Variable Temperature Raman Spectra Research Articles

Optical and structural properties of AlN thin films deposited on different faces of sapphire substrates

We report the comprehensive spectroscopic results of AlN thin films deposited on the A-, R- and C-surfaces of sapphire substrates by radio frequency magnetron sputtering. The optical and structural properties of the epitaxial-grown AlN films were characterized using various techniques of high-resolution x-ray diffraction spectroscopy, x-ray photoelectron spectroscopy, Raman scattering spectroscopy, spectroscopic ellipsometry and associated analytical tools. Our large number of measurement results clearly show that sapphire substrates of different polarities have effects on the surface roughness, dislocation density, grain size, microstrain, and surface oxygen binding capacity of the film grown on its surface. The results obtained from Ellipsometry measurements show that the thickness, band gap and roughness of AlN films grown on C-plane sapphire are the smallest among the three samples. After careful analyses of the variable temperature Raman spectra, as the temperature rises from 80 K to 800 K, the AlN film has always exhibited tensile stress. In the same temperature range, the tensile stress of the AlN film grown on the C-plane sapphire has the greatest effect with temperature. The lifetime of E2 (high) phonons gradually decays with the increase of temperature.

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.

Synthesis, Computational Studies, Inelastic Neutron Scattering, Infrared and Raman Spectroscopy of Ruthenocene

In this work we report a new and simplified synthesis of ruthenocene that prevents the formation of unwanted by‐products. We have also revisited the vibrational spectroscopy of this iconic molecule and in addition to infrared and variable temperature Raman spectra, we present the first inelastic neutron scattering (INS) spectrum, a technique that is little used in inorganic chemistry. The Raman spectra in the low energy range (< 200 cm–1) clearly show that the mode assigned as the ring–Ru–ring torsion is a librational mode. By generating the INS spectra predicted by previous assignment schemes we are able to show that they are all, at least partially, wrong because they fail to correctly predict the experimental INS spectrum. In the case of ruthenocene, the addition of INS data, in combination with periodic‐DFT calculations, has enabled the first correct assignment of the internal modes of ruthenocene. This straightforward means to test proposed assignments is one of the great strengths of vibrational spectroscopy with neutrons.

Chemical interpretation, enhanced ferroelectric property and dielectric breakdown strength of CuO-added Sr2Nb2O7 ceramics

The structural interpretation and electrical properties of perovskite layer structured (PLS) Sr2Nb2O7-xwt%CuO ceramics prepared by solid-state reaction method are investigated. The chemical interpretation of enhanced piezoelectricity is confirmed to be attributed to the rotation and/or distortion of oxygen octahedron caused by possible Cu2+ substitution at the A-site of Sr2Nb2O7 by XRD refinement and variable-temperature Raman spectra. Sr2Nb2O7-xwt%CuO (x = 0.3, 0.5 and 0.7) ceramics shows enhanced ferroelectric properties with a larger Pr of ∼4.1 μC/cm2 and a smaller Ec of ∼63.1 kV/cm. This study further explains the cations in A-site play a major structural role in the polarization process for PLS system. It was found that dielectric breakdown strength increases up to 258.8 kV/cm and then decreases gradually with the increase of CuO content. Impedance spectroscopy indicated that CuO addition could be helpful in increasing the grain boundary resistance then dielectric breakdown strength.

Facile synthesis and electrical switching properties of V2O3 powders

V2O3 powders were synthesized with mercaptoacetic acid (C2H4O2S) as reducing agent and stabilizer via a facile hydrothermal approach. The crystalline structure, surface morphology, valence state of the derived V2O3 powders were characterized via X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy. It was found that the ratio and reaction time played a duel role in the formation and morphology of the V2O3 powders. The metal-insulator transition properties of V2O3 powders were studied by the differential scanning calorimetry curve and variable temperature Raman spectra. The change in electrical resistance due to the metal-insulator transition was measured from 80 to 240K using physical property measurement system. The results showed V2O3 samples had excellent electrical switching properties with resistance changes as large as 104. This simple and fast synthesis approach makes the V2O3 powders easily accessible for exploring their fundamental properties and potential applications in novel electronic devices.

Vibrational assignments and conformer stability determination of cyclobutyldichlorosilane by variable temperature Raman spectra in krypton solution

The infrared spectrum (3100–300cm−1) in the gas phase and Raman spectrum (3100–150cm−1) of the liquid of cyclobutyldichlorosilane (c-C4H7SiHCl2) have been recorded. Variable temperature (−143°C to −123°C) studies of the Raman spectra (3100–300cm−1) dissolved in liquid krypton have been carried out. From these data, the experimental stabilities of the conformer have been determined to be Eq-g>Ax-t>Eq-t>Ax-g. The enthalpy difference between Eq-g and Ax-t has been determined from band pairs to be 85±10cm−1 (1.01±0.11kJmol−1) with the Eq-g conformer the more stable form. The enthalpy difference between Eq-g and Eq-t is 116±43cm−1 (1.39±0.52kJmol−1) and the enthalpy difference between Eq-g and Ax-g has been determined to be 138±42cm−1 (1.66±0.51kJmol−1) with the Eq-g conformer as the more stable form. The percentage of the equatorial gauche conformer is estimated to be 47% at ambient temperature. To support the spectroscopic study, ab initio calculations by the Møller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The infrared intensities, Raman activities, vibrational frequencies and band contours have been predicted from MP2(full)/6-31G(d) calculations and these theoretical values are compared to the experimental ones when available. The conformational stabilities have been predicted from theoretical calculations with basis sets up to 6-311+G(2d,2p) from both MP2(full) and density functional theory calculations by the B3LYP method. The results are discussed and compared to the corresponding properties of some related molecules.

Infrared and Raman spectra, structural parameters, conformational stability, vibrational assignment and ab initio calculations of 2-cyanoethylphosphine

Infrared (3200–400cm−1) and Raman spectra (3050–50cm−1) of 2-cyanoethylphosphine, NCCH2CH2PH2, were recorded. Rotation about C–C bond gives Trans (T) and Gauche (G) conformers and rotation around C–P bond gives trans (t) and gauche (g) forms. Enthalpy differences in xenon solution by variable temperature Raman spectra between Gt and Tt gives 89±12cm−1 and Tg 331±18cm−1. Ab initio calculations by MP2(full) and DFT calculations were carried out. From previously reported microwave rotational constants and MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters were obtained for three conformers. The determined heavy atom parameters for Gt [Tt] {Tg}: distances (Å) P1–C2=1.860(3) [1.858(3)] {1.864(3)}, C2–C3=1.534(3) [1.535(3)] {1.535(3)}, C3–C4=1.464(3) [1.465(3)] {1.463(3)}, C4–N5=1.160(3) [1.160(3)] {1.160(3)} angles (°) ∠P1C2C3=117.1(5) [114.9(5)] {110.0(5)}, ∠C2C3C4=112.5(5) [112.1(5)] {111.8(5)}, τP1C2C3C4=63.4(5) [180.0(5)] {176.8(5)}. Vibrational assignments are reported, supported by ab initio predictions.

Conformational stabilities from variable temperature Raman spectra, r0 structural parameters and vibrational assignments of 1,2-diphosphinoethane

Infrared spectra (3600–500cm−1) of gas, amorphous, crystalline solid and Raman spectra (3000–200cm−1) of liquid and solid were recorded. Variable temperature Raman spectra (30 to −38°C) of liquid were recorded which indicates the presence of significant amounts of six conformers at ambient temperature. Rotation about CC bond gives Trans (T) and Gauche (G) conformers and rotation around both CP bonds gives trans (t) and gauche (g) forms. Enthalpy differences for five of the lower energy conformers w.r.t. the lowest energy tTt form were determined to be 272±39cm−1, 434±40cm−1, 443±30cm−1, 663±60cm−1 and 671±34cm−1 for g′Tt, g′Tg′, g′Tg, tG′g′ and tG′t conformers, respectively, which are in the increasing order of energy. Ab initio calculations with various basis sets up to aug-cc-pVTZ were performed and to support the spectroscopic studies MP2(full)/6-31G(d) was used to predict Raman activities, infrared intensities, vibrational frequencies and band contours. By utilizing the previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predictions, adjusted r0 parameters were obtained for two Trans and two Gauche (CC bond) conformers. The determined heavy atom structural parameters for g′Tt [g′Tg′] {tG′g′} |tG′t| conformers are: distances (Å) P1C2=1.870(3) [1.866(3)] {1.858(3)} |1.859(3)|, C2C3=1.535(3) [1.536(3)] {1.534(3)} |1.536(3)|, C3P4=1.863(3) [1.866(3)] {1.866(3)} |1.859(3)|, and angles (°) ∠P1C2C3=110.5(5) [110.4(5)] {117.0(5)} |118.5(5)|, ∠C2C3P4=115.5(5) [110.4(5)] {113.7(5)} |118.5(5)|. The results are discussed and compared to the corresponding properties of some similar molecules.

Polymorphism and cold crystallization in optically nonlinear N-benzyl-2-methyl-4-nitroaniline crystal studied by X-ray diffraction, calorimetry and Raman spectroscopy

Polymorphism of N-benzyl-2-methyl-4-nitroaniline (BNA), a terahertz emitter and frequency doubler, was detected and the crystal structures of orthorhombic and monoclinic polymorphs have been determined at room temperature. The differential scanning calorimetry (DSC) studies revealed monotropic phase transition of the monoclinic BNA crystal system at 362 K and during re-heating, after the molten BNA cooling, the cold crystallization of the both polymorphs. The variable-temperature Raman spectra in the 700–1700 cm −1 range also evidenced the polymorphic transformation from the monoclinic to the orthorhombic, the latter stable in the wide temperature range, crystal system. The comparison of structures, DSC curves, FT-IR and Raman spectra of two forms enabled to recognize the N H···O hydrogen bond in the orthorhombic and C H···O hydrogen bond in the monoclinic one as well as other intermolecular interactions. The role of dynamic disorder, intra- and intermolecular charge transfers in the molecular mechanism of optical nonlinearity has been discussed.

Vibrational spectra, conformational equilibrium, ab initio calculations and spectral assignments of ethylmethylgermane

The infrared spectra of ethylmethylgermane (CH 3CH 2GeH 2CH 3) have been studied as a vapour in the 4000–400 and 500–100 cm −1 regions and as an amorphous solid at 78 K. Raman spectra of the sample as a liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and small intensity changes of certain bands with temperature were detected. The sample was also investigated as an amorphous solid on a cold finger of copper or a CsI window at 78 K. No crystallization was ever obtained in the Raman or infrared cryostats in spite of extensive annealing. The compound exists a priori in two conformers, anti and gauche, and the variable temperature Raman spectra suggest an equilibrium in which the gauche conformer has 0.4 kJ mol −1 lower enthalpy than anti in the liquid. B3LYP and MP2 calculations with various basis sets and the CBS-QB3 and G2 models were employed, yielding an average conformational enthalpy difference Δ H( anti– gauche) = 0.24 kJ mol −1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the gauche and anti conformers were calculated. Instead of scaling the calculated wavenumbers in the harmonic approximation, performed in our previous publications, calculations from B3LYP/cc-pVTZ were derived in the anharmonic approximation. In most cases these values gave a good agreement with the experimental results for 34 observed modes of the gauche and 10 modes of the anti conformer.

Vibrational spectra, ab initio calculations and vibrational assignments of 3-butyn-1-ol

The infrared spectra of 3-butyn-1-ol, HC CCH 2CH 2OH, have been recorded as a vapour in the range 3600–50 cm −1 and as a liquid between 3600 and 400 cm −1. Additional spectra of the alcohol isolated in an argon matrix at ca. 5 K were obtained and spectra were recorded after annealing to various temperatures between 10 and 35 K. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 143 K. Spectra of an amorphous solid were recorded at 78 K. In spite of several attempts and many different annealing temperatures, the sample crystallized neither in the IR nor in the Raman cryostats. In the variable temperature Raman spectra, some bands of the liquid changed in relative intensity and were interpreted in terms of conformational equilibria between two of the five possible conformers. Complete assignments were made for all the bands of the most stable conformer gg, in which the OH group is approaching the triple bond, forming an intramolecular hydrogen bond. From various bands assigned to a second conformer aa, in which OH is oriented anti to the C C bond, or a third conformer ag, the conformational enthalpy difference was found to be Δ conf H( ag-gg) = 0.9 kJ mol −1 in the liquid. The two highest energy conformers g’g and ag were not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. The calculations revealed that gg was the low energy conformer and CBS-QB3 calculations suggested the gg conformer was more stable by 5.4 and 4.2 kJ mol −1 relative to ag and aa, respectively, in the vapour. Vibrational wavenumbers and infrared and Raman band intensities for the three low energy conformers are reported from B3LYP/cc-pVTZ calculations.

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH 3C CCH(OH)CH 3, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm −1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C 1( a Me). From various bands assigned to a second conformer in which OH is oriented anti to H gem( a H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol −1. The highest energy conformer with OH anti to C 3( a C) was not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a H conformer had slightly lower energy, all the calculations revealed that a Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a Me conformer as more stable by 0.8 and 8.3 kJ mol −1 relative to a H an a C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.

Raman spectra, ab initio calculations, phase transitions and conformations of 1,3-disilabutane

Raman spectra of 1,3-disilabutane (SiH3CH2SiH2CH3) were recorded as a liquid at room temperature and at 14 temperatures between 293 and 137 K. Spectra of an amorphous and annealed solid were recorded at 78 K, and very distinct changes occurred on crystallization. In the variable temperature Raman spectra, some bands changed in intensity and were interpreted in terms of conformational equilibria between the two possible conformers. Complete assignments were made for all the bands of the most stable conformer, anti. From three band pairs assigned to the anti and gauche conformers, the conformational enthalpy differences ΔH(gauche-anti) were found to be between 1.3 kJ mol−1 and 1.7 kJ mol−1, using peak heights, and the more stable anti conformer was present in the crystal. A lower value of 0 ± 0.3 kJ mol−1 was obtained employing band areas. Ab initio calculations in the RHF and MP2 approximations and DFT calculations were carried out pointing to the anti conformer as the low energy form.

Vibrational spectra, conformational equilibria, ab initio calculations and vibrational assignments of ethylmethylfluorosilane

The infrared spectra of ethylmethylfluorosilane (CH 3SiHFCH 2CH 3) have been recorded as a vapour, liquid and solid at 78 K in the 4000–50 cm −1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of two different solid phases were obtained after annealing to temperatures of 120 and 130 K, and recooling to 78 K. Although the IR spectra were quite similar in the MIR region, certain differences were noted in the FIR region below 400 cm −1. The most stable conformer MeMe was present after annealing to 130 K, but three bands belonging to MeH were detected after annealing to 120 K. Various infrared bands changed intensity when the argon matrix was annealed to temperatures between 20 and 35 K, and some of these were related to changes in the conformational abundance. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, various bands changed in intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer MeMe. From various bands assigned to the three conformers, the conformational enthalpy difference Δ H from MeMe to the intermediate energy conformer MeH was found to be 0.5 kJ mol −1 and to the highest conformer MeF was 0.7 kJ mol −1. At ambient temperature this leads to 39% MeMe, 32% MeH and 29% of the MeF conformer in the liquid. Ab initio calculations in the RHF, MP2, DFT approximations and very accurate G2 calculations were carried out. With one exception, the MeMe conformer had the lowest enthalpy in all these calculations, the MeH had the intermediate and the MeF the highest enthalpy, and the calculations were in good agreement with the measurements.

Conformational properties of the methylated disilanes MeX 2SiSiX 2Me (X=H, F, Br and I): a combined ab initio and vibrational spectroscopic study

Together with ab initio calculations, Raman spectra of the methylated disilanes MeX 2SiSiX 2Me (X=H, F, Br and I) unambiguously prove the presence of two rotamers, anti and gauche in the liquid state. Energy differences between rotational isomers have been determined from variable temperature Raman spectra employing Van't Hoff plots of the logarithm of intensity ratios vs. the inverse temperature. For the disilanes with X=F, Br and I the anti conformation is preferred in the liquid state. For MeH 2SiSiH 2Me the gauche rotamer is slightly stabilized by 0.43 kJ mol −1. Vibrational spectra have been assigned by the help of normal coordinate analyses using optimized geometries and ab initio symmetry force fields.

Conformational behavior of ethyloxirane from infrared and Raman spectra, temperature dependent FT-IR spectra of xenon solutions, and ab initio calculations

Infrared spectra (3500-400 cm −1) of gaseous ethyloxirane, c-OC 2H 3C 2H 5, (1,2-epoxybutane) and xenon solutions of this molecule at temperatures from −60 to −100°C have been recorded. In addition, Raman spectra of the liquid and polycrystalline phases have been obtained, and variable temperature Raman spectra (25 to −89°C) of the liquid were also recorded. Spectroscopic evidence for the co-existence of the gauche-1 and gauche-2 conformers of the title compound is found in the fluid phases, along with small amounts of the cis rotameric form. In xenon solutions, the enthalpy difference between the gauche-1 and gauche-2 conformers has been determined to be 217 ± 34 cm −1 (620 ± 97 cal mol −1), with the gauche-1 rotamer the more stable form, whereas in the liquid phase this quantity is estimated to be 95 ± 6 cm −1 (272 ± 17 cal mol −1). In the annealed solid phase, only the gauche-1 conformer is found. Ab initio calculations, have been carried with different basis sets up to MP2 6-31 G∗ from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions. A force field from these calculations is used to support the vibrational assignments and simulate the observed infrared and Raman spectra. These results are compared to the corresponding quantities for some similar molecules.

Raman spectroscopy studies of antiferromagnetic FeCO3 and related carbonates

Raman spectroscopy experiments were performed on antiferromagnetic siderite (natural FeCO3). Weak lines at room temperature (in addition to the expected vibrational lines) were found to be seven well defined excitations at liquid helium temperature. Polarization tensor components of these new lines were examined at temperatures varying between room and liquid helium temperature. Frequency decreases upon cooling were observed for three of the lines (the greatest change occuring near the Néel temperature, 38 K). By comparison with infrared spectra, variable temperature Raman spectra and impurity analysis of two related crystals (antiferromagnetic MnCO3 and CaMg(CO3)2 containing 6% iron), new explanations for two (741 and 1735 cm−1) of three previously observed lines and for one (870cm−1) of the remaining four are presented. The three variable frequency lines (440,1175 and 1225 cm−1) are considered magnetic excitations between trigonal field, spin-orbit, and exchange split states of the ferrous ion. The frequency decreases upon cooling may be due to unquenched orbital angular momentum resulting in an exchange interaction of a non-Heisenberg form. Symmetry distortion due to magnetic ordering upon cooling may cause the infrared 741 cm−1 vibration to become Raman-active.