The band structure of one-dimensional (tetrazaporphyrinato)cobalt(II). A semi-empirical self-consistent field crystal orbital analysis

The band structure of (tetraporphyrinato)cobalt(II) (2) has been investigated by means of a semi-empirical INDO (intermediate neglect of differential overlap) crystal orbital approach. The simplest stoichiometric unit of the title compound contains an uneven number of electrons. We have considered metallic and insulating (Mott) electronic distributions in the “half-filled” band. The Hartree-Fock energies for two spatial symmetries have been determined: (i) singly occupied Co 3d_z² band (a_1g) and (ii) singly occupied ligand-centered (a_1u) linear combination. The mean-field energies of insulating “Mott” states have been calculated by means of a grand canonical (GC) averaging procedure. The electronic ground state is of spatial A_1g symmetry (“half-filled” Co 3d_z² dispersion) and corresponds to an insulating band occupation. The calculated width of the a_1g dispersion amounts to 0.13 eV. It is argued that the band structure results of 2 are in qualitative agreement with available experimental results derived for (phthalocyaninato(cobalt(II).     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Study on structures and electron affinities of small potassium–silicon clusters Si n K (n = 2–8) and their anions with Gaussian-3 theory

The neutral Si _n K (n = 2–8) clusters and their anions have been systematically studied by means of the higher level of Gaussian-3 schemes. Equilibrium geometries and electron affinities have been calculated and are discussed for each considered size. For neutral Si _n K clusters, the ground state structure is found to be “attaching structure”, in which the K atom is bound to Si _n clusters. The most stable isomer for their anions, however, is found to be “substitutional structures”, which is derived from Si_(n+1) by replacing the Si atom with a K. The dissociation energies of K atom from the lowest energy structures of Si _n K have also been estimated to examine relative stabilities.     

Molecular orbital theory of the hydrogen bond. 24. Ground-state water–uracil complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen-bonding sites in the uracil molecular plane. Hydrogen-bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N₁? H and O₂, N₃? H and O₂, and N₃? H and O₄. Open structures form at O₄, N₁? H, and N₃? H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N₁? H and the cyclic one at N₁? H and O₂. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N₃? H and a cyclic structure at N₃? H and O₄. The third cyclic structure at N₃? H and O₂ collapses to the “wobble” dimer at N₃? H and O₄. The two “wobble” dimers are significantly more stable than the open dimer formed at O₄, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N₁? H than N₃? H, owing to a more favorable molecular dipole moment alignment when association occurs through H₁. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen-bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen-bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen-bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.     

Zur Definition von Selektivität,Spezifität und Empfindlichkeit von Analysenverfahren

The three metric, functional concepts named in the title can be defined without arbitrariness and their values may be calculated for any multicomponent-analytical procedure from the relation of the measurable physical quantities x _ito the contents c _kwhich are to be determined. This relation is mathematically a “mapping” (achieved by systems of functions). The system of “analytical functions” (x _i→ c _k) is the inverse of the system of “calibration functions” (c _k→ x _i) which alone can be directly gained by experiments. The ‘calibration matrix” (γik) whose elements γ_ik are the “partial sensitivities” \(\tfrac{{\partial x_i }}{{\partial c_k }}\) represents the system in first approximation but only locally, i.e. for the respective constitution of the sample. The “sensitivity” of the analytical procedure as a whole, is numerically given by the determinant of this matrix; the “selectivity” is derived from the condition that the inversion of the calibration system to the analytical system (of functions) shall be possible by an iteration process. “Specificity” is defined in analogy to selectivity. In the appendix it is explained why selectivity is the strongest means to reduce the expenditure for the complete calibration of multicomponent analyses to a realistic and tolerable degree.     

Synthesis, crystal structure and electrochemical behavior of supramolecular complex [4, 4′-bipyH2][4, 4′-bipyH]2.5(P2Wl8O62)[4, 4′-bipyH1.5] · 2H2O

A Dawson-type supramolecular complex [4, 4′-bipyH₂][4, 4′-bipyH]_2.5 (P₂W_l8O₆₂)[4, 4′-bipyH_{1. 5}] · 2H₂O (1) has been synthesized hydrothermally and determined by means of IR, UV, cyclic voltammetry (CV) and X-ray single-crystal diffraction methods. Single crystal X-ray structural analysis reveals that the compound 1 exhibits 1D chains constructed from polyanions and 4, 4′-bipy molecules connected together by hydrogen bonds and weak contact. The 1D chains are assembled into 2D networks by weak contacts, and the intermolecular interaction of “water chains” is robust enough to really act as a “supramolecular glue” to assemble the 2D networks into 3D arrays. CV study reveals that compound 1 exhibits four chemically reversible processes.     

The effect of ammonium thiocyanate and sodium chloride on loss and recovery of mercury from water during storage

The effect of inorganic complexing agents such as thiocyanate and chloride on the stability of distilled water and natural waters spiked with 1 μg Hg l^-1 in polyethylene containers is reported. Distilled water solutions can be stored for several months without significant losses of mercury if they contain HNO₃(0.05–0.1 M) + NH₄SCN(0.001–0.01 M) or HNO₃(0.1 M) + NaCl(higher than 0.01 M). For river and pond waters, addition of HNO₃(0.1 M) + NH₄SCN(0.01 M) not only has a pronounced effect on preventing mercury losses, but also gives quantitative recoveries from spiked sample solutions from which mercury has been “lost”. Thiocyanate ion-favors desorption of mercury from solid phases; chloride is less effective in this respect.     

Magnetic studies of vanadyl(IV) tartrate dimers

Temperature-dependent magnetic susceptibility data have been collected for solid sodium salts of the binuclearvanadyl(IV) complexes [(VO)₂(d-tart)₂]^4?, [(VO)₂(dl-tart)₂]^4?, [(VO)₂(dl-mmt)₂]^4?,and [(VO)₂(dl-dmt)₂]^4?. (“tart” = tartrate(4?), “mmt” = monomethyl-tartrate(4?), “dmt” = dimethyltartrate(4?).) Ferromagnetic intradimer- and antiferromagnetic interdimer exchange is found; however, both interactions are small and similar in magnitude and reliable exchange constants cannot be extracted. The intradimer interaction probably occurs by a superexchange mechanism.     

Effect of inorganic salts on the aggregation behavior of branched block polyether at air/water and n-heptane/water interfaces

The effect of inorganic salts (CaCl₂, MgCl₂, NaCl, NaI and NaSCN) on the aggregation behavior of a synthesized polyether with seven poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) (PEO-PPO-PEO) arms attached to a tetraethylenepentamine core (simplified AE73) at air/water and n-heptane/water interfaces has been investigated by interfacial tension and oscillating bubble methods. The additions of NaCl, CaCl₂, and MgCl₂ may facilitate the micellization of AE73 and increase its maximum interfacial excess concentration (Γ_max) due to the “salting out” effect, while NaSCN induces opposite effect and NaI exerts little influence. The adsorption kinetics of AE73 is controlled not only by the diffusion between the bulk solution and the interfacial layer but also by the energetic and steric barriers generated by the already adsorbed molecules. The adsorption relaxation time of AE73 is reduced with the addition of salts and this phenomenon is more prominent at the n-heptane/water interface. The “salting in” ions decrease the dilational modulus of AE73 while the “salting out” ions induce an opposite effect. The mechanisms of the interaction between inorganic ions and the polyether were discussed; the difference in aggregation behavior between linear and branched block polyethers were also compared.     

Simulation of EPR spectra of ThBr8:Pa4+ in the incommensurable phase

At T = 95 K β-ThBr₄ undergoes a phase transition from a “high-temperature” phase with space group I_41/amd to a structure with an incommensurable distortion wave along the crystallographic c axis. The modulation can be thought to consist of a combination of a “rotation” and a “twist” of the Br atoms in the plane perpendicular to the c axis. The EPR spectra at 4.2 K show the characteristic “edge singularities” of the incommensurable structure. A model is presented for the simulation of these spectra. It is based on the “twist” component of the modulation only.     

Characterization of electrically conductive vanadate glass containing tungsten oxide

New conductive glass with a composition of 20BaO·10Fe₂O₃·xWO₃·(70 ? x)V₂O₅ (x = 10–50) was investigated by means of Mössbauer spectroscopy. A marked decrease in quadrupole splitting (Δ) was observed after the isothermal annealing at 500 °C for 1,000 min, due to the structural relaxation of 3D-network composed of FeO₄, VO₄, and VO₅ units. After the isothermal annealing, a marked increase in the electrical conductivity (σ) was observed from 1.7 × 10^?5 to 1.0 × 10^?1 S cm^?1 when “x” was 10, whereas comparable σ values of 1.1 × 10^?4 and 2.0 × 10^?4 S cm^?1 were observed when “x” was 40. These results evidently show that structural relaxation of 3D-network structure involved with a marked increase in σ is intrinsic of “vanadate glass”. XRD pattern indicated several weak peaks due to needle-like BaFe₂O₄ and α-Fe₂O₃ when the glass sample with “x” of 20 was annealed at 500 °C for 1,000 min. SEM study proved the formation of needle-like BaFe₂O₄ just on the surface of the sample, whereas hexagonal BaFe₁₂O₁₉ were observed in the annealed sample with “x” of 40. Chemical durability of WO₃-containing vanadate glass was investigated by immersing each glass sample into 20 %-HCl solution for 72 h.     

Biosynthesis of thalicarpine

The incorporation of (±)-, norlaudanosoline, ?nor-reticuline, ?N-methylcoclaurine and ?norlaudanidine into thalicarpine in Cocculus laurifolius DC has been studied and specific utilization of (±)- reticuline is demonstrated. The evidence supports that both the “halves” of thalicapine are derived from reticuline. Parallel feedings of (S)-, and (R)-, reticulines showed that the stereospecifity is maintained in the biosynthesis of thalicarpine from the 1-benzyl-tetrahydroisoquinoline precursor.A double-labelling experiment with (±)-[1-³H, 4′-O¹⁴CH₃] nor-reticuline has shown that the 4′OMe group of a nor-reticuline unit is lost in the biotransformation into thalicarpine. Feeding experiments also revealed that the plants can convert (S)-bolding and (S)-isoboldine into thalicarpine.     

Crystal structures of two macrocyclic diterpenoid photoproducts

The crystal structures of two macrocyclic diterpenoid photoproducts, the chemistry of which has been described in the preceding paper have been determined at 295 K.Compounds I (C₂₀H₂₆O₃) and II (C₂₂H₃₀O₅) are both orthorhombic, P2₁2₁2₁. In I. a = 24.18(1), b = 11.841 c=5.984(2) Å. Z=4. the structure being refined by least squares to a residual of 0.063 for 1234 “observed” reflections. For II. a = 24.30(1), b = 9.413(5). c = 9.149(5) Å. Z = 4 (R = 0.070, 982 “observed” reflections).     

The Cl + Bk extrapolation method. Application to hydrogen fluoride1

The Cl + B_k extrapolation method has been applied to calculations on the ground state of the hydrogen fluoride molecule. Comparison with the “full” calculations shows that for applications not requiring high accuracy (? 0.1 eV ≈ kcal/mole) the Cl + B_k method can be an efficient means of extrapolating the results of a “selected” calculation to the full configuration space.     

Effet Jahn-Teller coopératif dans le systéme Mn3O4Mn2SnO4

Mn³⁺ ion (3d⁴, t³_2ge_g¹) is liable to induce, by a cooperative Jahn-Teller effect, a macroscopic distortion of the cubic spinel structure; it is so in haussmanite Mn₃O₄, a tetragonal structure. The effect of chemical composition on “tetragonal-cubic” spinel transformations in the system Mn₃O₄Mn₂SnO₄ has been studied by X-ray diffraction; the c and a′ unit-cell dimensions show an abrupt change at a critical composition beyond which the system has the cubic spinel structure. The cation distribution has been worked out from an analysis of the X-ray diffraction intensities, and a correlation between the number of Mn³⁺ ions in octahedral sites and the degree of distortion has been obtained; the values of “cation-anion” bond distances, in six coordination, show that, in this system, the oxygen octahedral distortion and the macroscopic cell distortion are in a direct relationship. The paramagnetic study always attributes the “high-spin” configuration t³_2ge_g¹ to manganese.     

Dimeric flavanols ofJuniperus sabina. I

Procyanidins B₂, B₃, and B₄ have been isolated fromJuniperus sabina for the first time. On the basis of chemical transformations and the PMR spectroscopy of their decaacetates, and absolute configurations of the asymmetric centers of each of the halves of the molecules of the dimer have been determined: 2R, 3R, 4R for the “upper” half and 2R, 3R for the “lower” half of B₂, 2R, 3S, 4S for the “upper” half and 2R, 3S, for the “lower” half of B₃; and 2R, 3S, 4S for the “upper” half and 2R, 3R for the “lower” half of B₄.     

Détermination, à 25°C,du produit ionique des solvants mixtes acétone+eau+perchlorate de sodium

The ionic products of different mixed solvents acetone+water with sodium perchlorate have been determined by a method of standardisation using solutions of “strong” acids and bases.The evolution of pk_i is plotted in function of molar fraction of acetone and concentration of sodium perchlorate.The approximate values of the ionic product of acetone and acetone+sodium perchlorate media (0.5 M, 1 M, 2 M, 3 M) have been achieved easily by graphical extrapolation.     

NMR relaxation investigation of water “ordering” in aqueous lysozyme solutions

Lysozyme in aqueous dilute solution has been chosen as a model system to investigate the state and the mobility of water molecules near macromolecular surfaces. The dependence of the water proton NMR T₁ on temperature and on denaturing processes is interpreted in terms of long-range interactions giving rise to“ordering” of water, as seen on the NMR time scale.     

Irreversible thermodynamics of transport across charged membranes : Part III - efficiency of energy conversion in separation processes with nafion 120 membrane from phenomenological transport coefficients

In separation processes with charged membranes, as in electrodialysis units or electrochemical cells, the efficiency of conversion of electrical energy into the concentration gradient of an electrolyte is depressed by the immediate flow of water in the direction opposite to the solute flow. The equations for energy conversion in transport of ions and water across a cation-exchange membrane are derived in the present paper treating the system as a three-flow process and employing phenomenological transport equations. With these equations, the two-flow (q_1E, q_wE, q_1w and overall (q̃_E) degrees of coupling and the total (η) and component (η_1E,η_wE) efficiencies of energy conversion have been computed for the system sodium chloride/Nafion 120 membrane at temperatures of 298 and 333 K and for solute concentrations between 0.05 and 4 M. Considering the continuous separation processes, the so-called “driving region” and the energy requirement to keep the concentration difference in the adjacent compartments constant (static head) have been calculated and discussed.     

Specific features of the radiolysis of water and aqueous solutions of H2 and O2 by mixed n,γ-radiation with a high portion of the neutron component

A previously unknown feature of the kinetics of the radiolysis of water and hydrogen, oxygen, and hydrogen peroxide solutions has been discussed. By calculation, it has been revealed that concentration oscillations of the radiolysis products can appear during irradiation of the solution with fast neutrons or mixed n,γ-radiation with a high portion of the neutron component. The period and amplitude of the oscillations depend on the temperature, the dose rate, and the ratio of n/γ radiation components. It has been shown that oscillations cannot be excited during γ-radiolysis under any conditions. It is suggested that the mechanism of the oscillations is similar to the Belousov-Zhabotinsky reaction mechanism. A chain reaction proceeds in the irradiated system, in which the reactants H₂O₂ (“reducing agent”), “oxidizing agent” OH radicals initiating the chain, and the “catalyst” are introduced from the outside. Hydrogen molecules produced by the action of radiation play the role of the “catalyst”, and H₂O molecules formed in the secondary reactions are the “deactivated form of the catalyst”. Hydrogen atoms and hydrated electrons propagate the chain. Oxygen formed in both spurs and the secondary reactions is the “inhibitor” terminating the chain reaction.