Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

First separation and characterization of cis and trans 1,2-bisaryloxy perfluorocyclobutanes

4-Bromotrifluorovinyloxybenzene (4-Br-TFVE) undergoes cyclodimerization upon heating at 150 °C affording cis and trans isomers of 1,2-bis(4-bromophenoxy)hexafluorocylcobutane. Stereoisomers were separated by selective crystallization, confirmed by single crystal X-ray, and further characterized by NMR. Remarkable difference in the solid state structures include aryl to aryl dihedral angles of 12° for the trans isomer and 88° for the cis isomer. Polymers containing roughly equal amounts of cis and trans fused perfluorocyclobutyl (PFCB) rings should be expected to have low crystallinity due to the marked difference in the two unit cells as is observed.     

Isolation of C60(CF3)n (n = 2, 4, 6, 8, 10) with high compositional purity

The high temperature reaction of C₆₀ with silver(I) trifluoroacetate followed by 500 °C sublimation and subsequent HPLC purification has led to the isolation of the five trifluoromethyl[60]fullerenes C₆₀(CF₃)_n (n=2, 4, 6, 8, 10). Four of them have >90% compositional purity. Two of the compounds, C₆₀(CF₃)₄ and C₆₀(CF₃)₆, were obtained as C₁-symmetry isomers with >90% isomeric purity, and a sample of C₆₀(CF₃)₂ also contained ca. 15-20% of a C_s-symmetry isomer of C₆₀(CF₃)₄. The new compounds were characterized by IR and EI mass spectrometry (all five compounds), NMR spectroscopy (C₆₀(CF₃)₂, C₆₀(CF₃)₄, and C₆₀(CF₃)₆), and 2D COSY NMR spectroscopy (C₆₀(CF₃)₄ and C₆₀(CF₃)₆). Calculations at the AM1 and DFT levels of theory have led to the prediction of the most likely structures for C₆₀(CF₃)₂, C₁-C₆₀(CF₃)₄, C_s-C₆₀(CF₃)₄, and the two most likely structures of C₁-C₆₀(CF₃)₆.     

Synthesis and properties of PPE-type conjugated polymers containing tricarbonyl(arene)chromium unit in the main chain

Novel π-conjugated polymers containing (η6-arene)Cr(CO)₃ in the main chain based on poly(p-phenylene-ethynylene) were prepared by the Sonogashira coupling reaction of η6-(1,4-diethynylbenzene)tricarbonylchromium with 1,4-diiodo-2,5-dialkoxybenzenes. The polymers were soluble in common organic solvents and characterized by NMR and FT-IR spectra. The polymers exhibited an absorption maximum at around 390 nm, which showed a red shift of approximately 30 nm compared to that of the model compound. Cyclic voltammogram showed a single reversible oxidation peak derived from the (η6-arene)Cr(CO)₃ unit.     

Synthesis and characterisation of bis(pentafluorophenyl)antimony(V) cations, [(C6F5)2SbL3]

Pentacoordinate complex cations of the general formula [(C₆F₅)₂SbL₃]³⁺ stabilized as solid salts in combination with tetraphenylborate (BPh₄), tetrafluorobroate (BF₄) anions, where L=DMSO, Ph₃AsO, PyO, DMF, α-, β- and γ-picoline have been isolated. The newly formed complexes were characterized by elemental analysis, molar conductance measurements, solid-state IR and and NMR. From these results, a five-fold coordination around antimony was required.     

A short, simple and general approach for the synthesis of (3S,4S)-3-methoxy-4-methylamino pyrrolidine and (3S,4R)-3-methoxy-4-methylamino pyrrolidine

A general and efficient stereoselective approach for the synthesis of (3S,4S) and (3S,4R)-3-methoxy-4-methylamino pyrrolidines, a part of the structure of AG-7352, a naphthyridine antitumor agent and quinoline antibacterial compounds has been described.     

Effect of doping and temperature on the crystal structure of (V1−xMox)2O3 above and below the metal/insulator transition

The crystal structure of new molybdenum-doped vanadium sesquioxides (V_1−xMo_x)₂O₃ (0?x?0.20) has been studied at low temperature (10 K) and up to room temperature, through neutron and X-ray powder diffraction. The transition from insulating I- to metallic M-type phases, either by doping or thermally driven, is accompanied by an abrupt decrease of all interatomic distances. Within each structural type however, at 10 K, the effect of doping is essentially the same as at room temperature: it increases cation-oxygen distances, and decreases cation-cation distances, making the cationic coordination octahedra more regular. Thermal effects differ for each phase type: all interatomic distances normally increase in the M-type phase (but with different octahedral modifications depending on doping), but they decrease or remain constant in the I-type phase. This produces an unusual negative thermal expansion coefficient up to 5% at low temperature for the doped compounds.     

Substitution of Co in YBa2Fe3O8

The accommodation of Co in the oxygen-saturated solid-solution phase YBa₂(Fe_1−zCo_z)₃O_8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by Mössbauer spectroscopy. Of the nominal composition range 0.00?z?1.00 tested, the solid-solution limit under syntheses at 950°C in is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co³⁺ replaces Fe³⁺. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe³⁺ and Co³⁺ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe³⁺ by the smaller Co³⁺ ion. A contrast is pointed out with the substitution of cobalt in YBa₂Cu₃O₇ where it is a larger Co²⁺ ion that replaces smaller Cu²⁺.     

Evidence of four light-induced metastable states in iron-nitrosyl complexes

Thermodynamic measurements on irradiated Na₂[Fe(CN)₅NO]⋅ 2H₂O reveal the existence of four light-induced long-lived metastable states, which decay exponentially in time. The activation energies and frequency factors of the four independent states are determined from isothermal calorimetric and differential scanning calorimetric measurements.     

{[K.18-Crown-6]Br3}n: a unique tribromide-type and columnar nanotube-like structure for the oxidative coupling of thiols and bromination of some aromatic compounds

Reaction of 18-crown-6 with KBr and then with bromine (Br₂) afforded a unique tribromide salt with a nanotube-like structure ({[K.18-crown-6]Br₃}_n). Oxidative coupling of thiols and bromination of some aromatic compounds is the first report on the chemistry of this reagent.     

Magic cluster ion distributions: (NO)3+ (N2O)n and (NO)3+ (CO2)n

Supersonic jet expansions of mixtures of nitric oxide with either nitrous oxide or carbon dioxide have been investigated over a wide range of relative concentrations. Mixed molecular cluster ions of the form (NO) _m ⁺ (N₂O)_n and (NO) _m ⁺ (CO₂)_n are detected following non-resonant two-photon ionization. Over a wide range of intermediate concentrations, the cluster ion distributions (NO) ₃ ⁺ (N₂O)_n and (NO) ₃ ⁺ (CO₂)_n with n30 are significantly more intense than clusters containing other numbers of nitric oxide molecules. The extra abundance of these species is attributed to their especially stable structures and several possible forms are discussed. An intriguing possibility involves a stable cyclic nitric oxide trimer (or ion) when combined with nitrous oxide or carbon dioxide clusters.     

Synthesis of lariat organochalcogenoethers based on azacalix[3]arenes for the potentiometric detection of [UO2] ions

The syntheses of organochalcogen-supported azacalix[3]arenes are described in a one-pot manner in satisfactory yields. A remarkably selective potentiometric response was accomplished for uranyl ions over a variety of other metal ions, including alkali (Na⁺, K⁺), alkaline-earth (Mg²⁺, Ca²⁺, Ba²⁺), transition and heavy metal ions (Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Fe³⁺, Zn²⁺, Cd²⁺ and Pb²⁺) using an ion-selective electrode based on compound 3 incorporated into a polymeric (PVC) membrane.     

Solvent effects on the O NMR chemical shifts of 4-dimethylsulfoximide-1,1,1-trifluoro-3-alken-2-ones

A multi-linear-regression analysis using the Kamlet-Abbout-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the chemical shifts of three 4-dimethylsulfoximide-1,1,1-trifluoro-3-buten-2-ones is reported. The predominant conformational structures of compounds has been determined by energy minimization calculations using the Austin Model (AM1) semi-empirical method, which has been used to explain the NMR chemical shifts for CO and SO groups. The chemical shifts of carbonyl and sulfoximide oxygen’s of molecules showed similar dependencies (in ppm) on the solvent polarity-polarizability and the solvent hydrogen-bond-donor (HBD) acidities. The influence of solvent hydrogen-bond-acceptor (HBA) basicities was little significant.     

Synthesis and NMR studies of 2- and 3-fluorosubstitued five-membered heterocycles

A set of 2-fluoro- and 3-fluoro-substituted thiophenes, pyrroles and furans has been synthesized by a treatment of the corresponding lithio derivatives with N-fluorodibenzenesulfonamide. For all these compounds, and coupling constants and NMR chemical shifts have been measured. In all cases, a dramatic increase of the couplings has been observed in 2-fluoro- and 3-fluoro-substituted compounds in comparison with those measured for the parent compounds. The same is valid for measured in 3-fluoro derivatives.The DFT calculations performed for 2- and 3-fluoro-substituted compounds reproduce very well the experimental coupling values and show that the Fermi contact contribution is the main factor determining their magnitude. Also the trends observed in the NMR shieldings are well reflected in the calculated DFT data.     

Molecular dynamics in polyethylene and ethylene-1-butene copolymer investigated by NMR methods

1H and 13C NMR spectra and 1H spin-lattice relaxation times T₁ and T_1ρ have been employed to study the structure and molecular dynamics in polyethylene and ethylene-1-butene copolymer in the temperature range from 100 to 370 K. Results are interpreted in terms of α, β and γ -relaxation, as well as methyl group rotation. The activation energies for all motions were established. The incorporation of 1-butene into ethylene chain leads to an increase of mobility in amorphous and crystalline phases as well as appearance the 13C resonance characteristic to the monoclinic structure in addition to the orthorhombic observed in both polymers. The crystallinity degree derived from T_1ρ in studied polymers is close to that determined using DSC method.     

Using modulated DSC to investigate the origin of multiple thermal transitions in frozen 10% sucrose solutions

Modulated differential scanning calorimetry (MDSC) was used to investigate the effect of annealing on multiple thermal transitions in frozen aqueous sucrose solutions. Two thermal transitions were detected from the reversing heat flow. Here, to maintain consistency with terminology used in the practice of freeze-drying [1], the higher temperature is denoted, T^′g, and the lower transition is given the symbol, T^″g. The transition at low temperature is usually believed to be a true glass transition. However, the origin of the transition at high temperature is still highly controversial. Based upon a study of 10% sucrose solutions with different cooling histories and annealing conditions, we suggest that the transition at high temperature is also a glass transition. We conclude that the lower transition is a glass transition of a phase plasticized by a higher than equilibrium amount of unfrozen water, and the higher transition, T^′g, corresponds to a glass transition in a maximally freeze-concentrated solute phase.     

High-energy optical absorption in f-compounds

Over a large energy domain, the electronic spectra of f-compounds arise mainly from three types of electronic transitions: f-f transitions, f-d transitions and ligand to metal charge transfer. The first transitions can be predicted quite successfully via an empirical Hamiltonian exploiting the symmetry of the system under consideration and evaluating the radial dependence of the various interactions felt by the f-electron through adjustable phenomenological parameters. The two other electronic transitions can be modelled via the refined spin pairing theory. Both similarities and differences in chemical and spectroscopic behavior of lanthanides and actinides will be briefly discussed.