Tuning intermolecular magnetic exchange interactions in the solids C(x)F(2)(x)(CNSSS)(2)(AsF(6))(2): structural, EPR, and magnetic characterization of dimeric (x = 2, 4) diradicals

A series of diradical containing salts CxF2x(CNSSS)2(**2+0(AsF6-)2 {x = 2, 1[AsF6]2; x = 3, 3[AsF6]2; x = 4, 2[AsF6]2} have been prepared. 1[AsF6]2 and 2[AsF6]2 were fully characterized by X-ray, variable-temperature magnetic susceptibility, and solid-state EPR measurements, further allowing us to extend the number of examples of the family of rare 7pi RCNSSS(*+) radical cations. 1[AsF6]2: a = 6.5314(7) A, b = 7.5658(9) A, c = 9.6048(11) A, alpha = 100.962(2) degrees , beta = 96.885(2) degrees , gamma = 107.436(2) degrees , triclinic, space group P, Z = 1, T = 173 K. 2[AsF6]2: a = 10.6398(16) A, b = 7.9680(11) A, c = 12.7468(19) A, beta = 99.758(2) degrees , monoclinic, space group P21/c, Z = 2, T = 173 K. In the solid-state, CxF2x(CNSSS)2(**2+) (x = 2, 4) formed one-dimensional polymeric chains of dications containing discrete centrosymmetric radical pairs in which radicals were linked by four centered two-electron pi*-pi* bonds [12+, d(S...S) = 3.455(1) A; 22+, d(S...S) = 3.306(2) A]. The exchange interactions in these bonds were determined to be -500 +/- 30 and -900 +/- 90 cm-1, by variable temperature magnetic susceptibility measurements, respectively, providing rare experimental data on the singlet-triplet gaps in the field of thiazyl radicals. For 2[AsF6]2, the thermally excited triplet state was unambiguously characterized by EPR techniques [/D/ = 0.0254(8) cm(-1), /E/ = 0.0013(8) cm(-1)]. These experimental data implied a weakly associated nature of the radical moieties contained in the solids 1[AsF6]2 and 2[AsF6]2. Computational analysis of the dimerization process is presented, and we show that the 2c 4 electron pi*-pi* bonds in 1[AsF6]2 and 2[AsF6]2 have ca. 50% and 40% diradical character, respectively. In contrast, 3[AsF6]2.SO2, containing diradical C3F6(CNSSS)2(**2+) with an odd number of CF2 spacers, showed magnetic behavior that was consistent with the presence of monomeric radical centers in the solid state.     

Differences and similarities among hypoxanthinium nitrate hydrate structures

Crystals of hypoxanthinium (6‐oxo‐1H,7H‐purin‐9‐ium) nitrate hydrates were investigated by means of X‐ray diffraction at different temperatures. The data for hypoxanthinium nitrate monohydrate (C₅H₅N₄O⁺·NO₃^?·H₂O, Hx1 ) were collected at 20, 105 and 285 K. The room‐temperature phase was reported previously [Schmalle et al. (1990). Acta Cryst. C 46 , 340–342] and the low‐temperature phase has not been investigated yet. The structure underwent a phase transition, which resulted in a change of space group from Pmnb to P2₁/n at lower temperature and subsequently in nonmerohedral twinning. The structure of hypoxanthinium dinitrate trihydrate (H₃O⁺·C₅H₅N₄O⁺·2NO₃^?·2H₂O, Hx2 ) was determined at 20 and 100 K, and also has not been reported previously. The Hx2 structure consists of two types of layers: the `hypoxanthinium nitrate monohydrate' layers (HX) observed in Hx1 and layers of Zundel complex H₃O⁺·H₂O interacting with nitrate anions (OX). The crystal can be considered as a solid solution of two salts, i.e. hypoxanthinium nitrate monohydrate, C₅H₅N₄O⁺·NO₃^?·H₂O, and oxonium nitrate monohydrate, H₃O⁺(H₂O)·NO₃^?.     

Effect of the environment on molecular properties: synthesis, structure, and photoluminescence of Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) nanoclusters in eight different supramolecular frameworks

By selection of different charge-balancing anionic frameworks and different host-to-guest ratios, the photosensitizer-dye cation [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) has been embedded in a series of three-dimensional host structures. It occurs with variable geometry in different states of aggregation, including weakly interacting monomers, isolated dimers, columns, and layers. A large variation in its emission lifetime is correlated with the relative energy level spacings of the guest- and host-framework components. In a fully saturated host framework, the lifetime exceeds values reported for a series of conventional Cu(dmp)2 salts.     

Synthesis, structures, and olefin polymerization capability of vanadium(4+) imido compounds with fac-N3 donor ligands

One-pot reactions of V(NMe2)4 with a range of primary alkyl- and arylamines RNH2 and Me3SiCl afforded the corresponding five-coordinate vanadium(4+) imido compounds V(NR)Cl2(NHMe2)2 [R = 2,6-C6H3(i)Pr2 (1a, previously reported), 2-C6H4(t)Bu (1b), 2-C6H4CF3 (1c), (t)Bu (1d), Ad (Ad = adamantyl, 1e)]. The crystal structures of 1b (two diamorphic forms) and 1c featured N-H...Cl hydrogen-bonded chains. Reaction of 1a-e with the neutral face-capping, N3 donor ligands TACN (TACN = 1,4,7-trimethyltriazacyclononane) or TPM [TPM = tris(3,5-dimethylpyrazolyl)methane] gave the corresponding six-coordinate complexes V(NR)(TACN)Cl2 (2a-e) and V(NR)(TPM)Cl2 (3a-e). The X-ray structures of 2b, 2c, 2d, 3b, 3c, and 3e were determined. When activated with methylaluminoxane, certain of the complexes V(NR)(TPM)Cl2 (3) formed moderately active ethylene polymerization catalysts, whereas none of the compounds V(NR)(TACN)Cl2 (2) were active.     

Spectroscopic properties of a Tm3+:Bi2TeO5 single crystal

Single crystal of bismuth tellurite Bi₂TeO₅ (BTO) doped with trivalent thulium (Tm³⁺) was grown by the Czochralski method and investigated by spectroscopic techniques (absorption, emission). Absorption and emission of the Tm:BTO crystal collected in the VIS‐IR spectral range are presented. Spectroscopic features were investigated as a function of the excitation light polarisation (E//X, Y, and Z axis of optical indicatrix) and sample temperature. The oscillator strength of the ³H₆ → ³F₄ transition at 1.7 μm was determined by numerical integration of the X, Y and Z polarised absorption spectra. The obtained values are: P_X = 4.7 × 10⁻⁶, P_Y = 6.8 × 10⁻⁶ and P_Z = 5.9 × 10⁻⁶ that gives a mean value P_mean = 5.8 × 10⁻⁶. Emission was observed from the ¹G₄ and ³H₄ levels. The ³H₄ and ¹G₄ excited state dynamics was studied. Decay curves and time constants of luminescence were collected at 5 and 300 K and analysed. The ³F₄ → ³H₆ luminescence at around 1.8 μm was too weak to be acquired from the low concentrated sample. However, based on optical data it was possible to estimate the radiative probability A_rad of this transition and the ³F₄ radiative lifetime. The resultant values are: A_rad = 668 ± 0.4 s⁻¹ and τ_rad = 1.5 ms. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure,microstructure and physicochemical properties of BaW<Subscript>1−x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>4−δ</Subscript> materials

Nb-doped BaWO₄ with the assumed formula BaW_1?xNb_xO_4?δ (x = 0, 0.005, 0.01, 0.02 and 0.05) were prepared by solid-state reaction method. Crystal structure and phase composition were determined by X-ray diffraction method. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) was used to describe microstructure and chemical composition of synthesised materials. It was found that solubility limit of niobium in the BaWO₄ structure is the range 0.5–1 mol%, as formation of second phase—Ba₅Nb₄O₁₅—was observed for samples with higher dopant content. For evaluation of the chemical stability of synthesized materials, the comparative CO₂/H₂O exposure test was performed. Samples were exposed to carbon dioxide- and water vapour-rich atmosphere (7% CO₂ in air, 100% RH) at 298 K for 700 h. During this exposition, the chemical reactions between the samples and the surrounding gaseous atmosphere resulting in formation of barium hydroxide and/or barium carbonate can process. Thermogravimetry (TG) method was used for chemical stability evaluation. The comparison of samples before and after the CO₂/H₂O exposure test was performed. To support the interpretation of TG results, the analysis of gaseous products evolved during thermal treatment of the samples was done using mass spectrometer. The effect of dopant on the BaWO₄ chemical stability improvement was observed. In order to determine the electrical properties of obtained materials, the DC resistance measurements in synthetic air atmosphere were taken. It was shown that niobium doping and the presence of second phase—Ba₅Nb₄O₁₅—leads to an increase in the total conductivity of synthesised materials.     

Multiple Binding Sites Contribute to the Mechanism of Mixed Agonistic and Positive Allosteric Modulators of the Cannabinoid CB1 Receptor

The cannabinoid CB1 receptor (CB1R) is an abundant metabotropic G‐protein‐coupled receptor that has been difficult to address therapeutically because of CNS side effects exerted by orthosteric drug candidates. Recent efforts have focused on developing allosteric modulators that target CB1R. Compounds from the recently discovered class of mixed agonistic and positive allosteric modulators (Ago‐PAMs) based on 2‐phenylindoles have shown promising functional and binding properties as CB1R ligands. Here, we identify binding modes of both the CP 55,940 agonist and GAT228, a 2‐phenylindole allosteric modulator, by using our metadynamics simulation protocol, and quantify their affinity and cooperativity by atomistic simulations. We demonstrate the involvement of multiple adjunct binding sites in the Ago‐PAM characteristics of the 2‐phenylindole modulators and explain their ability to compete with orthosteric agonists at higher concentrations. We validate these results experimentally by showing the contribution of multiple sites on the allosteric binding of ZCZ011, another homologous member of the class, together with the orthosteric agonist.     

Copolymers dispersions designed to shaping of ceramic materials

Journal of Thermal Analysis and Calorimetry - To study the effect of ionic liquids (ILs) of the microstructure on the surface of the coal, four ILs ([Emim][BF4], [Bmim][BF4], [Bmim][NO3], and...     

The photoinduced triplet of flavins and its protonation states

The photogenerated triplet states of riboflavin and flavin mononucleotide (FMN) have been examined by time-resolved electron paramagnetic resonance (EPR) spectroscopy at low temperature (T = 80 K). Because of the high time resolution of the utilized EPR instrumentation, the triplets are for the first time observed in the nonequilibrated electron-spin polarized state and not in their equilibrated forms with the population of the triplet sublevels governed by Boltzmann distribution. The electron-spin polarization pattern directly reflects the anisotropy of the intersystem crossing from the excited singlet-state precursor. Spectral analysis of the resulting enhanced absorptive and emissive EPR signals yields the zero-field splitting parameters, |D| and |E|, and the zero-field populations of the triplet at high accuracy. These parameters are sensitive probes for the protonation state of the flavin's isoalloxazine ring, as becomes evident by a comparison of the spectra recorded at different pH values of the solvent. The three protonation states of the flavins can furthermore be distinguished by the kinetics of the transient EPR signals, which are dominated by spin-lattice relaxation. The fastest decays are observed for the protonated FMN and riboflavin triplets, followed by the deprotonated flavin triplets. Slow decays are measured for the triplet states of neutral FMN and riboflavin. Because proton transfer is found to be slow on the time scale of spin-polarized triplet detection by transient EPR, the pH-dependent spin-relaxation and zero-field splitting parameters offer a novel approach to probe the protonation state of flavins in their singlet ground state through the characterization of their triplet-state properties.