Characteristic Raman Bands Research Articles

Optical spectroscopy on Se clusters and chains confined in zeolites.

The incorporation of selenium into the supercages of zeolite Y leads to the formation of Se(8) rings and distorted Se chains in a ratio which is influenced by the nature of the zeolitic cations. Here we review Raman (including resonance Raman) and UV/vis absorption spectroscopy results on Se encapsulated into a number of cation-exchanged faujasite zeolites. Both rings and chains give rise to characteristic Raman bands. In particular, low-frequency Raman bands are attributed to localized vibrations in ordered segments of distorted chains. The UV/vis absorption spectra indicate an opening of the band gap of selenium upon confinement in these zeolites. This can be reversed through electronic interaction with zeolite cations.

Fourier transform Raman and infrared spectroscopy of pectins

The FT-Raman and FT-IR spectra of polygalacturonic (pectic) acid, potassium pectate and its derivatives, as well as commercial citrus and sugar beet pectins were measured and interpreted. Methyl and acetyl esters of potassium pectate derivatives have several characteristic Raman and IR bands that allow both this groups to be distinguished. The very intense Raman band at 857 cm −1 is sensitive to the state of uronic carboxyls and to O-acetylation. The wavenumber of this band decreases with methylation (min. 850 cm −1) and increases with acetylation (max. 862 cm −1). The acetylation of potassium pectate, as well as its acetylation together with methylation, causes drastic changes in the Raman spectra in the region below 700 cm −1. Sugar beet pectin, but not citrus pectin, showed Raman bands at 1633 and 1602 cm −1 and IR band at 1518 cm −1. All these bands rise from feruloyl groups and can be used for identification of pectins containing feruloyl groups.

Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces

Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute solutions with silver and copper surfaces in situ. Using silver as a model surface which supports strong SERS with a 514.5 nm laser, it was possible to observe at the near monolayer level, and the spectra were compared to those from oxyhydroxide species and mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to solution, the characteristic Cu oxide Raman bands disappeared, and a band increased in intensity over a period of ∼20 h. The intensity of the band on Cu became self-limiting after the formation of several monolayers, as supported by chronoamperometry experiments. This spectrum was stable after was removed from the solution provided the potential remained negative of −200 mV vs. Ag/AgCl. The results support the conclusion that is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of formation is the substitution lability of compared to The bond can be broken much more rapidly than the substitution inert bond, making formation of mixed oxide kinetically favorable. Once reduced to however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile form to an insoluble, substitution inert oxyhydroxide. Furthermore, reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable film with a thickness of a few monolayers. © 2003 The Electrochemical Society. All rights reserved.

Raman Spectra, Crystal Polymorphism, and Structure of a Prototype Ionic-liquid [bmim]Cl

Abstract A prototype ionic-liquid [bmim]Cl shows crystal polymorphism, having two crystal forms, Crystal (1) and Crystal (2), characterized by distinct X-ray powder patterns and Raman spectra. Two sets of characteristic Raman bands of Crystals (1) and (2), which are mutually exclusive with each other, do coexist in the spectrum of liquid [bmim]Cl. It seems that two distinct structures of the [bmim]+ ion, one corresponding to that in Crystal (1) and the other to that in Crystal (2), exist simultaneously in the ionic liquid state.

Metal binding and oxidation of amyloid-beta within isolated senile plaque cores: Raman microscopic evidence.

Alzheimer's disease (AD) is characterized by the deposition of amyloid plaques in the parenchyma and vasculature of the brain. Although previous analytical studies have provided much information about the composition and structure of synthetic amyloid-beta fibrils, there is, surprisingly, a dearth of data on intact amyloid plaques from AD brain. Therefore, to elucidate the structure and detailed composition of isolated amyloid plaque cores, we utilized a high-resolution, nondestructive technique, Raman microscopy. The data are of very high quality and contain detailed information about protein composition and conformation, about post-translational modification, and about the chemistry of metal binding sites. Remarkably, spectra obtained for senile plaque (SP) cores isolated from AD brain are essentially identical both within and among brains. The Raman data show for the first time that the SP cores are composed largely of amyloid-beta and confirm inferences from X-ray studies that the structure is beta-sheet with the additional possibility that this may be present as a parallel beta-helix. Raman bands characteristic of methionine sulfoxide show that extensive methionine oxidation has occurred in the intact plaques. The Raman spectra also demonstrate that Zn(II) and Cu(II) are coordinated to histidine residues in the SP cores, at the side chains' N(tau) and N(pi) atoms, respectively. Treatment of the senile plaques with the chelator ethylenediaminetetraacetate reverses Cu binding to SP histidines and leads to a broadening of amide features, indicating a "loosening" of the beta-structure. Our results indicate that Abeta in vivo is a metalloprotein, and the loosening of the structure following chelation treatment suggests a possible means for the solubilization of amyloid deposits. The results also reveal a direct chemical basis for oxidative damage caused by amyloid-beta protein in AD.

Sensitization of TiO2 by Polypyridine Dyes : Role of the Electron Donor

Dye-sensitized photoelectrochemical cells (DSSC) are characterized by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy during their polarization. Cells realized with a dye recently synthesized in one of our laboratories, containing two terpyridyl (terpy) ligands, are compared with cells using commercial dyes (Ru535 and Ru620) containing isothiocyanates and either bipyridyl (bpy) or terpy ligands. Here, two points are emphasized, first, the role of the functional group (carboxylate or phosphonate) which ensures the linkage to and, second, the role of the redox couple present in the electrolyte which can react with the dye D to give unwanted intermediate species. Two species, each of them giving a characteristic Raman band in the low wavenumber range, are characterized by Raman spectroscopy. The first of these species is triiodide; the nature of the second one, which directly implies the oxidized form of dye, is discussed. During the DSSC functioning, EIS allows one to discriminate three potential ranges, the photocurrent plateau, the recombination range, and the direct current range when the voltage decreases from anodic to cathodic. The second intermediate exists only in the photocurrent plateau, while exists also in the recombination range. These results do not depend on the nature (bpy or terpy) of the ligand. © 2003 The Electrochemical Society. All rights reserved.

Inorganic-Organic Cross-Linking in UV Curable Hard Coats Based Upon Vinyltriethoxysilane-Tetraethoxysilane-Polyfunctional Acrylate Hybrid Polymers: A Raman Spectroscopic Study

Hydrolysis, polycondensation and UV-induced radical polymerization processes in binary and ternary mixtures of vinyltriethoxysilane (VTEOS), tetraethoxysilane (TEOS), and several polyfunctional acrylates as reactive diluents were studied by means of FT-Raman and confocal micro-Raman spectroscopy. The study comprises measurements on sols, gels, xerogels, and thin films applied on top of glass slides and polymeric substrates. Characteristic Raman bands are utilized to gain information about the structural evolution, inorganic network connectivity, and organic cross-linking reactions. Supplementary, 29Si-NMR spectroscopic data are considered and correlated with Raman data. Structure-property correlations based on spectroscopic and mechanical data are outlined and discussed. It is demonstrated that thin hybrid polymer films may be studied in-situ by means of confocal micro-Raman spectroscopy.

Characterization of a new non-centrosymmetric polymorph of diphenyl-1,3,4-oxadiazole

Diphenyl-1,3,4-oxadiazole (DPO) crystallization experiments from solutions clearly reveal the polymorphism of the substance. Besides the formerly known centrosymmetric monoclinic structure with space group P2 1/ c (DPO I) a new monoclinic structure with the non-centrosymmetric space group Cc is found (DPO II): a=2.4134(4) nm, b=2.4099(3) nm, c=1.2879(2) nm, β=110.048(3)°, and V=7.0363(17) nm 3. The asymmetric unit contains six independent molecules in a complex packing motif. A re-determination of the crystal structure of DPO I at room temperature gives lattice parameters a=0.51885(6) nm, b=1.8078(2) nm, c=1.21435(14) nm, β=93.193(3)°, and V=1.1373(2) nm 3. X-ray measurements at 363 K show a significant increase of the unit cell volume by 1.6%. Differences between both structures concerning morphology and characteristic Raman bands are outlined in detail. DSC investigations show an irreversible transition from DPO I to DPO II at 97 °C. DPO II does not show any transition in the temperature range up to the melting point at 141 °C. The non-centrosymmetric DPO II structure shows triboluminescence.

UV resonance Raman and NMR spectroscopic studies on the pH dependent metal ion release from pseudoazurin

UV resonance Raman (UVRR) and 1H NMR spectra are measured for native Cu(I)– and Cu(II)–pseudoazurin, its apo-protein, and a few metal-substituted derivatives. The pH titration experiments of 1H NMR enabled us to determine the pKa* values of three His residues (His6, His40, and His81). The UVRR band characteristic of a metal coordinated histidyl imidazole was observed at 1385 cm−1 for Cu(II)–pseudoazurin in D2O but not for Cu(I)–pseudoazurin. This frequency is consistent with the Nδ coordination of His. For the Cu(I)–pseudoazurin a characteristic band of histidyl imidazolium was detected at 1408 cm−1 at acidic pH. This is assigned to protonated His81, which is deligated from Cu(I) at low pH values. The imidazolium Raman band at 1408 cm−1 was also detected in the UVRR spectrum of apo-pseudoazurin at pH* 3.9, and the acidification was accompanied by a significant change in the X-Pro bands at 1467 cm−1. Pseudoazurin substituted with Zn2+ and Cd2+ gave the characteristic Raman bands of the metal coordinated imidazole at 1388 and 1384 cm−1, respectively, at neutral pH, but its intensity diminished upon lowering the pH, and instead the imidazolium band at 1408 cm−1 grew. The X-Pro bands of the pseudoazurins substituted with Zn2+ and Cd2+ exhibited the pH dependence very similar to that observed for apo-pseudoazurin. These findings indicate that the Zn2+ and Cd2+ ions are released from the active site at acidic pH and it is accompanied by a change in hydrogen bonding state of Pro80. This behavior is clearly absent in both the Cu(I) and Cu(II) proteins, meaning that pseudoazurin discriminates between copper and the other metal ions.

Raman spectra of high‐density, low‐density, and linear low‐density polyethylene pellets and prediction of their physical properties by multivariate data analysis

AbstractRaman spectra have been measured for pellets of five samples of high‐density polyethylene (HDPE), seven samples of low‐density polyethylene (LDPE), and six samples of linear low‐density polyethylene (LLDPE). The obtained Raman spectra have been compared to find out characteristic Raman bands of HDPE, LDPE, and LLDPE. Principal component analysis (PCA) was applied to the Raman spectra in the 1600–650 cm−1 region after multiplicative scatter correction (MSC) to discriminate the Raman spectra of the three different PE species. They are classified into three groups by a score plot of PCA factor 1 vs. 2. HDPE with high density and high crystallinity gives high scores on the factor 1 axis, while LDPE with low density and low crystallinity yields negative scores on the same axis. It seems that factor 1 reflects the density or crystallinity. A PC weight loadings plot for factor 1 shows six upward peaks corresponding to the bands arising from the crystalline parts or all‐trans (CH2)n groups and seven downward peaks ascribed to the bands of the amorphous or anisotropic regions and those arising from the short branches. Partial least‐squares (PLS‐1) regression was applied to the Raman spectra after MSC to propose calibration models that predict the density, crystallinity, and melting points of the polyethylenes. The correlation coefficient was calculated to be 0.9941, 0.9800, and 0.9709 for the density, crystallinity, and melting point, respectively, and their root‐mean‐square error of cross validation (RMSECV) was found to be 0.0015, 3.3707, and 2.3745, respectively. The loadings plot of factor 2 for the prediction of melting point is largely different from those for the prediction of density and crystallinity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 443–448, 2002

Electronically conducting polymer of manganese halide complex bearing 2,6-bis(imino)pyridyl ligands

A novel class of electronically conducting polymer produced from a manganese halide complex bearing 2,6-bis(imino)pyridyl ligands has been electrochemically synthesized in acetonitrile (AN) electrolyte. The conducting polymer has been physically and chemically characterized by SEM, UV–Vis spectroscopy, FT-Raman spectroscopy and cyclic voltammetry. The morphology of the polymer is characterized by many lamellae, quite different from the usual granular appearance of conducting polymers. A blue shift in the UV–Vis spectra indicates that the polymer is less planar and less conjugated than the monomer. The characteristic Raman bands of the spectrum of the polymer are similar to those of its monomeric counterpart. The electrochemical behavior in AN solution containing tetrabutylammonium perchlorate electrolyte of the polymer is characterized by two redox couples and a large pseudo-capacitance behavior.

Resonance Raman Spectroscopic Investigation of Directly Linked Zinc(II) Porphyrin Linear Arrays

We have investigated the resonance Raman (RR) spectra of the directly linked porphyrin arrays in order to elucidate the relationship between excitonic interactions and molecular geometry depending on the number of pigments in the arrays. The RR spectra obtained by photoexcitation at the high-energy exciton Soret bands in the arrays are mainly composed of Raman modes localized on the constituent porphyrin monomers. In contrast, the RR spectra of the arrays with photoexcitation at the low-energy exciton split Soret bands reveal some characteristic Raman bands arising from strong excitonic interactions between the adjacent porphyrins in the arrays. Based on the RR measurements of the isotope labeled analogues and the normal-mode analysis of the dimer, it is suggested that the photoexcitation at the high-energy Soret band produces an electronically excited state largely localized within a monomer unit and that at the low-energy exciton split Soret band the excited state is in a way delocalized throughout the ...

Density Functional Theory Study on the Raman Spectra of Negative Polarons and Negative Bipolarons in Na-Doped Poly(p-phenylene)

Density functional theory with the 6-31G* basis set and the BYLP nonlocal exchange-correlation functional has been used for the normal coordinate calculations of the neutral, radical anion, and dianion species of p-terphenyl to p-sexiphenyl. The bands attributed to negative polarons and negative bipolarons in the reported Raman spectra of Na-doped poly(p-phenylene) have been successfully assigned on the basis of the present calculations. The calculated atomic displacements have indicated that the Raman bands not corresponding to the zone-center modes of neutral poly(p-phenylene) are due to negative polarons and negative bipolarons. The Raman bands characteristic of negative polarons and negative bipolarons have been identified. The frequency of the inter-ring CC stretch is sensitive to the structural changes in going from benzenoid to quinoid structure in negative polarons and negative bipolarons.

Detection of pesticide model compounds in ethanolic and aqueous microdroplets by nonlinear Raman spectroscopy.

Pesticide model compounds are detected in microdroplets by means of in situ nonlinear Raman spectroscopy. The chloro- and nitro-substituted aromatics are dissolved in ethanol as well as in water. Their vibrational spectroscopic fingerprint, that is, Raman bands characteristic of the functional groups attached to the aromatic system, is determined in bulk medium by linear Raman spectroscopy. As a result of the concept of group vibrations, the chosen compounds can be regarded as representatives for a whole class of pesticides containing these residues. External seeding of stimulated Raman scattering (SRS) is applied for the detection of these group vibrations of the pesticide model compounds. This technique can be utilized for in situ diagnostics and a noninvasive vibrational spectroscopic analysis of the chemical composition of microdroplets with natural or anthropogenic origin.

Micro-Raman and FT-IR spectroscopy study of the thermal transformations of St. Claire dickite

Abstract The thermal transformations of St. Claire dickite up to 1300°C were studied by micro-Raman and FT-IR spectroscopy methods. Spectra recorded after the thermal treatment at 700°C show drastic changes from those recorded at 600°C, which are compatible with the dehydroxylation and formation of a pseudo-amorphous phase of meta-dickite. Some silica is expelled together with the beginning of Al/Si ordering at 1000°C, as observed by FT-IR spectroscopy through the splitting of the combined Al–O and Si–O deformation band into two modes at 553 and 465 cm −1 . A separate phase of amorphous silica is detected by micro-Raman spectroscopy at 1100°C, as observed by the appearance of characteristic Raman bands at 432, 489 and 601 cm−1. Presence of minor amounts of corundum is detected through luminescence spectroscopy at 1100°C, and is indicated by the appearance of R-lines of Cr+3 impurity at 692 and 694 nm. Crystallization of some mullite is observed at 1200–1300°C by the appearance of characteristic Raman bands at 308, 413, 960, 1035 and 1127 cm−1 and by the appearance of IR band at 815 and 906 cm−1.

Matrix-isolated Al2OF6(2-) ion in molten and solid LiF/NaF/KF.

A Raman spectrum consistent with that expected from an Al2OF6(2-) ion was observed when Na2O was dissolved in a eutectic LiF/NaF/KF (FLINAK) melt at 500 degrees C, which contained a low concentration of either AlF3 or Na3AlF6. Furthermore, it was possible to trap the Al2OF6(2-) ion in the frozen solid and to measure its Raman and IR spectra at 25 degrees C. A number of bands have been detected; among those, the two most characteristic bands of the Al2OF6(2-) ion at 494 (polarized) and 265 cm-1 in the FLINAK melt at 500 degrees C, and those at 509 and 268 (Raman) and approximately 780 to approximately 900 (IR) cm-1 for the compound matrix isolated in solid FLINAK at 25 degrees C. In the absence of added oxide, the dissolved aluminum fluoride was in the form of the octahedral AlF6(3-) ion, which has characteristic Raman bands at 542 and 325 cm-1 in the FLINAK melt at 500 degrees C. Whereas alumina, Al2O3, was found to be essentially insoluble in FLINAK melts, it was possible to dissolve sufficient amounts of Na2O to convert most of the AlF6(3-) to the oxyfluoroaluminate, Al2OF6(2-). These solutions appeared to be metastable with respect to formation of insoluble alumina at higher temperatures. The present results can be compared to previous measurements on alumina dissolved in pure molten cryolite at much higher temperatures, where alumina solubility is low and broad bands due to oxide species are difficult to detect due to overlap with bands from AlF6(3-) and AlF4-.

Raman study of the hexafluoroaluminate ion in solid and molten FLINAK.

Raman spectra have been obtained for matrix-isolated AlF6(3-) in an LiF/NaF/KF (FLINAK) eutectic mixture. Three Raman bands characteristic of the hexafluoroaluminate ion were identified in the solids formed from FLINAK melts which contained small amounts (5-11 mol%) of either AlF3 or Na3AlF6. The three allowed Raman-active bands of the matrix-isolated octahedral complex ion, nu 1(A1g), nu 2(Eg), and nu 5(F2g), were observed at 560.5, 380, and 325 cm-1, respectively, for the solid sample at 25 degrees C. Wavenumbers and relative intensities were similar to those of Na3AlF6 (cryolite), K3AlF6, and K2NaAlF6 (elpasolite) and other crystals known to contain discrete, octahedral AlF6(3-) ions. Peak positions, half-widths, and relative intensities for the bands were measured for samples at temperatures different from room temperature through the melting transition and into the molten state. The transition from high-temperature solid to molten salt at about 455 degrees C occurred gradually without perceptible change in the peak positions, half-widths, or relative intensities. For a sample in molten FLINAK at 455 degrees C, the nu 1(A1g), nu 2(Eg), and nu 5(F2g) modes of the AlF6(3-) ion were observed at 542, 365, and 324 cm-1, respectively. Raman depolarization experiments were consistent with these assignments, and the low value of the depolarization ratio of the nu 1(A1g) mode at 542 cm-1 indicated that the sample was molten above 455 degrees C. Differential thermal analysis also indicated that the FLINAK samples melted at about 455 degrees C. Raman measurements were performed for samples at temperatures from 25 to 600 degrees C in a silver dish, on a hot stage, in an argon-filled atmosphere, under a microscope. Additional Raman experiments were performed on samples at temperatures from 25 to 750 degrees C in a conventional graphite windowless cell, in an argon-filled quartz tube, in a standard furnace. Over the concentration range 4.8-11 mol% AlF3 (CR 23-8.0) in FLINAK, only bands due to the AlF6(3-) ion were detected. There was no evidence to support the presence of other aluminum complexes in these melts.

AOTF Raman spectrometer for remote detection of explosives

A spectrometer based on acousto-optic tunable filters is developed for use in measuring Raman spectra as part of a detection system that is low-cost, reliable, and field-portable. The system is coupled with a fiber optic bundle to carry the excitation laser light to the sample and to collect the Raman scattered light. Spectra of the explosives trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX) and nitroguanidine (NQ) were obtained in very short times and are in good agreement with those taken with conventional Fourier transform Raman spectrometers. Spectra of mixtures of explosives were also obtained and show no overlap of their characteristic Raman bands.

IR and polarized Raman spectra of anilinium hydrogenphosphite, C6H5NH3+ HPO3H−

The IR and polarized Raman spectra of anilinium hydrogenphosphite, C6H5NH3+ HPO3H−, were recorded and analysed. All the characteristic IR and Raman bands of the monosubstituted benzene ring, C6H5NH3+, and the hydrogenphosphite ion, HPO3H−, were identified. Both the ions occupy lower site symmetry in the crystal lattice. Strong hydrogen bonding in the compound is indicated by ABC bands. Copyright © 2000 John Wiley & Sons, Ltd.

Evidence for π−π Interactions in the S1 State of Zinc Porphyrin Dimers Revealed by Picosecond Time-Resolved Resonance Raman Spectroscopy

The S1 states of Zn(II)−porphyrin dimers have been investigated with picosecond time-resolved resonance Raman spectroscopy. The transient absorption and Raman spectra of porphyrin dimers, in which two Zn(II)−porphyrins are covalently linked at the ortho or meta positions of phenylene spacers, are compared with those of their component monomer unit. Although the Q-band of the ortho dimer was definitely different from those of the meta dimer and reference monomer, the ground-state Raman spectra of the ortho and meta dimers are nearly the same as that of the monomer, suggesting that the porphyrin π−π interactions strongly appear in the excited state but little in the ground state in the ortho dimer. Several characteristic Raman bands were observed for the S1 excited state at 2 ps after photoexcitation. The monomer S1 state gave the marker bands at slightly lower frequencies (by 3−4 cm-1) than the corresponding ground state, and they showed no frequency shifts with time between 2 and 300 ps. On the contrary, in the case of the ortho dimer S1 state, two characteristic bands (ν2, ν4) appeared at frequencies significantly lower (by 10−13 cm-1) than the corresponding ground-state bands, and in addition, the frequency of the ν4 band exhibited an upshift around 10−20 ps following photoexcitation. This frequency shift of the ortho dimer was appreciably perturbed by steric hindrance between the two porphyrins groups when bulky tert-butyl groups at the para position were incorporated. The behaviors of transient Raman bands of the meta dimer appeared intermediate between the monomer and the ortho dimer. These observations give the first clear evidence for the presence of π−π interactions in the S1 excited state of porphyrin dimers with phenylene spacer and the occurrence of geometric relaxation toward the monomer-type structure in several tens of picoseconds.