Raman Band Shapes Research Articles

Non-destructive distinction between geogenic and anthropogenic calcite by Raman spectroscopy combined with machine learning workflow.

Here, we demonstrate, for the first time, the possibility of distinguishing between geogenic and anthropogenic calcite in a non-destructive and effective way. Geogenic calcite derives from natural sedimentary and metamorphic rocks whereas anthropogenic calcite is formed artificially due to the carbonation process in mortars and plaster lime binders. Currently, their distinction is a major unaddressed issue although it is crucial across several fields such as 14C dating of historical mortars to avoid contamination with carbonate aggregates, investigating the origins of pigments, and studying the origins of sediments, to name a few. In this paper, we address this unmet need combining high-resolution micro-Raman spectroscopy with data mining and machine learning methods. This approach provides an effective means of obtaining robust and representative Raman datasets from which samples' origins can be effectively deduced; moreover, a distinction between sedimentary and metamorphic calcite has been also highlighted. The samples, chemically identical, exhibit systematic and reliable differences in Raman band positions, band shape and intensity, which are likely related to the degree of structural order and polarization effects.

A new model for the calculation of homogeneously broadened Raman spectral bandwidths and bandshapes

AbstractRaman spectral bandwidths and bandshapes can provide detailed information on the physics and chemistry of a sample. The Raman band reported by a spectrometer (observed Raman band [ORB]) is a result of contributions from the true, intrinsic Raman band (IRB) and the spectrometer slit function. To obtain the IRB spectral width and shape, the effects of the spectrometer slit function must be removed from the ORB by measuring the spectrometer slit function and modeling the ORB as a convolution of the IRB and slit function. Though this model has proven to be useful, it is not based on physical parameters of the measurement and is limited. Thus, new ways to model Raman bandshape and bandwidth are needed. I previously developed a new method of modeling Raman bands using spectrometer geometric optics to examine the dependence of Raman band intensity on slit width. The model calculates the intensity of the ORB from the contributions of a continuum of dispersed slit functions. Here, I extend the use of this model for examining ORB spectral widths and shapes. I show that the model produces the same result as the convolution model when using Lorentzian Raman and Gaussian slit functions, confirming the validity of the model. However, in contrast to the convolution model, the new model is based on spectrometer geometric optics allowing one to account for various optical effects when modeling Raman spectra. This is demonstrated by modeling the effects of slit width on the Raman spectrum. I use the model to provide a detailed, physical understanding of the effects of slit width and IRB width on Raman bandshapes.

Micro‐Raman study of crichtonite group minerals enclosed into mantle garnet

AbstractWe report the first comprehensive micro‐Raman study of crichtonite group minerals (CGM) as inclusions in pyropic garnet grains from peridotite and pyroxenite mantle xenoliths of the Yakutian kimberlites as well as in garnet xenocrysts from the Aldan shield lamprophyres (Russia). The CGM form (i) morphologically oriented needles, lamellae, and short prisms and (ii) optically unoriented subhedral to euhedral grains, either single or intergrown with other minerals. We considered common mantle‐derived CGM species (like loveringite, lindsleyite, and their analogues), with Ca, Ba, or Sr dominating in the dodecahedral A site and Zr or Fe in the octahedral B site. The Raman bands at the region of 600–830 cm−1 are indicative of CGM and their crystal‐chemical distinction, although the intensity and shape of the bands appear to be dependent on laser beam power and wavelength. The factor‐group analysis based on the loveringite crystal structure showed the octahedral and tetrahedral cation groups with 18f and 6c Wyckoff positions, namely, dominantly TiO6 and to a lower extent CrO6, MgO4, and FeO4 groups, to be the major contributors to the Raman spectral features. The ionic groups with dodecahedral (M0) and octahedral (M1) coordination are inactive for Raman scattering while active in infrared absorption. A number of observed Raman modes in the CGM spectra are several times lower than that predicted by the factor group analysis. The noticed broadening of modes in the CGM Raman spectra may result from a combining of bands at the narrow frequency shift regions. Solid solution behavior, luminescence, and partial metamictization of the CGM may exert additional influence on the Raman band shape. The Raman spectral features showed CGM to be accurately identified and distinguished from other Ti‐, Fe‐, Cr‐, and Zr‐containing oxides (e.g., ilmenite or those of spinel and magnetoplumbite groups) occurring as accessory mantle minerals.

On the Regular Behavior of a Binary Mixture of Ionic Liquids.

Binary mixtures of the ionic liquids 1-ethyl-3-methylimidazolium dicyanamide and tributyl(methyl)ammonium dicyanamide, [C2C1im]x[N4441](1-x)[N(CN)2], are studied by means of their excess properties (enthalpy and volume), their viscosity, and their Raman spectra. The mixtures exhibit positive values of excess volume VE and excess enthalpy HE. The plot of the logarithm of viscosity as a function of composition is consistent with the finding HE > 0. The excess thermodynamic properties of the ionic liquid mixtures are compared with well-known results for binary mixtures of (high-temperature) molten salts. The asymmetry of the curve HE versus composition is reproduced by considering the volumetric fraction of the [C2C1im]x[N4441](1-x)[N(CN)2] mixtures according to basic results of the theory of regular solutions. The anion totally symmetric stretching mode νs(C≡N) was used to probe the local environment around the anion in a Raman spectroscopy investigation of the [C2C1im]x[N4441](1-x)[N(CN)2] mixtures as a function of the composition. The Raman band shape (peak frequency and bandwidth) of the νs(C≡N) mode changes as expected from the fluctuation of concentration around the probe oscillator according to models for vibrational spectroscopy of liquid mixtures. This thermodynamic and spectroscopic study leads to the classification of the [C2C1im]x[N4441](1-x)[N(CN)2] mixture as a regular solution. The application of the regular solution equation for HE to other ionic liquid binary mixtures is discussed.

Raman bandshape analysis and DFT study of C[dbnd]O and C[sbnd]H stretching modes of NN-Dibutyl formamide in DMSO solvent`

The study reports the inter-molecular interactions and dynamics of CO and CH stretching modes of NN- Dibutyl Formamide (DBF) in DMSO solvent using Raman spectroscopic technique. The Raman band of CO and CH stretching modes have been deconvoluted into two distinct bands for neat as well as in DMSO solvent. Peak wavenumbers of CO stretching modes show red shift while CH stretching modes shows blue shift with the increase in solvent concentrations. The optimized geometric parameters, vibrational wavenumbers, Mulliken atomic charges and natural bond orbitals of the molecule has been computed using Density Functional Theory (DFT) method with basis set 6-31 +G (d, p). In addition, the same basis set has been carried out with counterpoise keyword accounting BSSE calculation on monomer and dimer states with DMSO solvent to present an appropriate interacting environment. IEF-PCM solvation model has also been computed using the same basis set and compares the geometrical parameters and vibrational wavenumbers of the molecules and in their complexes. In order to get a complete study of the DBF + DMSO complexes, explicit solvation model has also been calculated for Monomer DBF in two solvent molecules. Theoretical calculations of frequencies have been compared with the experimental findings and the results are found in good agreement.

Raman spectroscopy as a tool of early dental caries detection–new insights

The aim of this study was to investigate the effectiveness of Raman spectroscopy in determination of changes in composition and structure of human tooth enamel with caries by use of new parameters such as the Raman band position, the full width at half maximum of Raman band and line shape of Raman band. The values of spectroscopic parameters were obtained by analysis of Raman scattering spectra. We proposed to base the diagnosis on several parameters determined by simultaneous analysis of Raman scattering, Rayleigh scattering and fluorescence emission. We verified the correlations between the values of the few selected parameters and the severity of dental caries. Raman band spectral parameters, such as position and line shape, can be useful to indicate the enamel caries changes. Moreover, it is possible to detect early caries by analysis of background in the Raman scattering spectrum related to the Rayleigh scattering and fluorescence emission. We demonstrated that a few independent parameters obtained by simultaneous analysis of three different physical phenomena could allow unambiguous determination of the early caries occurrence in the enamel. The published results give new insights into early caries detection and demonstrate the new possibilities of Raman spectroscopy as a diagnostic tool to recognize the caries disease. Copyright © 2017 John Wiley & Sons, Ltd.

Raman band shape analysis of cyanate-anion ionic liquids

Raman band shape analysis of the C≡N stretching mode was carried out for ionic liquids based on [SCN]−, [N(CN)2]−, [C(CN)3]−, and [B(CN)4]−, with a common cation, 1-ethyl-3-methylimidazolium, [C2C1im]+. Vibrational and reorientational time correlation functions were obtained by Fourier transforming isotropic and anisotropic Raman spectra. Comparison is provided between results for [C2C1im][SCN] and literature data available for molten alkali thiocyanates. Vibrational dephasing of the ωCN mode in [C2C1im][SCN] belongs to the regime of inhomogeneous broadening as it is determined by the distribution of vibrational frequencies, <Δω2>. Vibrational dephasing in [N(CN)2]−, [C(CN)3]−, and [B(CN)4]−, depends on both the <Δω2> and the correlation time of frequency fluctuation. Decay rates of reorientational time correlation functions within the range of 1.0ps do not correlate with the viscosity of these ionic liquids. Molecular dynamics simulations of these ionic liquids provide support to the experimental finding that the reorientational time correlation function of [SCN]− decays faster than the other anions. The Raman band shape analysis suggests that the short-time dynamics of these ionic liquids is structure-limited because of steric hindrance.

Ultraviolet Raman spectra of few nanometer thick silicon-on-insulator nanofilms: Lifetime reduction of confined phonons

We show that in Si-on-insulator (SOI) nanofilms (NFs), phonon-confinement-induced longitudinal optical (LO) phonon Raman band asymmetric broadening is much weaker than thickness-inversely-proportional symmetric broadening associated with phonon lifetime reduction. We discuss the origin of the effect, determine LO phonon lifetime in SOI NFs and modify Richter-Campbell-Fauchet model describing the confined-phonon Raman band shape by taking the symmetric broadening into account. We also discuss connection of the effect with SOI NF thermal conductivity reduction.

Raman bandshape analysis on C[sbnd]H and CSC stretching modes of dimethyl sulfoxide in liquid binary mixture: Comparative study with quantum-chemical calculations

The interacting nature of dimethyl sulfoxide (DMSO) in binary mixtures has been carried out on CH and CSC stretching modes of vibration using chloroform (CLF), chloroform-d (CLFd), acetonitrile (ACN) and acetonitrile-d3 (ACNd) solvents. Peak frequencies of both the stretching modes show blue shift with the increase in solvent concentration. Variation of Raman bandwidth with the solvent concentration was discussed using different mechanisms. Ab initio calculation for geometry optimization and vibrational wavenumber calculation have been performed on monomer and dimer structures of DMSO to explain the experimentally observed Raman spectra. Theoretically calculated values are found in good agreement with the experimental results. Vibrational and reorientational relaxation times have been studied corresponding to solvent concentrations to elucidate the interacting mechanisms of binary mixtures.

Communication: three-dimensional model for phonon confinement in small particles: quantitative bandshape analysis of size-dependent Raman spectra of nanodiamonds.

Raman spectroscopy of nano-scale materials is facing a challenge of developing a physically sound quantitative approach for the phonon confinement effect, which profoundly affects the phonon Raman band shapes of small particles. We have developed a new approach based on 3-dimensional phonon dispersion functions. It analyzes the Raman band shapes quantitatively in terms of the particle size distributions. To test the model, we have successfully obtained good fits of the observed phonon Raman spectra of diamond nanoparticles in the size range from 1 to 100 nm.

Model based variable selection as a tool to highlight biological differences in Raman spectra of cells.

In vitro Raman spectroscopy used for non-invasive, non-destructive characterization of single cells and tissues has proven to be a powerful tool for understanding the complex biochemical processes within these biological systems. Additionally it enables the comparison of a wide range of in vitro model systems by discriminating them based on their biomolecular differences. However, one persistent challenge in Raman spectroscopy has been the highly complex structure of cell and tissue spectra, which comprise signals from lipids, proteins, carbohydrates and nucleic acids, which may overlap significantly. This leads to difficulty in discerning which molecular components are responsible for the changes seen between experimental groups. To address this problem, we introduce a technique to highlight the significant biochemical changes between sample groups by applying a novel approach using Partial Least Squares - Discriminant Analysis (PLS-DA) Variable Importance Projection (VIP) scores normally used for variable selection as heat maps combined with group difference spectra to highlight significant differences in Raman band shapes and position. To illustrate this method we analyzed single HeLa cells in their live, fixed, fixed and ethanol dehydrated, to the fixed, dehydrated and then rehydrated states respectively. Fixation, ethanol dehydration and rehydration are known to induce molecular changes in the lipids and proteins within each cell.

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6](-) anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], which crystallizes above ~0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, <Δω(2)>, and the relaxation time of frequency fluctuation, τc, as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

Reversible Loss of Bernal Stacking during the Deformation of Few-Layer Graphene in Nanocomposites

The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (∼0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.

Solvent dependent Raman bandshape analysis on C[dbnd]O containing molecules: Vibrational relaxation study

Solvent dependent Raman band studies of some carbonyl containing molecules have been carried out in chlorobenzene (CLB) and chloroform (CLF) solvents. The variation of bandwidth with the solvent concentration was discussed using different mechanisms. The vibrational and reorientational correlation times were studied corresponding to solvent concentrations. The reorientational correlation time follows with the liquid viscosity in molecules under study. It reflects about the nature of solvent molecules hindering to the orientational motion of solute in liquid solution.

Environmental Effects on the Lignin Model Monomer, Vanillyl Alcohol, Studied by Raman Spectroscopy

Structural analysis of plant materials, i.e., lignin, cellulose, hemicellulose, etc., supports the growing interest of their uses, e.g., as sources for biofuels or materials. Lignin is a main polymer formed from three phenolic presursors, containing none, one, or two OMe groups, i.e., H, G, and S units, respectively. Raman spectroscopy gives valuable knowledge on lignin and has a large potential for further developments. Thus in the present work we show how the use of electronic structure theory can support the study of environmental effects on lignin Raman bands. Raman spectra of the lignin model monomer, vanillyl alcohol (G type), dissolved in different solvents were compared to investigate such effects on the Raman band shapes and positions. Density functional theory combined with the polarizable continuum model were applied to assign the observed bands and tested for prediction accuracy. Two ring deformation modes at ∼1600 cm(-1) showed strong dependency on solvent ability to act as hydrogen bond donor, and this has to be considered in addition to substitutional effects on these modes.

Raman spectroscopy of sol–gel derived titanium oxide thin films

AbstractRaman spectra of TiO2 films prepared via the sol–gel process were studied by UV and visible Raman spectroscopy. The evolution of the phases of TiO2 films during annealing was investigated, and the relative intensities of the Raman bands excited with 325 nm were found to be distinct from those of the bands excited with 514 nm. The transmittance and FTIR spectra of the films annealed at different temperatures were characterized. The crystallization process of the powders and thin films treated by different annealing methods were also studied with Raman spectroscopy. The results show that the change in the relative intensities is caused by the resonance Raman effect. The anatase to rutile transition of the powder occurs at 700 °C, while that of the thin film occurs at 800 °C. The analysis of Raman band shape (peak position and full width at half‐maximum) after conventional furnace annealing and rapid thermal annealing indicates the influence of the non‐stoichiometry and phonon confinement effect. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Raman spectra of acetonitrile in imidazolium ionic liquids

AbstractRaman spectra of dilute solutions of acetonitrile in ionic liquids reveal the characteristic features of ionic liquids' polarity. This is accomplished by investigating the Raman bandshape of the ν (CN) band, corresponding to the CN stretching mode of CH3CN, which is a very sensitive probe of the local environment. The amphiphilic nature of the CH3CN molecule allows us to observe the effect of electron pair acceptor and electron pair donor characteristics on ionic liquids. It has been found that the overall polarity of nine different ionic liquids based on 1‐alkyl‐3‐methylimidazolium cations is more dependent on the anion than cation. The observed wavenumber shift of the ν (CN) band of CH3CN in ionic liquids containing alkylsulfate anions agrees with the significant different values previously measured for the dielectric constant of these ionic liquids. The conclusions obtained from the analysis of the ν (CN) band were corroborated by the analysis of the symmetric ν1 (CD3 ) stretching mode of deuterated acetonitrile in different ionic liquids. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Double vibrational collision-induced Raman scattering by SF6–N2: Beyond the point-polarizable molecule model

Collision-induced Raman bandshapes and zeroth-order spectral moments are calculated both for the depolarized spectrum and for the extremely weak isotropic spectrum of the SF${}_{6}$(${\ensuremath{\nu}}_{1}$) $+{\mathrm{N}}_{2}$(${\ensuremath{\nu}}_{1}$) double-Raman-scattering band. A critical comparison is made with experiments conducted recently by the authors [Phys. Rev. A 81, 012702 (2010); 81, 042705 (2010)]. The study of this transition, hitherto restricted to the model framework of two point-polarizable molecules, is now completed to incorporate effects beyond the point-molecule approximation. Whereas the extended model offers a few percent improvement in the depolarized spectrum, it reveals a huge $80%$ increase in the isotropic spectrum and its moment, owing essentially to the polarizability anisotropy of ${\mathrm{N}}_{2}$. For both spectra, agreement between quantum-mechanical calculations and our experiments is found, provided that the best ab initio data for the (hyper)polarizability parameters are used. This refined study shows clearly the need to include all mechanisms and data to a high level of accuracy and allows one to decide between alternatives about difficult and controversial issues such as the intermolecular potential or the sensitive Hamaker force constants.

Why the Extended-Spectrum β-Lactamases SHV-2 and SHV-5 Are “Hypersusceptible” to Mechanism-Based Inhibitors

Extended-spectrum beta-lactamases (ESBLs) are derivatives of enzymes such as SHV-1 and TEM-1 that have undergone site-specific mutations that enable them to hydrolyze, and thus inactivate, oxyimino-cephalosporins, such as cefotaxime and ceftazidime. X-ray crystallographic data provide an explanation for this in that the mutations bring about an expansion of the binding pocket by moving a beta-strand that forms part of the active site wall. Another characteristic of ESBLs that has remained enigmatic is the fact that they are "hypersusceptible" to inhibition by the mechanism-based inactivators tazobactam, sulbactam, and clavulanic acid. Here, we provide a rationale for this "hypersusceptibility" based on a comparative analysis of the intermediates formed by these compounds with wild-type (WT) SHV-1 beta-lactamase and its ESBL variants SHV-2 and SHV-5, which carry the G238S and G238S/E240K substitutions, respectively. A Raman spectroscopic analysis of the reactions in single crystals shows that, compared to WT, the SHV-2 and SHV-5 variants have relatively higher populations of the stable trans-enamine intermediate over the less stable and more easily hydrolyzable cis-enamine and imine co-intermediates. In solution, SHV-2 and SHV-5 also form larger populations of an enamine species compared to SHV-1 as detected by stopped-flow kinetic experiments under single-turnover conditions. Moreover, a simple Raman band shape analysis predicts that the trans-enamine intermediates themselves in SHV-2 and SHV-5 are held in more stable, rigid conformations compared to their trans-enamine analogues in WT SHV-1. As a result of this stabilization, more of the trans-enamine intermediate is formed, which subsequently lowers the K(I) values of the mechanism-based inhibitors up to 50-fold in SHV-2 and SHV-5.

Ultrafast Protonation/Deprotonation Dynamics of N,N-Dimethylacetamide in Hydrochloric Acid As Studied by Raman Band Shape Analysis

The protonation/deprotonation dynamics of N,N-dimethylacetamide (DMAA) in hydrochloric acid has been studied with a Raman band shape analysis based on the two-state dynamic exchange model. The observed Raman band shape changes of the symmetric and antisymmetric CC/CN stretch modes with acid concentration have been successfully interpreted in terms of the ultrafast dynamic exchange between DMAA and its protonated form DMAAH+. It is confirmed that the protonation takes place on the oxygen atom rather than on the nitrogen atom. Quantitative information on the exchange dynamics between DMAA and DMAAH+ has been obtained from the curve fitting of the observed band shapes. The protonation process follows the first-order kinetics with a rate constant of k1 = (10.1 ± 0.2) × 1010 s−1 M−1, which is slightly larger than the value expected for a simple diffusion-controlled reaction and which can be explained in terms of the proton transfer through the hydrogen-bonding network. The mean lifetime of the protonated form DMAAH+ is strongly dependent on the acid concentration; it is several picoseconds in concentration lower than 0.5 mol dm−1 but is prolonged to a few tens of picoseconds in concentrations higher than 2 mol dm−1. It is also shown that the static equilibrium model, which corresponds to the slow exchange limit, fails to explain the observed band shape changes with acid concentration and that they can be explained if and only if the effect of ultrafast exchange is taken into account.