Determination of the reaction heat and studies of thermodynamic functions

The hydrolytic polymerization of Cr³⁺ at relatively high concentrations was studied by microcalorimetry. The thermal curves were determined with a 2277 thermal activity monitor. From the curves, the identified reaction heats (δrH _m ^σ ), hydrolysis constants (K) and thermodynamic functions (δrS _m ^σ ,δrH _m ^σ ) were calculated at different temperatures.     

Critical micelle concentration and self-aggregation of hexadecyltrimethylammonium bromide in aqueous glycine and glycylglycine solutions at different temperatures

Conductivities, densities and ultrasonic speeds measurements of hexadecyltrimethylammonium bromide (HTAB) in aqueous solutions of glycine (Gly) and glycylglycine (Gly-Gly) have been made at various temperatures. The critical micelle concentration (CMC), the degree of ionization (??) of the micelles, standard free energy, enthalpy, and entropy of the micellization process (??G _m ^° , ??H _m ^° , and ??S _m ^° ) for the present systems were estimated at different temperatures. The CMC values of HTAB in aqueous Gly and Gly-Gly were also evaluated by density and ultrasonic speed measurements. Apparent molar volumes, (V _?), apparent molar volumes at infinite dilution, (V _? ^° ), apparent molar compressibilities, (K _?), of HTAB in the pre- and post-micellar regions, and volume change on micellization (??V _? ^m ) were also estimated. Large positive values of T??S _m ^° and small negative values of ??H _m ^° suggest that micellization process is driven primarily by entropy increase. The increase in ??V _? ^m and K _? with rise in temperature is indicative of less compact micellar structure of HTAB in presence of amino acid additives. These data suggest that amino acids are solubilised probably in the palisade layer of the micelle.     

Ab initio study of small He cluster ions He n + ,n=2, 3, 4, 5, and low-lying Rydberg states of He4

SCF and CEPA calculations are applied to study the structure of small He cluster ions, He _n ⁺ ,n=2, 3, 4, 5 and some low-lying Rydberg states of He₄. The effect of electron correlation upon the equilibrium structures and binding energies is discussed. He ₃ ⁺ has a linear symmetric equilibrium geometry with a bond length of 2.35a ₀ and a binding energyD _e =0.165 eV with respect to He ₂ ⁺ +He (experimentally:D ₀=0.17 eV which corresponds toD _e ≈0.20 eV). He ₄ ⁺ is a very floppy molecular ion with several energetically very similar geometrical configurations. Our CEPA calculations yield a T-shaped form with a He ₃ ⁺ centre (R _e = 2.35a ₀) and one inductively bound He atom (4.39a ₀ from the central He atom of He ₃ ⁺ ) as equilibrium structure. Its binding energy with respect to He ₃ ⁺ +He is 0.031 eV. A linear symmetric configuration consisting of a He ₂ ⁺ centre with a bond length of 2.10a ₀ and two inductively bound He atoms (4.20a ₀ from the centre of He ₂ ⁺ ) is only 0.02–0.03 eV higher in energy. We expect that in larger He cluster ions structures with He ₂ ⁺ and He ₃ ⁺ centres andn?2 orn?3 inductively bound He atoms have nearly the same energies. In He₄ a low-lying metastable Rydberg state (³ Π symmetry for linear He ₄ ^* ,³ B ₁ for the T-shaped form) exists which is slightly stronger bound with respect to He ₃ ^* +He than the corresponding ion.     

Solvation numbers and hydration constant for thorium(IV) in ethanol-water medium

The solvation number and hydration constant of Th⁴⁺ in ethanol-water medium were determined at 25°C using UV-spectral and electrochemical measurements. A solvate formation equilibrium is demonstrated and characterized. Three molecules of ethanol (S) can bond to the metal cation with strengths comparable to that for H₂O to form ThS₃(H₂O) ₃ ⁴⁺ Formation of thorium monochelate with lawsone (2-hydroxy-1,4-naphthoquinone) eliminates bonding with alcohol molecules. The dissociation constant of the chelating agent_sK_a and the formation contant of the monochelated metal ion_sK _f ^* that are essentially independent of the solution composition are evaluated. Hydration titrations involving thorium-lawsone monochelate are performed and the data obtained from the changes of pH with solvent composition are analyzed. The solution independent constant,_sK _f ^* for thorium-lawsone complex formation in mixed aqueous ethanol is given by $$log_s K_f * = vpK_a + log_s K_h - log[LH] - vpH + 3logv$$ where vpK_a is the dissociation constant of the chelating agent LH in the solvent system of ν volume fraction of water and_sK_h is the solution-independent hydration constant of thorium (IV) in the solvent system. Log-values for the constants_sK_h,_sK _f ^* and_sK _a ^* are found to be 7.8±0.02, 11.38±0.04 and ?0.753, respectively.     

Niobium-chloride octahedral clusters as building units for larger frameworks

A systematic theoretic study on clusters containing edge-bridged octahedral metal units [Nb₆Cl₁₂] ⁿ (n?=?2, 3, or 4) and a large variety of ligands has been performed. The benchmark results on the [Nb₆Cl ₁₂ ⁱ ] ⁿ⁺ and [Nb₆Cl ₁₂ ⁱ Cl ₆ ^a ] ⁿ (n?=?2, 3 or 4) cluster units demonstrated the reliability of GGA PBE functional in combination with ZORA TZP basis set for the Nb-containing coordination compounds. The geometrical, electronic, and vibrational properties of large variety of substituted Nb₆Cl ₁₂ ⁱ Y ₆ ^a clusters have been provided. One- and two-dimensional structures with a [Nb₆Cl ₁₂ ⁱ (Bipyr) _x Cl _6?x ^a ] (x?=?2 and 4) building blocks have been proposed as good and stable candidates for new coordination polydimensional materials.     

Features of the Diels-Alder reaction between 9,10-diphenylanthracene and 4-phenyl-1,2,4-triazoline-3,5-dione

The Diels-Alder reaction between substituted anthracenes 1a?1j and 4-phenyl-1,2,4-triazoline-3,5 (2) is studied. In all cases except one, the reaction proceeds on the most active 9,10-atoms of substituted anthracenes. The orthogonality of the two phenyl groups at the 9,10-position of diene 1a is found to shield 9,10-reactive centers. No dienophiles with C=C bonds are shown to participate in the Diels-Alder reaction with 1a; however, the reaction 1a + 2 proceeds with the very active dienophile 2,4-phenyl-1,2,4-triazoline-3,5-dione. It is shown that attachment occurs on the less active but sterically accessible 1,4-reactive center of diene 1a. The structure of adduct 3a is proved by ¹H and ¹³C NMR spectroscopy and X-ray diffraction analysis. The following parameters are obtained for reaction 1a + 2 ? 3a in toluene at 25°C: K _eq = 2120 M^?1, ΔH _f ^≠ = 58.6 kJ/mol, ΔS _f ^≠ = ?97 J/(mol K), ΔV _f ^≠ = ?17.2 cm³/mol, ΔH _b ^≠ = 108.8 kJ/mol, ΔS _b ^≠ = 7.3 J/(mol K), ΔV _b ^≠ = ?0.8 cm³/mol, ΔH _r-n = ?50.2 kJ/mol, ΔS _r-n = ?104.3 J/(mol K), ΔV _r-n = ?15.6 cm³/mol. It is concluded that the values of equilibrium constants of the reactions 1a?1j + 2 ? 3a?3j vary within 4 × 10¹?10¹¹ M^?1.     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Molecular structure of a water-polyethylene glycol-KOH system, according to densitometry and viscometry data

The dynamic viscosity and density of a water-polyethylene glycol-KOH system are measured at temperatures of 293.15 to 323.15 K in concentrations ranging from 0.00001 and 0.001 (mole fractions). The activation parameters of viscous flow (ΔG _η ^≠ , ΔH _η ^≠ , and ΔS _η ^≠ ), structural temperature (T ₀), the partial molar volume of polyethylene glycol (PEG) in solution $\left( {\tilde V} \right)$ , intrinsic viscosity([η]), and the Huggins constant (K _H), are calculated. It is found that PEG has a structuring effect on water in water-PEG and water-PEG-KOH systems, with the PEG structuring effect in the latter being somewhat attenuated by the destructuring influence of KOH.     

Anionic charge effects on the crystal structures of Re6 octahedral cluster based compounds: The structure of the cubic bromide K2Re6S6Br8

The title compound synthesized by solid state reaction crystallizes in the cubic system (Pn-3) witha=13.433 Å. The structure has been determined by singlecrystal X-ray diffraction and shows that the compound is based on isolated |(Re₆S ₆ ⁱ Br ₂ ⁱ )Br ₆ ^a |^2? units. The K⁺ cations occupy two different sites. Steric effects lead to the switching from the cubic symmetry present in KRe₆Se₅Cl₉ to the monoclinic one in the case of KRe₆S₅Br₉ and again the former cubic symmetry in the new compound K₂Re₆S₆Br₈.     

The thermodynamic parameters of oxygen nonstoichiometry defects in lanthanum cobaltite doped with acceptor impurities (Sr and Ni)

The oxygen nonstoichiometry δ of lanthanum cobaltite doped with acceptor impurities (Sr and Ni), La_{1 ? x} Sr_xCo_0.9Ni_0.1O_{3 ? δ} (x = 0.1, 0.3), was studied by high-temperature thermogravimetry over the temperature and pressure ranges 723 K ≤ T ≤ 1373 K and 10^?3 atm ≤ $p_{O_2 } $ ≤ 1 atm. The partial replacement of cobalt with nickel and lanthanum with strontium increased the oxygen nonstoichiometry δ. The partial molar enthalpies $\Delta \bar H^\circ _O $ and entropies $\Delta \bar S^\circ _O $ of solution of oxygen in the solid phase were calculated. Models of point defect formation were suggested and analyzed. The equilibrium constants of formation and concentrations of predominant point defects, ionized oxygen vacancies V _o ^.. , holes Me _Co ^. (Co _Co ^. and Ni _Co ^. ), and electrons Me _Co ^′ (Co _Co ^′ and Ni _Co ^′ ) localized on 3d transition metals, were determined by nonlinear regression from the experimental and theoretical logp $p_{O_2 } $ ?δ dependences.     

Thermodynamics of extraction of tetravalent plutonium,uranium, thorium and zirconium

Thermodynamic treatment of the experimental data on the extraction of quadrivalent Pu, U, Th and Zr with tri-n-butyl phosphate (TBP) from nitric acid solutions is presented. It is shown that the extraction of all the quadrivalent metals studied is going according to the same mechanism: M(OH)^4?i+(4?i)NO ₃ ^? +2TBP?M(OH)_i(NO₃)_4?i·2 TBP. For Zr, i=0, 1, and 2; for the remaining M(IV), i=0 and 1. The thermodynamic constants of extraction of M(IV) with the kerosene solutions of TBP according to the above mentioned equation are as follows: Zr: K ₀ ⁰ =0.6; K ₁ ⁰ =14; K ₂ ⁰ =5. Pu: K ₀ ⁰ =380; K ₁ ⁰ =4.8·10⁴. U: K ₀ ⁰ =300; K ₁ ⁰ =1.8·10⁴. Th: K ₀ ⁰ ～150. It has been established that Zr and Pu(IV) are extracted into 2-thenoyltrifluoracetone (HA) from perchloric acid solutions under the formation of MA₄ and M(ClO₄)A₃ species. For the extraction from nitric acid solutions, the species formed are ZrA₄ and Zr(NO₃)A₃ in the case of Zr, PuA₄ and Pu(OH)A₃ in the case of Pu. The differences in the qualitative and quantitative characteristics of the extraction of M(IV) with TBP and HA from nitric and perchloric acids are explained by the effect of the character of the acid and of ionic potential upon the structure of the hydration shell of M _aq ⁴⁺ .     

Dynamic solvation effects on surface-impact dissociation of I 2 - (CO2)n

Surface-impact dissociation of I ₂ ^- (CO₂)n was studied by a molecular dynamics simulation in comparison with the experimental results. The branching fraction, ? _dis, of the I ₂ ^- dissociation was calculated as a function of the parent cluster size, n. This computational result reproduces the experimental one. We calculated a number of the I ₂ ^- dissociation events starting from given initial orientations. The most favorable molecular orientation obtained supports the wedge effect in which a CO₂ molecule located at the waist position of the I ₂ ^- core ion splits the I ₂ ^- bond as if a piece of wood is split by a mechanical thrust against a wedge. The time profile of the wedge action calculated for the I ₂ ^- (CO₂) impact shows that more than 20 % of the collision energy is converted to the vibrational energy of the I ₂ ^- .     

Amino acid hydration in aqueous ammoniu chloride and sodium chloride solutions

Partial volumes $\bar V^0$ of amino acids in aqueous NH₄Cl and NaCl solutions are discussed. The salts have different effects on water structure. The contributions of the charged NH ₃ ⁺ and COO^? groups of amino acids are found. Structural characteristics of hydrated complexes are calculated: partial volumes of water inside and outside the hydration sphere and hydration numbers. The same value of $\bar V^0$ (NH ₃ ⁺ , COO^?) is achieved at a higher NH₄Cl concentration. The two salt systems with the same $\bar V^0$ (NH ₃ ⁺ , COO^?) have similar values of the partial volumes of water and hydration numbers.     

Determination of the surface acidity of a free-base corrole in a self-assembled monolayer

Mixed self-assembled monolayers (SAMs) containing a corrole moiety have been prepared to examine their electrochemical properties and surface acidity, with the eventual goal of biosensor development. Mixed SAMs consisting of 6-mercapto-hexanol (6-MHO) and 8-amino-1-octanethiol (8-AOT) in varying ratios were modified with a free-base corrole and characterized via Osteryoung square-wave voltammetry and contact angle measurements. The surface acidity of the free-base corrole was determined using an electrochemical titration method, with pK _a values established at 6.4 when using Fe(CN) ₆ ^4? /Fe(CN) ₆ ^3? as a redox probe and at 6.7 when using iodide, and assigned to the CorH₄ ⁺ ? CorH₃ + H⁺ equilibrium.     

Velocity cross correlations in binary mixtures of simple fluids

The velocity cross correlation integrals $$D_{{\text{ab}}}^{\text{J}} = (N/3)\mathop \smallint \limits_{\text{o}}^\infty< {\text{v}}_{{\text{1a}}} ({\text{t}}) \cdot {\text{v}}_{{\text{2b}}} (0) > {\text{dt,}} {\text{a}} {\text{ = }} {\text{1,2;}} {\text{b}} {\text{ = }} {\text{1,2}}$$ can be estimated from the intradiffusion coefficients D ₁ ^° and D ₂ ^° at each mole fraction x₁ of component 1 on the basis of the exact relations among the Onsager phenomenological coefficients together with an assumed equation relating the joint diffusion coefficients D _ab ^J . The results from several such equations are compared with experimental data and with similar results derived by Hertz in a different way to represent the behavior of f _ab ≡D _ab ^J x _b in ideal reference systems. In some cases the agreement with experimental data for relatively ideal systems is even better than given by Hertz's results. For greater accuracy in predicting the D _ab ^J from D _a ^dg data one would need a prediction of the limiting value of D _aa ^J at x_a=0 for a=1,2. Presently known theory does not give a basis for estimating this limit reliably.     

Guided ion beam studies of the reactions of O+, O 2 + , N+ and N 2 + with silane

Guided ion beam mass spectrometry is used to measure the cross sections as a function of kinetic energy for reaction of SiH₄ with O⁺(⁴S), O ₂ ⁺ (²Π_g,v=0), N⁺(³P), and N ₂ ⁺ (²Σ _g ⁺ ,v=0). All four ions react with silane by dissociative charge-transfer to form SiH _m ⁺ (m=0?3), and all but N ₂ ⁺ also form SiXH _m ⁺ products where (m=0?3) andX=O, O₂ or N. The overall reactivity of the O⁺, O ₂ ⁺ , and N⁺ systems show little dependence on kinetic energy, but for the case of N ₂ ⁺ , the reaction probability and product distribution relies heavily on the kinetic energy of the system. The present results are compared with those previously reported for reactions of the rare gas ions with silane [13] and are discussed in terms of vertical ionization from the 1t ₂ and 3a ₁ bands of SiH₄. Thermal reaction rates are also provided and dicussed.     

Analysis of optical absorption spectra of transition metal cluster ions by the spin-polarized DV-Xα method

Optical absorption spectra of cobalt cluster ions, Co _n ⁺ , and vanadium cluster ions, V _n ⁺ , were analyzed by a theoretical calculation based on the spin-polarized DV-Xα method, and their electronic and geometric structures were obtained. Relative absorption cross section associated with each electronic transition was calculated; the calculation enables a qualitative comparison of calculated spectrum with a measured one not only in its transition energy but also in its intensity profile. This analysis shows that Co ₄ ⁺ , Co ₃ ⁺ , and V ₄ ⁺ have, respectively, a tetrahedral structure with a bond distance of 2.00Å, an equilateral triangle with a bond distance of 2.30Å, and a distorted tetrahedral structure with five bonds having a distance of 2.34 Å and one of 2.89Å. The differences in the population between majority and minority spins (spin-difference) evaluated from the electronic structure thus obtained were 2.0, 1.7, and zero per atom in Co ₃ ⁺ , Co ₄ ⁺ , and V ₄ ⁺ , respectively. These spin differences indicate a ferromagnetic and an antiferromagnetic spin-coupling in the cobalt and vanadium cluster ions, respectively.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.     

Thermodynamic transfer functions for urea and thiourea from water to water-tetrahydrofuran mixtures from precise vapor-pressure measurements

The rate of change of the standard chemical potential with solvent composition, \(\partial \bar G_0 /\partial Z\) , has been calculated from precise vapor-pressure measurements for urea at three temperatures and for thiourea at 298.15°K in water-tetrahydrofuran (THF) mixtures. From these results the standard free energy of transfer ΔG _t ^o of the solutes from water to various water-THF mixtures has been obtained together with the standard molar entropy ΔS _t ^o and the standard molar enthalpy ΔH _t ^o of transfer at 298.15°K in the case of urea. The quantity ΔG _t ^o for urea is negative in the water-rich region and positive for mole fractions THF>0.2. There is a nearly complete compensation between ΔH _t ^o andTΔS _t ^o at 298.15°K up to mole fraction THF=0.5. These phenomena can be partly related to the structure in H₂O-THF mixtures.     

Formation of helium cluster ions HHe x + (x≦14) and H3He x + (x≦13) in a very low temperature drift tube

The formation of cluster ions when hydrogen molecular ions H ₂ ⁺ and H ₃ ⁺ are injected into a drift tube filled with helium gas at 4.4 K has been investigated. When H ₂ ⁺ ions are injected, cluster ions HHe _x ⁺ (x≦14) are produced. No production of H₂He _x ⁺ ions is observed. When H ₃ ⁺ ions are injected, cluster ions HHe _x ⁺ (x≦14) are produced as well as H₃He _x ⁺ (x≦13), and very small signals corresponding to H₂He _x ⁺ (3≦x≦10) are observed. Information on the stability of HHe _x ⁺ and H₃He _x ⁺ is derived from the drift field dependence of the cluster size distributions. The cluster sizex=13 is found to be a magic number for HHe _x ⁺ , and for H₃He _x ⁺ ,x=10 and 11.