EPR Equilibration Study of the O–H Bond Dissociation Enthalpy in a Benzyl Phenolic Antioxidant

The equilibrium of the reaction galvinoxyl radical + 4,4′-methylene-bis(2,6-di-tert-butyl-phenol) (IOH) ⇆ hydrogalvinoxyl + 4,4′-methylene-bis(2,6-di-tert-butyl-phenoxyl radical) was monitored by electron paramagnetic resonance spectroscopy between 213 and 293 K. The equilibrium constant was calculated for each temperature point and correlated to T ⁻¹ applying the Van't Hoff relationship lnK _R = (−ΔH _R ⁰/RT) + (ΔS _R ⁰/R). The reaction enthalpy was found to be 12.0 ± 0.9 kJ/mol and the bond strength in IOH (BDE_(O–H)) = 340.7 ± 3.0 kJ/mol. The thermochemical stabilization of the involved phenoxyls is discussed. Authors' address: Sara N. Mendiara, Departamento de Química, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes 3350, Mar del Plata 7600, Argentina     

Aggregation of 1,3,7-trimethylxanthine with methylene blue in aqueous solution

We have studied self-association of aromatic molecules of the thiazine dye methylene blue in aqueous solution, using a dimer model. We have determined the dimerization equilibrium constant for the dye molecules K_D = 3900 ± 800 M⁻¹ at T = 293 K. We have decomposed the experimental spectrum into dimer and monomer components. Using the ratio of the molar absorption coefficients for two absorption bands of the dimer spectrum, we obtained the “average” value of the angle between the electronic transition moments of the molecules in the dimers, α = 48°. We have studied heteroassociation of methylene blue (MB) and 1,3,7-trimethylxanthine (caffeine) molecules in aqueous solution. We have calculated the heteroassociation constant as 200 ± 34 M⁻¹. We conclude that heteroassociation of methylene blue and caffeine molecules leads to a lower effective dye concentration in solution, which hypothetically may affect its biological activity. We have determined the values of the Gibbs free energy, the enthalpy, and the entropy for dimerization of methylene blue molecules: ΔG₂₉₃ = −(20 ± 3) kJ/mol, ΔH = −(25 ± 9) kJ/mol, Δ S₂₉₃ = −(17 ± 6) J/mol·K; and for methylene blue-caffeine heteroassociation: ΔG₂₉₃ = −(13 ± 3) kJ/mol, ΔH = −(14 ± 10) kJ/mol, ΔS₂₉₃ = −(2.4 ± 0.2) J/mol·K, respectively. We have shown that the methylene blue aggregates and the heteroassociates with caffeine are predominantly stabilized by dispersion interactions between the chromophore molecules in the associates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 158–163, March–April, 2006.     

Kinetic regularities and mechanism of polymerization of perfluoromethylvinyl ether at high pressures

The kinetics of the thermal polymerization of perfluoromethylvinyl ether (PFMVE) is studied at pressures of 3–13 kbar (300–1300 MPa) and temperatures of 80–260°C. The activation energy (E _act = (76 ± 3) kJ/mol) and activation volume (ΔV₀^⪼ = −(27 ± 2) cm³/mol) for the overall polymerization rate are determined. The inhibition method is used to estimate the activation energy of thermal initiation (E _in = (79.9 ± 3) kJ/mol). The quantity E _p − (1/2)E _t was calculated to be 36.6 ± 3 kJ/mol. The limiting polymerization temperature was evaluated: T _lim = (180 ± 3)°C. A mechanism of PFMVE polymerization is proposed on the assumption that the reaction is bimolecular.     

Spin-orbit coupling,spin-lattice relaxation,and spin-memory studies ofF-center-molecular-ion pairs in alkali halides

Summary The spin-orbit parameter Δ, spin-lattice relaxation timeT ₁, and spin-mixing parameter ɛ ofF _H(OH⁻) andF _H(CN⁻) centers in several alkali halides have been studied with magnetic circular dichroism at ∼2 K. A close comparison of the experimental results before and after optically induced association of theF center with the molecular ion has been made. In crystals of NaCl structure the negative spin-orbit parameter Δ changes little betweenF andF _H centers in the same host. For CsCl and CsBr two values of Δ have been derived forF _H(CN⁻) centers with axis parallel or perpendicular to the magnetic field. In all studied systems, the spin-lattice relaxation timeT ₁ is already shorter before aggregation compared toF formation. The spin-mixing parameter ɛ decreases slightly forF _H(OH⁻) compared toF centers, while it increases drastically forF _H(CN⁻) defects and reaches its maximum possible value ɛ=0.5 in cesium halides. First attempts to interpret these magneto-optical results will be presented. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

The study of lithium fast ion conductors of Li3xLa0.67−xScyTi1−2yNbyO3 system

Perovskite-type lithium fast ion conductors of Li_3xLa_0.67−xSc_yTi_1−2yNb_yO₃ system were prepared by solid state reaction. X-Ray powder diffraction shows that perovskite solid solution form in the ranges of x=0.10, y≤0.10. AC impedance measurements indicate that the bulk conductivities and the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ and 10⁻⁵ S·cm⁻¹ at 25 °C respectively. The compositions have low bulk activation energies of about 17 kJ/mol in the temperature ranges of 298 – 523 K and total activation energies of about 37 kJ/mol in the temperature ranges of 298 – 523 K.     

Electrical conductivity of BaCe0.9Nd0.1O3−α in H2+H2O+Ar gas mixture

Electroconductivity of BaCe_0.9Nd_0.1O_3−α was studied as a function of the composition of the H₂+H₂O+Ar mixture and temperature in the interval from 873 to 1173 K. It was shown that the electroconductivity was independent of P_H2 (0.97 to 0.10 atm) and P_O2 (10⁻²¹ to 10⁻²⁶ atm), but depended on P_H2O (0.08 to 0.005 atm). A mathematical processing of the P_H2O dependencies of the electroconductivity, which was performed in terms of a classical model of defect formation in high-temperature proton-conducting solid electrolytes, yielded equilibrium constants of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α and mobilities of protons and oxygen ions. The temperature dependencies of these quantities were used to determine the mobility activation energies of protons (E_a=34±7 kJ/mole) and oxygen ions (E_a=72±8 kJ/mole), and also the enthalpy (ΔH=−150±25 kJ/mole) and the entropy (ΔS=153±26 kJ/mole·K) of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α.     

Magnetoresistance and magnetic ordering in praseodymium and neodymium hexaborides

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB₆ and NdB₆) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB₆ and NdB₆ compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H ²), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB₆ and NdB₆ compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB₆ and NdB₆, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χ_loc has been estimated. It has been demonstrated that, in the temperature range T _N < T < 20 K, the behavior of the local magnetic susceptibility χ_loc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χ_loc ∝ (T − Θ _p )⁻¹.     

Adsorption and thermodynamics studies of U(VI) by composite adsorbent in a batch system

The adsorption of U(VI) from aqueous solutions onto composite adsorbent (algistar) has been studied using a batch adsorber. The parameters that affect the U(VI) sorption, such as contact time, solution pH, initial U(VI) concentration, and temperature, have been investigated and optimized conditions determined. Equilibrium isotherm studies were used to evaluate the maximum sorption capacity of composite adsorbent, and experimental results showed this to be 43.10 mg/g. The adsorption patterns of metal ions on composite adsorbent followed the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms. The Freundlich, Langmuir, and D–R models have been applied, and the data correlate well with Langmuir model, and that the sorption is physical in nature (the sorption energy E _ads = 12.90 kJ/mol). Thermodynamic parameters (∆H _ads^o = −41.08 kJ/mol, ∆S _ads^o = −68.00 J/mol K, ∆G _ads (298.15 K) = −20.81 kJ/mol) showed the exothermic heat of adsorption and the feasibility of the process. The results suggested that the composite adsorbent is suitable as a sorbent material for recovery and biosorption/adsorption of uranium ions from aqueous solutions.     

Crystallographic and magnetic phase transition in TlMnCl3

The results of our NMR, EPR and magnetic susceptibility measurements in the paramagnetic state of TlMnCl₃ are reported here. The NMR paramagnetic shift of thallium is found to be small but positive. Mn²⁺ EPR line is exchange narrowed. The susceptibility measurements indicate an antiferromagnetic transition. The heat of crystallographic phase transition ΔH, in TlMnCl₃ has been measured using differential scanning calorimetry. The crystallographic phase transition appears to be first order and ΔH is unusually low viz. 10 cal mole⁻¹. In the case of KMnF₃ Δ_H, which is reported here for the first time, is determined to be 2 cal mole⁻¹.     

Probing the field-induced variation of the chemical potential in Bi2Sr2CaCu2Oy via magneto-thermopower measurements

Approximating the shape of the magneto-thermoelectric power (TEP) ΔS(T,H) measured in Bi₂Sr₂CaCu₂O_y by an asymmetric linear triangle of the form ΔS(T,H)≃S _p (H)±B ^±(H)(T _c −T) with positive B ⁻(H) and B ⁺(H) defined below and above T _c , we observe that B ⁺(H) ≃2B ⁻(H). To account for this asymmetry, we explicitly introduce the field-dependent chemical potential μ(H) of holes into the Ginzburg-Landau theory and calculate both an average ΔS _av(T,H) and fluctuation contribution ΔS _fl(T,H) to the total magneto-TEP ΔS(T,H). As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around T _c , viz. Δ μ(H)∝S _p (H). Zh. éksp. Teor. Fiz. 116, 257–262 (July 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Solid Substrate–Room Temperature Phosphorimetry for the Determination of Trace Terbutaline Sulfate Based on Its Inhibition Oxidation of Rhodamine 6G by Sodium Periodate

When 1.00 mol l⁻¹ I⁻ is used as ion perturber, rhodamine 6G (Rh 6G) can emit strong and stable room temperature phosphorescence (RTP) on filter paper substrate in KHC₈H₄O₄–HCl buffer solution (pH = 3.50), heated at 70 °C for 10 min. NaIO₄ can oxidize Rh 6G, which makes the RTP signal quench. Terbutaline sulfate (TBS) can inhibit NaIO₄ from oxidizing Rh 6G, which makes the RTP signal of Rh 6G enhance sharply. The content of TBS is linear correlation to ΔIp of the system. Based on the facts above, a new inhibition solid substrate-room temperature phosphorimetry (SS-RTP) for the determination of trace TBS has been established. The linear range of this method is 0.0104–2.08 pg spot⁻¹ (corresponding concentration: 0.026–5.2 ng ml⁻¹, with a sample volume of 0.4 μl) with a detection limit (L.D.) of 2.6 fg spot⁻¹ (corresponding concentration: 6.5 × 10⁻¹² g ml⁻¹), and the regression equation of working curve is ΔIp = 2.040 + 54.54 m_TBS (pg spot⁻¹), n = 6, correlation coefficient is 0.9994. For the samples containing 0.0104 pg spot⁻¹ and 2.08 pg spot⁻¹ TBS, the relative standard deviation (RSD) are 3.8% and 2.3% (n = 8), respectively, indicating good precision. This method has been applied to determination of trace TBS in the practical samples with satisfactory results. The reaction mechanism of NaIO₄ oxidizing Rh 6G to inhibit SS-RTP for the determination of trace TBS is also discussed.     

Silicon and transition metal-silicides implanted with57Fe

Si(111) single crystals were implanted with⁵⁷Fe in a broad dose range in order to overlap the concentration range of bulk amorphous Fe _x Si_1−x samples. At high (≥10¹⁶ atoms/cm²) doses the measured hyperfine interaction values were found to be the same as in the bulk amorphous samples, suggesting the same Fe−Si bonding and a very similar structure for the two amorphous phases produced by different methods. A comparison of the isomer shift (δ) and quadrupole splitting (ΔE) values with the values of the stoichiometric crystalliine phases showed the same δ but different δE values indicating similar Fe−Si bonding but different atomic arrangement around the iron atom.     

Magnetooptical properties and FMR in granular nanocomposites (Co84Nb14Ta2) x (SiO2)100? x

For nanodimensional magnetically inhomogeneous amorphous granular films of the system (Co₈₄Nb₁₄Ta₂) _x (SiO₂)_100−x , 30≤x≤60 at.% the concentration dependences of the magnetooptical Kerr effect (MOKE) spectra and FMR have been investigated. The observed changes in the MOKE and FMR spectra are associated with transformations of microstructure and topology of the nanocomposites. For the compositions within the percolation region the transversal Kerr effect increases by an order of magnitude.     

A Remark on Lp-Boundedness of Wave Operators¶for Two Dimensional Schrödinger Operators

Let H=−Δ+V be a two dimensional Schr?dinger operator with a real potential V(x) satisfying the decay condition , δ > 6. Let H ₀=−Δ. We show that the wave operators are bounded in L ^p (R ²) under the condition that H has no zero resonances or bound states. In this paper the condition , imposed in a previous paper (K. Yajima, Commun. Math. Phys. 208, 125–152 (1999)), is removed. Received: 13 September 2001 / Accepted: 15 October 2001     

Semiempirical study of perturbations of the Landé g factors of electronic-vibrational-rotational levels of hydrogen: II. i 3 g − and j 3Δ g − states of the H2, HD,and D2 molecules

The values of the Landé g factors of the i ³Π _g ⁻ , v, N and j ³Δ _g ⁻ , v, N states of the H₂, HD, and D₂ molecules have been found semiempirically for the following vibrational and rotational quantum numbers: v≤3; N≤7 for H₂; N≤5 for HD; and N≤11 for D₂. These values were obtained in terms of the nonadiabatic model, which takes into account the interaction between the 3dπ³Π_g and 3dδ³Δ_g states with the same values of v and N in the approximation of pure precession, with the use of semiempirical values of the expansion coefficients of the wave functions in the Born-Oppenheimer basis determined by us previously and the results of numerical calculation of the overlap integrals of the vibrational wave functions of these states. The results obtained for the H₂ molecule are in good agreement with the data in the literature. For the i ³Π _g ⁻ and j ³Δ _g ⁻ states of the HD and D₂ molecules, the g factors were found for the first time. This made it possible to study for the first time the role of the isotopic effect in the perturbation of the dependences of the g factors of rovibrational levels on v and N for the triplet electronic states of the hydrogen molecule. It was found that the interference effects of interaction between the 3dπ³Π_g and 3dδ³Δ_g states lead both to significant differences—up to 8, 6, and 11 times for H₂, HD, and D₂, respectively (the i ³Π _g ⁻ state), 20 times for H₂ and HD, and two orders of magnitude for D₂ (the j ³Δ _g ⁻ state)—between the nonadiabatic values of the g factors and the corresponding adiabatic values for some isotopomers of the hydrogen molecule and to significant differences—up to 9 and 1.5 times for the j ³Δ _g ⁻ and i ³Π _g ⁻ states, respectively—in the nonadiabatic values of the g factors of rovibrational levels of different isotopomers of the hydrogen molecule. __________ Translated from Optika i Spektroskopiya, Vol. 96, No. 1, 2004, pp. 42–54. Original Russian Text Copyright ? 2004 by Astashkevich.     

Third neighbour hyperfine field shift in a diluteNiAl alloy

In the binary Ni_98.5Al_1.5 alloy we observed well resolved satellite hyperfine fields at¹¹¹Cd nuclear probes. The hyperfine shifts derivated are −4.0(3); −8.5(3) and ⃛14.0(3)% relative to the pure nickel value. From the relative intensity of the four components as well as from the quadrupole interaction data we identify the satellites as impurity-probe configurations containing zero, one, two or three Al impurities in the third neighbour shell only. The third neighbour contribution to the hyperfine field at Cd in nickel equals ΔH ₃= +4.5kG/μ _B-atom, allowing an estimate ΔH ₁≃ −17.8 kG/μ _B-atom for the first shell contribution, confirmed by a similar experiment on Ni_98.5Pd_1.5.     

Confrontation of nucleus deformation and multipole mixture parameters in (2+0 n ?2+01) transitions with rotational band structure for K π = 0 2+ and 0 3+

The difference of the energies of levels Δ _n = E _lev(2⁺0 _n ) − E _lev(0⁺0 _n ) at n = 1, 2, and 3 and the multipole-mixture parameter δ for (2⁺0₂−2⁺0₁) and (2⁺0₃−2⁺0₁) transitions are contrasted against the structure of the K ^π = 0₂⁺ and 0₃⁺ rotational bands that was calculated on the basis of the quasiparticle-phonon model. The values of (Δ₂ − Δ₁), (Δ₃ − Δ₁), and (Δ₂ − Δ₃) are found to correlate with the sign of the parameter δ and with the calculated structure of the K ^π = 0₂⁺ and 0₃⁺ bands. Original Russian Text ? A.M. Demidov, L.I. Govor, V.A. Kurkin, I.V. Mikhailov, 2009, published in Yadernaya Fizika, 2009, Vol. 72, No. 2, pp. 236–240.     

On Lieb-Thirring Inequalities for Schrödinger Operators with Virtual Level

We consider the operator H=−Δ−V in L₂(ℝ^d), d≥3. For the moments of its negative eigenvalues we prove the estimate Similar estimates hold for the one-dimensional operator with a Dirichlet condition at the origin and for the two-dimensional Aharonov-Bohm operator.     

Thermodynamic properties of the most stable gaseous small silicon-carbon clusters in their ground states

The most stable structures of gaseous Si _m C _n (3 ⩽ n+m ⩽ 6) clusters in their ground electronic states are determined with the high level electronic correlation method QCISD(T)/g3large. Thermodynamic properties on heat capacity (C _p,m ^Θ), entropy (S _m ^Θ), Gibbs energy function (−[G ^Θ −H ^Θ(T _r )]/T) and enthalpy function (H ^Θ−H ^Θ(T _r )) are predicted with standard statistical thermodynamics using the structure parameters and vibrational frequencies obtained with B3PW91/6-31G(d) method combined with the electronic excitation energies determined with time dependent density functional (TD DFT) method at B3PW91/6-31G(d) level. The electronic energies are calculated with the accurate model chemistry method at G3(QCI) level of theory and the Δ _f H _m ^Θ (0 K), Δ _f H _m ^Θ (298.15 K) and Δ _f G _m ^Θ (298.15 K) values are predicted. The heat capacities C _p,m ^Θ(T) as a function of temperature within 298.15-2000 K are fitted into analytical equations. The thermodynamic functions at higher temperatures are determined classically by using these equations. Most of the results obtained in this work are consistent with the available experiments.     

Correlation between superheated liquid fragility and potential energy landscape in Gd-and Pr-based glass-forming alloys

The kinetic viscosities of superheated liquids on the Gd-based bulk glass-forming alloys are measured by an oscillating viscometer in a high vacuum atmosphere. According to the viscosity data, the parameters of superheated liquid fragility, M, are calculated. Based on the values of M in Gd-and Pr-based (cited from the literature) glass-forming alloys, we find that there is a linear correlation between M and the absolute value of mixing enthalpy, |ΔH _mix|, in an alloy system with the same base element, and the larger M, the smaller |ΔH _mix|. The alloy with larger M exhibits the larger height of energy barriers separating the minima on the potential energy landscape. Supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB613901), the National Natural Science Foundation of China (Grant No. 50231040), the Natural Science Foundation of Shandong Province of China (Grant No. Z2004F02), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050422024)