Synthesis of (-)-epibatidine

The synthesis of (-)-epibatidine has been accomplished utilizing a highly exo-selective asymmetric hetero Diels-Alder reaction. The key steps employed to transform the resulting bicycle into the natural product include a fluoride-promoted fragmentation and a Hofmann rearrangement. Reaction: see text.     

Three-dimensional open frameworks based on cobalt(II) and nickel(II) m-pyridinecarboxylates

Three-dimensional open frameworks [Co2(nicotinate)4(mu-H2O)]-CH3CH2OH-H2O, 1, and [Ni2(nicotinate)4(mu-H2O)]-CH3CH2OH-H2O, 2, were obtained by hydro(solvo)thermal reactions between 3-cyanopyridine and cobalt(II) nitrate and nickel(II) perchlorate, respectively. Both 1 and 2 exhibit complicated 3-D structures based on [M2(nicotinate)4(mu-H2O)] (M = Co or Ni) building blocks and possess open channels that are occupied by removable solvent molecules. 3-D open frameworks [M2L4(mu-H2O)]-HL-(H2O)x (where M = Co, x = 2, 3, and M = Ni, x = 1, 4, and L = trans-3-(3-pyridyl)acrylate) were similarly prepared with trans-3-(3-pyridyl)acrylic acid in place of 3-cyanopyridine. Compounds 3 and 4 are isostructural and exhibit network topologies similar to that of 1 with open channels occupied by disordered trans-3-(3-pyridyl)acrylic acid and water guest molecules. Crystal data for 1: triclinic space group Ponebar, a = 10.534(1) A, b = 11.907(1) A, c = 14.046(1) A, alpha = 106.645(1) degrees, beta = 101.977(1) degrees, gamma = 112.078(1) degrees, and Z = 4. Crystal data for 2: tetragonal space group P4/ncc, a = 20.089(1) A, c = 14.016(1) A, and Z = 4. Crystal data for 3: monoclinic space group C2/c, a = 14.082(2) A, b = 15.278(2) A, c = 18.537(2) A, beta = 105.360(2) degrees, and Z = 2. Crystal data for 4: monoclinic space group C2/c, a = 14.082(1) A, b = 15.250(1) A, c = 18.606(1) A, beta = 106.747(1) degrees, and Z = 2.     

Energy isosbestic points in third-row transition metal alloys

The total electronic energies of the six electrons per atom (e per atom) alloys W, TaRe, HfOs, and YIr and the seven electrons per atom alloys Re, WOs, TaIr, HfPt, and YAu have been calculated in the local density approximation of density functional theory. When one considers common alloy structures such as atomically ordered variants of the body-centered cubic, face-centered cubic, or hexagonally closest packed structures and plots the total electronic energy as a function of the unit cell parameter, one finds for both the six and seven electrons per atom series energetic isosbestic points. An energetic isosbestic point corresponds to a critical value of the size parameter for which all members of the 6 or 7 e per atom series of compounds have nearly identical total electronic energy. Just as in spectroscopy, where the existence of such isosbestic points is the hallmark of two compounds present in the mixture, an energy isosbestic point implies there are just two separate energy curves. For both series it is found that the total electronic energy can be viewed as the weighted sum of a purely covalent term and a purely ionic term. Two semi-quantitative models are proposed to account for these two separate energies. In the first model the total energy is viewed as the sum of the elemental structural energy plus an ionic energy based on the Born-Mayer ionic model. In the second model one considers within the confines of mu2-Hückel theory the evolution of the total electronic energy as the Coulombic Hii integrals change in value.     

Bis­[4‐(2‐pyridyl­methyl­ene­amino)­phenyl] ether

The title compound, C₂₄H₁₈N₄O, is a bis‐bidentate Schiff base ligand exhibiting pseudo‐C₂ symmetry. The mol­ecule is twisted about the central ether linkage and exhibits an imine E configuration. In the crystal, the mol­ecules are linked by weak intermolecular C—H?N hydrogen bonds.     

Sodium hydrogen bis­(phenoxy­acetate)

In the title compound, Na⁺·H⁺·2C₈H₇O₃⁻, the anion contains a short Speakman-type hydrogen bond [O⃛O = 2.413 (2) Å]. The anions and the Na atoms lie across twofold axes.     

Spectroscopic and time-dependent density functional investigation of the role of structure on the acid-base effects of citrinin detection

A time-dependent density functional theory (TD-DFT) study was carried out on tautomers of the mycotoxin citrinin in the neutral, anionic, and cationic forms to gain insight into the role of chemical structure on detection. Steady-state fluorescence studies of citrinin in micellar aqueous solutions produced unusual results for ionic surfactants and the neutral Triton X-100 enhanced fluorescence emission. Ground-state and time-dependent density functional studies were carried out on five tautomers of citrinin using the B3LYP density functional and the 6-311+G(2df,2p) basis set. The investigation revealed that deprotonation is a major factor governing the shifts in fluorescence excitation and emission maxima. Moreover, the position of the lowest unoccupied molecular orbitals is removed from the fluorophore moiety of citrinin in the dianionic state which is consistent with the diminished fluorescence of the toxin in basic solutions. The ionic characteristics of certain chemosensors can influence the structure and intrinsic spectroscopic properties of citrinin.     

Evaluation of Vanadium(IV) as a Non-radioactive Surrogate for Technetium(IV) by Comparison of Stability Constants for Polyamino Polycarboxylate Ligand Complexation

The stability constants for the Tc(IV) and V(IV) complexation with the polyamino polycarboxylate ligands IDA, NTA, HEDTA and DTPA were determined using liquid–liquid extraction techniques. These stability constants were then used to evaluate the validity of using V(IV) as a chemical analogue for Tc(IV). Results suggest that Tc(IV), as TcOOH⁺, will form β _1?11 complexes with the selected ligands, while V(IV), as VO²⁺, will form β ₁₀₁ complexes. The values for these determined stability constants are (in log₁₀ unit) 10.9 ± 0.1, 11.4 ± 0.1, 14.9 ± 0.1, and 20.1 ± 0.1 for Tc(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for IDA, NTA, HEDTA and DTPA, respectively, they are 9.3 ± 0.1, 11.6 ± 0.2, 15.8 ± 0.1, and 20.8 ± 0.1 for V(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for the same suite of ligands. The incorporation of a hydroxide into the metal ligand complexes formed by Tc(IV) is proposed as the largest factor differentiating the apparent stability constants of Tc(IV) and V(IV). This work shows that V(IV) is a poor analog for Tc(IV); however, despite the differences in complexation mechanism between V(IV) and Tc(IV), V(IV) still appears to have some use for predicting Tc(IV) complexation behavior.     

Surface oxides on porous carbon

Wet oxidation of BPL porous carbon by H₂O₂ and HNO₃ changes the pore structure and also increases the concentration of surface oxides. KOH uptake is increased by oxidation and is seen to be dependent on KOH concentration and to a lesser extent on inert electrolyte concentration. Enthalpy of immersion measurements have allowed the estimation of the enthalpy of hydration of the surface oxides and the enthalpy of neutralization of the easily accessible acid oxides.     

Unusual features of micellar diffusion

Diffusion coefficients of tetradecyltrimethylammonium bromide in aqueous solutions have been determined at 25, 95 and 135 °C using the Taylor dispersion technique. The diffusion coefficient exhibits a minimum at a surfactant concentration above the critical micelle concentration (CMC). The results are interpreted in terms of electrostatic coupling and rapid exchange between micelle, surfactant monomer and counterions.