Dynamics and mechanism of the Crystal II → smecticG phase transition in TB7A by a temperature‐dependent micro‐Raman study and DFT calculations

The solid to smecticG (SmG) phase transition in a Schiff base liquid crystalline compound, terepthal‐bis‐heptylaniline (TB7A), is monitored in situ by temperature‐dependent Raman microspectroscopy, using the band of a C H in‐plane bending mode as a marker. Contrary to the earlier report of a sudden wavenumber shift, the in situ measurement shows very clearly that a new Raman band at ∼1160 cm⁻¹ appears at the Crystal II → SmG transition. The dynamics of this phase transition is discussed in terms of a triple well potential below 210 K and a double well potential above 210 K. The phase transition essentially takes place as a result of intra‐molecular rotation about the long molecular axis. The optimization energy at various fixed dihedral angles, ( C C CN ) are calculated using density functional theory (DFT) at the B3LYP/6‐31G* level of theory. The relative energy at each dihedral angle is calculated relative to optimization energy obtained without any constraints and plotted as a function of dihedral angle (Φ) between the adjacent phenyl ring planes, which also shows a double well potential at room temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Changes in the crystalline structure of chlorpropamide at high pressures and temperatures

The crystalline structure of chlorpropamide is studied by the X-ray diffraction method at high pressures up to 4.2 GPa and in the temperature range 300–450 K. At normal pressure and upon heating to its melting point T = 396 K no phase transitions are found in chlorpropamide. When the initial α form of chlorpropamide is recrystallized, the appearance of a polymorphic ε phase is observed. After recrystallization, the high pressure effect causes partial amorphization of chlorpropamide at pressures of P ～ 3 GPa. Baric and temperature coefficients are obtained for the α and ε forms of chlorpropamide.     

Disulfide linkage Raman markers: a reconsideration attempt

During the last decades, Raman spectroscopy has been routinely used for probing the conformational features of disulfide linkages in peptides and proteins. However, the interpretation of disulfide Raman markers is currently performed by a simple rule derived from the earliest observations on dialkyl disulfides. More precisely, this rule consists of the following: (1) in analyzing the Raman bands in the 550–500 cm⁻¹ region ascribed to disulfide bond stretch motion, namely, ν(S‐S), and (2) assigning the three types of Raman markers observed at ~500, ~520, and ~540 cm⁻¹ to three families of rotamers defined along the three successive bonds of the ‐C‐S‐S‐C‐ moiety, referred to as ggg, ggt, and tgt. In this report, we attempt to show that an accurate analysis of disulfide vibrational features needs the use of the five torsion angles (χ₁, χ₂, χ₃, χ₂', and χ₁') along the five successive bonds joining the two α‐carbon atoms in the cystine (Cys‐Cys) unit. The present work is inspired by the disulfide conformational investigations performed by a statistical scan of numerous protein crystal and nuclear magnetic resonance data, taking into account the handedness (right and left) of a disulfide bridge, its spatial shape (Staple, Hook, and Spiral), as well as the signs of the two extreme torsion angles χ₁ and χ₁'. It appears that the combined use of the old and recent conformational notations allows a more accurate structural and vibrational analysis of disulfide linkage. Copyright © 2014 John Wiley & Sons, Ltd.     

Insights into phase stability of anhydrous/hydrate systems: a Raman‐based methodology

FT‐Raman spectroscopy turns out to be a powerful technique to evaluate the amount of polymorphic and pseudopolymorphic forms in crystalline samples—which is particularly relevant in pharmaceutical sciences. This paper presents a methodology that allows successful quantitative evaluation of the solid‐state hydration and dehydration processes, using FT‐Raman spectroscopy. All the steps required for a reliable evaluation of the hydration/dehydration process are illustrated for the caffeine system, a particularly challenging system presenting limited spectral differences between the pseudopolymorphs. The hydration process of caffeine was found to occur in a single‐step process with a half‐life time of ca 13 h, while the dehydration occurs through a two‐step mechanism. The critical relative humidity was found to be at ca 81 and 42% for anhydrous and hydrate caffeine forms, respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Geometric classification of the torsion tensor of space‐time

Torsion appears in literature in quite different forms. Generally, spin is considered to be the source of torsion, but there are several other possibilities in which torsion emerges in different contexts. In some cases a phenomenological counterpart is absent, in some other cases torsion arises from sources without spin as a gradient of a scalar field. Accordingly, we propose two classification schemes. The firstone is based on the possibility to construct torsion tensors from the product of a covariant bivector and a vector and their respective space‐time properties. The secondone is obtained by starting from the decomposition of torsion into three irreducible pieces. Their space‐time properties again lead to a complete classification. The classifications found are given in a U ₄, a four dimensional space‐time where the torsion tensors have some peculiar properties. The irreducible decomposition is useful since most of the phenomenological work done for torsion concerns four dimensional cosmological models. In the second part of the paper two applications of these classification schemes are given. The modifications of energy‐momentum tensors are considered that arise due to different sources of torsion. Furthermore, we analyze the contributions of torsion to shear, vorticity, expansion and acceleration. Finally the generalized Raychaudhuri equation is discussed.     

Adsorption of 1,2,4‐triazole on a silver electrode: surface‐enhanced Raman spectroscopy and density functional theory studies

The pH‐dependent surface‐enhanced Raman scattering (SERS) of 1,2,4‐triazole adsorbed on silver electrode and normal Raman (NR) spectra of this compound in the aqueous solutions were investigated. The observed bands in the NR and SERS spectra were assigned with the help of density functional theory calculations for model molecules in the neutral, anionic, and cationic forms and their complexes with silver. The Raman wavenumbers and intensities were computed at the optimized molecular geometry. Vibrational assignments of the SERS and NR spectra are provided by calculated potential energy distributions. The combination of experimental SERS results and density functional theory calculations provide an insight into the molecular structure of adlayers formed by 1,2,4‐triazole on a silver surface at varying pH values and enable the determination of molecular orientation with respect to the surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman spectroscopic investigations on intermolecular interactions in aggregates and crystalline forms of trans‐astaxanthin

Astaxanthin is a carotenoid naturally found in microbial organisms, microalgae, and many crustaceans. Its consumption has led to beneficial effects such as pigmentation of marine animals, and it favorably addresses several human health issues as a result of its high important antioxidant property. Several companies produce synthetic trans‐astaxanthin for dietary purposes in aquaculture, where it is mainly used for pigmentation. It is known that trans‐astaxanthin manifests itself as a monomer in organic solvents, as aggregates in aqueous solutions of organic solvents, or as crystalline solids. These forms display unique optical and structural properties, which have an impact on biological systems. In this work, we report on detailed Raman investigations, in conjunction with optical absorption spectroscopy, of monomer, aggregates, and crystalline forms of trans‐astaxanthin. The Raman and optical absorption spectroscopic investigations of trans‐astaxanthin aggregates were performed as a function of time, showing the formation of card‐packed aggregates after 2 h, and head‐to‐tail aggregates after 24 h in a 10% acetone–water astaxanthin solution. For the crystalline trans‐astaxanthin, a pointwise Raman mapping evidenced the presence of two distinct crystal structures. The Raman modes of these crystal structures (A and B) were correlated with the intermolecular interactions present in chloroform solvated (AXT‐Cl) and unsolvated (un‐AXT) trans‐astaxanthin single crystals. Both crystal structure A and the card‐packed aggregates have similar intermolecular π stacking interactions as AXT‐Cl. The crystal structure B and the head‐to‐tail aggregates showed linear chain features as in un‐AXT. This work also clearly demonstrates that Raman spectroscopy is a powerful tool to distinguish the crystal structures present in crystalline powder of trans‐astaxanthin. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman spectroscopy as a probe of molecular order,orientation, and stacking of fluorinated copper‐phthalocyanine (F16CuPc) thin films

We report Raman scattering measurements on azimuthally ordered thin films of F₁₆CuPc, prepared by organic molecular beam deposition on A‐plane sapphire substrates. The observed peak frequencies have been compared both to the results of a model calculation for the vibrational modes of the free molecule and to those reported by other authors in related materials. This analysis provides a plausible identification of the modes responsible for the strongest spectral features. Detailed evaluation of the spectra reveals that some observed modes, which correspond to vibrations of the macrocycle inner ring, largely retain the intramolecular character and their polarisation properties can be used to study the orientation and stacking configuration of the molecules. We provide structural parameters deduced either in molecular or crystal symmetry considering the simpler possibilities, i.e. a single column molecular stacking and a herringbone‐like structure. The results suggest that the thicker and most ordered film is structurally close to the recently reported crystal organisation of bulk ribbon samples of this compound. The crystalline quality of the ordered films is mainly reflected in some other Raman peaks which are related to the motion of peripheral atoms and dominate the high wavenumber part of the spectra. These modes are affected by intermolecular interactions inducing Davydov splittings that are unequivocally identified by the observed Raman selection rules. The performed analysis also provides quantitative estimates of the degree of in‐plane ordering. Copyright © 2013 John Wiley & Sons, Ltd.     

Amorphous,nanocrystalline and crystalline calcium carbonates in biological materials

Raman spectroscopy is a powerful tool in identifying different calcium carbonate polymorphs. Here, the method is applied to cultured pearls from freshwater (genus Hyriopsis) and marine bivalve species (Pinctada maxima) as well as to shells of Diplodon chilensis patagonicus bivalves. Raman spectra for vaterite, detected for the first time in an adult shell, and amorphous calcium carbonate (ACC) are discussed. Results for ACC are compared with those of synthetically produced ACC and with the Raman spectroscopic features of stable biogenic ACC from the crustacean Porcellio scaber. Decomposition of the most intense signal of all calcium carbonate polymorphs—the ν₁ symmetric stretching mode of the carbonate ion—leads to the identification of two polymorphs within the ACC areas: a mixure of an amorphous and a crystalline fraction. The amorphous phase is characterised by a broad peak in the region of the lattice modes, which is composed of two distinct lattice modes with very high full‐widths at half‐maximum (FWHMs). The FWHMs of most of the crystalline fractions (in the range of 6.3–10.7 cm⁻¹) are too high for well‐crystallised materials and support reports of nanocrystalline calcium carbonate polymorph clusters in ACC. Crystallinity indices of different samples are calculated and found to be useful to describe roughly the state of crystallisation in the ACC areas. Copyright © 2010 John Wiley & Sons, Ltd.     

Vibrational spectroscopic study of budesonide

The Raman spectrum of budesonide is reported for the first time, and molecular assignments are proposed on the basis of ab initio BLYP DFT calculations with a 6‐31 G* basis set and vibrational wavenumbers predicted on a quasi‐harmonic approximation. Comparison with previously published infrared data has explained several spectral features, and the relative band intensities in the CO and CC stretching regions are interpreted. The results from this study provide data that can be used for the preparative process monitoring of budesonide, an important steroidal pharmaceutical in various dosage forms, and its interaction with excipients and other components. Copyright © 2007 John Wiley & Sons, Ltd.     

Characteristic Raman lines of phenylalanine analyzed by a multiconformational approach

Thanks to a considerable modulation of electronic polarizability, six phenylalanine (Phe) vibration modes located at ca. 1606, 1586, 1207, 1031, 1004 and 622 cm⁻¹ appear as intense or medium bands in the Raman spectra of peptides and proteins, as confirmed by the Raman data collected from free amino acid, somatostatin and bovine serum albumin (BSA). To get information on the nature and location of these lines, we resorted to a multiconformational analysis which consists in a systematic investigation of the structural and vibrational features of hydrated Phe in a conformational space depending on four angular variables: φ, ψ, χ₁ and χ₂. The first two variables correspond to the Phe backbone torsion angles, whereas the latter two refer to its side chain. Based on a protocol described in an accompanying report on glycine and its protonated and deprotonated species, we have prepared an initial set of 123 initial clusters of Phe + 5H₂O, including all plausible values of the above mentioned conformational angles. The results of their geometry optimization, by means of the density functional theory using the B3LYP hybrid functional, were first analyzed through the comparison between the E(χ₁, χ₂) energy maps obtained either by an explicit or by an implicit hydration model. Then, a set of nine doubly minimized clusters corresponding to the deepest local minima were used for further structural and vibrational analysis. Beyond providing a reliable assignment for the above mentioned characteristic Raman lines, the theoretical spectrum allowed us to carry out an overview of the whole observed data of Phe in aqueous solution. Copyright © 2013 John Wiley & Sons, Ltd.     

Polarized Raman and photoluminescence studies of a sub‐micron sized hexagonal AlGaN crystallite for structural and optical properties

The polarized Raman spectroscopy is capable of giving confirmation regarding the crystalline phase as well as the crystallographic orientation of the sample. In this context, apart from crystallographic X‐ray and electron diffraction tools, polarized Raman spectroscopy and corresponding spectral imaging can be a promising crystallographic tool for determining both crystalline phase and orientation. Sub‐micron sized hexagonal AlGaN crystallites are grown by a simple atmospheric pressure chemical vapor deposition technique using the self catalytic vapor–solid process under N‐rich condition. The crystallites are used for the polarized Raman spectra in different crystalline orientations along with spectral imaging studies. The results obtained from the polarized Raman spectral studies show single crystalline nature of sub‐micron sized hexagonal AlGaN crystallites. Optical properties of the crystallites for different crystalline orientations are also studied using polarized photoluminescence measurements. The influence of internal crystal field to the photoluminescence spectra is proposed to explain the distinctive observation of splitting of emission intensity reported, for the first time, in case of c‐plane oriented single crystalline AlGaN crystallite as compared with that of m‐plane oriented crystallite. Copyright © 2016 John Wiley & Sons, Ltd.     

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

The optimized geometry and structural features of the most prospective electro‐optic crystal 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST), and the vibrational spectral investigations have been comprehensively described with the near infrared Fourier transform (NIR FT) Raman and Fourier transform infrared (FT‐IR) spectra supported by the density functional theoretical (DFT) computations to elucidate the contribution of vibrational modes to the linear electro‐optic (LEO) effect. Mulliken population analysis and natural bond orbital (NBO) analysis have also been carried out to analyze the effects of intramolecular charge transfer (ICT), intramolecular hydrogen bonding and hyperconjugative interactions on the geometries. The influence of CT interaction between the phenyl ring and the dimethylamino group of the nonlinear optical (NLO) chromophore on the endocyclic and exocyclic angles, and the electronic effects such as hyperconjugation and back‐donation on the methyl hydrogen atoms have been examined. The concurrent intense activation of Raman and IR activities of the effective conjugation vibrational coordinate, which significantly contributes to the LEO effect resulting from the strong electron–phonon (e/ph) coupling, has been analyzed in detail. The effects of frontier orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), transition of electron density (ED) transfer and the influence of planarity in the stilbazolium ring on the first hyperpolarizability are also discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Molecular orientation analysis of organic thin films by z‐polarization Raman microscope

Polarization‐dependent Raman microscopy is a powerful technique to perform both structural and chemical analyses with submicron spatial resolution. In conventional Raman microscopy, the polarization measurements are limited only in the direction parallel to the sample plane. In this work, we overcome the limit of conventional measurements by controlling the incident polarization by a spatially modulated waveplate. In this method, the polarization perpendicular to the sample surface (z‐polarization) can be detected together with the parallel polarization (xy‐polarization). Because of this unique polarization control, our Raman microscope has the ability to image the molecular orientation, together with the molecular analysis. Here, we have investigated thin films of pentacene molecules that are widely studied as an organic semiconductor material. The orientations of pentacene molecules are imaged with a spatial resolution of 300 nm. Our results clearly indicate that the lamellar grains show the lower tilt angles compared to the neighboring islands, which has not been proved in conventional methods. The substrate effects and the thickness dependence of the film are also studied. These results provide knowledge about the relationship between the devise performance and the film structures, which is indispensable for future device exploitations. Copyright © 2012 John Wiley & Sons, Ltd.     

Tobacco alkaloids analyzed by Raman spectroscopy and DFT calculations

The tobacco alkaloids: nicotine, nornicotine, cotinine, anabasine, their protonated forms, and salts were analyzed by means of Raman spectroscopy supported by the density functional theory/B3LYP/aug‐cc‐pVDZ calculations. The analyses were performed based on Raman marker bands of neutral, monoprotonated, and diprotonated forms of tobacco alkaloids because in different surroundings various forms have been suggested to either dominate or to coexist. The form and distribution of nicotine directly in a plant and in phytopharmaceutical products were investigated by in situ Raman mapping. For the first time, the Raman optical activity spectrum of (−)‐nicotine in aqueous solution was measured and interpreted by means of the density functional theory calculations. The study provides a clear evidence that Raman spectroscopy techniques are powerful in efficient quality control and forensic and bioanalytical analyses of tobacco alkaloids. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental determination of vibrational anharmonic contributions

This article deals with the experimental determination of separate vibrational anharmonic contributions to the self‐energy of phonons in any crystals based on temperature dependence of their Raman spectra. We propose a new approach to find each anharmonic contribution by using special temperature points. We apply the approach to diamond, silicon, and crystalline α‐S₈ and show that our results for summarized anharmonicity in diamond and silicon are in good agreement with the values obtained previously for these systems by other researchers. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of crystalline perfection degree of multi‐walled carbon nanotubes: correlations between thermal kinetic analysis and micro‐Raman spectroscopy

Commercially available and laboratory‐prepared multi‐walled carbon nanotubes (MWCNTs) are systematically investigated by the use of micro‐Raman spectroscopy (MRS), thermogravimetric analysis (TGA) and complementary techniques (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) with the aim of establishing a standardised post‐growth diagnostic protocol for the assessment of their overall crystalline quality. By studying a set of ‘reference’ samples, clear correlations are evidenced between the Raman graphitisation indexes (D/G, G′/G and G′/D intensity ratios) commonly adopted to describe the crystalline arrangement of nanotubes, and their reactivity towards oxygen, as measured by the apparent activation energy needed for their oxidation, inferred from the kinetic analysis in quasi‐isothermal conditions. The higher the crystalline perfection degree, the higher the energy needed for oxidising them. The efficacy of the found correlations in indirectly assessing the reactivity of nanotubes prepared under different conditions is successfully demonstrated by the use of a second set of samples. The physical meaning and range of validity of the shown correlations are further discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Self‐assembly and structure formation in liquid crystalline phthalocyanine thin films studied by Raman spectroscopy and AFM

This work presents an investigation of films prepared by doctor blade casting, the formation of self‐assembled microstructures of a liquid crystalline phthalocyanine with highly oriented molecules. Raman Spectroscopy in combination with atomic force microscopy is applied to study the structures within the films. By keeping the substrate at room temperature or at 353 K during coating, different geometric structures namely rods and islands form. Rod‐like structures are growing in coating direction, whereas directional growth of the islands is not observed. The distribution of the rod lengths varies widely, whereas the width appears more uniform. Annealing of the samples shows a different behavior of the two textures. Islands tend to melt, and rods smooth their structural form, which is extracted from Raman imaging in combination with atomic force microscopy. Additionally, Raman imaging gives insight into laterally different relative crystallinity. These observations are discussed in the context of the molecular orientation as probed by polarized Raman spectroscopy. These polarized Raman spectra indicate azimuthal alignment of the molecules within the rods (edge on alignment). This alignment occurs along and also perpendicular to the growth direction. In contrast to the alignment in the rods, the molecules inside the islands occurring at higher temperature do not show preferential molecular orientation. After annealing, no preferential molecular orientation is observed in rods because of the loss of anisotropy, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.