Low-frequency Raman Spectra Research Articles

Cholesterol Conformational Structures in Phospholipid Membranes.

Cholesterol is a major component of biomembranes that impacts membrane order, permeability, and lateral organization, but the precise molecular mechanisms of cholesterol's actions are still under investigation. Density functional theory (DFT) calculations have opened the fingerprint vibration bands of large molecules to detailed spectral analysis. For cholesterol, Raman spectral interpretation for conformational structure and hydrogen bonding is now possible. Here, DFT calculations of cholesterol conformers identify 10 structure types that also have unique low-frequency Raman spectra. By fitting experimental spectra to these types, the distribution of cholesterol structures present in phospholipid (PL) membrane vesicles was measured. The distributions reveal that the cholesterol iso-octyl chain tends to align with saturated PL chains and shifts to a thermal distribution for unsaturated PL chains. The results agree with the templating effect of cholesterol on PL membranes and show that the top of the iso-octyl chain is rigid like the rings. It is also shown that the inclusion of water molecules hydrogen bonded to the cholesterol hydroxy group in the DFT calculations may improve the spectral fitting for future studies.

Raman spectroscopy of atomically thin HfX2 (X=S, Se)

Abstract We investigated interlayer modes of few-layer HfX2 (X=S, Se) by using low-frequency micro-Raman spectroscopy with three excitation energies (1.96 eV, 2.33 eV, 2.54 eV) under vacuum condition (10−6 Torr). We observed interlayer modes in HfSe2 when the 2.54-eV excitation energy was used. The low-frequency Raman spectra reveal a series of shear and breathing modes (<50 cm−1) that are helpful for identifying the number of layers. The in-plane Eg and out-of-plane A1g modes of HfSe2 are located at ~150 cm−1 and ~200 cm−1, respectively. In HfS2, in-plane Eg and out-of-plane A1g optical phonons are observed at ~260 cm−1 and ~337 cm−1, respectively. The in-plane and out-of-plane force constants of atomically thin HfSe2 are obtained to be 1.87 ×1019 N/m3 and 6.55×1019 N/m3, respectively, by fitting the observed interlayer modes using the linear chain model. These results provide valuable information on materials parameters for device designs using atomically-thin layered HfX2 (X=S, Se).

Low-frequency Raman spectrum of methamphetamine hydrochloride and its alterations induced by impurities

Methamphetamine is one of the most abused drugs worldwide. Forensic laboratories have developed various methods to analyze methamphetamine for identifying and comparing seizures. These methods basically focus on the physicochemical properties of the methamphetamine molecule. Because methamphetamine is commonly distributed in its hydrochloride salt form, information on the crystalline state of methamphetamine could give new insight for forensic drug analysis. To grasp this information, we applied low-frequency Raman spectroscopy to methamphetamine hydrochloride. A laboratory-built low-frequency Raman microspectrometer was used for measuring low-frequency Raman spectra of optically pure and racemic methamphetamine hydrochloride. A mixture of methamphetamine hydrochloride with dimethyl sulfone, which is frequently added as a diluent to illicit methamphetamines, was also measured. An ab initio calculation was performed to assign peaks in the low-frequency spectra. The phonon modes of methamphetamine hydrochloride, and their changes induced by impurities are discussed. To the best of our knowledge, this is the first reported application of low-frequency Raman spectroscopy technique to methamphetamine hydrochloride.

Low-Frequency Raman Spectroscopy on Amorphous Poly(Ether Ether Ketone) (PEEK).

Low-frequency peaks in the Raman spectra of amorphous poly(ether ether ketone) (PEEK) were investigated. An amorphous sample with zero crystallinity, as confirmed by wide-angle X-ray diffraction, was used in this study. In a previous study, two peaks were observed in the low-frequency Raman spectra of the crystallized samples. Among these, the peaks at 135 cm-1 disappeared for the amorphous sample. Meanwhile, for the first time, the peak at 50 cm-1 was observed in the crystallized sample. Similar to the peak at 135 cm-1, the peak at 50 cm-1 disappeared in the amorphous state, and its intensity increased with increasing crystallinity. The origins of the two peaks were associated with the Ph-CO-Ph-type intermolecular vibrational modes in the simulation. This suggests that the Ph-CO-Ph vibrational mode observed in the low-frequency region of PEEK was strongly influenced by the intermolecular order.

Study of Raman Scattering of Light in Liquid Arenes and Their Halogen-Substituted ones in the Low-Frequency Region of the Spectrum

Purpose. Investigation of Raman scattering of light in liquid arenas and their halogenated ones in the low-frequency range of the spectrum, taking into account the clustering processes in their structure.Methods. Raman spectroscopy methods were used, as well as a modeling method using cluster representations of the structure of liquids. Raman spectra of light in the low-frequency range from 17 to 500 cm–1 were obtained using a LabRAM HR Evolution spectrometer at room temperature (23°C).Results. The analysis of theoretical and experimental work on the properties of dimeric configurations of benzene and on the effect of clustering processes in substances on their IR and Raman spectra is carried out. Low-frequency Raman spectra of liquid benzene, o-xylene, ethylbenzene, fluorobenzene, chlorobenzene, brombenzene, toluene, o-fluorotoluene, m-fluorotoluene, p-fluorotoluene, o-chlorotoluene, m-chlorotoluene, p-chlorotoluene, 2,4-dichlorotoluene, 2,6-dichlorotoluene were obtained. The model of formation and disintegration of cluster formations has been tested. Formulas have been obtained for estimating the minimum frequency of libration oscillations of the ωmin dimer formation in the cluster structure and the position of the maxima of spectral bands in the low-frequency region of the Raman spectrum, depending on the number of particles included in the cluster formations. The theoretical results obtained by ωmin are in satisfactory agreement with the experimental data within the total error.The proposed model makes it possible to predict the position of some spectral bands of the Raman spectrum. The presence of other spectral bands is obviously related to the multiparticle interaction in the cluster structure and requires additional research.Conclusion. The proposed model of formation and decay of cluster formations and the relations following from it allow us to estimate the value of the minimum frequency in the low-frequency range of the Raman spectrum of liquid arenes and their halogenated ones from the known values of the enthalpy of formation and the moment of inertia of the dimer. Conversely, using known values of the minimum frequency, the values of the enthalpy of formation and the moment of inertia of the dimer can be estimated without resorting to complex quantum mechanical calculations.

Visualization of isothermal crystallization and phase separation in poly[(R)-3-hydroxybutyrate]/poly(L-lactic acid) by low-frequency Raman imaging

The visualization of the variation of the inter/intra molecular interaction (C = O⋯CH3) between poly[(R)-3-hydroxybutyrate] (PHB) and poly-L-lactic acid (PLLA) in the PHB/PLLA miscible blend during phase separation and crystallization process was successfully investigated using Raman imaging. Images of the blend were developed using high- and low-frequency Raman spectra acquired during the isothermal crystallization of the blend, and both of them were compared. The low-frequency region allowed to observe the changes in the hydrogen bonds between the molecular chains in the blend during phase separation and crystallization via a band at 75 cm−1 derived from PHB. The imaging results obtained using the band at 75 cm−1 due to hydrogen bonding (C = O⋯CH3) between molecular chains were in good agreement with the results obtained using the C = O stretching band at 1720 cm−1. Herein, we demonstrated that the low-frequency region of the Raman spectrum is more sensitive to detecting the start of the phase separation and crystallization of PHB than the corresponding high-frequency region.

Lifting Hofmeister's Curse: Impact of Cations on Diffusion, Hydrogen Bonding, and Clustering of Water.

Water plays a role in the stability, reactivity, and dynamics of the solutes that it contains. The presence of ions alters this capacity by changing the dynamics and structure of water. However, our understanding of how and to what extent this occurs is still incomplete. Here, a study of the low-frequency Raman spectra of aqueous solutions of various cations by using optical Kerr-effect spectroscopy is presented. This technique allows for the measurement of the changes that ions cause in both the diffusive dynamics and the vibrations of the hydrogen-bond structure of water. It is found that when salts are added, some of the water molecules become part of the ion solvation layers, while the rest retain the same diffusional properties as those of pure water. The slowing of the dynamics of the water molecules in the solvation shell of each ion was found to depend on its charge density at infinite dilution conditions and on its position in the Hofmeister series at higher concentrations. It is also observed that all cations weaken the hydrogen-bond structure of the solution and that this weakening depends only on the size of the cation. Finally, evidence is found that ions tend to form amorphous aggregates, even at very dilute concentrations. This work provides a novel approach to water dynamics that can be used to better study the mechanisms of solute nucleation and crystallization, the structural stability of biomolecules, and the dynamic properties of complex solutions, such as water-in-salt electrolytes.

Explanation and prediction for the product of trehalose dihydrate selective dehydration process using mid-frequency Raman difference spectra

Explanation and prediction for the product of trehalose dihydrate dehydration process was realized in this work. β form is the thermodynamic dehydration product of trehalose dihydrate. And α form is the kinetic dehydration product of trehalose dihydrate, which was analyzed by low-frequency Raman spectra, mid-frequency Raman difference spectra and IR difference spectra, and the analysis results confirmed that the crystal structure of trehalose dihydrate and α form are similar. The selective dehydration process from trehalose dihydrate to α form is due to their similar short-range orders. Amorphous trehalose is the uncontrollable dehydration product of trehalose dihydrate due to the collapse of water channels. The dehydration product of trehalose dihydrate by freeze-drying was a mixture of α form and amorphous phase, and the content of amorphous trehalose in the freeze-drying product increases with the decrease of the particle size of dihydrate. Study on the dehydration principle of organic hydrates will guide the preparation, storage and desolvation of drugs and foods.

Anisotropic and Finite Effects on Intermolecular Vibration and Relaxation Dynamics: Low-Frequency Raman Spectroscopy of Water Film and Droplet on Graphene by Molecular Dynamics Simulations.

The structural and dynamical properties of water can be greatly altered by the anisotropic interfacial environment. Here, we study the intermolecular vibration and relaxation dynamics of a water film and a water droplet on a graphene surface based on low-frequency Raman spectra calculated from molecular dynamics simulations. The calculated Raman spectra of the interfacial water systems show a weakened libration peak and an enhanced intermolecular hydrogen bond (HB) stretching peak compared to the spectrum of bulk water, which are attributed to softened orientation motion. We also find that the collective polarizability relaxation in the droplet is much slower than that in the film and bulk, which is completely different from the collective dipole relaxation. The slow relaxation is due to a positive correlation between the induced polarizabilities of distinct molecules caused by the global and anisotropic structural fluctuations of the water droplet. Furthermore, we find that the two-dimensional HB network by the orientation-ordered interfacial water molecules leads to different intermolecular vibration dynamics between the parallel and perpendicular components. The present theoretical study demonstrates that low-frequency Raman spectroscopy can reveal the anisotropic and finite effects on the intermolecular dynamics of the water film and droplet.

Angular resolved ultra-low frequency Raman spectroscopy of few-layer black phosphorous

Few-layer black phosphorus (FLBP) exhibits valuable mechanical, optical, and electronic properties for potential applications in nanodevices, ranging from insulators to superconductors. In particular, their band structure and optical properties are closely related to layer numbers, stacking sequences, and twist angles. However, the mechanism behind these factors is not fully understood, as it is quite difficult to obtain stacking details using non-destructive probes. This article provides direct experimental evidence of the weak interlayer van der Waals (vdW) interaction of FLBP using a state-of-the-art, angular resolved, ultra-low frequency Raman spectroscopy system. A modified linear-chain model is presented to evaluate the layer number dependence on the extremely low-frequency breathing modes (Ag3 and Ag4). The polarization analysis illustrates that most bi-layer BP samples prefer the AB stacking sequence. Furthermore, the stacking angle of bi-layer BP was assigned based on its minimum intensity of Ag3 and Ag4 modes. This investigation on angular resolved extremely low-frequency Raman spectra sheds light on a new approach for analyzing the stacking details of FLBP, which is of great significance for developing novel FLBP-based nanodevices.

Native state of wood cellulose: evidence that further supports its non-crystalline nature

Abstract Although highly desirable, the nature of wood-cellulose in its native state has remained difficult to comprehend. Contrary to the traditional acceptance of wood-cellulose being crystalline, in 2016, the authors’ research found that the cellulose was not crystalline. Here, additional evidence is presented that further supports the non-crystalline model. One of the key pieces of evidence was obtained by 64% H2SO4 hydrolysis of tension- and opposite-aspen woods (TW and OW, respectively). The TW (G-layer rich) yielded significant amount of CNCs (TW-CNCs, 20.7%), the OW yielded a much lower amount (OW-CNCs, 5.4%). Although a higher yield of TW-CNCs was expected due to the presence crystalline cellulose in the G-layer, the lower yield of the OW-CNCs was a surprise because, assuming absence of G-layer, based on the authors’ earlier findings no CNCs were expected to be generated. To explain this anomaly, anatomical examination of the woods using stains was carried out which showed that some OW fibers also contained the crystalline G-layer and therefore, provided an explanation as to why the OW-CNCs were produced. The results clearly showed that the acid hydrolysis did not destroy the crystalline cellulose and therefore, in the case of a normal (G-layer free) wood which, as previously reported had not generated CNCs, the cellulose must have been non-crystalline. An additional indication of the wood’s S2 cellulose being not crystalline was the absence of the 93 cm−1 Raman band in the low frequency spectrum of the TW S2 layer. Further evidence was obtained by comparing low frequency Raman spectra of TW-CNCs, TW-holopulp, and aspen-holopulp as well as the mixture-samples of crystalline cellulose and xylan at the concentration levels of their occurrence in these holopulps. Overall, these findings provided further support to the contention that the native wood-cellulose is non-crystalline.

Systematic study of the effect of small-molecules on the low-frequency Raman spectrum of water

Systematic study of the effect of small-molecules on the low-frequency Raman spectrum of water

Quantitative Monitoring of Cocrystal Polymorphisms in Model Tablets Using Transmission Low-Frequency Raman Spectroscopy

Cocrystallization is a technique for improving the physical properties of active pharmaceutical ingredients. However, cocrystals can transform into more stable polymorphs as well as dissociate to original materials. Therefore, an analytical technique is required to determine the polymorphic transformation quickly and accurately in tablets. The purpose of this study is to develop a method to monitor cocrystal polymorphs in model tablets using transmission low-frequency Raman spectroscopy. The tablets, consisting of only metastable polymorphs of caffeine-glutaric acid cocrystals, were stored under various relative humidity levels. The composition of the cocrystal polymorphs were calculated from a calibration curve relating the actual composition to the predicted values calculated by partial least squares regression processing of low-frequency Raman spectra. The metastable form gradually converted to a stable form, and polymorphic phase transformation occurred with increasing relative humidity. Ninety-six percent of the metastable form converted into a stable form stored at 25 °C after 3 h at 95% RH. In conclusion, transmission low-frequency Raman spectroscopy can be used to quantitatively monitor cocrystal polymorphs. This technique is one of the candidate techniques to quantifiably evaluate the physico-chemical stability of cocrystal polymorphs in tablets.

Intermolecular Dynamics of Positively and Negatively Charged Aromatics and Their Isoelectronic Neutral Analogs in Aqueous Solutions.

In this study, we investigated the temperature dependence of intermolecular vibrations and orientational dynamics in the aqueous solutions of imidazole hydrochloride, imidazole, sodium triazolide, and triazole using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and steady-state Raman spectroscopy. The difference low-frequency Raman spectra under 250 cm-1 of the aqueous solutions relative to the neat water showed that the spectral shoulder in the high-frequency region at 60-100 cm-1, assigned to the libration of an aromatic ring, was higher in frequency for the imidazolium cation but lower for the triazolide anion than those of the respective neutral aromatics. The results of the ab initio quantum chemistry calculations of the clusters of the aromatics and water molecule(s) were consistent with the experimental spectra of the aqueous solutions. Further, the results of the temperature-dependent experiments showed that the signal intensity in the low-frequency region below 50 cm-1 increased for all solutions with an increase in temperature. In contrast, the spectral density in the high-frequency region above 80 cm-1 exhibited almost no shift for the 1.0 M solutions, while a significant red shift was observed for the 5.0 M solutions. In addition, the temperature-dependent densities, viscosities, and surface tensions were characterized for the aqueous aromatic solutions from 293 to 353 K.

Low- versus Mid-frequency Raman Spectroscopy for in Situ Analysis of Crystallization in Slurries.

Slurry studies are useful for exhaustive polymorph and solid-state stability screening of drug compounds. Raman spectroscopy is convenient for monitoring crystallization in such slurries, as the measurements can be performed in situ even in aqueous environments. While the mid-frequency region (400–4000 cm–1) is dominated by intramolecular vibrations and has traditionally been used for such studies, the low-frequency spectral region (<200 cm–1) probes solid-state related lattice vibrations and is potentially more valuable for understanding subtle and/or complex crystallization behavior. The aim of the study was to investigate low-frequency Raman spectroscopy for in situ monitoring of crystallization of an amorphous pharmaceutical in slurries for the first time and directly compare the results with those simultaneously obtained with mid-frequency Raman spectroscopy. Amorphous indomethacin (IND) slurries were prepared at pH 1.2 and continuously monitored in situ at 5 and 25 °C with both low- and mid-frequency Raman spectroscopy. At 25 °C, both spectral regions profiled amorphous IND in slurries as converting directly from the amorphous form toward the α crystalline form. In contrast, at 5 °C, principal component analysis revealed a divergence in the detected conversion profiles: the mid-frequency Raman suggested a direct conversion to the α crystalline form, but the low-frequency region showed additional transition points. These were attributed to the appearance of minor amounts of the ε-form. The additional solid-state sensitivity of the low-frequency region was attributed to the better signal-to-noise ratio and more consistent spectra in this region. Finally, the low-frequency Raman spectrum of the ε-form of IND is reported for the first time.

Exploring the energy storage density in 60Bi2O3–10SrO-30Fe2O3 lead-free relaxor glass for designing energy storage devices

Strontium doped glass was synthesized by the conventional melt-quenching technique. Systematically measured dielectric properties revealed relaxor ferroelectric-like behavior in a broad range of frequency and temperature. The glassy nature of the present sample was investigated by X-ray diffraction (XRD) measurements. glass transition temperature, crystallization temperatures and melting temperature are determined by differential thermal analysis (DTA). The low frequency Raman spectra displays a Boson peak at 72 cm−1, which confirms polar clusters (PCs) formation. The present sample depicts a wide as well as diffuse peak of dielectric permittivity ε'(ω) and tangent loss tan δ. wide as well as diffuse peak shifted to the higher temperatures (from 538 K to 553 K), a typical relaxor behavior is indicated by this type of diffuse transition. The presence of polar clusters (PCs) imbedded in the glass matrix indicates the presence of relaxor ferroelectric (RFE) behavior in this glass. The measured energy storage density of the strontium doped glass was 4 mJ.cm−3 with efficiency of 70% under an applied electric field of 17 kV.cm−1 at room temperature. Magnetic properties via vibrating-sample magnetometer (VSM) show a typical anti-ferromagnetic behavior. These results depict that the present glass can be regarded a promising candidate for designing energy storage devices.

Comparison of Proton Acceptor and Proton Donor Properties of H2O and H2O2 in Organic Crystals of Drug-like Compounds: Peroxosolvates vs. Crystallohydrates.

Two new peroxosolvates of drug-like compounds were synthesized and studied by a combination of X-ray crystallographic, Raman spectroscopic methods, and periodic DFT computations. The enthalpies of H-bonds formed by hydrogen peroxide (H2O2) as a donor and an acceptor of protons were compared with the enthalpies of analogous H-bonds formed by water (H2O) in isomorphic (isostructural) hydrates. The enthalpies of H-bonds formed by H2O2 as a proton donor turned out to be higher than the values of the corresponding H-bonds formed by H2O. In the case of H2O2 as a proton acceptor in H-bonds, the ratio appeared reversed. The neutral O∙∙∙H-O/O∙∙∙H-N bonds formed by the lone electron pair of the oxygen atom of water were the strongest H-bonds in the considered crystals. In the paper, it was found out that the low-frequency Raman spectra of isomorphous crystalline hydrate and peroxosolvate of N-(5-Nitro-2-furfurylidene)-1-aminohydantoin are similar. As for the isostructural hydrate and peroxosolvate of the salt of protonated 2-amino-nicotinic acid and maleic acid monoanion, the Raman spectra are different.

Revisiting the phase transition sequence in L-methionine: Description of the disordering mechanism in an essential amino acid.

Raman spectroscopy investigations on L-methionine (L-Met) performed in a large temperature range (170-420K) and in a wide spectral window (5-3600cm-1) have revealed an extended disordering mechanism triggered by thermally activated motions of the terminal side-chain atoms, from 250 up to 390K. This very progressive disordering process is characterized by two thermodynamic features, the first corresponding to a broad endotherm (250 → 310K) marking the beginning of the process, while the second ending the disordering transformation is a sharp endothermic peak at 390K. These thermodynamic events are correlated with the softening of lattice vibrations and the increase of the quasielastic scattering, considered as the signatures of displacive phase transitions. The amorphous-like band-shape of the low-frequency Raman spectrum collected above 390K, resulting from the strong anharmonicity of local motions, is contrasting with the detection of additional Bragg peaks above 390K by x-ray diffraction, consistent with the Cp jump accompanying the endothermic peak. These observations suggest that L-Met is progressively dynamically disordered adopting additional configurations in the crystalline lattice through rotations of CH3 and the side-chain flexibility not clearly detected by x-ray diffraction. These results should be crucial for considering the stability of dried proteins composed of methionine residues.

Molecule-like and lattice vibrations in metal clusters.

We report distinct molecule-like and lattice (breathing) vibrational signatures of atomically precise, ligand-protected metal clusters using low-temperature Raman spectroscopy. Our measurements provide fingerprint Raman spectra of a series of noble metal clusters, namely, Au25(SR)18, Ag25(SR)18, Ag24Au1(SR)18, Ag29(S2R)12 and Ag44(SR)30 (–SR = alkyl/arylthiolate, –S2R = dithiolate). Distinct, well-defined, low-frequency Raman bands of these clusters result from the vibrations of their metal cores whereas the higher-frequency bands reflect the structure of the metal–ligand interface. We observe a distinct breathing vibrational mode for each of these clusters. Detailed analyses of the bands are presented in the light of DFT calculations. These vibrational signatures change systematically when the metal atoms and/or the ligands are changed. Most importantly, our results show that the physical, lattice dynamics model alone cannot completely describe the vibrational properties of ligand-protected metal clusters. We show that low-frequency Raman spectroscopy is a powerful tool to understand the vibrational dynamics of atomically precise, molecule-like particles of other materials such as molecular nanocarbons, quantum dots, and perovskites.

Influence of Nd3+ modifying on 80TeO2–xZnO–(20−x)Na2O ternary glass system

In this study, the ternary system of TeO2–ZnO–Na2O (TZN) with a fixed TeO2 content of 80 mol. % and with a varied ZnO and Na2O content of totally 20 mol. % was examined for thermal/mechanical properties, Raman and x-ray absorption fine structure (XAFS) spectra, and the evolution of medium distance (correlation length) of order, called blob size here, given from low-frequency Raman spectra and longitudinal/transverse sound velocities as a function of ZnO concentration. The TZN glasses were doped with Nd2O3, and the effects of Nd doping on the above-mentioned properties were reported. High-temperature in situ x-ray diffraction experiments were also conducted to know possible oxygen coordination numbers of these main-constructive cations from crystalline phases precipitated at elevated temperatures. According to the information, the evolution of blob size of Nd-doped and non-doped TZN glasses was attempted to be predicted with a term of theoretical volume of their molar unit constructed with cation–oxygen polyhedra of TeO3, TeO4, ZnO4, ZnO5, NaO4, and NaO5. A transition region due to the structural change owing to the coordination numbers of Te–O and Zn–O was elucidated concerning density, molar volume, and Poisson ratio. XAFS spectroscopy revealed that the ZnO component had different polyhedra of ZnO4 and ZnO5, whose ratio was dependent not only on ZnO concentration but also on whether Nd3+ ions were doped or not. Based on the information taken from a variety of observations, our preliminary analysis suggested that the different mechanical properties between the doped and non-doped glasses were explained by five-coordinated zinc in the glasses. Possible molecular models shall be given for lower and higher ZnO concentrations in TZN glasses.