cm−1 is mainly composed of vibration modes of rocking/twisting types (see Table 2) and exhibits a high number of modes due to the chain configuration of Bm form. The shape of CH2-deformations’ modes of the scissoring type within 1400–1500 cm−1 spectral region is the only difference observed between the calculated Raman spectra at 300 K and 100 K. Another important point can be noticed in the low intensity broad bands nearly 1645 and 1646 cm−1 corresponding to the CO stretching vibration which was well corroborated by DFT calculations.
Fig. 7b shows the high-wavenumber region which is mainly related to the large bands associated with stretching vibrations due to the CH, CH2, and CH3 units. This region presents a good agreement between experimental Raman spectra and calculated Raman spectra in both 300 K and 100 K temperatures. It is worth noting that in the 2800–3000 cm−1 range no difference were evidenced between calculated Raman spectra at different temperatures, which supports the good stability of AA chain under low temperature.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2177008 and 2177009. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ra05388a |
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