Article pubs.acs.org/JPCA
Polarized Raman and Infrared Spectroscopy and ab Initio Calculation of Palmitic and Stearic Acids in the Bm and C Forms L. F. L. da Silva,† T. Andrade-Filho,‡ P. T. C. Freire,§ J. Mendes Filho,§ J. G. da Silva Filho,§ G. D. Saraiva,∥ S. G. C. Moreira,† and F. F. de Sousa*,‡ †
Faculdade de Física, Universidade Federal do Pará, CEP 66075-110, Belém, PA, Brazil Instituto de Ciências Exatas, Universidade Federal do Sul e Sudeste do Pará, CEP 68505-080, Marabá, PA, Brazil § Departamento de Física, Universidade Federal do Ceará, CEP 60455-970, Fortaleza, CE, Brazil ∥ Faculdade de Educaçaõ Ciências e Letras do Sertão Central, Universidade Estadual do Ceará, CEP 63900-000, Quixadá, CE, Brazil ‡
ABSTRACT: A complete experimental study on the vibrational properties of palmitic and stearic acids crystallized in the Bm and C forms, both belonging to the monoclinic system with the P21/a 5 (C2h ) space group, through polarized Raman and infrared spectroscopy, is reported in this paper. Density functional theory calculations were also performed to assign the normal modes and to help in the interpretation of the experimental data. The different polarizations were compared and their influence on the spectral profiles, in both the lattice and the internal mode regions, was discussed. In general, the Raman and infrared spectra exhibit accentuated differences among the polymorphic forms, which are associated with the different molecular modifications, defined as gauche and all-trans conformations. Insights about interaction among different groups are also furnished.
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INTRODUCTION Characterization of the polymorphic properties of organic systems via spectroscopic techniques is particularly important to better comprehend their potentialities.1−3 As organic materials, fatty acids have undoubted biological importance, being fundamental compounds for cellular metabolism. In terms of their physical and chemical properties, they have a high melting enthalpy that makes them good candidates for application to energy-storage and phase-change materials technologies.4,5 Fatty acids are largely found in nature, being present in all living organisms. In addition, they participate in most fundamental biological mechanisms, including several aspects of metabolism.6−8 In the middle of the last century, many efforts were devoted to the synthesis and characterization of fatty-acid crystals.9−11 One interesting aspect observed in the studies of fatty acids was their complex polymorphism.12,13 They exhibit a high number of crystalline forms with similarity among their structures. These compounds can be grown in different structures depending on a variety of crystallization conditions, e.g., the temperature and the rate of crystallization and the nature of the solvent, as well as the purity of the acid and the number of carbon atoms in the chain.10,11,13 Fatty-acid molecules are composed of a long carbon chain bound to a carboxylic group at one of its ends. As with most chain molecules, they crystallize in a thin plate shape and, in particular, in thin lozenge-shaped crystals with a hydrogen© 2017 American Chemical Society
bounded bimolecular-layer structure stacked perpendicular to the surface plate.14 Several possibilities for molecular arrangement in the stacking process give rise to polymorphism15,16 and polytipism.17 Polymorphism is a solid-state phenomenon related to the different possibilities for molecular packing in the crystallization process. This effect is observed frequently in organic molecules such as proteins, amino acids, and paraffins, among others.18 The study of polymorphism in fatty acids is a complicated task due to the difficulty in identifying the real origin of polymorphic behavior. The diversity of possible crystalline forms depends on the chain length, parity, and number of unsaturated carbons in the alkyl chain.17 With regard to the carbon-chain parity, the forms A1, A2, A3, Asuper, C, Bo/m, and Eo/m were observed for even normal fatty acids and the forms A′, B′ C′, C″, and D′ were observed for odd ones.19−24 The B and E forms exhibit two polytypes, one being monoclinic (P21/c) and the other being orthorhombic (Pbca).25,26 According to the literature, the A forms are characterized by triclinic symmetry and growth morphology as needle-like crystals. These forms show methyl and carboxylic groups coexisting in the same plane.23 The other forms, in general, exhibit the growth morphology of tiny crystalline platelets with a lozenge shape. It is very important to remember that crystals Received: May 2, 2017 Revised: June 4, 2017 Published: June 6, 2017 4830
DOI: 10.1021/acs.jpca.7b04117 J. Phys. Chem. A 2017, 121, 4830−4842
Article
The Journal of Physical Chemistry A
289 K). Single crystals were obtained in the form of tiny transparent platelets in a lozenge shape, and their polymorphic phases were identified by X-ray powder-diffraction (XRPD) measurements. The XRPD patterns were obtained using a Rigaku powder diffractometer with a Bragg−Brentano geometry. Cu Kα radiation (λ = 1.540 56 Å) was used and operated at a voltage of 40 kV and current of 25 mA. The XRD measurements were taken in the 2θ range 3−45° using a stepscan procedure (0.02°) for counting times of 1 s/step. The polarized-Raman spectra were recorded with a Jobin Yvon T6400 spectrometer equipped with a N2-cooled chargecoupled device detection system. The spectral resolution was set to ±2 cm−1. The laser’s excitation was supplied by the 514.5 nm line of an argon-ion laser. An Olympus microscopic lens with a focal distance of 20.5 mm and a numeric aperture of 0.35 was used to focus the laser on the sample surface. The infrared (IR) spectra were measured using the powder in each sample. The IR spectra were measured in the 3000−100 cm−1 range (mid-infrared) in the transmittance mode using a Nicolet 5ZPX FT-IR spectrometer. An accessory-setting A225/ Q platinum, attenuated total reflectance (ATR), and a detectorsetting RT-Dla TGS wide-range MIR-FTIR with an aperture of 6 mm were used at the sample, allowing measurement of up to 100 cm−1 with a spectral resolution of about 4 cm−1 for 120 scans.
with this last morphology are found in the C form, whose symmetry is monoclinic; in this case, the acute angle of the lozenge is 54°. The B and E forms are considered conformational polymorphs, and cannot be differentiated from their morphological characteristics. In this case, the crystals grow as tiny platelets in lozenge shape, having an acute angle of about 75°. Another important point to mention is that these forms show polytypism. Depending on the molecular stacking, monoclinic (B m or E m) or orthorhombic (Bo or Eo) modification is observed.1,27 The monoclinic and orthorhombic phases are best characterized by molecular packing, in such a way that they possess separate planes containing only methyl or carboxylic groups. These characteristics can explain the morphology of growth, taking into account the interaction forces between the adjacent molecules.3,23 Moreover, the B forms have gauche conformation, a change in the orientation of the carboxylic groups with regard to the carbon chain, instead of other forms that show all-trans conformation (for example, the polymorphs of the C form).13 Different polymorphic modifications will present different vibrational properties.13 Thus, by studying such properties, one can obtain rich information about the polymorphic behaviors of these materials because various properties depend crucially on the specific polymorphic species. Among several examples, we cite the property solubility, an important characteristic used by the pharmaceutical industry.23,28 Due to their rich polymorphism, fatty acids have been subject to various structural and vibrational investigations, including measurements of X-ray diffraction (XRD), calorimetric measurements (DSC), Raman spectroscopy, Fourier-transform Raman (FT-Raman) and infrared (FT-IR) spectroscopy, and scanning-electron microscopy.29−38 These different techniques have been used in the past few years to study the polymorphism and phase transitions of these substances in the crystalline phase.28,39−42 In this paper, we report a detailed study of the vibrational properties of single crystals of palmitic (16:0, hexadecanoic acid) and stearic (18:0, octadecanoic acid) acids grown in the Bm and C forms, both with monoclinic symmetry. The experiments were carried out using polarized Raman and infrared absorption at ambient-temperature and -pressure conditions. For each sample, polarized-Raman spectra were obtained between 20 and 3000 cm−1 for two scattering geometries [z(yy)z and z(xx)z]. All infrared-absorption spectra were measured from 100 up to 3000 cm−1, allowing us to evaluate the polymorphism of the crystals. We highlight that the spectral region of the infrared-active modes corresponding to the crystalline lattice vibrations (
