Isothermal kinetic analysis of the thermal decomposition of magnesium hydroxide using thermogravimetric data

In the present study, the kinetics of the thermal decomposition of magnesium hydroxide is investigated, using isothermal methods of kinetic analysis. For this purpose, experiments in thermogravimetric analyser were carried out in standard values of temperature (350°, 400°, 450° and 500°C) which resulted in weight loss percent as a function of time. The data were further modified to give fraction reacted ‘' versus time to be tested in various forms of ‘' functions. In order to determine the mechanism of the magnesium hydroxide decomposition and the form of the conversion function which governs the dehydroxylation of Mg(OH)₂, four different methods of isothermal kinetic analysis were used. Applying each of these methods to the data, it was concluded that the nucleation mechanism predominates the Mg(OH)₂, decomposition for all values of temperature tested; at 350°C the kinetic model which represents the experimental data is that of reaction at phase boundaries (random nucleation), F₁: ln(1−)=kt) while for the higher temperatures 400°, 450° and 500°C the kinetic equation of nucleation and development in two dimensions, A₂: [−ln (1−)]^1/2=kt was found to fit better the experimental results. The activation energy was evaluated applying two alternative methods; the Arrhenius plot, using maximum rates of reaction, from which the activation energy was evaluated to be 20.54 kcal/mol. An alternative method based on plots of ln t versus 1/T corresponding to the same value of ‘' gave values of 10.72, 13.82 and 16.31 kcal/mol for ‘' values of 0.25, 0.50 and 0.75, respectively.     

Comprehensive study of positronium formation in polymer blends between polyethylene and ethylene-vinylacetate copolymer

The intensity I₃ of ortho-positronium (o-Ps) in a polymer blend system consisting of polyethylene (PE) and ethylene-vinylacetate (EVA, random copolymer with a vinylacetate content of about 14%) was measured as functions of EVA weight content (Φ=0–100%), electric field (E=0–60 kV/cm ), positron irradiation time (t=0–200 h) and temperature (T=100–300 K). It was found that the addition of small amounts of EVA to PE significantly alters the electric field, positron irradiation time and temperature dependence of I₃. Positron trapping on polar EVA is suggested to be responsible for the sensitive effects of EVA.     

Density functional theory analysis of CaRgn complexes: (Rg=He, Ne, Ar; n=1–4)

CaRg_n⁺ (Rg=He, Ne, Ar) complexes with n=1–4, are investigated by performing using the B3LYP/6-311+G (3df) density functional theory calculations. The CaHe_n⁺ (n=1–4) complexes are found to be stable. In the case of CaNe_n⁺ and CaAr_n⁺, stable structures and stationary point were found only for n=1 and 2. For n=3 in the C_3V and the D_3h point group as well as for n=4 in the T_d (tetrahedral) point group a saddle point (imaginary frequency) is observed and global minimum could be obtained along the potential energy surface.     

Thermodynamic stabilities of NiTeO3 and Ni3TeO6 by solid oxide electrolyte e.m.f. method

The e.m.f. of the galvanic cells Pt,C,Te(l),NiTeO₃,NiO/15 YSZ/O₂ (P_o2 = 0.21 atm),Pt and Pt,C,NiTeO₃,Ni₃TeO₆,NiO/15 YSZ/O₂ (P_o2 = 0.21 atm),Pt (where 15 YSZ=15 mass% yttria-stabilized zirconia) was measured over the ranges 833–1104 K and 624–964 K respectively, and could be represented by the least-squares expressions E₍₁₎±1.48 (mV) = 888.72 − 0.504277 (K) and E_(II) ±4.21 (mV) = 895.26 − 0.81543T (K).

After correcting for the standard state of oxygen in the air reference electrode, and by combining with the standard Gibbs energies of formation of NiO and TeO₂ from the literature, the following expressions could be derived for the ΔG^°_f of NiTeO₃ and Ni₃TeO₆: ΔG_f^°(NiTeO₃) ± 2.03 (kJ mol⁻¹) = −577.30 + 0.26692T (K) and ΔG^°_f(Ni₃TeO₆)±2.54 (kJ mol⁻¹) = −1218.66 + 0.58837T (K).     

Kinetics and mechanisms of the redox reaction of hexaaquotitanium(III) and the ethylenediamine tetraacetato titanium(III) complex by aqueous solutions of bromine

At 25°C, I = 1.0 M (CF₃SO₃⁻Li⁺+CF₃SO₃H), [H⁺] = 0.034–0.274 M and λ = 453 nm, the rate equation for the oxidation of Ti(H₂O), ₆³⁺ by bromine was found to be: −d/[Br₂]T/dt=kK/[Br₂][Ti^III]/[H⁺]+K+kK/[Br₃⁻][Ti^III]/[H⁺+K, where k = 9.2 × 10⁻³ M ⁻¹ s ⁻¹ and K = 4.5 × 10⁻³ M. At [H⁺] = 1.0 M, [Br⁻] = 0.05–0.4 M, the apparent second-order rate constant decreases as [Br⁻] increases.

The pH-dependence of the oxidation of Ti^III-edta by bromine is interpreted in terms of the change in identity of the Ti^III-edta species as the pH of the reaction medium changes. The second-order rate constants were fitted using a non-linear least-square computer program with (1/k₀^edta)² weighting into an equation of the form: k₀^edta =k₁+k₂K₁[H⁺]⁻¹+k₃K₁K₂[H⁺]^−2/1+K₁[H⁺[H⁺⁻¹+K₁K₂[H⁺]⁻², with K₁ and K₂ fixed as earlier determined at 9.55 × 10⁻³ and 2.29 × 10⁻⁹ M, respectively, for the oxidation of bromine. k₁=k₂=(3.1±0.32)×10³M⁻¹s⁻¹ k₃=(2.3±0.45)×10⁶N⁻¹s⁻¹.

It is proposed that these electron transfer reactions proceed by univalent changes with the production of Br₂^.− as a transient intermediate. An outer-sphere mechanism is proposed for these reactions. The homonuclear exchange rate for Ti^III-edta+Ti^IV-edta is estimated at 32 M⁻¹ s⁻¹.     

Rates of the reactions CN + H2CO and NCO + H2CO in the temperature range 294–769 K

The rate coefficients of the reactions: (1) CN + H₂CO → products and (2) NCO + H₂CO → products in the temperature range 294–769 K have been determined by means of the laser photolysis-laser induced fluorescence technique. Our measurements show that reaction (1) is rapid: k₁(294 K) = (1.64 ± 0.25) x 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; the Arrhenius relation was determined as k₁ = (6.7 ± 1.0) x 10⁻¹¹ exp[(−412 ± 20)/T] cm³ molecule⁻¹ s⁻¹. Reaction (2) is approximately a tenth as rapid as reaction (1) and the temperature dependence of k₂ does not conform to the Arrhenius form: k₂ = 4.62 x 10⁻¹⁷T^1.71 exp(198/T) cm³ molecule⁻¹ s⁻¹. Our values are in reasonable agreement with the only reported measurement of k₁; the rate coefficients for reaction (2) have not been previously reported.     

Photochemical E/Z isomerization of aryl-substituted methanimines. AM1-CI potential energy curves along the CN double bond twisting and the N atom in-plane inversion

Potential energy curves of aryl-substituted methanimines along the C = N bond twisting and the in-plane inversion of the N atom were obtained by AM1-SDCI calculations, and the photochemical E/Z isomerization paths were elucidated. An aromatic group introduced at the C atom of the C = N group has little effect on the S₀ surface for the inversion and rotation paths, while it has a significant effect on the shape of the T₁ curve along the rotation path. It is suggested that phenylmethanimine and 2-anthrylmethanimine undergo photoisomerization by the rotation mechanism. The methoxy group introduced at the N atom raises the inversion barrier on the S₀ state, while it lowers the 90°-twistedT₁ energy. TheT₁ potential energy curves of N-methoxy-1-phenylethanimine and N-methoxy-1-(2-anthryl)ethanimine indicate that the former undergoes a two-way isomerization and the latter a one-way isomerization by the rotation mechanism, which is in accordance with experiment.     

Molecular motion and phase transition in perovskite-type (CH3)nNH4−nSnCl3 (n=1–4) studied by proton NMR spin–lattice relaxation

Powder X-ray diffraction, ¹¹⁹Sn NMR spectra, and ¹H NMR spin–lattice relaxation times, T₁, were measured for (CH₃)_nNH_4−nSnCl₃ (n=1–4). From the Rietveld analysis, it is shown that all four compounds crystallize into deformed perovskite-type structures at room temperature. The temperature dependence of ¹H T₁ was analyzed in terms of the CH₃ reorientation and other motions of the whole cation. Except for the phase transition in CH₃NH₃SnCl₃, which is from monoclinic to rhombohedral at 331 K, ¹H T₁ was continuously changed at other phase transitions in this compound as well as in the n=2–4 compounds, suggesting that the transitions are not caused by the change of the motional state of the cation but by an instability of the [SnCl₃]_nⁿ⁻ perovskite lattice.     

Structural and theoretical studies of Xe(OChF5)2 and [XeOChF5][AsF6] (Ch = Se, Te)

The XeOSeF₅⁺ cation has been synthesized for the first time and characterized in solution by ¹⁹F, ⁷⁷Se and ¹²⁹Xe NMR spectroscopy and in the solid state by X-ray crystallography and Raman spectroscopy with AsF₆⁻ as its counter anion. The X-ray crystal structures of the tellurium analogue and of the Xe(OChF₅)₂ derivatives have also been determined: [XeOChF₅][AsF₆] crystallize in tetragonal systems, P4/n, a=6.1356(1) Å, c=13.8232(2) Å, V=520.383(14) Å³, Z=2 and R₁=0.0453 at −60°C (Te) and a=6.1195(7) Å, c=13.0315(2) Å, V=488.01(8) Å³, Z=2 and R₁=0.0730 at −113°C (Se); Xe(OTeF₅)₂ crystallizes in a monoclinic system, P2₁/c, a=10.289(2) Å, b=9.605(2) Å, c=10.478(2) Å, β=106.599(4)°, V=992.3(3) Å³, Z=4 and R₁=0.0680 at −127°C; Xe(OSeF₅)₂ crystallizes in a triclinic system, , a=8.3859(6) Å, c=12.0355(13) Å, V=732.98(11) Å³, Z=3 and R₁=0.0504 at −45°C. The energy minimized geometries and vibrational frequencies of the XeOChF₅⁺ cations and Xe(OChF₅)₂ were calculated using density functional theory, allowing for definitive assignments of their experimental vibrational spectra.     

Three-dimensional polymer gel dosimetry: basic physical properties of the dosimeter

This study concentrated on assessment of the basic physical properties of a polymer gel dosimeter evaluated by NMR. For this, BANG-2 type polymer gel was prepared. The dosimeters were irradiated by ⁶⁰Co gamma photons and by 4, 6 and 18 MV X-ray photons for doses in the range 0–50 Gy. The multi-echo CPMG sequence was used for the evaluation of T₂-relaxation times in the irradiated gel dosimeters. Dependence of 1/T₂ in terms of the following factors was studied: absorbed dose, energy of applied radiation, temperature during NMR evaluation, time between irradiation and NMR evaluation and strength of the magnetic field. An exponential dependence of the 1/T₂ response on absorbed dose in the range 0–50 Gy was observed, while in the range 0–10 Gy the data could be fitted by a linear function. This paper also describes the dependence of 1/T₂ response on: radiation energy, strength of magnetic field and time from irradiation of the dosimeters to NMR evaluation. Increase of gel dosimeter 1/T₂ response with the decrease of the temperature during NMR evaluation has been qantitatively described. The polymer gel dosimetry system used in this study proved that it is a reliable system for three-dimensional dose distribution measurement.     

Kinetic analysis of thermogravimetric data obtained under linear temperature programming—a method based on calculations of the temperature integral by interpolation

A new technique, called interpolation method, with general application in the kinetic analysis of processes studied by thermogravimetry (TG) under linear temperature programming is developed. It is based on the linear relationship, with slope 1, between log g() and log I(γ, θ) for the appropriate kinetic function, where I(γ, θ) is the normalized temperature integral, θ the normalized temperature (θ=T/T₀) and γ a dimensionless activation energy (γ=E/RT₀). Values of log I(γ, θ) are calculated by linear interpolations in a pre-built table. This method can easily be programmed and implemented in a personal computer, where the results (kinetic parameters and quality of regressions for the kinetic functions considered) are typically obtained in a very short time. The method is validated by analyzing different simulated thermogravimetric curves and comparing the results with those determined with some classic methods taken from the literature. In addition, the results are compared with the values obtained by a similar method, also developed and explained in this paper, which involves the evaluation of all the values of the temperature integral by numerical integration, therefore, demanding a much larger calculation time. The interpolation method is found to be more accurate than other published methods, particularly in the case of thermogravimetric curves corresponding to processes with low activation energies. The results obtained are always similar to those determined by the integration method, which is taken as reference. Application of the technique to experimental data for various types of reactions shows that the results are in agreement with the published parameters and kinetic laws.     

Transport phenomenon as a function of counter and co-ions in solution: chronopotentiometric behavior of anion exchange membrane in different aqueous electrolyte solutions

Anion exchange membrane has been investigated in different electrolyte solutions by chronopotentiometry to explore the influence of co-ion and counterion of the exchange group of the membrane, on the transport phenomena. Chloride, nitrate, sulfate and acetate in sodium salts were used as counterions and sodium, potassium, calcium and ammonium in chloride salts were used as co-ions. The membrane showed a potential drop (E₀) in all these electrolytes when a constant current was applied across it, which remained constant for a period less than τ, called the transition time and rose gradually to a maximum (E_max) value. The parameters such as τ, E₀ and E_max and the potential jump (ΔE) and τ and the inflection zone (Δt) along the time axis have been measured and compared at an applied current density (I) of 10 mA cm⁻² in 10 mM solutions. The values of τ^1/2/z_A[A₀] or τ^1/2/z_C[C₀], with or , E₀ and ΔE with or (where r_A and r_C are the ionic radii of counter and co-ions, respectively) have been correlated. Permselectivity (P) and transference number of the membrane with respect to each one of the above electrolytes have been evaluated and discussed.     

Vibrational dependence of the NH3 (v2)+NO and NO(v)+NH3 charge transfer cross sections

A tandem quadrupole mass spectrometer is used to study the charge transfer reactions NH₃⁺ + NO and NO⁺ + NH₃ over a collision energy range 1.5–13 eV. The vibrational state of the reagent ions is selected by resonance-enhanced multiphoton ionization. For the 0.9 eV exothermic process NH₃⁺ + NO → NH₃ + NO⁺ excitation of the v₂ umbrella bending mode (v₂ = 0–12) causes no marked change in the charge transfer cross section, while in the reverse process NO⁺ + NH₃ → NO + NH₃⁺ excitation of the NO⁺ vibration (v = 0–6) strongly enhanced the charge transfer cross section.     

Gas-phase stability of cluster ions SF m + (SF6) n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SF⁺_m (SF₆)_n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF⁺_m (SF₆)₁ were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF⁺_m (SF₆)_n with m = 0−4. The structures of SF⁺₅, SF⁺ (SF₆)₁, SF⁺₃ (SF₆)₁, and SF⁺₅ (SF₆)₁ were investigated by ab initio molecular orbital calculations. For SF⁺₅, the D_3h geometry is found to be most stable andC_4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.     

Photoionization mass spectrometry of kinetic energy-selected ions. The translational energy distribution of N /N2 in the inner-shell ionization energy range

An ion retarding potential difference (IRPD) method has been used to investigate the ion yield and kinetic energy distributions of N⁺/N₂ produced by photoionization mass specrometry using synchrotron radiation. Photoion yield curves of energy selected N⁺ ions are deduced. Translational energy distribution of N⁺ at energies of the N(1s)→π*, N(1s)→(nl)¹ and above the N(1s)⁻¹ threshold are determined. Comparison is made with previous photoin-photoelectron coincidence work using time-of-flight (TOF) measurements.     

Oxygen abstraction from tetrahydrofuran by molybdenum-iodide complexes. X-ray molecular structure of [Mo2(μ-O)(μ-I)(μ-O2CCH3)I2(THF)4][MoOI4(THF)] (THF = tetrahydrofuran)

The interaction between Mo₂(O₂CCH₃)₄, Me₃SiI and I₂ in THF resulted in oxygen abstraction from the solvent and formation of [Mo₂(μ-O)(μ-I)(μ-O₂CCH₃) I₂(THF)₄]⁺[MoOI₄(THF)]⁻ and I---(CH₂)₄---I. The molybdenum complex has been characterized by X-ray diffractometry. Crystal data: triclinic, space group P , a = 13.827(3) Å; b = 15.803(7) Å; c = 9.950(3) Å; = 93.34(4)°; β = 102.40(2)°; γ = 90.09(2)°; V = 2120(2) Å₃; Z = 2; d_calc = 2.559 g cm⁻³; R = 0.0476 (R_w = 0.0613) for 370 parameters and 3938 data with F₀²> 3σ(F₀²). The metal-metal distance in the cation is 2.527(2) Å and indicates a strong interaction. The magnetic behavior is consistent with the assignment of one unpaired electron to the Mo₂⁷⁺ core of the cation and one to the d¹ Mo(V) center of the anion. The interaction between Mo(CO)₆ and I₂ in THF also results in the formation of 1,4-diiodobutane.     

Molecular structure of 3-methylthiophene studied by gas electron diffraction combined with microwave spectroscopic data

The molecular structure of 3-methylthiophene has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r_{g(S---C2) = 1.719(2) Å}, r_{g(C2=C3) = 1.370(3) Å}, r_{g(C3---C6) = 1.497(6) Å}, r_{g(C2---H) = 1.101(5) Å}, CSC = 91.6(2)°, SC₂C₃ = 113.3(5)°, SC₅C₄ = 111.3(3)°, C₂C₃C₆ = 123.2(11)° and C₃C₆H = 112(2)°. The values of r(S---C₂) - r(S=C₅) and r(C₂=C₃)-r(C₄ =C₅) were fixed at the 3–21G* value of 0.002 Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Rotational excitation of ions produced by the He(2 S) Penning ionization of CO and N2

Rotational-state distributions of the CO⁺ (A–X, B–X) and N₂⁺(B–X) emissions produced by the collisions of He(2 ³S) with CO and N₂ were studied in the collision energy (E_R range 100–200 meV. The rotational populations of the emitting states can be fitte by single Boltzmann temperatures (T_R. The T_R (320 ± 30 K) for the ν′ = 3 and 4 levels of the CO⁺ (A²Π) state are nearly independent of, or slightly increase with, E_R, while T_R for the CO⁺(B²Σ⁺, ν′ = 0) state increases rapidly with E_R.The T_R (430 ± 20 K) for the N₂⁺(B²Σ⁺, ν′ = 0) state is nearly independent or slightly decreases with increasing E_R. Interactions providing these trends are discussed.     

Molecular vibrations of complexes with trigonal ligands. Part V. Symmetry coordinates for tetrahedral complexes

Ligands of C_3V symmetry are assumed to be a part of a T_d complex. Suitable combinations of C_3v (a₂ + e) symmetry coordinates, which fit into the T_d model, are studied. The results constitute a useful supplement to the C_3V (a₁ + e) type coordinates studied previously. Torsional coordinates in the Ni(PF₃)₄ type model are treated in particular.     

A novel method for determining the viscosity of polymer solution

The relative viscosity η_r and, thus, the reduced viscosity η_sp/C of polymer solution could be obtained by recording the flow times of the polymer solution and the pure solvent in a capillary viscometer. Our experimental results indicated that the measurement of the flow time of the pure solvent was unnecessary. In particular, if the recorded flow time of the pure solvent was used to determine the viscosity of polymer solution, the reduced viscosity η_sp/C exhibited either a drastic increase or a significant decrease in an extremely dilute solution, depending upon the properties of the polymer solution investigated. In this research work, a new method for determining the viscosity of polymer solutions is reported. In the proposed method, the flow time of polymer solution at zero concentration, t₀^*, instead of the measured flow time of the pure solvent, was used to determine the viscosity of polymer solution. The reduced viscosity η_sp/C determined by the new method is proportional to concentration C even in an extremely dilute solution. The relative viscosity η_r vs. C plot also indicated clearly that t₀^*, instead of the measured flow time of the pure solvent, should be used for determining the viscosity of polymer solution. At low concentrations, the flow time of the polymer solution was proportional to C. As a result, t₀^* could be determined by extrapolating the flow time of the polymer solution to C=0.