A comparison study on Raman scattering properties of α- and β-MnO2

In this comment to a recent paper [Anal. Chim. Acta 585 (2007) 241-245], we report a comparison study on Mn oxide-related compounds with different crystallographic forms, which distinguish between β-MnO₂ and α-MnO₂ type materials via Raman scattering (RS) spectroscopy. The tetragonal rutile-type β-MnO₂ is characterized by a RS band at ∼667 cm⁻¹ of symmetry A_1g, whereas the α-MnO₂ type materials feature two main RS contributions at about 574 and 634 cm⁻¹, belonging to A_g spectroscopic species of a tetragonal hollandite-type framework. These data represent a clear signature for identifying β-MnO₂ and α-MnO₂ type materials via RS spectroscopy.     

The crystal structure of Yb2(SO4)3·3H2O and its decomposition product, β-Yb2(SO4)3

Yb₂(SO₄)₃·3H₂O, synthesised by hydrothermal methods at 220(2) °C, has been investigated by single crystal X-ray diffraction. Yb₂(SO₄)₃·3H₂O crystallises in space group Cmc2₁ and is isostructural with Lu₂(SO₄)₃·3H₂O. The crystal structure has been refined to R₁=0.0145 for 3412 reflections [F_o>3σ(F)], and 0.0150 for all 3472 reflections. The structure of Yb₂(SO₄)₃·3H₂O is a complex framework of YbO₆ octahedra, YbO₈ and YbO₅(H₂O)₃ polyhedra and SO₄ tetrahedra. Thermal data shows that Yb₂(SO₄)₃·3H₂O decomposes between 120 and 190 °C to form β-Yb₂(SO₄)₃. The structure of a twinned crystal of β-Yb₂(SO₄)₃ was solved and refined using an amplimode refinement in R3c with an R₁=0.0755 for 8944 reflections [F_o>3σ(F)], and 0.1483 for all 16,361 reflections. β-Yb₂(SO₄)₃ has a unique structural topology based on a 3D network of pinwheels.     

Structural and vibrational characterization of [KrF][AuF6] and α-[O2][AuF6] using single crystal X-ray diffraction, Raman spectroscopy and electron structure calculations

The salt [KrF][AuF₆] has been prepared by the direct oxidation of gold powder in anhydrous HF at 20 °C using the potent oxidative fluorinating agent KrF₂. The KrF⁺ salt readily oxidizes molecular oxygen at ambient temperature to yield [O₂][AuF₆]. Variable temperature Raman spectroscopy has been used to identify a reversible phase transition in [O₂][AuF₆], which occurs between −114 and −118 °C. Single crystal X-ray diffraction has been used to characterize the low-temperature, α-phase of [O₂][AuF₆]. The phase transition is attributed to ordering of the O₂⁺ cation in the crystal lattice, which is accompanied by minor distortions of the AuF₆⁻ anion. The α-phase of [O₂][AuF₆] crystallizes in the triclinic space group , with a=4.935(6) Å, b=4.980(6) Å, c=5.013(6) Å, α=101.18(1)°, β=90.75(2)°, γ=101.98(2)°, V=342.97 Å³, Z=1, and R₁=0.0481 at −122 °C. The structure of the precursor, [KrF][AuF₆], has also been determined by single crystal X-ray diffraction and crystallizes in the monoclinic space group Cc with a=7.992(3) Å, b=7.084(3) Å, c=10.721(4) Å, β=105.58(1)°, V=584.8(4) Å³, Z=4 and R₁=0.0389 at −125 °C. The KrF⁺ and AuF₆⁻ ions interact by means of a FKr---FAu fluorine bridge that is bent by 125.3(7)° about the bridge fluorine. The KrF_t and Kr---F_b bond lengths in [KrF][AuF₆] were determined to be 1.76(1) and 2.15(1) Å, respectively. The energy minimized structures of the [KrF][AuF₆] ion-pair and the AuF₆⁻ anion have been determined at the Hartree-Fock (HF), MP2 and local density functional (LDF) levels of theory. These calculations have also been used to assign the vibrational spectrum of the [KrF][AuF₆] ion-pair in greater detail and to reassign the vibrational spectrum of the AuF₆⁻ anion.     

X-ray diffraction study of crystal and molecular structure of acetylacetone azocompound — [C11H11N2O2F1]

The 3-[4-fluorophenylazopentandion]-2,4 reagent is synthesized based on acetylacetone, and its crystal and molecular structure is studied by X-ray diffraction. It is found that [C₁₁H₁₁N₂O₂F] crystals 1 belong to the monoclinic symmetry, C2/c space group. The unit cell parameters of 1 are a = 18.827(3) ?, b = 12.839(2) ?, c = 26.475(5) ?, β = 104.834(2)°. The analysis shows that the reagent is present in the solution as well as in the crystal. A comparative analysis of these three structures with the reported data is carried out.     

A preparatory and X-ray diffraction study of the SrCl2NdCl3 system

The SrCl₂NdCl₃ system was examined over the full composition range by the Guinier powder X-ray diffraction technique. A solid solution, Sr_(1−x)Nd_xCl_(2+x), was found for the composition region 0 < x < 0.18. Beyond the solid solution region two intermediate chloride phases were identified: Sr_0.80Nd_0.20Cl_2.20 (Sr₄NdCl₁₁) and Sr_0.643Nd_0.357Cl_2.357 (Sr₉Nd₅Cl₃₃). Orthorhombic Sr₄NdCl₁₁ is isostructural with vernier-type Sr₄DyCl₁₁; lattice parameters are a = 7.230(5); b = 35.292(18), and c = 6.826(4)Å. The phase Sr₉Nd₅Cl₃₃ exhibits hexagonal symmetry with lattice parameters a = 12.908(6) and c = 24.823(10), Å and is isostructural with Nd₁₄Cl₃₃.     

High-pressure Raman study of the β-BaB2O4 crystal and pressure-induced phase transitions

A high-pressure Raman study of β-BaB₂O₄ reveals the occurrence of four pressure-induced phase transitions near 48, 70, 80 and 96 kbar, respectively. Above 96 kbar, β-BaB₂O₄ becomes amorphous and the transition is irreversible. A Raman quartet originating from Davydov splitting and the pressure-scanned Fermi resonance effect is observed.     

Synthesis and X-ray crystal structure of the dinuclear copper(I) complex [Cu2((Me-Pk)2En)(PPh3)4](ClO4)2 · 2CHCl3

The new dinuclear copper(I) complex, [Cu₂((Me-Pk)₂En)(PPh₃)₄](ClO₄)₂ · 2CHCl₃ (I), where (Me-Pk)₂En = N,N′-bis(1-pyridin-2-yl-ethylidene)ethane-1,2-diamine), has been synthesized and characterized by elemental analyses, FT-IR, and single-crystal X-ray diffraction method. In this complex, two Cu(PPh₃)₂ units are connected by one (Me-Pk)₂En bridging ligand. The coordination geometry around each copper(I) atom is a distorted tetrahedron formed by two N atoms from (Me-Pk)₂En and two P atoms from the PPh₃ ligands. The distance between two copper atoms is 7.06(1) ?.     

Phase formation in Li2MoO4-Rb2MoO4-MMoO4 (M = Ca, Sr, Ba, Pb) systems and the crystal structure of α-Rb2Pb(MoO4)2

A subsolidus triangulation of Li₂MoO₄-Rb₂MoO₄-MMoO₄ (M = Ca, Sr, Pb, Ba) systems is performed. The RbLiMoO₄-Rb₂M(MoO₄)₂ (M = Pb, Ba) joins, where 11 mol.% long Rb₂M(MoO₄)₂-based solid solutions are found, are studied in most detail. Ternary molybdates do not form in the systems, which is confirmed by spontaneous flux crystallization. The α-Rb₂Pb(MoO₄)₂ crystals are obtained and their crystal structure is solved (a = 20.9724(15) ?, b = 12.1261(8) ?, c = 16.1171(10) ?, β = 115.728(13)°, C2/m space group, R = 0.0695, Z = 16), which is a monoclinic superstructure of the palmierite type and has the largest cell volume and the most complex structure among lead-containing palmierites. One of the MoO₆ tetrahedra is orientationally disordered over two sites; lead atoms are shifted from the centers of their coordination polyhedra to one of their faces and have cn = 6–8; for rubidium cations cn = 10–12.     

Geometric distortions of octahedral cations and tetrahedral anions in disordered [Cu(bpy)3]CrO4·7.5H2O crystal – A comparative study

A novel copper(II) chromate complex of the formula [Cu(bpy)₃][CrO₄]·7.5H₂O (1) (bpy = 2,2′-bipyridine) was prepared in the crystalline form and characterized by X-ray diffraction and spectroscopic methods (EPR, FIR-IR, NIR–Vis–UV). Two non-equivalent [Cu(bpy)₃]²⁺ groups in the form of tetragonally elongated octahedron are built into the hydrogen-bonded network. The tetragonality parameter T is 0.9830 and 0.9876 for Cu(1) and Cu(2) centers, respectively, points to the significant Jahn–Teller (JT) distortion in the tris-bipyridine copper(II) cations. The EPR parameters g₁, g₂ and g₃ are equal to 2.168, 2.159 and 2.072, respectively, correspond to the Cu–N bond directions oscillating between long and short distances. The chromate moiety is a non-coordinated to the metal center and forms a very distorted tetrahedron with three bond lengths in the range 1.617(2)–1.629(2) Å. The O(3A) and O(3B) atoms execute reorientational motion between two equilibrium arrangements with equal probability. The related geometric distortion, Δ_r, is 0.097 Å. The Gaussian analysis of single crystal electronic spectra was based on the four d–d bands assigned for Cu(II) center to the ²A_1g(dx2−y2) → ²A_2g(dz2), ²B_1g(dxy), ²B_2g(dxz) and ²B_3g(dyz) transitions in D_2h symmetry.     

Raman scattering study of the β-BaB2O4 crystal under low temperature

We recorded the Raman spectra at the low temperature (84 K) and determined the directional dispersion of extraordinary phonons of β-BaB₂O₄ crystal. The vibration—intensity relations between the (B₃O₆)³⁻ ring and the β-BaB₂O₄ crystal are analysed. The G—F matrix method was used to calculate the vibrational frequencies of the (B₃O₆)³⁻ ring in the β-BaB₂O₄ crystal. The assignment of internal vibration modes and vibrational symmetry are presented.     

Soft activation of the C-S bond: X-ray diffraction and spectroscopic study of the cluster Ru4(μ4-S)(μ,η3-C3H5)2(CO)12

The complex Ru₄(μ₄-S)(μ,η³-C₃H₅)₂(CO)₁₂ is prepared and examined by IR and NMR spectroscopy; its crystal structure is determined (an automatic Bruker-Nonius X8 Apex four-circle diffractometer equipped with a 2-D CCD-detector, 100 K, graphite-monochromated molybdenum source, λ = 0.71073 ?). The crystal belongs to the orthorhombic crystal system with unit cell parameters a = 19.3781(9) ?, b = 12.2898(7) ?, c = 10.1726(4) ?, V = 2422.6(2) ?³, space group Pnma, Z = 4, composition C₁₈H₁₀O₁₂Ru₄S, d _x = 2.343 g/cm³. The molecule of point symmetry C ₁ is situated on the mirror plane of the space group Pnma, two carbonyl groups at Ru2 and Ru3 atoms overlapping with the allylic ligand with a weight of 50% so that carbon atoms coincide. Thus, we have a racemic structure with two overlapping enantiomers of the molecule of Ru₄(μ₄-S)(μ,η³-C₃H₅)₂(CO)₁₂. Original Russian Text Copyright ? 2008 by I. Yu. Prikhod’ko, V. P. Kirin, V. A. Maksakov, A. V. Virovets, and A. V. Golovin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 49, No. 4, pp. 748–752, May–June, 2008.     

Surface enhanced Raman spectroscopic study of α-bithiophene and α-quaterthiophene molecules adsorbed on silver sols

Surface enhanced Raman scattering (SERS) has been observed from α-bithiophene (2T) and α-quaterthiophene (4T) adsorbed on Ag sols. Experimental results suggest that the adsorbed oligothiophene molecules are anti but twisted and the twist angle is smaller in the surface adsorbed state than in solution. It has been observed that the enhancement factor decreases with the increase of the chain length and maximum enhancement is obtained at a higher concentration for the longer oligomer. The Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to the SERS of oligothiophenes in Ag sols     

A study of the structure of (HfO2)x(Al2O3)1?x/Si films by X-ray photoelectron spectroscopy

By X-ray photoelectron spectroscopy (XPS), using the technique of layer-by-layer analysis, the films of (HfO₂) _x (Al₂O₃)_1−x solid solutions synthesized by chemical vapor deposition are studied. The possibility to determine the structure of solid binary solutions based on the analysis of the XPS spectra is demonstrated.     

Synthesis and crystal structure study of 2′-Se-adenosine-derivatized DNA

The selenium derivatization of nucleic acids is a novel and promising strategy for 3D structure determination of nucleic acids.Selenium can serve as an excellent anomalous scattering center to solve the phase problem,which is one of the two major bottlenecks in macromolecule X-ray crystallography.The other major bottleneck is crystallization.It has been demonstrated that the incorporated selenium functionality at the 2′-positions of the nucleosides and nucleotides is stable and does not cause significant st...     

Ni7−δSnTe2: Modulated crystal structure refinement, electronic structure and anisotropy of electroconductivity

Single crystals of Ni_7−δSnTe₂ were grown during re-crystallization of the presynthesized powder in a two zone furnace. The modulated structure was solved and refined in the (3+2)-dimensional superspace group I4/mmm(0-α0, α00)0.ss.mm with lattice parameters a=3.759(1) and c=19.410(2) Å (measured at 153 K) and Z=2. Satellite reflections observed in the diffraction images can be assigned to the incommensurate modulation vectors q₁=da* and q₂=db* with d=0.410(1). The composition resulting from X-ray structure refinement is Ni_5.81SnTe₂. The structure model has been also developed in the orthorhombic (3+1)-dimensional superspace group Immm(α00)00s assuming twinning according to [110], giving thus the composition Ni_5.79SnTe₂. The origin of the modulation can be attributed to a variation of the occupancy of the Ni(3) site in Ni/Te slabs of the structure. Band structure calculations on a commensurate approximant and single crystal electrical resistivity measurements reveal anisotropic metallic conductivity for this compound.     

Synthesis and reactions of cycloheptadienyl and cyclooctadienyl tungsten complexes: X-ray crystal structure of [W(CO)2(PPh3)2(η-C7H9)][BF4]

Protonation of the cycloheptatriene complex [W(CO)₃(η⁶-C₇H₈)] with H[BF₄] · Et₂O in CH₂Cl₂ affords the cycloheptadienyl system [W(CO)₃(η⁵-C₇H₉)][BF₄] (1). Complex 1 reacts with NaI to yield [WI(CO)₃(η⁵-C₇H₉)], which is a precursor to [W(CO)₂(NCMe)₃(η³-C₇H₉)][BF₄], albeit in very low yield. The dicarbonyl derivatives [W(CO)₂L₂(η⁵-C₇H₉)]⁺ (L₂=2PPh₃, 4, or dppm, 5) were obtained, respectively, by H[BF₄] · Et₂O protonation of [W(CO)₂(PPh₃)(η⁶-C₇H₈)] in the presence of PPh₃ and reaction of 1 with dppm. The X-ray crystal structure of 4 (as a 1/2 CH₂Cl₂ solvate) reveals that the two PPh₃ ligands are mutually trans and are located beneath the central dienyl carbon and the centre of the edge bridge. The first examples of cyclooctadienyl tungsten complexes [WBr(CO)₂(NCMe)₂(1-3-η:5,6-C₈H₁₁)] (6) and [WBr(CO)₂(NCMe)₂(1-3-η:4,5-C₈H₁₁)] (7) were synthesised by reaction of [W(CO)₃(NCR)₃] (R=Me or Prⁿ) with 3-Br-1,5-cod/6-Br-1,4-cod or 5-Br-1,3-cod/3-Br-1,4-cod (cod=cyclooctadiene), respectively. Complexes 6 and 7 are precursors to the pentahapto-bonded cyclooctadienyl tungsten species [W(CO)₂(dppm)(1-3:5,6-η-C₈H₁₁)][BF₄] and [W(CO)₂(dppe)(1-5-η-C₈H₁₁)][BF₄] · CH₂Cl₂.     

Synthesis, crystal growth, and structure of Ta3Al2CoC—An ordered quarternary cubic η-carbide and the first single crystal study of a η-carbide

Single crystals of the new carbide Ta₃Al₂CoC were synthesised from metallic melt and characterized by XRD, EDX and WDX measurements. The crystal structure of Ta₃Al₂CoC was refined on the basis of single crystal data (cF112, Fd3¯m, a=11.6153(13) Å, Z=16, 169 reflections, 13 parameters, R₁(F)=0.0315, wR₂(F²)=0.0857). Ta₃Al₂CoC belongs to the great family of η-carbides or which are important components for cermets. Its crystal structure is characterised by TaC₆-octahedra, which are connected to a three-dimensional net. Co and Al have icosahedral surroundings without contacts to C-atoms. All positions show full occupation. Ta₃Al₂CoC represents the first η-carbide with a complete structure refinement on the basis of single crystal data.     

X-ray diffraction and electron microscope study of the Bi2S3PbBi2S4 system

An electron microscope and X-ray diffraction phase analytical study of the Bi₂S₃-galenobismutite (∼PbBi₂S₄) system has been made on samples rapidly quenched from the melt and samples prepared by sintering Bi₂S₃ and PbS at a temperature of 996 K, 10 K below the eutectic temperature. Five well-ordered phases were found, Bi₂S₃, galenobismutite, and three twinned phases, V-1, V-2, and V-3. The X-ray powder data of these materials are given as well as the refined lattice parameters. Electron microscope examination of the samples using a technique involving slight misalignment of the crystal fragments allowed the structures of disordered materials in the phase V region to be determined.