Kossel diagrams of blue phases

CB15/E9 mixtures submitted to an electric field exhibit a tetragonal phase BPX, having a D¹⁰₄(I4₁22) symmetry and two hexagonal phases BPH^3d and BPH^2d The Kossel diagram technique allows us (a) to confirm the hexagonal symmetry of BPH^3d and to determine precisely its space group D²₆ (P6₂22) and (b) to study the field-induced phase transitions between BP II, BPX and BPH^3d. We show that the BP II → BPH^3d transition is a continuous deformation involving a dilatation in the field direction and a shear perpendicular to this direction. The BP II → BPX and BPX → BPH^3d transitions are discontinuous.     

Observation of a hexagonal blue phase in systems with negative dielectric anisotropy

Abstract

The occurrence of a three dimensional hexagonal blue phase BPH (possible space groups P6₂22 and P6₄22) in materials with negative dielectric anisotropy ?_a = ?_∥ - ?_? has been detected by the investigation of Kossel diagrams. This is the first observation of a field-induced transition between different blue phase modifications for systems with ?_a < 0. BP I as well as BP II can be transformed to BPH at a sufficiently high field strength E _BP-BPH For neither transitions has any discontinuity of the wavelength of selective reflection in the field direction been observed. Investigations for field strengths E < E _BP-BPH show continuous deformation and reorientation of BP I and BP II. Decreasing (increasing) lattice constants in the field direction are found for BP I oriented with a two-fold (four-fold) axis parallel to E and for BP II oriented with either a four-fold or two-fold axis parallel to E. The preferred orientation in the field is the same as that found for ?_a > 0 (four-fold axis parallel to the field direction).     

Effects of temperature and anion type on the two-dimensional condensation of the 4,4′-bipyridine cation radical on mercury

The reduction of 4,4′-bipyridine (BPH²⁺₂) on mercury in an acid medium gives a very narrow sharp tail-less reversible voltammetric peak that can be ascribed to the formation of a two-dimensional (2D) phase of the cation radical BPH^·+₂ at the electrode according to the reaction

BPH²⁺₂ + e⁻ |BPH^·+₂|_2D

The corresponding oxidation peak possesses similar properties and arises from the destruction (fusion) of the 2D phase.In this work we studied the influence of some experimental variables, namely the type of anion present in the medium, the concentration of 4,4′-bipyridine and temperature on the 2D phase transition peaks. Also, we tested various analytical criteria to validate this assignation and fitted both voltammetric peaks numerically to the theoretical model developed for this purpose.     

Theoretically derived absorption and MCD spectral data for the cellular species produced in the hydrolysis of cis-diamminedichloroplatinum(II)

The cellular species formed in the hydrolysis of cis-Pt(NH₃)₂Cl₂ (DDP), namely, cis-[Pt(NH₃)₂XY]ⁿ⁺ (X, Y = Cl^?, H₂O, OH^?; n = 0, 1, 2) have been investigated theoretically using the relativistic and nonrelativistic extended Huckel molecular orbital method. Molecular orbital (MO) results for trans-DDP and its hydrolysis products are also reported for comparison. Transition energies, molar absorption coefficients (?), and B terms from magnetic circular dichroism (MCD) derived from theory are presented for each of the species studied. The electronic absorption and MCD spectra of all the complexes are predicted to exhibit ligand field transitions arising primarily from excitations between the occupied Pt 5d orbitals and the unoccupied Pt 5d and 6p_z orbitals, respectively. The 5d → 6p_z transitions are expected to yield intense absorptions in the UV spectral region. Some intensity is generated in the d → d transitions as a result of the low symmetry of these complexes. Correlation of available experimental data with theory allows spectral assignments to be made and predicted. Substituent effects in the cis- and trans-isomeric species are discussed. Finally, the applicability of the EHMO method to these systems is examined.     

Préparation,sites de symétrie et photoluminescence de l'ion Bi3+ dans les antimoniates de terres rares Y2BSbO7 (B = Ga,Lu, Y)

New rare-earth antimonates Y₂BSbO₇(B = Ga, Lu, Y) have been prepared. Excitation and emission spectra of these host lattices doped with Eu³⁺ reveal two symmetry sites for Eu³⁺ (Y and B sites). The symmetry of these sites is not D_3d as in the pyrochlore structure.The photoluminescence of the Bi³⁺ ion is analysed in these compounds. The spectra are ascribed to the ³¹S₀ → ³P₁ and ³P₁ → ¹S₀ transitions with a perturbation due to the ³P₀ metastable level. The decay curve consists of two exponentials in agreement with the presence of Bi³⁺ in two symmetry sites.     

Polarized crystal spectra of the mixed metal salt [(CH3)3NH]MnxCu1−xCl3·2H2O

The electronic absorption spectra in two polarizations are reported for crystals of the dichroic salt, TMAMn_xCu_1?xCl₃·2H₂O where TMA represents the trimethylammonium cation, (CH₃)₃NH⁺. Although TMACuCl₃·2H₂O is monoclinic, the mixed metal salts in which x ≥ 0.20 adopt the orthorhombic structure of TMAMnCl₃·2H₂O. The bands observed in the near ir region are adequately explained as d-d transitions of the Cu(II) ion in D_2h symmetry. Other polarized bands which occur in the visible region and are neither Mn(II) nor Cu(II) d-d transitions are discussed.     

Sensitization of Tb3+ luminescence with Ce3+ in LaOBr: Tb3+, Ce3+

Luminescence emission and uv-excitation properties of LaOBr: Tb³⁺, LaOBr: Ce³⁺, and LaOBr: Tb³⁺, Ce³⁺ phosphors were studied. The visible emission spectra of La_0.995Tb_0.005OBr consists of⁵D_3,4 →⁷F_3–6 transitions in the wavelength range of 410–630 nm. The excitation of the Tb³⁺ ion gives a broad 4f → 5d transition band at 254 nm and weaker4f → 4f transition lines above 300 nm. The uv-excitation and emission of La_0.995Ce_0.005OBr at 290, 315, 355 (excitation), and 440 nm (emission) originate from transitions between the 4f-ground state and the four crystal field components of the5d²D excited state. The sensitization of Tb³⁺ luminescence in LaOBr with Ce³⁺ at varying concentrations is described and discussed. With increasing Ce³⁺ concentration the ⁵D₃ →⁷F transitions of Tb³⁺ quench totally and the⁵D₄ →⁷F transitions begin to quench gradually. The excitation spectrum of the⁵D₄ →⁷F₅ transition of Tb³⁺ consists of four bands due to Tb³⁺ and Ce³⁺, of which the three Ce³⁺ bands increase in intensity and the Tb³⁺ band decreases as the Ce³⁺ concentration is increased.     

Air-Stable Hexagonal Bipyramidal Dysprosium(III) Single-Ion Magnets with Nearly Perfect D6h Local Symmetry

Eight-coordinated Dy^III centres with D_6h symmetry are expected to act as high-performance single-molecule magnets (SMMs) due to the simultaneous fulfilment of magnetic axiality and a high coordination number (a requisite for air stability). But the experimental realization is challenging due to the requirement of six coordinating atoms in the equatorial plane of the hexagonal bipyramid; this is usually too crowded for the central Dy^III ion. Here a hexaaza macrocyclic Schiff base ligand and finetuned axial alkoxide/phenol-type ligands are used to show that a family of hexagonal bipyramidal Dy^III complexes can be isolated. Among them, three complexes possess nearly perfect D_6h local symmetry. The highest effective magnetic reversal barrier is found at 1338(3) K and an open hysteresis temperature of 6 K at the field sweeping rate of 1.2 mT s⁻¹; this represents a new record for D_6h SMMs.     

X‐ray and neutron powder diffraction analyses of Gly·MgSO4·5H2O and Gly·MgSO4·3H2O,and their deuterated counterparts

We have identified a new compound in the glycine–MgSO₄–water ternary system, namely glycine magnesium sulfate trihydrate (or Gly·MgSO₄·3H₂O) {systematic name: catena‐poly[[tetraaquamagnesium(II)]‐μ‐glycine‐κ²O:O′‐[diaquabis(sulfato‐κO)magnesium(II)]‐μ‐glycine‐κ²O:O′]; [Mg(SO₄)(C₂D₅NO₂)(D₂O)₃]_n}, which can be grown from a supersaturated solution at ∼350 K and which may also be formed by heating the previously known glycine magnesium sulfate pentahydrate (or Gly·MgSO₄·5H₂O) {systematic name: hexaaquamagnesium(II) tetraaquadiglycinemagnesium(II) disulfate; [Mg(D₂O)₆][Mg(C₂D₅NO₂)₂(D₂O)₄](SO₄)₂} above ∼330 K in air. X‐ray powder diffraction analysis reveals that the trihydrate phase is monoclinic (space group P2₁/n), with a unit‐cell metric very similar to that of recently identified Gly·CoSO₄·3H₂O [Tepavitcharova et al. (2012). J. Mol. Struct. 1018 , 113–121]. In order to obtain an accurate determination of all structural parameters, including the locations of H atoms, and to better understand the relationship between the pentahydrate and the trihydrate, neutron powder diffraction measurements of both (fully deuterated) phases were carried out at 10 K at the ISIS neutron spallation source, these being complemented with X‐ray powder diffraction measurements and Raman spectroscopy. At 10 K, glycine magnesium sulfate pentahydrate, structurally described by the `double' formula [Gly(d₅)·MgSO₄·5D₂O]₂, is triclinic (space group P, Z = 1), and glycine magnesium sulfate trihydrate, which may be described by the formula Gly(d₅)·MgSO₄·3D₂O, is monoclinic (space group P2₁/n, Z = 4). In the pentahydrate, there are two symmetry‐inequivalent MgO₆ octahedra on sites of symmetry and two SO₄ tetrahedra with site symmetry 1. The octahedra comprise one [tetraaquadiglcyinemagnesium]²⁺ ion (centred on Mg1) and one [hexaaquamagnesium]²⁺ ion (centred on Mg2), and the glycine zwitterion, NH₃⁺CH₂COO⁻, adopts a monodentate coordination to Mg2. In the trihydrate, there are two pairs of symmetry‐inequivalent MgO₆ octahedra on sites of symmetry and two pairs of SO₄ tetrahedra with site symmetry 1; the glycine zwitterion adopts a binuclear–bidentate bridging function between Mg1 and Mg2, whilst the Mg2 octahedra form a corner‐sharing arrangement with the sulfate tetrahedra. These bridged polyhedra thus constitute infinite polymeric chains extending along the b axis of the crystal. A range of O—H…O, N—H…O and C—H…O hydrogen bonds, including some three‐centred interactions, complete the three‐dimensional framework of each crystal.     

Theoretical investigations of Fe porphyrins. V. Low-lying electronic states of bisammineporphinatoiron(III)

Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [Fe^III(P)(NH₃)₂]⁺. The low-spin ground state is calculated as (d_xy² (d_π)³ with d_xy(d_π)⁴ higher by 0.15 eV. The near-ir bands at ～1 eV observed in low-spin ferriheme proteins are attributed to (π → d_π) transitions. The lowest high-spin state is ⁶A_1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ～1.2 eV are attributed to higher ⁶[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on Fe^III(TPP)CI [TPP = tetrapbenylporphyrin] may be an ¹[tripsextet] state.     

Interpretation der Absorptionsspektren der Komplexionen [Mo(CN)8]4−, [Mo(CN)8]3−, [W(CN)8]4− und [W(CN)8]3−

Zusammenfassung Die Absorptions- und Reflexionsspektren der Oktocyanokomplexe desMo(IV) undW(IV) sowie die Absorptionsspektren der Oktocyanotomplexe desMo(V) undW(V) werden mitgeteilt. Die Spektren werden unter Zugrundelegung der durch Raman- und IR-spektroskopische Untersuchungen gefordertenD _4d-Symmetrie dieser Verbindungen interpretiert. Die beobachteten Banden niedriger Intensität (log<3) werden Übergängen in einem Termsystem zugeordnet, das für die Konfigurationend ² undd ₁ und die SymmetrieD _4d berechnet worden ist. Banden hoher Intensität (log>3) werden auf Übergänge in antibindende Zustände zurückgeführt, an denen höherep-Zustände des Zentralions sowie Ligandenzustände beteiligt sind. Die erhaltenen Werte des Feldparameters stimmen mit ligandenfeldtheoretischen Erwartungen überein.

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Absorption and reflection spectra of the octacyanides ofMo(IV) andW(IV) and the absorption spectra of the octacyanides ofMo(V) andW(V) are presented. The spectra are interpreted in terms of theD _4d symmetry of the compounds supported by investigations of Raman and infrared spectra. Bands of low intensity (log<3) correspond to transitions between levels obtained in the case of the configurationsd ² andd ¹ respectively, in a field ofD _4d symmetry. Bands of high intensity (log>3) are attributed to transitions into antibonding levels in which p-orbitals of the central ion and ligand orbitals participate. The values of the field parameter obtained are in accord with ligand field theory.

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Résumé Les spectres d'absorption et de réflexion des complexes octocyanurés duMo(IV) et duW(IV) ainsi que les spectres d'absorption des mêmes complexes deMo(V) et de W(V) sont présentés. Les spectres sont interprétés en supposant la symétrieD _4d des molécules indiquée par des analyses des spectres Raman et infrarouges. Les bandes de faible intensité (log<3) sont attribuées à des transitions dans un système de niveaux, calculé pour les configurationsd ² etd ¹, respectivement, en symétrieD _4d. Des bandes de forte intensité (log>3) sont attribuées à des transitions vers des niveaux antiliants auxquels participent des fonctions élevéesp de l'ion central et des fonctions des groupes liés. Les valeurs obtenues pour le paramètre de champ sont en accord avec les prévisions de la théorie.

     

Charge‐transfer transitions and optic spectra of chromites: A model computation

In the cluster approach, we consider the peculiarities of charge‐transfer (CT) states and CT O 2p → Cr 3d transitions in the octahedral (CrO₆)^9? complex. We have computed the reduced matrix elements of electric‐dipole transition operator on many‐electron wave functions — the initial and final states of CT transitions. We have parameterized the obtained results and computed the relative intensities of various allowed CT transitions in the absence of the mixing of CT configurations having the same symmetry. Using the Tanabe‐Sugano technique, we have taken into account this mixing and obtained the energies of many‐electron CT transitions and their actual intensities as well. We have also allowed for the Coulomb interaction between the 2p‐electrons of the O^2? ligands and the 3d‐electrons of the central Cr³⁺ ion in the (CrO₆)^9? cluster. This interaction proved insignificant for the optic spectra. Modeling the optic spectrum of chromium‐based oxides has yielded a complicated CT band consisting of 33 lines with the main maximum at about 7 eV and satellites in the range of 4–5 and 8–9 eV. The total extent of the CT band is about 8 eV. The model spectrum is in satisfactory agreement with experimental data, which shows the limited validity of the generally accepted notion of a simple structure of CT spectra. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Structures of Rydberg transitions in the absorption spectra of methyl-substituted derivatives of bis(η6-benzene)chromium

The effect of methylation of ligands in bis(η⁶-benzene)chromium (1) on the structure of Rydberg transitions in absorption spectra has been studied. A detailed analysis and interpretation of all Rydberg elements of the vapor-phase spectra of bis(η⁶-benzene)chromium (2), bis(η⁶-o-xylene)chromium (3), bis(η⁶-m-xylene)chromium (4), and bis(η⁶-mesitylene)chromium (5) was carried out. The vapor-phase electronic absorption spectrum of bis(η⁶-p-xylene)chromium (6) was measured, and the assignment of the Rydberg bands was made for the first time. The first ionization potentials of complexes 2–5 were refined. The energy of detachment of the 3d_z ² electron and the parameters of the Rydberg excitations for molecule 6 were determined. The vibronic components of the 3d_z ²→R4p _x,y transition in the spectra of complexes 2 and 6 were assigned. The differences in the Rydberg structure of the spectra of compounds 2–6 were analyzed in terms of the selection rules for optical transitions in the corresponding symmetry groups. The vapor-phase spectra correspond to conformers with the symmetry groupsC _2v andC ₂ for complexes 2–4, with the symmetry groupsD _3h andD ₃ for compound 5, and with the symmetry groupD _2d for complex 6. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 897–903, May, 1998.     

Single crystal circular and linear dichroism spectra and absolute configuration of M(en)3(NO3)2 (M = Zn(II), Cu(II), Ni(II), Co(II), Mn(II) and Ru(II)

The isomorphous single crystals of M(ethylenediamine)₃(NO₃)₂, where M is Zn(II), Ni(II) and Co(II), exhibit macroscopic optical activity as predicted by their acentric space group. Axial circular dichroism measurements on these pure crystals show conclusively that spontaneous resolution has occurred. The axial circular dichroism and orthoaxial linear dichroism spectra of these pure crystals, and of Cu(II), Ni(II), Co(II), Mn(II) and Ru(II) doped into the Zn(en)₃(NO₃)₂ crystal have been measured at ambient and cryogenic temperatures in the range from 7 to 35 kK. The first NO₃^? transition at 32.5 kK is assigned as ¹A ← ¹A based on its linear polarization and sign of rotational strength. The d-d transitions are assigned in the context of D₃ symmetry and reveal a small negative crystal field parameter k, consistent with theoretical prediction. A positive R for all d-d transitions is found to be associated with the Λ configuration for all of the complex ions, by correlation with the crystal and solution circular dichroism of Ru(en)₃²⁺.     

The electronic structure of CrO3+ and MoO3+ complexes

The results of ab initio molecular orbital calculations for [CrOf_s]^2? and polarised single crystal electronic spectra of [MoOCl₃(Op(NMe₂)₃)₂] and Ph₄As[MoOCl₄(H₂O)] are presented. These data are consistent with the electronic transitions of the MO³⁺ moieties, O_2pπ → M_dxy and M_dxy → Md_dxy,dyz being the lowest energy transitions in the spectra of their respective complexes, both these transitions being of low intensity.     

First-principles calculation of temperature-dependent electronic transitions mechanism in V or Nb substituted BiFeO3

Here, we present a simulation study of temperature-dependent electronic transitions in BiVO₃ (BVO) and BiNbO₃ (BNO) using density functional theory (DFT) together with generalized gradient approximation (GGA) and two-dimensional correlation analysis (2D-CA). The results indicate that heat accumulation can accelerate the degeneracy of V-3d orbital in BVO and the splitting of Nb-4d orbital in BNO at 750 K. We found changes in the type of d–p hybrid orbital as follows, for BVO: V-d_x²_+y² + d_Z²-O-2p_z → V-d_x²_+y²-O-2p_z; and for BNO: Nb-d_x²_+y²-O-2p_z → Nb-d_x²_+y² + d_Z²-O-2p_z. Furthermore, we found changes in the type of hybrid orbital leading to the following electron–electron interactions, for BVO: t_2g (V-d_Z²-O-2p_z) + e_g (V-d_x²_+y²-O-2p_z) → t_2g (V-d_x²_+y²-O-2p_z); and for BNO: t_2g + e_g (Nb-d_x²_+y² + d_Z²-O-2p_z) → t_2g (Nb-d_x²_+y²-O-2p_z) + e_g (Nb-d_z²-O-2p_z). The electronic transitions are determined by a charge-transfer from the occupied O-2p⁴ orbitals to the unoccupied V-3d³ (or Nb-4d³) and Bi-6p³ orbitals. Due to the temperature-dependent electronic structure closely related to these electronic transitions, this study provides a new perspective for the design and improvement of BFO-based temperature-sensitive devices.     

Some continuous geometry transitions and the consequences for a transition‐metal atom

By considering a transition‐metal ion in the field of some symmetrically placed ions, we illustrate the consequences of a continuous change of the structure of the ligands on the energies of the 3d electrons of the transition‐metal atom. Both a rigorous mathematical derivation and an analysis based on group theory are presented. For six ions surrounding the transition‐metal atom, a continuous D_∞h → D_3d → O_h→ D_3d→ D_6d transition is observed, whereas for four ions, a continuous D_{∞ h}→ D_2d→ T_d→ D_2d→ D_4d transition results. Although the two systems possess many similarities, interesting differences are found as well. Finally, we demonstrate that when including orbital interactions between the ligands and the transition‐metal atom, quantitative differences occur. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Synthesis and phase behaviour of poly(propylene imine) dendromesogens of lowest generations

Liquid crystal poly(propylene imine) dendrimers of the zero (D₀) and the first (D₁) generations were synthesised and structurally characterised by elemental analysis, FT-IR and nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry (MALDI-ToF). The phase behaviour of all the compounds was studied using polarisation microscopy and differential scanning calorimetry. Their structures were investigated by X-ray diffraction method by direct magnetic field influence. Analysis of diffraction patterns was carried out using the paracrystal model of Hosemann and the Fonk cluster model. Structural simulation and diffraction calculations on the models were used for determining the layer structure parameters of the dendrimers. Temperature behaviour of the structural parameters was considered in the interval from room temperature to isotropic melting temperature. It was determined that only the dendrimer of zero generation can be oriented by magnetic field. The schemes of phase transformations of the dendrimers look like: Cr→159^°С→Sm-C→274^°С→I for D₀ and G→55–66^°С→Sm-F→108^°С→Sm-C→274^°С→I for D₁.     

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, ³(n→π^*), in formamide (HCONH₂) and N,N-dimethyl formamide (HCONMe₂) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the ³(π→π^*) and ³(n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me₂CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/dE) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.     

Analysis of autoionizing Rydberg series Cu I 3d 9 4snl with the multichannel quantum defect theory

Using atomic beam technique, a combination of collisional and laser excitation, and photoion detection, autoionizing Cu I states in the region of the ionization limits Cu II 3d ⁹ 4s(^3,1 D) were investigated. In spite of the complicated structure of the signals due to the four different ionization limits³ D ₃,³ D ₂,³ D ₁ and¹ D ₂ and the large number of possible (LSJ)-states, which can be reached by this experimental technique, the majority of the signals could be attributed to definite Rydberg series 3d ⁹ 4s(³ D ₃,³ D ₂,³ D ₁,¹ D ₂)nl (LSJ). Perturbations were analyzed by the three- and four-channel quantum defect theory and by Hartree-Fock calculations. General formulas for the calculation of the photoionization cross section by the four-channel quantum defect theory in the case of two closed and two open channels are given.