Isostructural bisdithiazolyl and bisthiaselenazolyl radicals: trends in bandwidth and conductivity

Reaction of N-alkylated pyridine-bridged bisdithiazolylium cations [1]+ (R1 =Me, Et; R2 =Ph) with selenium dioxide in acetic acid provides a one-step high-yield synthetic route to bisthiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = Ph). The corresponding radicals 1 and 2 can be prepared by chemical or electrochemical reduction of the cations. Structural analysis of the radicals has been achieved by a combination of single-crystal and powder X-ray diffraction methods. While the two sulfur radicals 1 adopt different space groups (P3(1)21 for R1 = Me and P(-)1 for R1 = Et), the two selenium radicals 2 (space groups P3(1)21 for R1 = Me and P3(2)21 for R1 =Et) are isostructural with each other and also with 1 (R1 = Me, R2 = Ph). Variable-temperature magnetic measurements on all four compounds confirm that they are undimerized S = 1/2 systems, with varying degrees of weak intermolecular antiferromagnetic coupling. Variable-temperature electrical conductivity measurements on the two selenium radicals provide conductivities sigma(300 K) = 7.4 x 10-6 (R1 = Et) and 3.3 x 10-5 S cm-1 (R1 = Me), with activation energies, E(act), of 0.32 (R1 = Et) and 0.29 eV (R1 = Me). The differences in conductivity within the isostructural series is interpreted in terms of their relative solid-state bandwidths, as estimated from Extended Hückel band-structure calculations.     

Antitumour metal compounds: more than theme and variations

Triggered by the resounding success of cisplatin, the past decades have seen tremendous efforts to produce clinically beneficial analogues. The recent achievement of oxaliplatin for the treatment of colon cancer should, however, not belie the imbalance between a plethora of investigated complexes and a very small number of clinically approved platinum drugs. Strategies opening up new avenues are increasingly being sought using complexes of metals other than platinum such as ruthenium or gallium. Based on the chemical differences between these metals, the spectrum of molecular mechanisms of action and potential indications can be broadened substantially. Other approaches focus on complexes with tumour-targeting properties, thereby maximizing the impact on cancer cells and minimizing the problem of adverse side effects, and complexes with biologically active ligands.     

Ferromagnetic long range ordering in copper(II) maleate monohydrate

Summary The magnetic properties of copper(II) maleate monohydrate, Cu(C₄H₂O₄)·H₂O, have been measured in the 1.8–66 K range. Magnetic susceptibility and isothermal magnetization measurements on powdered samples reveal a transition to a ferromagnetic state in this layered compound. The transition to the ordered state occurs at T_c=4.0 K. The positive value of 8.0 K for the Weiss constant in the paramagnetic region confirms the ferromagnetic nature of the interactions. The magnetic susceptibility data above 3T_c may be fitted with a Heisenberg quadratic layer expression with the best-fit parameters J=+4.3 cm^–1, g=2.0, and zJ=+0.42 cm^–1. The e.p.r. spectrum at room temperature indicates orthorhombic symmetry. A complex containing the isomeric form of the ligand, copper(II) fumarate dihydrate, has also been prepared and its magnetic properties determined in the 80–300 K range. The data may be accomodated by two-dimensional theory with J=+1.05 cm^–1, g=2.09, and zJ=+0.10 cm^–1.     

Ferromagnetic ordering in a new nickel polyborate NiB12O14(OH)10

A new nickel polyborate, NiB12O14(OH)10 was synthesized using boric acid as a flux. This material has two-dimensional borate layers with a quasi-square lattice of Ni2+. The Ni2+ ions locate in the plane of the two-dimensional layer, bridged through BnO(n+1) chains in the plane and connected with the three-membered ring borate groups out of the plane. The dc and ac magnetic susceptibility, magnetization and specific heat measurements show that this material undergoes a weak ferromagnetic phase transition at Tc = 5.8 K. At T < Tc, a metamagnetic phase transition is observed at about 5 T, associated by a spin-flop, suggesting the ferromagnetic ordering is induced by an antiferromagnetic interaction. A broad maximum in the ac susceptibility at TM = 23-24 K indicates an intermediate short-range ordering.     

From magnets to metals: the response of tetragonal bisdiselenazolyl radicals to pressure

The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).     

Magnetic ordering and metal‐atom site preference in tetragonal CrMnAs: Electronic correlation effects

The electronic and magnetic structures of tetragonal, Cu₂Sb‐type CrMnAs were examined using density functional theory. To obtain reasonable agreement with reported atomic and low‐temperature magnetic ordering in this compound, the intra‐atomic electron–electron correlation in term of Hubbard U on Mn atoms are necessary. Using GGA + U, calculations identify four low‐energy antiferromagnetically ordered structures, all of which adopt a magnetic unit cell that contains the same direct Cr Cr and Cr Mn magnetic interaction, as well as the same indirect Mn⋅⋅⋅Mn magnetic interaction across the Cr planes. One of these low‐energy configurations corresponds to the reported case. Effective exchange parameters for metal–metal contacts obtained from SPRKKR calculations indicate both direct and indirect exchange couplings play important roles in tetragonal CrMnAs. © 2018 Wiley Periodicals, Inc.     

Enhanced conductivity and magnetic ordering in isostructural heavy atom radicals

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P42(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E2---E2' contacts. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E2 position, with sigma(300 K) reaching a maximum (for E1 = E2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E(act) follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E2 position, order ferromagnetically, with Curie temperatures of T(c) = 12.8 and 17.0 K, respectively. The coercive fields H(c) at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Hückel Theory band structure calculations.     

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF(3) from first-principles calculations

KCrF(3) has been systematically investigated by using the full-potential linearized augmented plane wave plus local orbital method within the generalized gradient approximation and the local spin density approximation plus the on-site Coulomb repulsion approach. The total energies for ferromagnetic and three different antiferromagnetic configurations are calculated in the high-temperature tetragonal and low-temperature monoclinic phases, respectively. It reveals that the ground state is the A-type antiferromagnetic in both phases. Furthermore, the ground states of the two phases are found to be Mott-Hubbard insulators with the G-type orbital ordering pattern. In addition, our calculations show the staggered orbital ordering of the 3d(x(2) ) and 3d(y(2) ) orbitals for the tetragonal phase and the 3d(z(2) ) and 3d(x(2) ) orbitals for the monoclinic phase, which is in agreement with the available data. More importantly, the relationship between magnetic structure and orbital ordering as well as the origin of the orbital ordering are analyzed in detail.     

Copper-linked hexaniobate lindqvist clusters-variations on a theme

Four new one-dimensional materials and one dimer complex based on the linkage of [Nb6O19] clusters and [CuLx] (L=ethylenediamine (en), NH3, H2O) assemble under ambient conditions. These phases include the following: Rb4[Cu(en)2(H2O)2]3[(Nb6O19H2)2Cu(en)2].24H2O (1), space group P; [Cu(en)2(H2O)2]2[(Nb6O19H2)Cu(en)2].14H2O (2), space group P; Rb2[Cu(NH3)2(H2O)4][Cu(NH3)4(H2O)2]2{[Nb6O19][Cu(NH3)]2}(2).6H2O (3), space group P; {[Nb6O19][Cu(NH3)2(H2O)]2[Cu(H2O)4]2}.3H2O (4), space group P2/n; and {[Nb6O19][Cu(NH3)2(H2O)]2[Cu(H2O)4]2} (5), space group C2/m. All structures have been solved by single-crystal methods, and compounds 1-5 were characterized by thermogravimetric analysis, Fourier transform IR, chemical analysis, and magnetic measurements. It has been demonstrated that the conformation, charge, and geometry of the [Nb6O19]-[CuLx] chains can be modulated by varying the type and amount of the [CuLx]2+ species. The charge balance is provided by mixed Rb+/[CuLx]2+ or [CuLx]2+ cations only for structures 1-3, whereas 4 and 5 are neutral chains with no counterions. There are weak antiferromagnetic Cu2+-Cu2+ interactions in all phases. Compounds 2-5 represent the first examples in which the [Nb6O19] Lindqvist ion forms extended solids rather than dimers or decorated monomers when reacted with transition-metal, cationic complexes.     

Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Synthesis of multiply substituted, ion channel forming octi(p-phenylene)s: theme and variations

[reaction: see text] To facilitate the access to unique models for biological processes, we examined six different synthetic routes to octi(p-phenylene) rods with lateral and terminal substituents R(L) and R(T). This systematic study allowed us to increase to overall yield for the synthesis of a new class of oligo(p-phenylene) ionophores about 20 times and to provide general insights into the practicability of synthetic routes to multiply substituted molecular rods.     

Ferromagnetic ordering in bis(benzene)chromium(+) Bis(4,5-dimethoxybenzene-1,2-dithiolato)nickel(-)

The compound bis(benzene)chromium(+) bis(4,5-dimethoxy-1,2-benzenedithiolato)nickel(-), [Cr(bz)(2)](+)[Ni(dmox)(2)](-), was prepared as well-defined crystals by a metathesis reaction. X-ray single-crystal structure confirms the expected symmetries of the cations (D6h) and anions (D2h). The Ni-S bond length is typical for a monoanionic bis(arene)-1,2-dithiolato nickel complex thus confirming the ascribed charges. The structure reveals interpenetrating cation and anion sublattices where all ions are well separated from other ions of the same charge and with almost perpendicular symmetry planes of cations and anions. Above ca. 10 K the compound is paramagnetic and follows the Curie-Weiss law for S = 1/2 on each ion with a Weiss temperature of 8.5 K. Magnetic susceptibility and magnetization studies reveal the transition to a ferromagnetically ordered phase at 3.4 K. The ferromagnetic interaction is likely to occur through short contacts between anion heteroatoms (O and Ni) and cation H atoms, forming a 2-D network. A third dimension is possible through somewhat longer interionic S-H contacts. The Curie-Weiss law behavior can be modeled for 1-D chains with a ferromagnetic interaction of J = 11.6 K.     

Pressure-induced ordering in adamantane: a Monte Carlo simulation study

Isothermal-isobaric ensemble Monte Carlo simulation of adamantane has been carried out with a variable shape simulation cell. The low-temperature crystalline phase and the room-temperature plastic crystalline phases have been studied employing the modified Williams potential. We show that at room temperature, the plastic crystalline phase transforms to the crystalline phase on increase in pressure. Further, we show that this is the same phase as the low-temperature ordered tetragonal phase of adamantane. The high-pressure ordered phase appears to be characterized by a slightly larger shift of the first peak toward a lower value of r in C-C, C-H, and H-H radial distribution functions as compared to the low-temperature tetragonal phase. The coexistence curve between the crystalline and plastic crystalline phase has been obtained approximately up to a pressure of 4 GPa.     

One-dimensional Intrinsic Ferromagnetic Semiconductor CuCl_2: a First-principles Study

One-dimensional nanowires with robust magnetism are desirable for spintronic applications. Herein, on the basis of the first-principles calculations, systematic investigations on the electronic and magnetic properties of the CuCl2 nanowire were performed, which can be potentially tailored from its bulk form. The CuCl2 nanowire exhibits a ferromagnetic ground state. The band structures indicate that the CuCl2 nanowire is a ferromagnetic semiconductor. The spin flip gap is large enough for avoiding spin flip. Phonon dispersion and Born-Oppenheimer molecular dynamics simulation manifest that the CuCl2 nanowire is stable. In addition, distinct magnetic properties of the CuCl2 nanowires inside two types of carbon nanotubes were obtained. The study broadens the family of the existing one-dimensional materials with promising applications for spintronics.     

Theoretical study of the variations in the β-protons hyperfine splitting parameter of hydrocarbon radicals

The proton hyperfine splitting constants of a large number of positive, negative, and neutral radicals, have been examined in term of the Heller-McConnell relation a _H=B_c cos² whose validity is discussed. B is taken as a function of the energy of the singly occupied orbital and values are calculated by first order perturbation theory for the cases of a methyl, methylene, and dimethylene group attached to the system. Substantial agreement is found between theory and experiment indicating the correctness of the postulated cause of the B behaviour.

---

Zusammenfassung Die Konstante der Hyperfeinaufspaltung für -Protonen einer großen Anzahl positiver, negativer und neutraler -Radikale wurde mit der Beziehung von Heller und McConnell als Grundlage untersucht; die Gültigkeit dieser Beziehung wird diskutiert. B wird als von der Energie des einfach besetzten Orbitals abhängig angenommen und seine Werte mit Hilfe der Störungstheorie erster Ordnung für die Fälle von Methyl-, Methylen- und Dimethylengruppe am -System berechnet. Aus der guten Übereinstimmung zwischen Theorie und Experiment wird auf die Richtigkeit der postulierten Ursachen für das Verhalten von B geschlossen.

     

Side-chain and backbone ordering in a polypeptide

We report results from multicanonical simulations of polyglutamic acid chains of length of ten residues. For this simple polypeptide we observe a decoupling of backbone and side-chain ordering in the folding process. While the details of the two transitions vary between the peptide in gas phase and in an implicit solvent, our results indicate that, independent of the specific surroundings, upon continuously lowering the temperature side-chain ordering occurs only after the backbone topology is completely formed.     

Antiferromagnetic ordering in a two-dimensional electron liquid

The possibility of an antiferromagnetic phase in a two-dimensional electron gas is investigated within the density functional formalism of phase transitions. © 1995 John Wiley & Sons, Inc.     

Silicon radicals in silicon oxynitride: a theoretical ESR study

Quantum mechanical calculations are performed on a series of silicon radical defects. These are the upward arrow Si[triple bond]O(3-x)Nx, upward arrow Si[triple bond]N(3-x)Si(x), and upward arrow Si[triple bond]Si(3-x)Ox defects, where x takes on values between 0 and 3. The defects under study constitute a central silicon radical, upward arrow Si, with differing first-nearest-neighbor substitution, as may be found at a Si/SiOxNy interface. These first-nearest neighbor atoms are connected to the silicon radical via three single covalent bonds, denoted as " [triple bond] ". A hybrid defect, upward arrow Si[triple bond]ONSi, is also included. Calculations are performed on gas-phase-like cluster models, as well as more-constrained hybrid quantum and molecular mechanical (QM/MM) models. The isotropic hyperfine coupling constants of these defects are calculated via density functional theory (DFT). Trends in these calculated hyperfines are consistent between the different models utilized. Analysis of the electronic structure and geometries of defects correlate well with trends in the electronegativity of the first-nearest-neighbor atoms. Changes in radical hybridization, induced by changes in the first-nearest-neighbor composition, are the primary factor that affects the calculated hyperfines. Furthermore, comparisons to experimental results are encouraging. Agreement is found between experiments on amorphous to crystalline materials.     

Photolysis and oxidation of azidophenyl-substituted radicals: delocalization in heteroatom-based radicals

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the corresponding nitrenophenyl radicals with the following zero-field splitting parameters: |D/hc| = 0.277 cm(-1), |E/hc| < or = 0.002 cm(-1) (7 from 14); |D/hc| = 0.256 cm(-1), |E/hc| < or = 0.002 cm(-1) (8 from 16); |D/hc| = 0.288 cm(-1), |E/hc| < or = 0.002 cm(-1) (9 from 19); |D/hc| = 0.352 cm(-1), |E/hc| = 0.006 cm(-1) (10 from 21); |D/hc| = 0.336 cm(-1), |E/hc| = 0.004 cm(-1) (11 from 25). UB3LYP/6-31G computations and ESR spectroscopic analyses suggest that these are nitreno radicals, even para-linked systems with possible quinonoidal resonance forms. Neat samples of azidophenyl radicals 14 and 21 showed bulk paramagnetic behavior, consistent with the lack of close contacts in their crystal structures. Efforts to make photolabile coordination complexes of 14 and 21 with paramagnetic transition metal ions were unsuccessful: Cu(ClO4)2 x 6H2O instead oxidized them to the corresponding diamagnetic nitrosonium perchlorate salts.