Thermal stability of alkali metal borates and alkaline earth metal borates

The correlation between the enthalpies of formation of alkali metal and alkaline earth metal borates and the composition of the vapor in equilibrium with their melts is considered. The thermal stabilities of the studied borates have been estimated. A method is suggested for determination of the relative composition of the vapor over borate melts on the basis of their enthalpies of formation.

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Zusammenfassung Es wurde die Beziehung zwischen den Bildungsenthalpien von Alkali- (Erdalkali-)metallboraten und der Zusammensetzung der sich mit der Schmelze im Gleichgewicht befindlichen Gasphase betrachtet. Die thermische Stabilität der untersuchten Borate wurde geschätzt. Es wurde ein Verfahren entwickelt, die relative Zusammensetzung der Gasphase über Boratschmelzen auf Grund ihrer Bildungsenthalpien zu bestimmen.

     

Manifestation of circular photogalvanic current by dynamic holography in BaTiO3

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO₃: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10^–9 A/W using a nonstationary measurement technique at a wavelength of 0.632 m.Professor N. Kukhtarev is on leave from the Institute of Physics in the Ukraine and is supported by the NSFDr. G. Dovgalenko is on leave from the IRPA Rotor-Control research laboratory of the Ukraine and is supported by the University of Arkansas     

Time evolution studies of the H2O/quartz interface using sum frequency generation, atomic force microscopy, and molecular dynamics

Many interfacial studies on solid surfaces, for example, quartz/water, assume that a standard cleaning procedure regenerates the surface reproducibly. In the reported work, the results of two surface specific techniques, sum frequency generation (SFG) spectroscopy and atomic force microscopy, show that the effects of prolonged exposure to Nanopure water and to pH 10 NaOH are distinctly different. In conjunction with the experimental data, molecular mechanics is used to correlate the SFG spectral frequencies to the hydrogen stretching vibrations of the surface-bound water molecules. It is found that after 17 days of soaking in water, water molecules penetrate into the SiO2 matrix to produce a swollen and amorphous layer; it is likely that broken Si-O bonds from the polishing process serve as nucleation sites for hydration and swelling. Disorder introduced in the interfacial water layer is detected by the rising intensity of the weakly hydrogen-bonded SFG peak at 3450 cm(-1). Dominance of the 3450 cm(-1) is absent in a pH 10, NaOH-soaked quartz disk, indicating that the strong hydrogen-bonded network in water remains intact.     

Trends in metal-biradical exchange interaction for first-row M(II)(nitronyl nitroxide-semiquinone) complexes

We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes.     

A [2 + 2] cycloaddition route to dimethylaminomethylene vinamidinium salts

[reaction: see text] Trifluoropropanoic acid reacts with 1 equiv of POCl3 in DMF to generate the trifluoromethyl enamine (7). At this stage, two reaction manifolds are available. The expected reaction with additional POCl3 generates the 2-trifluoromethyl vinamidinium salt (3c). However, thermally driven loss of fluoride generates an iminium ion, which sets the stage for a [2 + 2] cycloaddition to ultimately generate the dimethylaminomethylene vinamidinium salt (1).     

Melting temperatures of poly(2,6-dimethyl-1,4-phenylene oxide) in methylene chloride solutions: Thermodynamic analysis

Melting temperatures were observed visually for poly-(2,6-dimethyl-1,4-phenylene oxide) in methylene chloride at nine concentrations (polymer weight fractions ranging from 0.0042 to 0.2362). The data were analyzed upon the assumptions that ΔH_u and ΔS_u, the molar heat and entropy of fusion per polymer unit, are constant over the temperature range studied, and that a Flory-Huggins chemical potential expression with a concentration-independent pair interaction parameter, χ₁ = (0.5 + ψ₁) + ψ₁Φ/T, satisfactorily describes the polymer unit activity in the binary solutions. Computation gave ΔH_u = 1404 cal/mole of units (therefore Δh = 11.7 cal/g), ψ₁ = ?0.569₁, and Φ = 342.₄°K. The effect of using various combinations of data points upon the values of these three parameters, as determined by least-squares linear regression treatment of the melting temperature expression, is indicated.     

Asymmetric hydrogenation of N-sulfonylated-alpha-dehydroamino acids: toward the synthesis of an anthrax lethal factor inhibitor

A novel and highly enantioselective Ru-catalyzed hydrogenation of N-sulfonylated-alpha-dehydroamino acids has been discovered and demonstrated in the synthesis of an anthrax lethal factor inhibitor (LFI). Herein, this methodology is used to prepare N-sulfonylated amino acids in up to 98% ee. This unprecedented hydrogenation uses a chiral Ru catalyst rather than Rh as typical for acylated dehydroamino acids and esters, and this work reports the first asymmetric hydrogenation of a tetrasubstituted dehydroamino acid derivative using a Ru catalyst. [reaction: see text]     

Trends in exchange coupling for trimethylenemethane-type Bis(semiquinone) biradicals and correlation of magnetic exchange with mixed valency for cross-conjugated systems

A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states.