Two isomeric phosphacarboranes 2,1- and 6,1-PCB(8)H(9), the first representatives of the 10-vertex closo phosphacarborane Series

The reaction between arachno-4-CB(8)H(14) and PCl(3) in the presence of PS (PS = proton sponge = 1,8-dimethylamino naphthalene) (dichloromethane, rt, 24 h) produced the neutral phosphacarborane closo-2,1-PCB(8)H(9) (35% yield), while a similar reaction of nido-1-CB(8)H(12) gave the isomeric compound closo-6,1-PCB(8)H(9) (27% yield). The structures of both compounds were derived on the basis of the combined ab initio/GIAO/NMR ((1)H, (11)B, (13)C) approach. The optimized structures at a correlated level of theory (MP2) with 6-31G* basis set were used as a basis for calculations of the (11)B and (13)C chemical shifts at GIAO-SCF/II and GIAO-MP2/II, the latter showing excellent agreement with experimental data.     

A sensitive spectrophotometric method for lead determination by flow injection analysis with on-line preconcentration

A new flow injection (FI) system for the determination of Pb(II) at trace level with a preconcentration step and spectrophotometric detection is proposed. It is based on preconcentration of lead ions on chitosan and dithizone-lead complex formation in aqueous medium (pH 9). The chemicals and FIA variables influencing the performance of the system were optimized and applied to the determination of lead in natural, well, and drinking water samples. It is a simple, highly sensitive, and low cost alternative methodology. The method provided a linear rage between 25 and 250 μg l⁻¹, a detection limit of 5.0 ng ml⁻¹ and a sample throughput of 15 h⁻¹. The obtained results of spiked samples are in good agreement between the proposed method and ICP-AES.     

Transformation of dynamic DSC results into isothermal data for the curing kinetics study of the resol resins

Kinetics of thermosetting polymers curing is difficult to study by isothermal methods based on the differential scanning calorimetry (DSC) technique. The difficulty is due to the low sensitivity of the equipment for total reaction heat measurements during high temperature process. The aim of this paper is to display the equivalence between a dynamic model, the Ozawa method, and an isothermal isoconversional fit, which allows predicting the isothermal behavior of the resol resins cure through dynamic runs by DSC. In this work, lignin–phenol–formaldehyde and commercial phenol–formaldehyde resol resins were employed. In addition, the isothermal kinetic parameters for both resins were performed by means of transformation of the data obtained from the dynamic Ozawa method.     

Energetics of hydroxybenzoic acids and of the corresponding carboxyphenoxyl radicals. Intramolecular hydrogen bonding in 2-hydroxybenzoic acid

The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental).     

The luminescence efficiency of YAG: Ce phosphors

The luminescence intensity of Yttrium aluminium garnet activated by the Ce³⁺ (YAGCe³⁺) under cathode-ray excitation depends on the preparation method and content of some dopants. Samples containing Fe, Pt, Ir or some colour centre, respectively, show relatively low luminescence. The maximum luminescence intensity was obtained with the Czochralski method grown single crystals in 98% Ar+2% H₂ atmosphere which were sensitized by oxygen annealing followed by treatment at 1500 °C in molybdenum container using a wet hydrogen atmosphere.     

Corpuscular diagnostics of a hollow-cathode discharge-III. The metal-ions-regime evolution

The variations in time of mass spectra of ions, extracted from the hollow-cathode discharge in the regime of the intensive sputtering of the cathode material were measured. The mass spectrum changes in time conspicuously. From the results of measurements a distinct anisotropy of ion beam is inferred. The discharge turns into a metal-ions regime.The helpful discussions with Dr. J. Musil are gratefully acknowledged.     

Periodic solutions of a nonlinear propagation equation via a fixed point argument

In this work the initial value problem for the equation $$u_t + \beta u_x + yf(u)_x - \delta u_{xxt} = g,\forall x \in R, \forall t \in [0,T],$$ with periodic boundary conditions is interpreted in the sense of periodic distributions and studied via fixed point arguments. Weak solutions exist iff∈C ⁰ (R) andg∈L ^∞(L ²(0,1)). Moreover, regularity inf, g and the initial data implies regularity of solutions.     

Interdiffusion in the Co-Ni system-I concentration penetration curves and interdiffusion coefficients

The interdiffusion in the Co-Ni system has been investigated in the temperature range of 950 to 1150 °C, by means of diffusion specimens of pure Co and Ni. The concentration curvesN(x, t) obtained with the aid of an JXA-3 A JEOL electron microprobe were evaluated using the smoothing cubic spline method by means of the Boltzmann-Matano equation. The obtained interdififusion coefficient values ¯D increase with the increasing Ni concentration and satisfy the Arrhenius equation at temperaturesH 990 °C. At lower temperatures the influence of high diffusivity paths may be effective, resulting in higher ¯D values. No expressive influence of atomic (Ni₃Co) or magnetic order on the interdiffusion has been detected. The activation enthalpyH values were found almost concentration independent. A Kirkendall effect study has been carried out with positive results which are presented in Part II of this paper. A new method for the determination of diffusants concentration in the Kirkendall plane was proposed. With the use of this method and of Darken equations the intrinsic diffusion coefficients were calculated. These results are given in Part III.     

A model of oxide film originating at thermal oxidation of GaAs

In this paper an approximative procedure to determine of the most probable model of oxide film originating at thermal oxidation of GaAs single-crystal samples within the temperature range 480–530 °C is proposed. It is shown that the system presented is more complicated if compare with that presumed previously.     

Optimization of the Microwave-Assisted Saponification and Extraction of Organic Pollutants from Marine Biota Using Experimental Design and Artificial Neural Networks

Several improvements in sample pretreatment for the determination of organic pollutants (i.e. n-alkanes, polycyclic aromatic hydrocarbons, PAHs) in marine biota (mussels) are presented. The use of liquid nitrogen and homogenization of the samples are shown to be an alternative to the time consuming liophilization step required for the analysis of biota samples. Microwave-assisted hydrolysis and extraction are combined to isolate organic pollutants (19 n-alkanes and 27 PAHs) from biota matrices. Experimental design (ED) and artificial neural networks (ANNs) were used to optimize the experimental conditions. NIST-CRM 2978 was used to test the validity of the developed method which shows a good agreement with certified values.