Phase transitions of L-histidinium hydrogen oxalate crystal under high pressures investigated by Raman spectroscopy.

L-histidinium hydrogen oxalate (L-HisH)(HC2O4) crystal is formed from amino acid. L-histidine with oxalic acid whose vibrational high pressures behavior have not yet been investigated in the literature. Here we synthesized (L-HisH)(HC2O4) crystal by slow solvent evaporation method in a 1:1 ratio of L-histidine and oxalic acid. In addition, a vibrational study of (L-HisH)(HC2O4) crystal as a function of pressure was performed via Raman spectroscopy in the pressure range of 0.0-7.3 GPa. From analysis of the behavior of the bands within 1.5-2.8 GPa, characterized by the disappearance of lattice modes, the occurrence of a conformational phase transition was noted. A second phase transition, now from structural type, close to 5.1 GPa was observed due to the incidence of considerable changes in lattice and internal modes, mainly in vibrational modes related to imidazole ring motions.

Pressure-dependence Raman spectroscopy and the lattice dynamic calculations of Bi2(MoO4)3 crystal.

This work reports a pressure-dependent Raman spectroscopic study and the theoretical lattice dynamics calculations of a Bi2(MoO4)3 crystal. The lattice dynamics calculations were performed, based on a rigid ion model, to understand the vibrational properties of the Bi2(MoO4)3 system and to assign the experimental Raman modes under ambient conditions. The calculated vibrational properties were helpful to support pressure-dependent Raman results, including eventual structural changes induced by pressure changes. Raman spectra were measured in the spectral region between 20 and 1000 cm-1 and the evolution of the pressures values was recorded in the range of 0.1-14.7 GPa. Pressure-dependent Raman spectra showed changes observed at 2.6, 4.9 and 9.2 GPa, these changes being associated with structural phase transformations. Finally, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were performed to infer the critical pressure of phase transformations undergone by the Bi2(MoO4)3 crystal.

Phase changes of tris(glycinato)chromium(III) monohydrate crystal systematically studied by thermal analyses, XRPD, FTIR, and Raman combined with ab initio calculations.

Tris(glycinato)chromium(III) monohydrate [Cr(C2H4NO2)3·H2O] crystals were grown through the slow solvent evaporation method. The crystals were studied by Fourier transform infrared (FTIR) and Raman spectroscopy at room temperature. The assignments of vibration modes were performed using the Density Functional Theory (DFT). Thermal analyses (TGA, DTA, and DSC), X-ray diffraction (XRD), and Raman were used to study the phase changes on the crystals under high- and low-temperature conditions. Temperature-dependent XRPD measurements were carried out in the interval of 473-12 K. Several changes were observed in the patterns, like the appearance of new peaks and the disappearance of peaks occurring within 373-393 K due to water loss. In addition, the Raman measurements were performed in the 423-10 K interval. Several changes on the inter and intramolecular vibration bands during the cooling, such as decreasing bands' intensities, the appearance of vibration modes, and discontinuities on the modes' behavior, were observed. These spectral modifications occurred at about 370 K and within 120-220 K, thus, confirming that the crystals undergo two phase changes, one being structural and the other one conformational, respectively, at high- temperature and low-temperature conditions. Finally, thermal investigations corroborated the structural and vibrational results under high temperatures.

Structural, thermal, electronic, vibrational, magnetic, and cytotoxic properties of chloro(glycinato-N,O)(1,10-phenanthroline-N,N')­copper(II) trihydrate coordination complex.

Chloro(glycinato-N,O)(1,10-phenanthroline-N,N')­copper(II) trihydrate complex was synthesized through the slow evaporation method. The crystal's structural, thermal, magnetic, and vibrational properties were obtained by X-ray powder diffraction (XRPD), thermal analyses, magnetization, Raman, and Fourier-transform infrared (FT-IR) spectroscopy. XRPD results showed that the crystalline complex belongs to a monoclinic system (P21/n). Thermal analyses revealed that around 333 K, the material undergoes a thermodynamically irreversible process. Magnetic data showed a paramagnetic behavior with weak ferromagnetic interactions. Moreover, all the Raman- and infrared-active bands were assigned from computational calculations based on the density functional theory (DFT) to analyze intra-molecular vibrational modes. In addition, the cytotoxic assay on colorectal cancer cells was performed to evaluate the antitumor activity of this ternary compound. Therefore, the antineoplastic activity of [Cu(1,10-phenanthroline)(glycine)Cl]•3H2O complex in HCT-116 cells was confirmed, showing a potent cytotoxic effect.

A temperature-dependent Raman scattering and X-ray diffraction study of K2Mo2O7·H2O and ab initio calculations of K2Mo2O7.

This study reports a temperature-dependent Raman scattering and X-ray diffraction study of K2Mo2O7·H2O. The high-temperature Raman scattering analysis shows that the material remains structurally stable, with triclinic symmetry, in a temperature range from 300 to 413 K and undergoes a structural phase transition between 413 and 418 K. This phase transition is most likely connected with the dehydration process of K2Mo2O7·H2O. The temperature-dependent X-ray diffraction patterns are measured from 30 to 573 K. The results show that the discovered phase transition occurs between 419 and 433 K, in good agreement with the Raman scattering results. According to the Raman data, with increasing temperature, the dehydrated crystal of K2Mo2O7 undergoes a new phase transformation at 603 K and melts at ~843 K. Principal component and hierarchical cluster analyses are performed based on the treatment of the raw spectral data to infer the phase transformations occurring in the material. Assignments of the Raman modes for the K2Mo2O7 system at ambient conditions are studied through first-principles calculations based on density functional perturbation theory. These calculations are applied to understand the electronic properties, including the band structure and the associated projected density of states, of K2Mo2O7 under the local density approximation.

Pressure-induced phase transition in Glycinium maleate crystal.

The multicomponent glycinium maleate single crystal was grown by the slow evaporation method. The crystal was submitted to pressures ranging from 1 atm to 5.6 GPa and Raman spectroscopy was used as a spectroscopic probe. The modifications of relative intensity bands related to the lattice modes at 0.3 GPa were associated with rearrangements of hydrogen bonds. Moreover, between 1.7 and 4.8 GPa the Raman results indicate that the crystal experience a long structural phase transition, which was confirmed by PCA analysis. DFT calculations gave us more precision in the assignments of modes. The behavior of the internal modes under pressure showed that the maleic acid molecule undergoes greater modifications than glycine amino acid. All observed modifications were reversible when the pressure was released.

Computational structural, electronic and optical properties of the palmitic acid in its C form.

In this work, we report a theoretical study of the structural, electronic, and optical properties of palmitic acid crystal in its C form under DFT calculations level. Palmitic acid is a fatty acid that constitutes the large majority of vegetable oils with recognized potential applications in medicine, pharmaceuticals, cosmetics technology, foods, and fuel. As a main result, we have found that the electronic bandstructure reveals an indirect gap given by 3.713 eV (E→B andE→Γ), as a main bandgap, while the secondary bandgaps found were 4.175 eV (γ1→Γ) and 4.172 eV (γ2→B). It behaves like a wide bandgap semiconductor, which points to potential applications in optoelectronic devices.

Raman spectroscopy of captopril crystals under low-temperature conditions.

The polymorphism is a characteristic of several active principles, and can affect the bioavailability of a drug. Among the drugs used in the treatment of heart diseases, captopril is one of the most widely used in the world. Despite the knowledge of vibrational properties of captopril under high temperature and under high pressure, a lack of information impedes the understanding of the substance in the crystal form at low temperatures. In this research, we investigated the vibrational properties of captopril crystals under cryogenic conditions in the 300-8 K interval using Raman spectroscopy. By observing the behavior of the inter- and intra-molecular vibrations it was possible to infer that the captopril molecules suffered a rearranging into the unit cell due slight orientational changes mainly involving CH⋯O hydrogen bonds. The phenomenon occurs in a large temperature range. However, the observed changes do not suggest the occurrence of a structural phase transition and the Raman spectra indicate that the trans conformation is recorded down to the lowest temperature available in the experiments.

Effect of Fe (III) on L-asparagine monohydrate investigated under low- and high-temperature conditions.

Raman spectra of Fe-doped L-asparagine monohydrate (LAM:Fe) crystal were studied under several temperatures varying from 17 to 490 K. The effect of Fe (III) ion on the stability of the crystal in changing temperature through the vibrational spectra was discussed. The behavior of inter and intra-molecular vibration modes has indicated two phase transitions and an amorphous transformation. These effects were also clarified by X-ray powder diffraction measurements which corroborate very well the Raman data. In addition, we have determinated the lattice parameters of all phases and verified that under low temperature conditions the crystal undergoes a conformational transition whereas under high temperatures its structure transforms from the orthorhombic (P212121-space group) to the monoclinic (P21-space group) symmetry and, after this process, it goes to an amorphous phase due to the start of the decomposition. Finally, differential scanning calorimetry analysis was utilized as complementary technique to investigate the structural stability of LAM:Fe and results are in a good agreement with the Raman and the X-ray diffraction data.

Lattice dynamics calculations and high-pressure Raman spectra of the ZnMoO4.

We report here the analysis of vibrational properties of the ZnMoO4 by using theoretical and experimental approaches, well as results of high pressure experiments in this system. The analysis of the lattice dynamics calculations through the classical rigid ion model, was applied to determine the mode assignment in the triclinic phase of the ZnMoO4. Additionally, the experimental high-pressure Raman spectra of the ZnMoO4 were carried out from 0 GPa up to 6.83 GPa to shed light on the structural stability of this system. The pressure-dependent studies showed that this crystal undergoes a first order phase transition at around 1.05 GPa. The Raman spectrum analysis of the new phase shows a significant change in the number of modes for the spectral range of 20-1000 cm-1. The instability of this phase occurs due to the decrease of the MoO bond lengths in the high-pressure phase, connected with tilting and/or rotations of the MoO4 tetrahedra leading to a disorder at the MoO4 sites. The second and third phase transformations were observed, respectively, at about 2.9 GPa and 4.77 GPa, with strong evidences, in the Raman spectra, of crystal symmetry change. The principal component analysis (PCA) and the hierarchical cluster analysis (HCA) were used in order to infer the intervals of pressure where the different phases do exist. Discussion about the number of non equivalent sites for Mo ions and the kind of coordination for molybdenum atoms is also furnished.

Temperature dependence Raman spectroscopy and DFT calculations of Bi2(MoO4)3.

This work reports a theoretical and experimental study on the electronic and vibrational properties of Bi2(MoO4)3. First-principle calculations were applied to increase the understanding on the properties of the chemical composition through the energy bands. The conduction band minimum (CBM) is found at the high symmetric Γ-point, while the valence-band maximum (VBM) is located between the Z and the Γ-points. Therefore, these facts confirm that the Bi2(MoO4)3 crystal is a semiconductor compound with an indirect band-gap of about 2.1 eV. Moreover, lattice dynamic properties were calculated using density functional perturbation theory (DFPT) in order to assign the experimental Raman bands. In addition, we performed temperature-dependent Raman spectroscopic studies in the Bi2(MoO4)3 crystals to obtain information on structural changes induced by effects of the temperature change. From the changes observed in the Raman spectra phase transitions at ∼ 668 and 833 K were inferred, with the last one possibly related to the disorder due to the heating process.

High-pressure studies on l,l-dileucine crystals by Raman spectroscopy and synchrotron X-ray diffraction combined with DFT calculations.

The vibrational properties of the dipeptide l-leucyl-l-leucine hydrate were investigated through Raman and infrared spectroscopy. With the aid of first principle calculations using the density functional theory, the assignment of the vibrational modes from the material was furnished. In addition, the behavior of the crystal under high pressure was investigated using Raman spectroscopy (~8 GPa) and synchrotron X-ray diffraction (~26 GPa). The results show significant changes in both the X-ray diffractogram and the Raman spectra, suggesting that l-leucyl-l-leucine hydrate undergoes a phase transition between 2.3 and 2.9 GPa. Finally, for pressures above 16 GPa the broadening of X-ray peaks suggests a disorder in the crystal lattice induced by high-pressure effects.

New structural phases of [bis(L-alaninato) diaqua] nickel(II) dihydrate crystal.

The study of [bis(L­alaninato) diaqua] nickel(II) dihydrate crystal using Raman scattering and X-ray diffraction as a function of temperature is reported in this paper. Thermal analysis (TGA and DSC) complementary measurements were also performed in order to obtain information on structural changes and mass loss occurred in this material. It was identified that the crystal undergoes loss of water at two different temperatures: ~340 and 393 K. X-ray diffraction measurements showed two phase transformations related to these two water loss events. After heating up to 423 K, the sample was cooled down to 298 K and its diffraction pattern presented the same pattern at 423 K, evidencing an irreversible phase transformation. The diffraction results also showed that crystal goes to monohydrate and anhydrous phases. Furthermore, cell lattice parameters and space groups of both phases were determined by applying Rietveld refinement through Le Bail method, demonstrating that their structures belong to the P21 and C2/c space groups, both with monoclinic symmetry. In addition, assignments of Raman spectra vibrational bands (at 300 K) are provided. The high-temperature Raman spectra were obtained in the 100-3500 cm-1 range, where it was observed several abrupt changes in the intensity of low-wavenumber bands and the appearance/disappearance of some vibrational modes that have coupling with OH⋯O hydrogen bonds. These spectral changes are in good agreement with X-ray diffraction and thermal analyses data. Finally, we obtained Raman measurements at low temperatures, from which we identified that the crystal structure is extremely stable throughout the temperature range of 293-10 K.

Phase transformation in the C form of myristic-acid crystals and DFT calculations.

In this study, the vibrational frequencies of myristic acid (CH3(CH2)12COOH) were obtained using density functional theory calculations, and the results were compared with experimental Raman and infrared data. Additionally, Raman spectra of crystalline myristic acid were recorded in the 300-20 K range. Raman spectroscopy gives important insights into the effect of low temperatures on its monoclinic phase. X-ray diffraction was performed from 298 to 133 K to provide additional information about the cryogenic behavior of the crystals. These undergo a phase transformation, which was confirmed by differential scanning calorimetry through an enthalpy anomaly observed at low temperatures. Raman spectra and X-ray diffraction refinement of the cell parameters in combination with differential scanning calorimetry at low temperatures revealed slight modifications, confirming a conformational change in the myristic acid molecules involving rearrangement of dimers within the unit cell.

Structural, vibrational and thermal studies on bis(l-glutaminato)copper(II).

Copper(II) complexes of amino acids have been widely studied as potentials medicines and dietary supplementation, so the knowledge about the metal-ligand sites, thermal stability and behavior of these complexes is an important subject of study. Although the Raman spectroscopy could help to elucidate the nature of the interactions into crystal there are only few information about vibrational study of this compound in the literature and no data depending on the temperature. In addition, there is no temperature-dependent X-ray diffraction study of this material. We report here Raman Spectroscopy and Powder X-ray Diffraction measurements, both as a function of temperature and as a way of studying the thermal stability of the material. After the synthesis of the sample and confirmation of its crystal structure by Powder X-ray Diffraction, Raman measurements were performed in the 70-3600 cm-1 spectral region as a function of temperature from 10 up to 300 K. Some peaks become more evident during the cooling, due to a decrease in width and an increase in intensity. There is a discontinuity in the wavenumbers evolution around 110 K, that should be associated with a conformation of the structure. Optimized geometry and vibrational frequencies were obtained by means of Density Functional Theory and for the first time the analysis of the vibrational modes was done in terms of the Potential Energy Distribution. X-ray diffraction measurements as a function of temperature and Rietveld refinement showed discontinuities in the lattice parameters and degradation around 493 K (at air atmosphere) and 513 K (under vacuum). These results were corroborated by the thermal analysis which indicates that the compound is stable up to 493 K.

Polarized Raman and Infrared Spectroscopy and ab Initio Calculation of Palmitic and Stearic Acids in the Bm and C Forms.

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 (C2h5) 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.

Pressure induced transformations in sorbic acid.

This research reports a pressure dependent Raman study of the sorbic acid between 0.0 and 10.0GPa. The unpolarized Raman spectra were measured in the spectral range of 20-3000cm-1. The high-pressure Raman scattering study of the sorbic acid showed that it underwent a gradual, disordering process. At the room temperature and at the ambient pressure conditions, the crystal structure of the sorbic acid belongs to the monoclinic system with a C2/c (C2h6) space group. The pressure increase induced a higher disorder in the monoclinic unit cell, since a single bending mode, and only very broad stretching Raman modes are present at pressure of ~10GPa. Upon pressure release the high-pressure phase transforms directly into the ambient-pressure phase. The presence of the internal vibrational modes is a guarantee that the molecular structure is maintained. Beyond this, the presence of external modes shows that the crystal has a memory to reverse the process and suggest that the crystal, which was in high disorder (broad Raman bands), does not suffer decomposition in the crystalline structure. The DFT calculations for the sorbic acid were performed in order to understand the vibrational properties. The theoretical study showed that the volume of the unit cell and beta angle decrease significatively when passing from the 0.0GPa to 8.0GPa. The decreases in the volume and beta angle of this particular unit cell were supposed to induce the larger increase in the bandwidths of the observed bands, pointing to some disorder in the monoclinic phase.

Conformational change in the C form of palmitic acid investigated by Raman spectroscopy and X-ray diffraction.

Fatty acids are substances found in most living beings in nature. Here we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid, a fatty acid with 16 carbon atoms. The Raman spectra were obtained in the temperature interval from 300 to 18K in the spectral range between 30 and 3100 cm(-1). The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements were obtained from 290 to 80K showing a slight modification in the lattice parameters at about 210K. Differential scanning calorimetry (DSC) measurements were recorded between 150 and 300K and no enthalpic anomaly in the DSC thermogram was observed. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.

Low-temperature phase transformation studies in the stearic acid: C form.

This paper reports the temperature-dependent measurements in the C form of stearic acid. Raman scattering, X-ray diffraction, and differential scanning calorimetry measurements were performed at low temperatures. The polarized Raman spectra were measured for temperatures ranging from 8 to 300 K over the spectral range of 30-3000 cm(-1). The spectral changes observed in both the lattice vibrational modes and the internal vibrational modes regions of the Raman spectrum, allowed to identify a phase transition undergone by the stearic acid crystal occurring between 210 and 170 K and a change in the structure continues to be observed down to 8 K. The anharmonicity of some vibrational modes and the possible space groups presented by the crystal at low temperatures were also discussed. Low-temperature X-ray diffraction measurements were performed from 290 to 80 K and the results showed slight changes in the lattice parameters at ∼200 K. Furthermore, the evidence of the phase transformation was provided by the differential scanning calorimetry measurements, which identified an enthalpic anomaly at about 160 K.

Temperature-dependent vibrational spectroscopic study and DFT calculations of the sorbic acid.

This work reports a temperature-dependent vibrational spectroscopic study of the sorbic acid (C6H8O2), as well as the mode assignment at ambient conditions, based on the density functional theory. Temperature-dependent vibrational properties have been performed in polycrystalline sorbic acid through both Raman and infrared spectroscopy in the 20-300 K and 80-300 K temperature ranges, respectively. These studies present the occurrence of some modifications in the Raman spectra that could be interpreted as a low temperature phase transition undergone by sorbic acid from the monoclinic phase to an unknown phase with conformational change of the molecules in the unit cell.