Low-temperature Raman Spectra Research Articles

Low Temperature Raman Spectroscopy of Tetrahydrofuran: Phonon Spectra Compared to Matrix Isolation Spectra in Air

The conformation of tetrahydrofuran (THF) molecules in vapor has been the subject of considerable computational and experimental studies, the most recent by Park and Kwon stated that the difference between the most stable, twisted C2 conformer and the bent Cs conformer is 17 ± 15 cm−1. Because of low symmetry, all modes from both conformers are allowed in the Raman and infrared spectra. In 1982, Aleksanyan and Antipov observed the emergence of two Raman bands at 249 and 303 cm−1 at 20 K, while only one band at 293 cm−1 was present in solid THF at 142. They assigned the 249 cm−1 band to the restricted pseudorotational motion of THF in the solid state, because on heating, the band diminishes and is too weak to be observed near melting point (at 142 K). Cadioli et al. reported a study of the vibrational spectrum of tetrahydrofuran, giving a complete assignment of all bands including those present in the low-temperature Raman spectrum at 85 K and infrared bands observed at 90 K. They assigned the band at 242 cm−1 in the Raman spectrum at 85 K as an overtone of the lowest normal mode (pseudorotational mode), while the 299 cm−1 band in the same spectrum was assigned as a radial mode. In the following, low-temperature Raman spectra of solid THF together with the Raman matrix isolated spectrum of THF in air will be presented and compared to published data. Our results indicate that the band observed at 245 cm−1 at 10 K is too strong to be assigned as an overtone, since its intensity is of the same magnitude as the 299 cm−1 band.

Vibrational spectra and molecular structure of sulfanilic acid: IR and low temperature Raman studies and DFT investigation of monomeric and dimeric forms

Room temperature IR and Raman spectra, low temperature Raman spectra and UV–vis spectrum of sulfanilic acid (SA) were measured. Molecular structures of monomeric SA in the OH and zwitter-ion (ZI) forms and dimeric SA were optimized at the b3lyp/6-311++g** level using the Gaussian 09 software. Computations of APT charges, IR, Raman and UV–vis spectra and related quantities were also carried out. MEP, ESP plots and pictorial representations of the HOMO-LUMO were drawn using the optimized structures of the different forms of the SA molecule. Barrier heights were calculated for different tops using one dimensional potential energy scan versus dihedral angle at the same level of theory. The IR, Raman and UV–vis spectra were interpreted in light of the computed quantities. Thermodynamic functions were computed in the temperature range 100–600 K. Bioactivity scores of the SA molecule were also determined. The present investigation suggests that the SA molecule exists in the ZI form as a monomer and in the OH form as a dimer. Appearance of the intense and broad IR band in the range 2200 to 3300 cm−1 could be explained only in terms of the OH stretching mode of the dimeric form of SA.

Multi-Centered Solid-Phase Quasi-Intramolecular Redox Reactions of [(Chlorido)Pentaamminecobalt(III)] Permanganate—An Easy Route to Prepare Phase Pure CoMn2O4 Spinel

We synthesized and structurally characterized the previously unknown [Co(NH3)5Cl](MnO4)2 complex as the precursor of CoMn2O4. The complex was also deuterated, and its FT-IR, far-IR, low-temperature Raman and UV-VIS spectra were measured as well. The structure of the complex was solved by single-crystal X-ray diffraction and the 3D-hydrogen bonds were evaluated. The N-H…O-Mn hydrogen bonds act as redox centers to initiate a solid-phase quasi-intramolecular redox reaction even at 120 °C involving the Co(III) centers. The product is an amorphous material, which transforms into [Co(NH3)5Cl]Cl2, NH4NO3, and a todorokite-like solid Co-Mn oxide on treatment with water. The insoluble residue may contain {Mn4IIIMnIV2O12}n4n−, {Mn5IIIMnIVO12}n5n− or {MnIII6O12}n6n− frameworks, which can embed 2 × n (CoII and/or CoIII) cations in their tunnels, respectively, and 4 × n ammonia ligands are coordinated to the cobalt cations. The decomposition intermediates decompose on further heating via a series of redox reactions, forming a solid CoIIMIII2O4 spinel with an average size of 16.8 nm, and gaseous N2, N2O and Cl2. The CoMn2O4 prepared in this reaction has photocatalytic activity in Congo red degradation with UV light. Its activity strongly depends on the synthesis conditions, e.g., Congo red was degraded 9 and 13 times faster in the presence of CoMn2O4 prepared at 550 °C (in air) or 420 °C (under N2), respectively.

Charge-density-wave melted superconductivity in 1T-TiSe2

Understanding the origin of superconductivity and the interplay between superconductivity and charge-density wave (CDW) in transition-metal dichalcogenides (TMDs) is one of fundamental interests in scientific research as well as in applications. In this work, we have explored the inherent physical mechanism for the superconducting transition in Ta-doped 1T-TiTa x Se2 single crystals by combining density functional theory (DFT) calculations and experiment approaches. The theoretical calculation results reveal that with Ta doping, the CDW is suppressed by the raising of chemical potential, which is further confirmed by x-ray absorption near-edge structure (XANES) and low-temperature Raman spectra. And then, the superconductivity is motivated by the enhancement of density of states near the Fermi level. Our present results shed new light on understanding the emergence of superconductivity within the CDW phase, as well as the potential applications in electronic devices.

Vibrational spectra and molecular structure of sulfanilamide: IR and Low temperature Raman studies and DFT investigation of monomeric and dimeric forms

Raman spectra of sulfanilamide were recorded at temperatures 80, 90, 120, 150 and 300 K in the range 50-4000 cm−1. The room temperature FTIR spectrum of this compound was investigated in the range 400−4000 cm−1. The molecular structures and vibrational parameters for the lowest energy conformer of sulfanilamideand its dimer were carried out at the B3LYP/6−311++G** levelusing the Gaussian 09 software and PEDs have been calculated using the GAR2PED software.Low temperature Raman spectra enabled to observe some modes which were not resolved at room temperature.The low temperature Raman spectra show splitting of bands around 1135 and 1595 cm−1 which could be associated with the (C)NH2---SO2(NH2) interaction, which is indicative of the existence of sulfanilamide in dimeric form. Based on the present investigation five Quantum Scaling Factors (QSFs) are proposed.These QSFs provide better agreement between the observed and computed scaled frequencies.

Investigation on ZnTe/CdxZn1-xTe heterostructure for photodetector applications

ZnTe/CdxZn1-xTe (0.2 ≤ x ≤ 1.0) heterostructures were fabricated by thermal evaporation method by using CdTe and ZnTe as source materials. The properties of heterostructures were analysed by X-ray diffraction (XRD), photoluminescence (PL), temperature dependent Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and I-V characterizations. Double diffraction peak in the diffractogram has confirmed bilayer with no or minimum diffusion across the junction. Effect of thermal expansion and an-harmonic coupling to the other phonon modes causing strain induced shift of the longitudinal optical modes were studied through low-temperature Raman spectra. The junction parameters such as barrier height, ideality factor and series resistance were extracted from I - V characteristics by applying various models. ZnTe/CdxZn1-xTe heterostructures having low barrier height and good photo response are explored in this work for the photodetector application. Sample having composition x = 0.8 showed higher response in the visible region with improved response at longer wavelengths.

Dielectric relaxation, magneto-dielectric coupling, and pyrocurrent anomaly in point defect controlled HoCrO3

HoCrO3, a member of the rare-earth chromate family, is known to exhibit an orthorhombic structure with Pbnm space group in bulk at room temperature. We have employed the pressure driven powder consolidation technique, spark plasma sintering, for synthesizing HoCrO3 powder-compact, which exhibits a superior quality with fewer defects over that synthesized by conventional solid-state sintering (resulting in a large reduction of charge carrier mediated extrinsic dielectric contribution). The powder compact shows magneto-dielectric coupling but does not exhibit a detectable ferroelectric phase in the range of 50 K to 300 K. Low temperature Raman spectra show structural distortions below 250 cm−1, indicating Ho ion displacements. Temperature evolution of the crystal structure using synchrotron x-ray diffraction reveals a probable structural phase transition below 240 K. Pyrocurrent measurement, together with synchrotron x-ray diffraction analysis, further points to a magneto-structural transition at 100 K.

Vibrational dynamics of 1,3-propanediol in liquid, polycrystalline and glassy states: A Raman spectroscopic study.

Vibrational transitions of 1,3-propanediol in liquid and crystal phases are assigned on the basis of Raman and infrared spectra of liquid and low temperature (295K-10K) Raman spectra which are presented here for the first time. In the crystal, there are four molecules per unit cell, each having a gGGg' conformation. The vibrations for the P21/n crystal phase having four molecules in the unit cell are obtained by an ab initio calculation using CRYSTAL09 program, and are in good agreement with the observed Raman bands. Observed bands in Raman spectrum of liquid are in proximity of those observed for crystal phase, and it is tempting to assign them as belonging mainly to gGGg' conformer. However, quantum chemical ab initio calculations provide basis to ascertain that tGG'g and tGGt are two conformers having lowest energy at the B3LYP/6-31++G(d,p) level of theory. Several bands that disappear from Raman spectrum of liquid on solidification (384, 872 and 1040cm-1) could give an insight into the nature of conformers present in liquid. Since the closest calculated vibrations are 386cm-1 for tGG'g and 1043cm-1 for tGG't conformer, we conclude that all three conformers: tGG'g, tGGt and gGGg', are present in the liquid.

A Known Iron(II) Complex in Different Nanosized Particles: Variable-Temperature Raman Study of Its Spin-Crossover Behavior.

The spin-crossover (SCO) polymorph B (complex 1) of the known compound [FeII{N(CN)2}2(abpt)2], where abpt is 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, has been prepared in three different particle sizes averaging ∼300 (sample 1a), ∼80 (sample 1b), and ∼20 nm (sample 1c). Two independent octahedral molecules possessing Fe1 and Fe2 were found to be present in the crystal of B. Magnetostructural relationships had established that at room temperature both FeII sites are in the high-spin state (HS-HS), whereas a decrease in the temperature to 90 K induces the complete high-spin to low-spin conversion of the Fe1 site, with Fe2 remaining in the high-spin state (LS-HS). The three samples have been characterized by elemental analyses, ATR spectra, solution UV/vis spectra (to exclude resonance Raman effects) and powder X-ray diffraction patterns, while their morphological characteristics have been examined by scanning electron microscopy (SEM). The SCO behavior of the originally prepared sample 1a has been monitored in detail by variable-temperature Raman studies in the 300-80 K range using mainly low-frequency ν(Fe-N) and δ(NFeN) modes and the ν(C≡N) mode of the axial dicyanamido groups as spin-sensitive vibrations. The new peaks that appear in the low-temperature Raman spectra of the LS-HS form of the complex are reproduced in the calculated spectrum of the LS state of [FeII{N(CN)2}2(abpt)2]. The influence of the average particle size on the SCO properties of 1 has also been studied by variable-temperature Raman spectra. The studies indicate that, during the HS-HS → LS-HS transition, the latter form of the complex appears at higher temperatures for the smaller particles; the T1/2 shift accomplished by manipulating the particle size within a range of roughly 1 order of magnitude (300-20 nm) may be as high as ∼30 K. The SCO features of 1, as deduced from the Raman study, are in excellent agreement with those derived from a traditional variable-temperature magnetic susceptibility study, indicating the utility of the former.

Order-disorder phase transition in the peroxidovanadium complex NH4[VO(O2)2(NH3)

Complex NH4[VO(O2)2(NH3)] (1) undergoes an order-disorder phase transition at Tc~258K. This transition is accompanied by change in the space group of the orthorhombic lattice and also by significant structural rearrangements of the constituent molecules, which are pertinent mostly to their NH4+ ions and their ammonia ligands. The low-temperature solid state IR and Raman spectra of 1 were corroborated by solid-state computations that employed Gaussian functions as the basis set. Results of these computations yielded excellent agreement with experimental data. On the curves of temperature dependence of vibrational modes, the phase transition is expressed by an abrupt change of the slope above Tc.

Synthesis and properties of selenium trihydride at high pressures

The chemical reaction products of molecular hydrogen $({\mathrm{H}}_{2})$ with selenium (Se) are studied by synchrotron x-ray diffraction (XRD) and Raman spectroscopy at high pressures. We find that a common ${\mathrm{H}}_{2}\mathrm{Se}$ is synthesized at 0.3 GPa using laser heating. Upon compression at 300 K, a crystal of the theoretically predicted Cccm ${\mathrm{H}}_{3}\mathrm{Se}$ has been grown at 4.6 GPa. At room temperature, ${\mathrm{H}}_{3}\mathrm{Se}$ shows a reversible phase decomposition after laser irradiation above 8.6 GPa, but remains stable up to 21 GPa. However, at 170 K Cccm ${\mathrm{H}}_{3}\mathrm{Se}$ persists up to 39.5 GPa based on XRD measurements, while low-temperature Raman spectra weaken and broaden above 23.1 GPa. At these conditions, the sample is visually nontransparent and shiny suggesting that metallization occurred.

Evidence of Polaron Excitations in Low Temperature Raman Spectra of Oxalic Acid Dihydrate.

Low temperature Raman spectra of oxalic acid dihydrate (8-300 K) for both the polycrystalline and single crystal phase show strong variation with temperature in the interval from 1200 to 2000 cm(-1). Previous low temperature diffraction studies all confirmed the stability of the crystal P21/n phase with no indications of any phase transition, reporting the existence of a strong hydrogen bond between the oxalic acid and a water molecule. A new group of Raman bands in the 1200-1300 cm(-1) interval below 90 K is observed, caused by possible loss of the center of inversion. This in turn could originate either due to disorder in hydroxyl proton positions or due to proton transfer from carboxylic group to water molecule. The hypothesis of proton transfer is further supported by the emergence of new bands centered at 1600 and 1813 cm(-1), which can be explained with vibrations of H3O(+) ions. The broad band at 1600 cm(-1) looses intensity, while the band at 1813 cm(-1) gains intensity on cooling. The agreement between quantum calculations of vibrational spectra and experimentally observed Raman bands of hydronium ions in oxalic acid sesquihydrate crystal corroborates this hypothesis.

Manifestation of Structure of Electron Bands in Double-Resonant Raman Spectra of Single-Walled Carbon Nanotubes.

Micro-Raman spectra of single-walled carbon nanotubes in the range of two-phonon 2D bands are investigated in detail. The fine structure of two-phonon 2D bands in the low-temperature Raman spectra of the mixture and individual single-walled carbon nanotubes is considered as the reflection of structure of their π-electron zones. The dispersion behavior of 2D band fine structure components in the resonant Raman spectra of single-walled carbon nanotube mixture is studied depending on the energy of excitating photons. The role of incoming and outgoing electron-phonon resonances in the formation of 2D band fine structure in Raman spectra of single-walled carbon nanotubes is analyzed. The similarity of dispersion behavior of 2D phonon bands in single-walled carbon nanotubes, one-layer graphene, and bulk graphite is discussed.

Raman spectra observation of silver nanoparticles in porous silicon fabricated by ion implantation

Porous silicon layers fabricated by the low-energy high-dose Ag+-ion implantation of crystalline silicon with doses from 7.5 × 1016 to 1.5 × 1017 ion/cm2 are studied by Raman spectroscopy. Pores with sizes from ∼100–180 nm formed on irradiated silicon surface are controlled by scanning electron microcopy. Synthesized silver nanoparticles are observed in the structure of porous silicon. The sizes of pour and nanoparticles are increased with an increase in implantation dose. Acoustic vibrations generated by laser irradiation in silver nanoparticles of various sizes are registered by low-temperature Raman spectra of composite material.

Influence of electric field and domain structure on the low-temperature Raman spectra of SrTiO3

The domain structure of SrTiO3 below its cubic to tetragonal phase transition depends not only on the stress-state of a particular sample but can also be modified very efficiently by the application of electric fields. Hence, the interpretation of field induced phenomena needs a careful consideration of the domain distribution which is often neglected in existing studies. In this paper, it is shown that the strong variation of polarized Raman spectra under external fields observed within both the paraelectric tetragonal phase and the field induced ferroelectric phase, is a direct consequence of the redistribution of domains. Raman data can therefore be used to characterize the domain structure of a particular sample.

Xenon(IV)-carbon bond of [C6F5XeF2]+; structural characterization and bonding of [C6F5XeF2][BF4], [C6F5XeF2][BF4]·2HF, and [C6F5XeF2][BF4]·nNCCH3 (n = 1, 2); and the fluorinating properties of [C6F5XeF2][BF4

The [C6F5XeF2](+) cation is the only example of a Xe(IV)-C bond, which had only been previously characterized as its [BF4](-) salt in solution by multi-NMR spectroscopy. The [BF4](-) salt and its new CH3CN and HF solvates, [C6F5XeF2][BF4]·1.5CH3CN and [C6F5XeF2][BF4]·2HF, have now been synthesized and fully characterized in the solid state by low-temperature, single-crystal X-ray diffraction and Raman spectroscopy. Crystalline [C6F5XeF2][BF4] and [C6F5XeF2][BF4]·1.5CH3CN were obtained from CH3CN/CH2Cl2 solvent mixtures, and [C6F5XeF2][BF4]·2HF was obtained from anhydrous HF (aHF), where [C6F5XeF2][BF4]·1.5CH3CN is comprised of an equimolar mixture of [C6F5XeF2][BF4]·CH3CN and [C6F5XeF2][BF4]·2CH3CN. The crystal structures show that the [C6F5XeF2](+) cation has two short contacts with the F atoms of [BF4](-) or with the F or N atoms of the solvent molecules, HF and CH3CN. The low-temperature solid-state Raman spectra of [C6F5XeF2][BF4] and C6F5IF2 were assigned with the aid of quantum-chemical calculations. The bonding in [C6F5XeF2](+), C6F5IF2, [C6F5XeF2][BF4], [C6F5XeF2][BF4]·CH3CN, [C6F5XeF2][BF4]·2CH3CN, and [C6F5XeF2][BF4]·2HF was assessed with the aid of natural bond orbital analyses and molecular orbital calculations. The (129)Xe, (19)F, and (11)B NMR spectra of [C6F5XeF2][BF4] in aHF are reported and compared with the (19)F NMR spectrum of C6F5IF2, and all previously unreported J((129)Xe-(19)F) and J((19)F-(19)F) couplings were determined. The long-term solution stabilities of [C6F5XeF2][BF4] were investigated by (19)F NMR spectroscopy and the oxidative fluorinating properties of [C6F5XeF2][BF4] were demonstrated by studies of its reactivity with K[C6F5BF3], Pn(C6F5)3 (Pn = P, As, or Bi), and C6F5X (X = Br or I).

Anisotropic lattice thermal expansion of PbFeBO4: A study by X-ray and neutron diffraction, Raman spectroscopy and DFT calculations

The lattice thermal expansion of mullite-type PbFeBO4 is presented in this study. The thermal expansion coefficients of the metric parameters were obtained from composite data collected from temperature-dependent neutron and X-ray powder diffraction between 10K and 700K. The volume thermal expansion was modeled using extended Grüneisen first-order approximation to the zero-pressure equation of state. The additive frame of the model includes harmonic, quasi-harmonic and intrinsic anharmonic potentials to describe the change of the internal energy as a function of temperature. The unit-cell volume at zero-pressure and 0K was optimized during the DFT simulations. Harmonic frequencies of the optical Raman modes at the Γ-point of the Brillouin zone at 0K were also calculated by DFT, which help to assign and crosscheck the experimental frequencies. The low-temperature Raman spectra showed significant anomaly in the antiferromagnetic regions, leading to softening or hardening of some phonons. Selected modes were analyzed using a modified Klemens model. The shift of the frequencies and the broadening of the line-widths helped to understand the anharmonic vibrational behaviors of the PbO4, FeO6 and BO3 polyhedra as a function of temperature.

Evidence of electron-phonon and spin-phonon couplings at the Verwey transition inFe3O4

Using low temperature Raman spectra on two-dimensional epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ thin films, the presence of electron-phonon (e-p) and spin-phonon (s-p) couplings is established in the vicinity of Verwey transition (${T}_{\mathrm{V}}$). The e-p coupling is linked to ${A}_{1g}$ and ${T}_{2g}$ Raman modes and s-p to the ${T}_{2g}$ mode, below isotropic point (${T}_{\mathrm{K}}$). The parabolic dependence of the characteristic spin-correlation eigenstates on the magnetization, ${\ensuremath{\Delta}\ensuremath{\omega}}_{s\ensuremath{-}\mathrm{phn}}\ensuremath{\propto}\ensuremath{\langle}{S}_{i}|{S}_{J}\ensuremath{\rangle}\ensuremath{\propto}{[\frac{M(T)}{{M}_{S}}]}^{2}$ observed between ${T}_{\mathrm{K}}$ and ${T}_{\mathrm{V}}$ demonstrates the presence of s-p coupling. This work provides experimental evidence of the onset of correlation between these couplings and charge-orbital ordering below ${T}_{\mathrm{K}}$ to govern the Verwey transition in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films.

Synthesis and Characterization of Li-Modified AgTaO3

(LixAg1−x)TaO3 (0 ≤ x ≤ 0.15) ceramics were fabricated by a mixed-oxide solid-state sintering route. Compositions with x ≤ 0.1 sintered into single-phase compounds and could be indexed according to the rhombohedral, \( R{\bar{3}}c \) or R3c structure. For x > 0.1, x-ray diffraction revealed second-phase formation, probably LiTaO3 and Ag2Ta8O21. It was noticed that the metastable and stable phase solubility limits for Li in AgTaO3 were x < 0.05 and 0.10 < x < 0.15, respectively. Raman and dielectric measurements confirmed the existence of a ferroelectric state in the compositions with x ≥ 0.05. This triggering of ferroelectricity supports the premise that substitution of relatively smaller ions increases the displacement of the A-site cation. The transition temperature was found to increase as a function of increasing Li concentration. Low-temperature (<100 K) Raman spectra and electron diffraction indicated that Li-doped AgTaO3 ceramics seem to adopt new tilt systems.

Vibrational spectra, quantum chemical calculations and spectral assignments of 1,1-difluoro-1-silacyclohexane

Raman spectra of 1,1-difluoro-1-silacyclohexane as a liquid, and as a solid at 78 K were recorded and depolarization data obtained. The infrared spectra of the vapour, liquid and amorphous and crystalline solids have been studied. In the low temperature IR and Raman spectra eight and three bands, respectively, were shifted a few cm(-1) when the sample crystallized. No bands vanished after crystallization in agreement with the assumption that only one conformer (chair) was present in all the states of aggregation. The compound exists in the stable chair conformation, whereas in the parent silacyclohexane a possible twist form should have more than 15 kJ mol(-1) higher energies than the chair, as derived from various calculations. The wavenumbers of the vibrational modes were calculated in the harmonic and anharmonic approximation employing B3LYP/cc-pVTZ calculations. The 27 A' and 21 A″ fundamentals were assigned on the basis of the calculations, infrared vapour contours, Raman depolarization measurements and infrared and Raman band intensities. An average, relative deviation of 1.5% was found between the observed and the anharmonic wavenumbers for the 48 modes.