Comparaison des champs de force des niobates,tantalates et antimoniates de structure columbite et trirutile

A comparison is made between the force fields calculated for the M^IINb^V₂O₆ and M^IITa^V₂O₆ columbite series and the M^IITa^V₂O₆ and M^IISb^V₂O₆ trirutile series. The trirutile structure is shown to be particularly more rigid than the columbite structure. For this reason, the trirutile structure exists only for divalent cations of small size. The stability of the two structures and their relative modifications are then compared. From a comparison of the vibrational spectra of the rutile FeSbO₄ with those of the trirutile MSb₂O₆ compounds, a short-range order in the rutile structure is proposed.     

The First Step of the Oxidation of Elemental Sulfur: Crystal Structure of the Homopolyatomic Sulfur Radical Cation [S8].+

The oxidation of elemental sulfur in superacidic solutions and melts is one of the oldest topics in inorganic main group chemistry. Thus far, only three homopolyatomic sulfur cations ([S₄]²⁺, [S₈]²⁺, and [S₁₉]²⁺) have been characterized crystallographically although ESR investigations have given evidence for the presence of at least two additional homopolyatomic sulfur radical cations in solution. Herein, the crystal structure of the hitherto unknown homopolyatomic sulfur radical cation [S₈]^.+ is presented. The radical cation [S₈]^.+ represents the first step of the oxidation of the S₈ molecule present in elemental sulfur. It has a structure similar to the known structure of [S₈]²⁺, but the transannular sulfur⋅⋅⋅sulfur contact is significantly elongated. Quantum-chemical calculations help in understanding its structure and support its presence in solution as a stable compound. The existence of [S₈]^.+ is also in accord with previous ESR investigations.     

A theoretical comparison of the lowest-lying singlet and triplet states of H2CBe and of HCBeH

The low-lying singlet and triplet states of H₂CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H₂CBe has C_2v symmetry and is a triplet with one π electron (³ B₁). The results presented here suggest that the lowest-energy singlet structure is the (¹B₁) open-shell singlet, also with C_2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C_2v structure with two π electrons (¹A₁) is 15.9 kcal/mol higher than ³B₁. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (³Σ) and singlet (¹Δ) states. The triplet is more stable than the singlet (¹Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H₂ CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H₂ CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.     

Exchange studies in a trigonal Cr3+ dimer system: the (4A2 × 2E) and (4A2 × 4A2) states in Rb3Cr2Br9

A study of the polarised optical absorption and Zeeman spectra of Cr²⁺ dimers in Rb₃Cr₂Br₉ is reported. In particular, transition between the (⁴A₂ × ⁴A₂) dimer ground manifold and the (⁴A₂ × ²E) singly excited manifold have been investigated, and the detailed structure of the (⁴A₂ × ²E) manifold is explained in terms of a recently proposed model for magnetic exchange in pair systems of D_3h symmetry. The ground manifold is shown to exhibit small departures from the interval rule structure, mainly due to magnetostrictive effects. Anomalously low splitting factors are found in applied magnetic field and are discussed in the context of similar effects observed in Cs₃Cr₂Br₉ but not in the chlorine isomorphs. An analysis of the vibronic structure for (⁴A₂ × ²E) is also presented.     

Structural Characterization of Divalent Magnesium-Dopedα-Fe2O3

The defect structure of divalent magnesium-dopedα-Fe₂O₃has been examined by Rietveld structure refinement of the X-ray powder diffraction data. The results show that the Mg²⁺ions occupy the vacant interstitial octahedral sites as well as substituting on the two adjacent octahedral Fe³⁺sites in the corundum-relatedα-Fe₂O₃structure. The structure therefore involves a linear cluster of three Mg²⁺ions replacing two Fe³⁺ions. Interatomic potential calculations indicate that this is the most energetically favorable defect cluster for the system.     

Sur de nouveaux borates mixtes des métaux de transition isotypes de la warwickite

The isomorphism of borates CaLn³⁺BO₄ where Ln³⁺ stands for a small rare earth cation of Y³⁺, with the mineral warwickite (Fe,Mg)₃Ti(BO₄)₂ is demonstrated. The octahedral sites of the warwickite structure seem thus to accommodate rather large cations, like trivalent Y³⁺ or Dy³⁺ and bivalent Ca²⁺. The synthesis of several new transition-metal borates with this structure is reported. From a survey of all the warwickite-type compounds one comes to the conclusion that the structure is only stable when the size of the divalent cation M²⁺ is larger than that one of the trivalent M³⁺.     

Photoionization of the C-1?C-4 monosubstituted alkyl benzenes: Thermochemistry of [C7H7]+ and [C8H9]+ formation

The appearance energies for the [C₇H₇]⁺ and [C₈H₉]⁺ fragment ions produced in the fragmentation of the C-1? C-4 monosubstituted alkyl benzenes have been measured by photon impact. The mean heat of formation calculated for [C₇H₇]⁺ is 205.3 ± 1.9 kcal mol^?1 which is consistent with a threshold tropylium structure. For [C₈H₉]⁺ the mean heat of formation is calculated to be 199.2 ± 1.3 kcal mol^?1 which can be equated with either a methyl tropylium or α-phenylethyl structure at threshold. Some evidence is provided for the existence of the α-phenylethyl ion.     

Ternäre Halogenide vom Typ A3MX6. II. Das System Ag3−xNaxYCl6: Synthese,Strukturen, Ionenleitfähigkeit

Ternary Halides of the A₃MX₆ Type. II. The System Ag_3?xNa_xYCl₆: Synthesis, Structures, Ionic Conductivity . The influence of the substitution of Ag⁺ by Na⁺ ions on the crystal structure and the ionic conductivity of Ag₃YCl₆ (stuffed LiSbF₆-type structure) has been investigated. The system Ag_3?xNa_xYCl₆ forms a complete solid solution. The stuffed LiSbF₆-type structure is stable for all compositions. For compounds with Na⁺ contents of x > 1.67, the cryolite-type structure is observed as the high-temperature form. The transition temperature decreases steadily with increasing Na⁺ content. The “end member” phase Na₃YCl₆ transforms at 243 K from the monoclinic cryolite-type structure to the stuffed LiSbF₆-type structure (trigonal, R3 ; a = 697.3(1), c = 1 868.4(14) pm, Z = 3; R = 0.094; R_w = 0.069). The crystal structures of Ag_1.3Na_1.7YCl₆ (trigonal, R3 ; a = 691.5(2), c = 1 853.7(6) pm, Z = 3; R = 0.099, R_w = 0.081) and AgNa₂YCl₆ (trigonal, R3 ; a = 691.7(1), c = 1 853.9(5) pm, Z = 3; R = 0.099, R_w = 0.064) have also been determined. Both chlorides crystallize like Ag₃YCl₆ and Na₃YCl₆-I in the stuffed LiSbF₆-type structure. The monovalent cations, Ag⁺ and Na⁺, are distributed over the five octahedral voids that are occupied by the Ag⁺ ions alone in Ag₃YCl₆. The ionic conductivity for compounds within the solid solution Ag_3?xNa_xYCl₆ decreases with increasing Na⁺ content. The values for Na₃YCl₆ (σ = 1 · 10^?6 Ω^?1 cm^?1 at T = 500 K) are by 2.5 to 3.5 orders of magnitude smaller than those for Ag₃YCl₆ (σ = 6 · 10^?4 Ω^?1 cm^?1 at T = 500 K).     

Defect structure and ionic conductivity in Bi3Nb0.8W0.2O7.1

Defect structure and conductivity behavior are discussed in the oxide ion conductor Bi₃Nb_0.8W_0.2O_7.1. Investigations were carried out using a combination of AC impedance spectroscopy and powder X-ray and neutron diffraction. Bi₃Nb_0.8W_0.2O_7.1 shows a defect fluorite type structure with evidence for superlattice ordering in the oxide ion sublattice. A detailed analysis of the diffraction results allow for proposed models for the defect structure and suggest vacancy trapping in the six coordinate environment of Nb⁵⁺/W⁶⁺ cations. The influence of the defect structure on ionic conductivity is discussed.     

Tetra-hydrides of the third-row transition elements: spin–orbit coupling effects on geometrical deformation in WH4 and OsH4

The dissociation energies of MH₄ (M =  La, Hf–Hg) were computed using full optimized reaction space (FORS) multi-configuration self-consistent field (MCSCF) and second-order multi-reference Møller–Plesset perturbation methods with the SBKJC basis sets augmented by a set of polarization functions (SBKJC(f,p)). It was shown that of the molecules examined, only four tetra-hydrides HfH₄, TaH₄, WH₄, and OsH₄ with T_d symmetry are lower in energy than the corresponding dissociation limits. For WH₄ and OsH₄, the potential energy surfaces from the D_4h to the T_d structure were explored from both theoretical calculations and symmetry arguments based on the pseudo-Jahn- Teller effect. As for WH₄, it is found that the ground state could be ³E_g, ³A_2g, or ³B_2g at the D_4h structure. The present calculations suggest that the ground state is ³E_g, and that this state is stabilized by the e_u deformation into a C_2v structure (³B₁) and then sequentially to the most stable T_d structure (³A₂). If the molecular system is promoted to the lowest ³B_2g state, the D_4h structure can directly deform into the most stable T_d structure along the b_2u vibrational mode. For OsH₄, the ground state (⁵B_1g) at the D_4h structure deforms into a D_2d structure and the resulting ⁵B₂ state strongly interacts with the lowest ³E and ¹A₁ states due to the spin-orbit couplings (SOCs). As a result, it was shown that the relativistic potential energy of the lowest spin-mixed state (ground state) monotonically decreases along the D_2d deformation path from the D_4h to the T_d structure.     

Relativistic MS Xα calculations of the electronic structure and related properties of PtCl62−

Relativistic MS Xα calculations have been performed on a PtCl₆^2? cluster in O_h symmetry. The electronic structure is discussed and compared with the electronic structure obtained recently for IrCl₆^2?. The theoretical photoionization and optical spectra are in very good agreement with experiment.     

New materials synthesis: Characterization of some metal-doped antimony oxides

In order to understand the chemistry of altermetal dopants in antimony oxide, the detailed structural characterization of two β-Sb₂O₄ compounds is reported, Mo-doped β-Sb₂O₄ (1.5 metal%) and V-doped β-Sb₂O₄ (5 metal%). The methods used to characterize these materials are X-ray and neutron diffraction, scanning electron microscopy, Mo K-edge extended X-ray absorption fine structure spectroscopy, and elemental analysis. The atomic position of each of these dopants in Sb₂O₄ is radically different as is the overall effect on the host structure. Molybdenum does not substitute for Sb atoms, rather the Mo atoms are found in channels of electron density formed by Sb³⁺ lone pairs. The two nearest Sb³⁺ are absent and the oxygen stoichiometry is preserved. The formula is Sb_1.97Mo_0.015O₄. Vanadium incorporates substitutionally for the Sb³⁺ atoms and there are random oxygen vacancies in the resultant structure. The formula is Sb_1.9V_0.1O_3.67. In each case the atomic positions of the host structure (Sb and O) are remarkably unaltered. The β-Sb₂O₄ structure can accommodate Mo and V simultaneously, presumably both means of metal incorporation are employed in this ternary oxide.     

Electron impact mass spectrometry of [C7H8N]+ and [C6H7]+ and [C6H7]+ fragment ions produced from o-toluidine and N-methylaniline

An energetic study of the production of [C₇H₈N]⁺ and [C₆H₇]⁺ fragment ions from o-toluidine and N-methylaniline is reported. The mechanisms for the formation of the ions are suggested. Metastable peaks associated with the formation and fragmentation of reactive [C₇H₈N]⁺ and [C₆H₇]⁺ ions were detected and kinetic energy released were determined. The results indicate that the [C₇H₈N]⁺ ion is formed at threshold from o-toluidine with an aminotropylium structure whereas for N-methylaniline the ion is formed with anN-phenylmethaniminium structure. [C₆H₇]⁺ ions are believed to be formed at threshold from the two precursors with a protonated benzene structure.     

The study of cis - and trans -2 butene using mass spectrometry

Using mass spectrometric technique, the effect of geometrical isomerism on the first and higher appearance energy values for C₄H₃ ⁺, C₄H₇ ⁺ and C₃H,₃ ⁺ ions obtained from cis-2-butene andtrans-2-butene is reported. The structure in the ionization efficiency curves (studied for 9 eV above threshold) for the same ions obtained from the two isomers is reported and compared. It is believed that at threshold C₄H₇ ⁺ fragment is formed from the two isomers as methallyl ion. For C₃H₃ ⁺ fragment formed from the cw-isomer at threshold the proposed structure is the propargyl ion with ΔH_f equal to 279-4 kcal/mole while for that ion obtained fromtransisomer the proposed structure is the allenyl ion with ΔH_f equal to 296.6 kcal/mole.     

Two-Dimensional Layered Metallic VSe2/SWCNTs/rGO Based Ternary Hybrid Materials for High Performance Energy Storage Applications

In this work, the ternary hybrid structure VSe₂/SWCNTs/rGO is reported for supercapacitor applications. The ternary composite exhibits a high specific capacitance of 450 F g⁻¹ in a symmetric cell configuration, with maximum energy density of 131.4 Wh kg⁻¹ and power density of 27.49 kW kg⁻¹. The ternary hybrid also shows a cyclic stability of 91 % after 5000 cycles. Extensive density functional theory (DFT) simulations on the structure as well as on the electronic properties of the binary hybrid structure VSe₂/SWCNTs and the ternary hybrid structure VSe₂/SWCNTs/rGO have been carried out. Due to a synergic effect, there are enhanced density of states near the Fermi level and higher quantum capacitance for the hybrid ternary structure compared to VSe₂/SWCNTs, leading to higher energy and power density for VSe₂/SWCNTs/rGO, supporting our experimental observation. Computed diffusion energy barrier of electrolyte ions (K⁺) predicts that ions move faster in the ternary structure, providing higher charge storage performance.     

A DFT quantum‐chemical study on the structures and active sites of polymethylaluminoxane

The electronic structure and geometry of polymethylaluminoxane (MAO) [—Al(CH₃)O—]_n with different size (n = 4–12) have been studied using quantum‐chemical DFT (density functional theory) calculations. It has been found: 1) Starting from n = 6, the three‐dimensional oxo‐bridged (cage) structure of MAO is more stable than the cyclic structure. 2) Both for cage structure and for cyclic structure the Lewis acidity of Al atoms characterized by their net positive charge amplifies with increasing size of MAO (n). 3) Trimethylaluminium (AlMe₃) reacts with the cage structure of MAO with cleavage of an Al‐O dative bond and formation of acidic tri‐coordinated Al^v and basic di‐coordinated O^v atoms in the MAO molecule. Two molecules AlMe₃ are associated with acidic Al^v and basic O^v centers. As the MAO increases in size, the acidity of Al^v centers amplifies and the distance Al^v‐(AlMe₃) shortens; on the contrary, interaction of AlMe₃ with O^v centers weakens and the distance O^v‐(AlMe₃) increases with increasing n value. The total heat of Al₂Me₆ interaction with MAO (sum interaction of Al^v‐(AlMe₃) and O^v‐(AlMe₃)) noticeably decreases as the size of MAO increases (from 50.9 kcal/mol for n = 4 to 20.2 kcal/mol for n = 12). It is proposed that acidic Al^v and basic O^v centers formed in the cage structure of MAO interact with zirconocene yielding ‘cation‐like’ zirconium active centers.     

ESR studies on the structure of C6F−6 anions

Rigid solution spectra for C₆F^?₆ and C₄F^?₈ are compared, and it is concluded that C₆F^]t-₆ cannot have the planar, σ^* structure previously postulated. Instead, a puckered-ring structure with σ and pseudo π delocalisation is postulated.     

The geometric and electronic structures of I3− and I5− from effective-potential calculations

The (ab initio) effective-potential theory developed by Ewig et al. is applied to the structures of the polyiodide ions, I₃^? and I₅^?. The bare ions I₃^? and I₅^? are found by optimization of the geometry to be symmetric and linear. The counterion environment, however, greatly influences the equilibrium structure. A symmetric, flexible counterion environment produces only a slightly altered symmetric, linear equilibrium structure for the I₃^? anion ; whereas an asymmetric, rigid counterion frame leads to unequal bond lengths and bending of the anion. For the I₅^? anion, the potential energy calculated for bending of the central I—I—I angle, α, is very small so that a slight interaction with the lattice will readily lead to the experimental bent structure with α = 94°. At this value of α, the two outer angles are found to be equal and close to the experimental value. The electronic structures of the I₃^? and I₅^? ions are also discussed.     

On the ionic states of vinylidene and acetylene

Electronic states of C₂H⁺₂ cation (with acetylenic and vinylidenic structure) are investigated by ab initio SCF and PNO CEPA calculations. The lowest excited state of the acetylene cation is calculated to be ⁴A₂ in a strongly bent cis conformation, and a qualitative explanation is given for the lack of detectable emission from theò∑_g⁺ state. Only the radical anion with the vinylidenic structure is bound.     

Structure and electronic structure of [(CF3)2PN]2NVCl2, a molecule involving a planar six-membered ring (N3P2V)

Summary The SCF method is applied to determine the (gas phase) structure of [(CF₃)₂PN]₂NVCl₂, which agrees with the solid-state X-ray structure within typical errors of 2 pm and 2° in bond distances and angles. The electronic structure of atoms forming the ring is best described in terms of divalent N^– and tetravlent P⁺ with appreciable declocalization of nitrogen lone pairs into low-lying empty orbitals of neighbouring atoms P and V. No evidence for aromaticity of the ring system is found.