Simulated Raman spectra of four tetraphenylbutadiene polymorphs

We report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4‐tetraphenyl‐1,3‐butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab‐initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4‐tetraphenyl‐1,3‐butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (C_i) or 2‐fold (C₂) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of C_i and C₂ conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra.     

Thermodynamics of reversible gas adsorption on alkali-metal exchanged zeolites--the interplay of infrared spectroscopy and theoretical calculations

Detailed understanding of weak solid-gas interactions giving rise to reversible gas adsorption on zeolites and related materials is relevant to both, fundamental studies on gas adsorption and potential improvement on a number of (adsorption based) technological processes. Combination of variable-temperature infrared spectroscopy with theoretical calculations constitutes a fruitful approach towards both of these aims. Such an approach is demonstrated here (mainly) by reviewing recent studies on hydrogen and carbon monoxide adsorption (at a low temperature) on alkali-metal exchanged ferrierite. However, the methodology discussed, which involves the interplay of experimental measurements and theoretical calculations at the periodic DFT level, should be equally valid for many other gas-solid systems. Specific aspects considered are the identification of gas adsorption complexes and thermodynamic studies related to standard adsorption enthalpy and entropy.     

PLS-BP法近红外光谱技术同时测定鲜乳中四种主成分

基于近红外光谱技术,将偏最小二乘法(Partial Least Squares,PLS)和单隐层的反向传播网络(Back-Propagation Network,BP)联用并测定了鲜乳中4种主成分含量.用PLS法将原始数据压缩为主成分,取前3个主成分的14个数据输入网络,以Kolmogorov定理为依据,经过实验确定中间层的神经元个数为29,初始训练迭代次数为1000,建立了脂肪、蛋白质、乳糖、牛乳总固体4种主成分含量的预测校正模型.PLS-BP模型对样品4个组分含量的预测决定系数(R2)分别为:0.961、0.974、0.951、0.997;本研究为近红外光谱技术在鲜乳多组分快速检测提供了新思路.     

High-resolution solid-state 2H NMR spectroscopy of polymorphs of glycine

High-resolution solid-state (2)H MAS NMR studies of the α and γ polymorphs of fully deuterated glycine (glycine-d(5)) are reported. Analysis of spinning sideband patterns is used to determine the (2)H quadrupole interaction parameters, and is shown to yield good agreement with the corresponding parameters determined from single-crystal (2)H NMR measurements (the maximum deviation in quadrupole coupling constants determined from these two approaches is only 1%). From analysis of simulated (2)H MAS NMR sideband patterns as a function of reorientational jump frequency (κ) for the -N(+)D(3) group in glycine-d(5), the experimentally observed differences in the (2)H MAS NMR spectrum for the -N(+)D(3) deutrons in the α and γ polymorphs is attributed to differences in the rate of reorientation of the -N(+)D(3) group. These simulations show severe broadening of the (2)H MAS NMR signal in the intermediate motion regime, suggesting that deuterons undergoing reorientational motions at rates in the range κ ≈ 10(4)-10(6) s(-1) are likely to be undetectable in (2)H MAS NMR measurements for materials with natural isotopic abundances. The (1)H NMR chemical shifts for the α and γ polymorphs of glycine have been determined from the (2)H MAS NMR results, taking into account the known second-order shift. Further quantum mechanical calculations of (2)H quadrupole interaction parameters and (1)H chemical shifts reveal the structural dependence of these parameters in the two polymorphs and suggest that the existence of two short intermolecular C-H···O contacts for one of the H atoms of the >CH(2) group in the α polymorph have a significant influence on the (2)H quadrupole coupling and (1)H chemical shift for this site.     

Localized quantification of anhydrous calcium carbonate polymorphs using micro-Raman spectroscopy

Micro-Raman spectroscopy is a powerful technique for qualitative and quantitative analysis of different mineral mixtures. In this paper, micro-Raman spectroscopy was used for quantification in local regions (180 × 180 μm area) of ternary mixtures of the synthetic calcium carbonate (CaCO₃) polymorphs (vaterite, aragonite, calcite) as well as CaCO₃ formed during the carbonation of nanolime suspension. The obtained results of localized quantification were in agreement with the detected concentrations obtained from bulk quantitative phase analysis of X-ray powder diffraction patterns. The detection limits were found to be below 0.5 wt.% for each CaCO₃ polymorphs. Through the use of 2D mapping, localized quantification of CaCO₃ polymorphs can be achieved. This information could be potentially useful for conservation of valuable Cultural Heritage objects, as it might influence the consolidation treatment chosen.     

Quantitative analysis of synthetic calcium carbonate polymorphs using FT-IR spectroscopy

Fourier Transform Infrared Spectroscopy (FT-IR) was used successfully for the simultaneous quantitative analysis of calcium carbonate phases (calcite, aragonite, vaterite) in ternary mixtures. From the FT-IR spectra of pure calcite, aragonite and vaterite powders with KBr, the absorptivities, α, of the absorption bands at 713 cm⁻¹ for calcite, 745 cm⁻¹ for vaterite, 713 and 700 cm⁻¹ for aragonite, were determined. In order to overcome the absorption band overlapping a set of equations based on Beer's law was developed. The detection limits were also established and found to be 1.1×10⁻⁴ mg calcite per mm² of pellet at 713 cm⁻¹, 3.6×10⁻⁴ mg aragonite per mm² of pellet at 700 cm⁻¹, 1.8×10⁻⁴ mg aragonite per mm² of pellet at 713 cm⁻¹ and 3.1×10⁻⁴ mg vaterite per mm² of pellet at 745 cm⁻¹. Analysis of a known ternary mixture of calcium carbonate polymorphs tested the validity of the method.     

Raman and infrared spectroscopy of selected vanadates

Raman and infrared spectroscopy has been used to study the structure of selected vanadates including pascoite, huemulite, barnesite, hewettite, metahewettite, hummerite. Pascoite, rauvite and huemulite are examples of simple salts involving the decavanadates anion (V10O28)6-. Decavanadate consists of four distinct VO6 units which are reflected in Raman bands at the higher wavenumbers. The Raman spectra of these minerals are characterised by two intense bands at 991 and 965 cm(-1). Four pascoite Raman bands are observed at 991, 965, 958 and 905 cm(-1) and originate from four distinct VO6 sites. The other minerals namely barnesite, hewettite, metahewettite and hummerite have similar layered structures to the decavanadates but are based upon (V5O14)3- units. Barnesite is characterised by a single Raman band at 1010 cm(-1), whilst hummerite has Raman bands at 999 and 962 cm(-1). The absence of four distinct bands indicates the overlap of the vibrational modes from two of the VO6 sites. Metarossite is characterised by a strong band at 953 cm(-1). These bands are assigned to nu1 symmetric stretching modes of (V6O16)2- units and terminal VO3 units. In the infrared spectra of these minerals, bands are observed in the 837-860 cm(-1) and in the 803-833 cm(-1) region. In some of the Raman spectra bands are observed for pascoite, hummerite and metahewettite in similar positions. These bands are assigned to nu3 antisymmetric stretching of (V10O28)6- units or (V5O14)3- units. Because of the complexity of the spectra in the low wavenumber region assignment of bands is difficult. Bands are observed in the 404-458 cm(-1) region and are assigned to the nu2 bending modes of (V10O28)6- units or (V5O14)3- units. Raman bands are observed in the 530-620 cm(-1) region and are assigned to the nu4 bending modes of (V10O28)6- units or (V5O14)3- units. The Raman spectra of the vanadates in the low wavenumber region are complex with multiple overlapping bands which are probably due to VO subunits and MO bonds.     

Rapid discrimination of Eurycoma longifolia extracts by Fourier transform infrared spectroscopy and two dimensional correlation infrared spectroscopy

Eurycoma longifolia is a herbal medicinal plant of South-East Asian origin, popularly recognized as ‘Tongkat Ali’. The root extracts have been used in indigenous traditional medicines for its unique antimalarial, anti-pyretic, antiulcer, cytotoxic and aphrodisiac properties. It is an important task that fast and effective analysis methods monitor the inherent qualities of traditional herbal medicines and its corresponding extracts products as a complicated mixture system. Owing to the unique fingerprint character and extensive applicability to test sample, infrared spectral method have been used in many research fields. In this paper, we use FT-IR, second derivative infrared spectroscopy and two-dimensional correlation infrared spectroscopy (2D-IR) step by step to analyze E. longifolia and its different extracts (extracted by hexane, ethyl acetate, dichloromethane and methanol in turn). The findings indicated that FT-IR and 2D-IR can provide many holistic variation rules of chemical constituents. The structural information of the samples indicated that E. longifolia and its extracts contain a large amount of quassinoids, since some characteristic absorption peaks of quassinoids, such as ∼1700 cm⁻¹, ∼1670 cm⁻¹, ∼1600 cm⁻¹,∼1500 cm⁻¹, and ∼1270 cm⁻¹ can be detected. This method, having its high resolution and excellent macroscopic fingerprint features, can not only supply lots of structural information of main components in the complicated system, but also can differentiate the tiny differences between the similar systems according to the macro-fringerprint characters. This method is highly rapid, effective, accurate and well repetitive for pharmaceutical research.     

Nanofoam porosity by infrared spectroscopy

Infrared spectroscopy has been used to determine the porosity of polymer nanofoams produced from block copolymers of an aromatic polyimide with either poly(propylene oxide) or poly(α-methyl styrene). It is shown that, with an independent measurement of the film thickness, both the absorption bands and the interference fringes can yield an accurate measure of the void content. The results obtained are in quantitative agreement with density gradient methods. © 1995 John Wiley & Sons, Inc.     

Diffuse-reflectance spectroscopy in the infrared

The theory, and the problems encountered in the development of diffuse-reflectance spectroscopy in the infrared region as an analytical technique, are reviewed. The introduction of Fourier-transform infrared spectrometers has eliminated the difficulty of detecting small scattered intensities, and diffuse-reflectance measurement is now a routine method. The first commercial instruments are now available.     

Polarization fourier transform infrared spectroscopy

Some Fourier transform infrared spectroscopic studies with linearly and circularly polarized radiation are reported. They were all carried out by means of the differential technique, consisting in the comparison of two single-channel spectra of opposite polarization. Thus, linear dichroic or circular dichroic spectra with positive and negative bands were recorded. Typical application examples of both techniques are presented.     

Characterization and quantitation of clarithromycin polymorphs by powder X-ray diffractometry and solid-state NMR spectroscopy

Characterization of clarithromycin polymorph was performed by solid-state cross polarization and magic angle spinning (CP/MAS) 13C-NMR spectroscopy. Two polymorphs, form II and form I, of clarithromycins indicated characteristic resonances of C1 carbonyl carbon at 176.2 and 175.2 ppm, respectively. Since each peak of C1 carbon was well separated in the spectrum of the two polymorphs, we performed quantitative analysis of the polymorphic fraction from the peak area of these peaks. The peak area of form I was found to linearly increase with an increase of its content, with a correlation coefficient of above 0.99. Solid-state NMR was found to be a useful technique to determine the characteristics of the polymorphic forms.     

Photoacoustic infrared spectroscopy of polymer beads

Photoacoustic (PA) spectra of four types of polymer resin beads, ranging in size from 35 to 150 μm, were acquired using a Fourier transform infrared spectrometer capable of both rapid- and step-scan mirror movement. Thermal diffusion lengths were on the order of the particle sizes of the beads. The PA magnitude spectra were similar to absorption spectra; both positive- and negative-going features occurred in the phase spectra. The frequency dependences of the total PA intensities of the polymer resins and carbon black differed by a factor of about f^−0.30. The intensities of the weak bands in the ratioed spectra (resin beads/carbon black) displayed a similar dependence. Partial saturation caused a more gradual variation for the stronger bands, where the intensity is proportional to f^−0.1–f^−0.2.     

Determination of hexacelsian by infrared spectroscopy

Hexacelsian has been determined by infrared spectroscopy with KBr discs and K(4)Fe(CN)(6) as internal standard. A KBr particle size of <40 mum gave better homogenization of the sample-KBr mixture than a particle size in the 40-70 mum range. For determinations of hexacelsian in synthetic samples containing amorphous phase or celsian, calibration curves were constructed. A least-squares fit yielded correlation coefficients of 0.998 and 0.997.     

Rotationally resolved infrared spectroscopy of adamantane

We present the first rotationally resolved spectra of adamantane (C(10)H(16)) applying gas-phase Fourier transform infrared (IR) absorption spectroscopy. High-resolution IR spectra are recorded in the 33-4500 cm(-1)range using as source of IR radiation both synchrotron radiation (at the AILES beamline of the SOLEIL synchrotron) as well as a classical globar. Adamantane is a spherical top molecule with tetrahedral symmetry (T(d) point group) and has no permanent dipole moment in its vibronic ground state. Of the 72 fundamental vibrational modes in adamantane, only 11 are IR active. Here we present rotationally resolved spectra for seven of them: ν(30), ν(28), ν(27), ν(26), ν(25), ν(24), and ν(23). The typical rotational structure of spherical tops is observed and analyzed using the STDS software developed in the Dijon group, which provides the first accurate energy levels and rotational constants for seven fundamental modes. Rotational levels with quantum numbers as high as J = 107 have been identified and included in the fit leading to a typical standard deviation of about 10(-3) cm(-1).     

Micro methods in infrared and Raman spectroscopy

Summary Employing the concept of the optical conductance [1–3] and taking into account the spectroscopic properties of the sample, firstly the features of sample arrangements are given, which are using minimal amounts of sample determined by the condition of complete illumination of the different types of spectrometers and using different techniques for infrared, near-infrared and Raman micro spectroscopy. Secondly, arrangements of a micro sample are discussed, the minimal size of which is determined by the laws of diffraction. In this case the spectrometers are usually not fully illuminated. The minimal amount of sample necessary is for infrared microscopy in the order of 1000 m³ (1 pg), for Raman microscopy of the order 100 m³ (0.1 pg). Examples demonstrate the state of the art and finally developments, which can be anticipated, are outlined.     

Fourier-transform infrared photoacoustic spectroscopy of zeolites

Photoacoustic detection is shown to be very valuable for obtaining infrared-spectroscopic data for samples with characteristics that involve very difficult sampling. The modification reactions on the zeolite mordenite, a highly scattering material, are investigated by this technique and by gas-adsorption experiments. The mechanism of the reactions between borane groups inside the channels of the mordenite and amines at different reaction temperatures is described. At low temperatures, amine- and amino-boranes are formed that polymerize at elevated temperatures if steric hindrance is absent. The implantation of boron-nitrogen compounds inside the channels of the mordenite allow a continuous and controlled variation of the accessibility of the mordenite for gas molecules such as krypton and xenon at 273 K.     

First-principles study of the four polymorphs of crystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

The electronic structure and vibrational properties of the four polymorphs of crystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) have been studied using density functional theory within the local density approximation. The results show that the states of N in the ring make more important contributions to the valence bands than these of C and N of NO2 and so N in the ring acts as an active center. From the low frequency to high-frequency region, the molecular motions of the vibrational frequencies for the four HMX polymorphs present unique features. It is also noted that there is a relationship between the band gap and impact sensitivity for the four HMX polymorphs. From the cell bond order per unit volume, we may infer the variation order of crystal bonding for the four polymorphs and so predict their impact sensitivity order as follows: beta-HMX < gamma-HMX < alpha-HMX < delta-HMX, which is in agreement with their experimental order.