Mg8Rh4B — A new type of boron stabilized Ti2Ni structure

The new magnesium rhodium boron compound Mg₈Rh₄B has been synthesized by reaction of the metal powders with crystalline or amorphous boron or the RhB precursor. The crystal structure of Mg₈Rh₄B was solved using single-crystal X-ray diffraction data (space group , , Z=8, 174 reflections, R_F=0.016). The crystal structure can be described as a filled Ti₂Ni type where the interstitial sites 8b (), located at the center of two nested Mg₄Rh₄ tetrahedra, are occupied by boron atoms. Taking into account the absence of the Ti₂Ni-type phase in the binary Mg-Rh system, the boron atoms can be considered as stabilizing this structural motif. From the bonding analysis with the electron localization function the crystal structure is described as covalently bonded [Rh₄B]³⁻ anions, embedded in a cationic magnesium matrix.     

Crystal growth, transport, and magnetic properties of Ln3Co4Sn13 (Ln=La, Ce) with a perovskite-like structure

Ln₃Co₄Sn₁₃ (Ln=La, Ce) have been synthesized by flux growth and characterized by single crystal X-ray diffraction. These compounds adopt the Yb₃Rh₄Sn₁₃-type structure and crystallize in the cubic space group (No. 223) with Z=2. Lattice parameters at 298 K are , , and , for the La and Ce analogues, respectively. The crystal structure consists of an Sn-centered icosahedron at the origin of the unit cell, which shares faces with eight Co trigonal prisms and 12 Ln-centered cuboctahedra. Magnetization data at 0.1 T show paramagnetic behavior down to 1.8 K for Ce₃Co₄Sn₁₃, with per Ce³⁺, while conventional type II superconductivity appears below 2.85 K in the La compound. Electrical resistivity and specific heat data for the La compound show a corresponding sharp superconducting transition at T_c∼2.85 K. The entropy and resistivity data for Ce₃Co₄Sn₁₃ show the existence of the Kondo effect with a complicated semiconducting-like behavior in the resistivity data. In addition, a large enhanced specific heat coefficient at low T with a low magnetic transition temperature suggests a heavy-fermionic character for the Ce compound. Herein, the structure and physical properties of Ln₃Co₄Sn₁₃ (Ln=La, Ce) are discussed.     

Hexachloroiridate(IV) as a redox probe for the electrochemical discrimination of B-DNA and M-DNA monolayers on gold

This paper demonstrates the effectiveness of using the redox couple to investigate DNA monolayers, and compares the potential advantages of this system to the standard redox couple. B-DNA monolayers were converted to M-DNA by incubation in buffer containing 0.4 mM Zn²⁺ at pH 8.6 and studied by cyclic voltammetry (CV), impedance spectroscopy (IS) and chronoamperometry (CA) with . Compared to B-DNA, M-DNA showed significant changes in CV, IS and CA spectra. However, only small changes were observed when the monolayers were incubated in Mg²⁺ at pH 8.6 or in Zn²⁺ at pH 6.0. The heterorgeneous electron-transfer rate (k_ET) between the redox probe and the surface of a bare gold electrode was determined to be 5.7 × 10⁻³ cm/s. For a B-DNA modified electrode, the k_ET through the monolayer was too slow to be measured. However, under M-DNA conditions, a k_ET of 1.5 × 10⁻³ cm/s was reached. As well, the percent change in resistance to charge transfer, measured by IS, was used to illustrate the dependence of M-DNA formation on pH. This result is consistent with Zn²⁺ ions replacing the imino protons on thymine and guanine residues. The redox couple was also effective in differentiating between single-stranded and double-stranded DNA during de-hybridization and rehybridization experiments.     

Bi2/3Ce1/3Rh2O5: A new mixed-valent Rh oxide with hitherto unknown structure

The new compound Bi_2/3Ce_1/3Rh₂O₅ has been discovered. It is currently the only known compound in the Bi-Ce-Rh-O system, and it crystallizes in a previously unknown structure type. The structure was established from single crystal X-ray diffraction data. Interatomic distances indicate the oxidation states as Bi_2/3³⁺Ce_1/3⁴⁺Rh₂^3.33+O₅. The structure indicates no ordering between Rh³⁺ and Rh⁴⁺. The lack of charge ordering is consistent with the metallic properties determined from electrical conductivity, Seebeck coefficient, and magnetic susceptibility measurements.     

Synthesis and high-temperature thermoelectric properties of Ni and Ti substituted LaCoO3

The effect of Ti and Ni substitution in LaCoO_3−δ was investigated by means of electrical resistivity and Seebeck coefficient properties in a broad temperature range. The studied compounds crystallize in a rhombohedral crystal structure within the whole substitution range. The Seebeck coefficient of most of the studied compounds is positive indicating predominant hole-type charge carriers. The electrical resistivity decreases with increasing temperature for all compositions. Increasing the Ni content results in a decrease of the electrical resistivity, while the resistivity increases with increasing Ti content. The power factor, PF, for the Ni substituted samples is PF=1.42×10⁻⁴ W/m² K for x=0.10 at and decreases with temperature. The LaCo_1−xTi_xO_3±δ compounds reveal an enhancement of the power factor with increasing temperature. Ti substitution leads to a higher power factor compared to that of Ni substitution at .     

Low-temperature heat capacities and standard molar enthalpy of formation of the solid-state coordination compound trans-Cu(Ala)2(s) (Ala = l-α-alanine)

Low-temperature heat capacities of the solid coordination compound trans-Cu(Ala)₂(s) have been measured by a precision automated adiabatic calorimeter over the temperature range from T = 78 K to 390 K. The experimental values of the molar heat capacities in the temperature region were fitted to a polynomial equation of heat capacities (C_p,m) with the reduced temperatures (X), [X = f (T)], by a least square method. The smoothed molar heat capacities and thermodynamic functions of the complex trans-Cu(Ala)₂(s) were calculated based on the fitted polynomial. The smoothed values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were tabulated with an interval of 5 K. Enthalpies of dissolution of {Cu(Ac)₂·H₂O(s) + 2Ala (s)} and 2:1 HAc (aq) in 100 ml of 2 mol dm⁻³ HCl, respectively, and trans-Cu(Ala)₂(s) in the solvent [2:1 HAc (aq) + 2 mol dm⁻³ HCl] at T = 298.15 K were determined to be , , and by means of an isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as from the enthalpies of dissolution and other auxiliary thermodynamic data using a Hess thermochemical cycle.     

Solubility and limiting activity coefficient of simvastatin in different organic solvents

Equilibrium mole fraction solubility of Zocor^® (simvastatin) a pharmaceutically important compound, was measured between 279 and 315 K, in fifteen different industrial-relevant organic solvents including: methyl acetate, ethyl acetate, propyl acetate, iso-propyl acetate, butyl acetate, iso-butyl acetate, sec-butyl acetate, tert-butyl acetate, and ethanol, propanol, 1-butanol, 2-butanol, 1-pentanol, 1-hexanol, and 1-octanol. Fusion enthalpy, Δ_fusH, melting point temperature, T_m, were measured to be 32,169 J/mol, 412.6 K, respectively; and the difference in the molar heat capacity (at constant pressure) of the liquid, and solid form of simvastatin, ΔC_P, was approximated (by extrapolation) to be 230 J/mol K. Dissolution of simvastatin was found to be endothermic, and entropically favorable. The activity coefficient at infinite dilution of simvastatin in each solvent was calculated from the experimental data, then fitted to Gibbs–Helmholtz equation to estimate the limiting partial molar excess enthalpies, , and the limiting partial molar excess entropies, . The data was also fitted to the non-random-two-liquid (NRTL) activity coefficient equation to generate the model interaction parameters for dissolution of simvastatin in the organic solvents studied here.     

A simple colorimetric sensor for biologically important anions based on intramolecular charge transfer (ICT)

A sensitive colorimetric sensor (1) based on 4,5-dinitrobenzene-1,2-diamine was designed and synthesized. Binding of anions such as AcO⁻, F⁻ and results in a notable change in the visible region of spectrum (an approximately 90 nm red shift), which can be detected by the ‘naked-eye’. Furthermore, the binding ability was evaluated by UV–vis titration experiments as following: AcO⁻ > F⁻ >   Cl⁻, Br⁻, I⁻. The nature of the color change of 1 induced by AcO⁻ was due to the intramolecular charge transfer (ICT) which was confirmed by X-ray crystal structure and ¹H NMR titration spectra.     

The photophysics of 7H-adenine: A quantum chemical investigation including spin–orbit effects

Vertical and adiabatic electronic spectra have been investigated by means of combined density functional and multi-reference configuration interaction methods. Spin–orbit coupling has been determined employing a non-empirical spin–orbit mean-field operator. In the vertical absorption spectrum of isolated 7H-adenine, the transitions to the lowest ¹ state, the optically bright ¹ state, and a so far unknown ¹(π_H → (Ryd, σ^*)) state are predicted to lie very close to each other. The strong ¹ transition at 4.8 eV is the lowest excitation of ¹(π → π^*) type in 7H-adenine. It is red shifted by about 0.3 eV with respect to the corresponding excitation in the 9H-tautomer. We find the global minimum on the S₁ potential energy hypersurface at about 4.2 eV for a ¹ electronic structure. A potential well with a minimum at 4.3 eV exhibits mixed ¹ character. A planar ¹ structure with a potential energy of 4.6 eV constitutes a stationary point on the S₁ surface. At the present stage it is unclear whether it corresponds to a minimum or a saddle-point. The lowest-lying ¹(π → (Ryd, σ^*)) state is metastable with respect to N₇–H₁₄ bond dissociation. Its inner (Rydberg) potential well with an adiabatic excitation energy of 4.6 eV represents another minimum on the S₁ PEH. From the theoretical results presented in this work, we conclude that isolated 7H-adenine will be able to emit photons for excitation energies below 4.7 eV(264 nm). Above this threshold singlet excited 7H-adenine can undergo ultrafast non-radiative relaxation to the electronic ground state, either by hydrogen detachment via the ¹(π → (Ryd, σ^*)) channel or via a conical intersection of the ¹ state along a ring puckering mode. The ³ T₁ state can be efficiently populated via intersystem crossing from one of the S₁ potential energy wells. Large-amplitude motions in the T₁ state along an out-of-plane distortional coordinate lead to significant configuration interaction of the ¹ and ¹ structures which lend intensity to the phosphorescence.     

High stable sandwich-type polyoxometallates based on . Synthesis, chemical properties and characterization of [(A-β-SiW9O34)2(MOH2)3CO3] (M = Y and Yb)

Highly hydrolytic and thermally stable sandwich-type polyoxometallates of [(A-β-SiW₉O₃₄)₂(MOH₂)₃CO₃]¹³⁻ (M = Y³⁺ and Yb³⁺) have been synthesized at room temperature by stoichiometric reactions of the trilacunary ligand with M³⁺ in 0.1 M carbonate solution. The new complexes were isolated as sodium and mixed sodium/potassium salts and were characterized by elemental analysis, IR, ¹³C and ²⁹Si NMR, UV–Vis spectroscopy, TGA, DSC and single crystal structure analysis. The crystal structure of the complexes consist of two lacunary Keggin moieties which are linked by a (H₂OMO)₃C belt into an assembly of virtual C₂ symmetry. Each M³⁺ ion adopts a mono-capped trigonal-prismatic coordination. The C₂ axis of the complexes and the local 3-fold axis of the MO₆ group lies in the (H₂OMO)₃C belt plane. The trigonal prismatic geometry is achieved by the two terminal oxygen atoms of an edge shared pair of WO₆ octahedra from each moiety and two oxygen from the belt, and the cap by one external water ligand. The hydrolytic and thermal stabilities of the complexes and the reasons that prove the retention of the isomeric form of the trilacunary ligand upon complexation are discussed.     

PrCo2Al8 and Pr2Co6Al19: Crystal structure and electronic properties

The praseodymium cobalt aluminides, PrCo₂Al₈ and Pr₂Co₆Al₁₉, were prepared by reaction of the elemental components in an arc-melting furnace, followed by heat treatment at 900 °C for several days. Their chemical composition was checked by scanning electron microscopy and energy dispersive spectroscopy, and their crystal structure was refined from single crystal X-ray diffraction data. PrCo₂Al₈ adopts the CaCo₂Al₈ type of structure, crystallizing with the orthorhombic space group Pbam, with four formula units in a cell of dimensions at room temperature: , , . Pr₂Co₆Al₁₉ crystallizes in the monoclinic space group C2/m, with four formula units in a cell of dimensions at room temperature: , , and β=103.903(1)°. Its structure belongs to the U₂Co₆Al₁₉ type. The crystal structures of both compounds studied can be viewed as three-dimensional structures resulting from the packing of Al polyhedra centred by the transition elements. Along the c-axis, the coordination polyhedra around the Pr atoms pack by face sharing to form strands, which are separated one from another by an extended Co-Al network. Magnetic measurements have revealed that PrCo₂Al₈ orders antiferromagnetically at , with a clear metamagnetic transition occurring at a critical field H_c=0.9(1) T. The temperature dependence of the susceptibility of Pr₂Co₆Al₁₉ does not provide any evidence for long-range magnetic ordering in the temperature domain 1.7-300 K. At low temperatures (T<10 K), the susceptibility saturates in a manner characteristic of a non-magnetic singlet ground state. At high temperatures, the magnetic susceptibility of each compound follows a Curie-Weiss law, with the effective magnetic moment per Pr atom of 3.48(5)μ_B and 3.41(2)μ_B for PrCo₂Al₈ and Pr₂Co₆Al₁₉, respectively. These values are close to the theoretical value of 3.58μ_B expected for a free Pr³⁺ ion and exclude any contribution due to the Co atoms. Both compounds exhibit in the temperature range 5-300 K metallic-like electrical conductivity, and their Seebeck coefficient is of the order of several μV/K.     

The electronic gas-phase spectrum of B3 radical revisited

The gas-phase electronic spectrum of cyclic-B₃ (D_3h) radical has been remeasured in a supersonic molecular beam using a mass-selective resonant 2-color 2-photon technique, leading to a revision of previously reported spectroscopic constants. The species was prepared by ablation of a boron nitride rod in the presence of helium. Ab intio calculations on the geometries and vertical electronic excitation energies, as well as mass identification, indicate that the detected band, centered at 21848.77(2) cm⁻¹, is the origin of the cyclic-¹¹B₃ system. A spectral fit yields the rotational constants as B″ = 1.2246(45) and C″ = 0.62131(72) cm⁻¹ in the ground state, and B′ = 1.1914(44) and C′ = 0.61173(69) cm⁻¹ in the excited 2 ²E′ state.     

Structure and properties of the CaFe2O4-type cobalt oxide CaCo2O4

The calcium cobalt oxide CaCo₂O₄ was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo₂O₄ crystallizes in the CaFe₂O₄ (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO₆ octahedral network. The structure of CaCo₂O₄ belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co³⁺, 3d, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10⁻¹ Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K²), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.     

Is Cd2 truly a van der Waals molecule? Analysis of rotational profiles recorded at the , transitions

Rotational profiles of the ²²⁸Cd₂ isotopomer recorded in the (υ′, υ″) = (26, 0), (27, 0), (42, 0), (45, 0), (46, 0), (48, 0) vibrational bands of the transition were analysed. As a result, the , , , , and excited- as well as the ground-state rotational constants of the (¹¹⁴Cd)₂ were determined. The analysis allowed determining the absolute values for the and excited- and ground-state bond lengths, respectively. The obtained result – the – distinctly shorter than that obtained with assumption of pure ground-state van der Waals bonding, supports a theoretical prediction of a covalent admixture to the bonding. Analysis of the partially-resolved rotational profile recorded in the (υ′, υ″) = (38, 0) band of the same isotopomer recorded at the transition allowed estimating the rotational constant in the B1_u state.     

2,4,6-Trimethylpyridinium perchlorate: Polar properties and correlations with molecular structure of organic–inorganic hybrid crystal

[(CH₃)₃C₅H₂NH][ClO₄] has been synthesized and characterized by X-ray (at 344, 245, 180 and 115 K), calorimetric, dilatometric, dielectric and pyroelectric measurements. At room temperature the crystal structure is polar, space group Pmn2₁. It consists of discrete disordered [ClO₄]^- anions and ordered trimethylpyridinium cations giving the 3D network of hydrogen bonds. The compound reveals a rich polymorphism in the solid state. It undergoes four solid–solid phase transitions: from phases I to II at 356/327 K (heating/cooling), II→III at 346/326, III→IV at 226 K and IV→V at 182/170 K. [(CH₃)₃C₅H₂NH][ClO₄] reveals a strong pyroelectric response over a wide temperature region (phases III, IV and V) with the spontaneous polarization changes (ΔP_s) of the order of . The spontaneous polarization is irreversible over all the polar phases, however, the magnitude of the ΔP_s in the vicinity of the phase transitions is characteristic of compounds with the ferroelectric order. The molecular mechanism of the successive phases transitions in the studied crystal is proposed.     

Crystal structure and thermochemical properties of 1-dodecylamine hydrochloride (C12H28NCl)(s)

Crystal structure of 1-dodecylamine hydrochloride (C₁₂H₂₈NCl)(s) has been determined by an X-ray crystallography. Lattice potential energy and the molar volumes of the solid compound and its cation were respectively obtained. The enthalpy of dissolution of the compound was measured by an isoperibol solution-reaction calorimeter at 298.15 K. The molar enthalpy of dissolution at infinite dilution was determined to be , and relative apparent molar enthalpies (Φ_L), relative partial molar enthalpies (L₂) of the compound and relative partial molar enthalpies (L₁) of the solvent (double distilled water) at different concentrations m (mol kg⁻¹) were obtained through fitted multiple regression equation by means of Pitzer's theory. Finally, hydration enthalpies of the substance and its cation were calculated by designing a thermochemical cycle in accordance with lattice potential energy and the molar enthalpy of dissolution at infinite dilution .     

The mechanism of nitrogen fluorescence inside a femtosecond laser filament in air

The visible and near-UV fluorescence spectroscopy of air generated inside a femtosecond laser plasma filament was studied. The primary reactions, , populates the electronic excited state N₂(C³П_u) of N₂. The N₂ fluorescence is not by direct electron–ion recombination of . Using a pump–probe method, the fluorescence lifetime of C³П_u of N₂ at atmospheric pressure was determined to be about 85 ps.     

Syntheses, crystal and electronic structure, and some optical and transport properties of LnCuOTe (Ln=La, Ce, Nd)

Three new compounds, LaCuOTe, CeCuOTe, and NdCuOTe, have been synthesized from the respective rare-earth elements, CuO, and a KI flux at 1023 K. The compounds, which have the ZrSiCuAs structure type, are isostructural to LaCuOS, and crystallize in space group P4/nmm of the tetragonal system with two formula units in cells of dimensions at 153 K of , , for LaCuOTe; , , for CeCuOTe; and , , for NdCuOTe. The structure of LnCuOTe (Ln=La, Ce, Nd) is composed of alternating PbO-like [Ln₂O₂] and anti-PbO-like [Cu₂Te₂] layers stacked perpendicular to [0 0 1]. The experimental optical band gaps of LaCuOTe and NdCuOTe are 2.31 and 2.26 eV, respectively. At 298 K the electrical conductivity of LaCuOTe is 1.65 S/cm and the Hall mobility is +80.6 cm² V⁻¹ s⁻¹. The positive values of the Seebeck and Hall coefficients indicate p-type electrical conduction. First-principles theoretical calculations were performed on LaCuOQ (Q=S, Se, Te). In LaCuOTe, Cu 3d and Te 5p orbitals dominate the states near the valence band maximum; the states near the conduction band minimum are composed of Cu 4s, Te 5p, and La 5d orbitals. The larger dispersion of Cu 3d orbitals and the presence of Te 5p orbitals near the valence band maximum are responsible for the larger hole mobility of LaCuOTe compared to LaCuOS and LaCuOSe.     

Synthesis, crystal structure and magnetic properties of an Os-containing pillared perovskite La5Os3MnO16

A new Os-containing, pillared perovskite, La₅Os₃MnO₁₆, has been synthesized by solid state reaction in sealed quartz tubes. This extends the crystal chemistry of these materials which had been known only for Mo and Re, previously. The crystal structure has been characterized by X-ray and neutron powder diffraction and is described in space group C-1 with parameters a=7.9648(9) Å; b=8.062(1) Å; c=10.156(2) Å, α=90.25(1)°, β=95.5(1)°; γ=89.95(2)°, for La₅Os₃MnO₁₆. The compound is isostructural with the corresponding La₅Re₃MnO₁₆ phase. A very short Os-Os distance of 2.50(1) Å was found in the dimeric pillaring unit, Os₂O₁₀, suggestive of a triple bond as demanded by electron counting. Nearly spin only values for the effective moment for Os⁵⁺ () and Mn²⁺ () were derived from magnetic susceptibility data. Evidence for magnetic transitions was seen near ∼180 and 80 K. Neutron diffraction data indicate that T_c is 170(5) K. The magnetic structure of La₅Os₃MnO₁₆ at 7 K was solved revealing that Os⁵⁺ and Mn²⁺ form ferrimagnetically coupled layers with antiferromagnetic interlayer ordering. The ordered moments are for Mn²⁺ and for Os⁵⁺, which are reduced from the respective spin only values of 5.0 and . The observation of net ferrimagnetic (antiparallel) intraplanar coupling between Os⁵⁺(t_2g³) and Mn²⁺(t_2g³e_g²) is interesting as it appears to contradict the Goodenough-Kanamori rules for 180° superexchange.