Analysis of isothermal kinetic data from solid-state reactions

In most solid state reactions the reaction velocity can be described as a product of two functionsK(T) andf(1?α) whereT is the temperature and α the degree of conversion of the solid reactant. The physical interpretation of these functions is discussed, and a systematic method is described by whichf(1?α) of a reaction is identified from its kinetic data.K(T) and the reaction mechanism are then determined. This method has been successfully applied to analyse the kinetics of the thermal decomposition of silver azide.     

Simulation of the curing process of an unsaturated polyester resin in adiabatic conditions

The exothermal process of curing of thermoset resins in adiabatic conditions cannot be monitored by differential thermal analysis techniques such as DSC. Starting from the specific reaction rate, heat capacity as function of the temperature and the heat of reaction at some reference temperature, it is possible to design any adiabatic operation. In this paper we apply the energy balance to the curing process in adiabatic conditions and solve the basic rate law for the two empirical kinetic functionsf(α) usually used:n ^th-order kinetics [f(α)=(1?α)ⁿ], and autocatalytic kinetics [f(α)=α^m(1?α)ⁿ], where α is the degree of conversion andn andm the reaction orders, in order to obtain the heat generation curve (dH/dt) as a function of time, as well as the change of temperature with time, the explosion time, the maximum adiabatic temperature and the rate of reaction as a function of the degree of conversion.     

Darstellung,Kristallstruktur, Schwingungsspektren und Normalkoordinatenanalyse von K2[OsCl5(CO)] · H2O

Synthesis, Crystal Structure, Vibrational Spectra, and Normal Coordinate Analysis of K₂[OsCl₅(CO)] · H₂O The X-ray structure determination of K₂[OsCl₅(CO)] · H₂O (monoclinic, space group P2₁/c a = 13.600(2), b = 7.122(1), c = 22.186(11) Å, β = 98.66(3)°, Z = 8) revealed two crystallographic independent bat very similar complex anions [OsCl₅(CO)]^2? with rough C_4v point symmetry. Due to the stronger trans influence of the carbonyl group the bond lengths in the Cl? Os? CO axis Os? Cl = 2.449(2), 2.430(2) Å are langer as compared with the octahedron basis Os? Cl = 2.340-2.370 Å. The water of crystallization is coordinated to potassium (K? OH₂ = 2.625-2.815 Å). Using the molecular parameters the IR and Raman spectra are assigned by normal coordinate analysis. The valence force constants are f_d(CO) = 15.30, f_d(OsC) = 3.88, f_d(OsCl) = 1.81, f_d(OsCl) = 1.36, f_d(OH) = 7.65, 7.82, 7.79 mdyn/Å. The strengthening of the Os? C bond by stronger back donation of the Os^III(d⁵) complex in comparison with the isostructural Os^IV (d⁴) compound is discussed.     

Rate constants of substantial initiation in free-radical polymerization: A new method for determination of conversion dependences

A new method for determination of the conversion dependence of substantial initiation rate constants k _i = f(C) in free-radical polymerization processes has been developed. On the basis of the known data on k _i1 = f(C) dependences for initiator I₁ and the kinetic analysis of a single trivial and simple experiment, this method allows one to calculate k _i2 = f(C) function for any other initiator I₂ under the same conditions (monomer, temperature). The reference experiment includes measurements of polymerization rates in the presence of initiator I₁ in a wide conversion range from 0 to 100% and in the presence of I₂, on the condition that the rates of initiation are equal w _i1 = w _i2, thus ensuring equal initial rates of polymerization. The above-described approach has been approved for the polymerization of styrene, methyl methacrylate, and vinyl acetate initiated with AIBN and benzoyl peroxide.     

Application of the ISM EoS for polymer melts

A method for predicting an analytical equation of state for polymer melts from surface tension and liquid state density at the freezing temperature (γ_f,ρ_f), as scaling constants, is presented. B₂(T) follows a promising corresponding-states principle. Calculation of α(T) and b(T), the two other temperature-dependent constants of the equation of state, are made possible by scaling. As a result, γ_f and ρ_f are sufficient for the calculation of thermophysical properties of polymer melts. We applied the procedure to predict specific volumes of polyethylene glycol (PEG), polypropylene glycol (PPG), polypropylene (PP) and polyvinylchloride (PVC) at compressed state with temperature range from 298.15 to 423.15 K and pressures up to 200 MPa. The experimental specific volumes were correlated satisfactorily with our procedure and the results are within 3%.     

Stark and Zeeman spectroscopies of 4f 66s6p 7G1-6 levels in Sm I under external electric and magnetic fields

Systematic study of hyperfine structures, Zeeman and Stark effects in Sm I is performed for the lowest ⁷G_1-6 levels belonging to the configuration 4f ⁶6s6p by atomic-beam laser spectroscopy with fluorescence detection. The hyperfine coupling constants of ⁷G_2-6 levels are determined. From the Zeeman splittings for the 4f ⁶6s ^{2 7}F_2-6 ? 4f ⁶6s6p ⁷G_2-6 transitions, g-values are determined for the ⁷G_2.6 levels and the precision is improved by several orders of magnitude. From the Stark splittings for the ⁷F_0-3 ? ⁷G_1-3 transitions, tensor polarizabilities α ₂(J) are determined for the upper ⁷G_1-3 levels. Particularly for the ⁷G₁ level (15 650.55 cm^?1) which has close-lying opposite-parity level, the isotope dependence of α ₂(J) is clearly observed for the first time.     

Kristallstruktur,Schwingungsspektren und Normalkoordinatenanalyse von K2[IrCl5(NH3)]

Crystal Structure, Vibrational Spectra, and Normal Coordinate Analysis of K₂[IrCl₅(NH₃)] The X-ray structure determination of K₂[IrCl₅(NH₃)] (orthorhombic, space group Pnma, a = 13.426(4), b = 10.015(2), c = 6.8717(7) Å, Z = 4) revealed the C_s point symmetry of the complex anion [IrCl₅(NH₃)]^2? (Ir? Cl = 2.337–2.365, Ir? N = 2.067(10); N? H = 0.73–0.79 Å). Using the molecular parameters the IR and Raman spectra are assigned by normal coordinate analysis. The valence force constants are f_d(NH) = 5.88, f_d(IrN) = 2.66, f_d(IrCl) = 1.68 mdyn/Å.     

Adsorption properties of GaAs-CdS system

The adsorption properties of GaAs-CdS solid solutions and the constituent binary systems with respect to CO and NH₃ were studies by piezoquartz microweighing, temperature-programmed desorption, and IR spectroscopy. On the basis of an analysis of the measured α _p = f(T), α _T = f(p), and α _T = f(t) dependences, the thermodynamic and kinetic characteristics of adsorption, earlier obtained acid-base and other physicochemical characteristics of adsorbents, and the electronic properties of the adsorbate molecules, the mechanism and regularities of the adsorption processes at various conditions and compositions of the system were established. A comparison of the adsorption properties of the GaAs and CdS individual binary compounds with their (GaAs)_x(CdS)_1?x solutions, multicomponent systems, revealed common and distinctive features. Optimal compositions of adsorbents suitable for manufacturing primary transducers in sensors for medical and environmental purposes were determined.     

Hydrothermal synthesis and characterization of (Na,K)Ti2(PO4)3 solid solutions

Na_1?x K_xTi₂(PO₄)₃ (0 ≤ x ≤ 1) solid solutions are synthesized through ion exchange under hydrothermal conditions and a sol-gel process. The unit cell parameters are calculated for (Na,K) titanium phosphates. Cation-exchange reactions in the NaTi₂(PO₄)₃-KTi₂(PO₄)₃-NaCl-KCl-H₂O system are studied at T = 973 K and p = 200 MPa. The solid phase with compositions in the range 0 ≤ x ≤ 0.7 is enriched with sodium; in the range 0.7 ≤ x ≤ 1.0, it is enriched with potassium. The excess functions of mixing for the solid solutions are described in terms of the Margules model. Titanium phosphates Na_1?x K_xTi₂(PO₄)₃ show greater nonideality than zirconium phosphates Na_1?x K_xZr₂(PO₄)₃ and lower thermodynamic stability in decay into pure components at high pressures and temperatures.     

Hydrothermal synthesis and structure of K6[(UO2)4(CrO4)7]·6H2O: A layered uranyl chromate with a new uranyl sheet topology

The layered uranyl chromate, K₆[(UO₂)₄(CrO₄)₇]·6H₂O (1), has been synthesized by reacting UO₃ with K₂Cr₂O₇ under mild hydrothermal conditions. The structure of 1 is formed from UO₂²⁺ cations that are bound by chromate anions to yield a pentagonal bipyramidal geometry around the uranium centers. These polyhedra are bridged by chromate anions to yield one-dimensional _∞¹[UO₂(CrO₄)₂]²⁻ chains. The chains are similar to the _∞¹[UO₂(CrO₄)₂(H₂O)]²⁻ chains found in the previously reported one-dimensional uranyl chromate, K₂[UO₂(CrO₄)₂(H₂O)]·3H₂O, a phase that forms concomitantly with 1. These chains are condensed with the loss of the coordinated water molecules into two-dimensional _∞²[(UO₂)₄(CrO₄)₇]⁶⁻ layers with additional one-dimensional _∞¹[(UO₂)₂(CrO₄)₃]²⁻ chains. These anionic layers form a new type of anionic sheet topology and are charge balanced by both intra- and interlayer K⁺ cations. Crystallographic data (193 K): 1, orthorhombic, space group P2₁2₁2, a=10.9583(5) Å, b=22.582(1) Å, c=7.9552(4) Å, Z=2, MoKα, λ=0.71073, R(F)=1.77% for 268 parameters with 4657 reflections with I>2σ(I).     

Separated projectile and targetK x-ray production in symmetric heavy ion collisions as a function of the target thickness

Cross sections for projectile and targetK x-ray emission have been measured as a function of the target thickness for the symmetric systems Ni-Ni, Cu-Cu, Nb-Nb, Ag-Ag at energies between 75 and 105 MeV. The projectileK x-rays were separated from the target ones by using the Doppler shift. K_α and K_β energy shifts and K_β to K_α intensity ratios were also determined and used to calculateM- andL-ionization and the correspondingK-fluorescence yield for both collision partners. At non vanishing small target thicknesses, the targetK-vacancy production cross section is generally larger than that of the projectile. By analysing the target thickness dependence of the cross sections with a two component model which takes also into account the evolution of the projectileM-,L- andK-shell population inside the solid, targetK-vacancy sharing and cross sections forKK electron capture in symmetric systems could be determined. These results are in good agreement with molecular orbital model predictions.     

Effet sterique sur les vitesses de transfert protonique: cyclohexanols substitues dans le dimethylsulfoxyde

Steric effects on proton transfer from, and to, hydroxylic oxygen have been studied in a series of seventeen α-methyl and a-benzyl cyclohexanols in anhydrous DMSO, under both acid and base catalysed conditions, using dynamic MNR techniques. The protonation rate constants (k₁ ? 10⁶ M^-1 s^-1 at 25°C) obey a Taft-Ingold relationship, containing only a steric contribution E_s = E_s^OH + E_s^α, where: E_s^OH = 0 or 0.15 for an axial or equatorial hydroxyl respectively and E_s^α = ?0.070 (or ?0.115) for substituting an α-hydrogen by a methyl (or benzyl) group. An equatorial hydroxylic function is therefore 40% more reactive than its axial homologue. These kinetic data are fairly consistent with structural information resulting from IR spectroscopy (v_co and v_OH vibrations) and from NMR (hydroxylic chemical shifts and coupling constants).     

Potassium-cationic conductivity in the mixed ferrite-aluminate solid electrolytes with five-charged cation additives

New potassium-conducting solid electrolytes in the mixed ferrite-aluminate K_1.80(Fe_{1 ? x} Al _x )_1.90V_0.10O₄ and K_1.85(Fe_{1 ? x} Al _x )_1.925P_0.075O₄ are synthesized and studied. In the first system, the conductivity only slightly depends on the composition; therefore, the composition of solid electrolyte can be optimized with respect to the conductivity-stability characteristics. In the second system, extremes and considerable negative deviations from the additivity are observed in the conductivity isotherms. Possible reasons for this phenomenon are discussed.     

Decreasing current ramp technique for the study of fast electrode reactions

A new technique for measuring rate constants of moderately fast electrode reactions is proposed. The input is a decreasing current ramp of the form i(t) = Δi(1 ? t/τ). The overpotential transient-response expected for the activation/diffusion model is confirmed through experiments on the system K₄Fe(CN)₆+K₃Fe(CN)₆/Pt. The rate constant and the transfer coefficient are obtaine presence of several interesting features (a maximum, a zero crossing etc.) in the η?t profile makes this technique almost unique.     

The study of decomposition reactions through the exchange of thermal energy

In many decomposition reactions, the reaction velocity can be described as a product of two functions: a temperature dependent part K(T) and the kinetic function f(1 – α), where T designates the temperature and α the fraction of reactant that has decomposed. The physical interpretation of these functions is discussed for both solid and homogeneous systems. A method is described by which f(1 – α) and K(T) can be determined from kinetic data. The mechanism of decomposition can subsequently be identified which should be consistent with the derived kinetic parameters. The method has been applied to analyze the kinetics of the thermal decomposition of nitromethane. © 1994 John Wiley & Sons, Inc.     

Constrained force constants for octahedral molecules

A set of constrained force constants has been derived from experimental vibrational frequency data for eighty three octahedral molecules. Superimposing the condition that the larger value for f_d—f_dd′ be used when f_dα—f_dα″ is a maximum on the six equations relating vibrational frequencies to force constants generates a seventh. This provides a uniform set of results for all 83 molecules. The values of the force constants have a simple rationale in terms of chemical bonding theory. Some preliminary calculations for SF₆ show that these force constants are suitable for use in generating reliable molecular dynamical trajectory data.     

The protonation constants and 31P NMR of tetra-, penta-, hexa- and heptapolyphospates

The first and second protonation constants of linear polyphosphates at 25°C and ionic strength 0.1 have been evaluated: log K₁ = 8.91, log K₁₂ = 6.13 for P₂O₇^4?(P₂); log K₁ = 8.88, log K₁₂ = 5.86 for P₃O₁₀^5?(P₃); log K₁ = 8.40, log K₁₂ = 6.58 for P₄O₁₃^6?(P₄); log K₁ = 8.15, log K₁₂ = 7.03 for P₅O₁₆^7?(P₅); log K₁ = 8.12, log K₁₂ = 7.16 for P₆O₁₉^8?(P₆); log K₁ = 8.07, log K₁₂ = 7.18 for P₇O₂₂^9?(P₇). The variations of these values with the polyphosphate chain length have been discussed. A simple identification for polyphosphate species utilizing ¹³P NMR signal ratio of middle to end P has been proposed. By applying a micro ion-exchange technique, a rapid concentration of each polyphosphate and the subsequent preparation of the magnesium salt have been accomplished.     

A Correlation Between Solvatochromic Solvent Polarity Parameters and the Ionization Constants of Various Phenols in 1,4-Dioxane–Water Mixtures

Precise thermodynamic ionization constants K for 3-nitrophenol, 3,4-dichlorophenol, and 4-cyanophenol have been obtained in 1,4-dioxane-water mixtures (0–70% volume fraction in dioxane) at 25°C using a potentiometric method. The same information for another twelve cationic, neutral, and anionic phenols were taken from the literature. Three different methods were used to study the effects of the solvents on the ionization constants: one involves a single polarity parameter, E _T(30); the next involves the Kamlet–Taft multiparametric method; and the last involves the preferential solvation model. The pK values follow the preferential solvation model, but the parameters obtained are highly correlated. Using the data for the phenol molecule as reference, a linear correlation between ΔpK and E _T(30) has been used to develop a new method of obtaining pK values for any binary solvent composition, with only the pK in water known. The pK(s) values correlate with the molecular parameters for the dipolarity/polarizability of the solvent π* and its hydrogen-bond donor ability α. The preferential solvation parameter, f _12/1, correlates with the parameter for the hydrogen-bond donor ability of the solvent. All the phenols follow Hammett's equation and the reaction constants have been calculated for the different water–dioxane mixtures.