bluuebunny/crossref_metadata_2025_split huggingface dataset:
Natural and synthetic compounds with kröhnkite-type chains. An update
Authors: Fleck Michel , Kolitsch U. 
Published: December 2003

AbstractThe crystal structures of five new triclinic double salt dihydrates have been determined from single-crystal X-ray diffraction data. The following four compounds all contain kröhnkite-[Na2Cu(SO4)2·2 H2O]-type tetrahedral-octahedral chains: the three isotypic synthetic compounds K2Mg(CrO4)2·2 H2O (a= 5.674(1),b= 6.462(1),c= 7.517(2) Å,α= 110.38(3),β= 95.24(3),γ= 109.86(3)°,V= 236.0(1) Å3,R(F) = 0.036, space groupP1̅, no. 2,Z= 1), K2Mg(MoO4)2·2 H2O (a= 5.884(1),b= 6.491(1),c= 7.700(1) Å,α= 111.67(2),β= 96.59(2),γ= 108.62(2)°,V= 249.8(1) Å3,R(F) = 0.019, space groupP1̅, no. 2,Z= 1) and K2Mn(SeO4)2·2 H2O (a= 5.674(1),b= 6.608(1),c= 7.523(2) Å,α= 110.31(3),β= 95.69(3),γ= 108.35(3)°,V= 244.10(9) Å3,R(F) = 0.024, space groupP1̅, no. 2,Z= 1), and the mineral messelite Ca2(Fe2+,Mn2+,Mg)(PO4)2·2 H2O (a= 5.480(1),b= 5.759(1),c= 6.569(1) Å,α= 90.18(3),β= 102.62(3),γ= 108.45(3)°,V= 191.3(1) Å3,R(F) = 0.022, space groupP1̅, no. 2,Z= 1). The fifth compound, the synthetic dichromate K2Zn(Cr2O7)2·2 H2O (a= 6.794(1),b= 7.735(2),c= 7.834(2) Å,α= 88.97(3),β= 80.90(3),γ= 64.57(3)°,V= 366.5(2) Å3,R(F) = 0.036, space groupP1̅, no. 2,Z= 1) is closely related to the kröhnkite-group; it contains Cr2O7groups assuming the same bridging role thatXO4groups have in kröhnkite-type oxysalts. The atomic arrangements of the title compounds are described and structural relations with other kröhnkite-type and -like compounds are discussed. In addition, related yavapaiite-type sheet structures ofAM′(XO4)2compounds are briefly reviewed, including the rod group symmetry of the underlying kröhnkite-chain building unit. Twelve different space groups have been found for these sheet structures. The previously reported space group symmetries for α-NH4Fe(CrO4)2(P21) and CsTa(PO4)2(P1̅) are obviously incorrect and should be revised toP21/nandC2/m, respectively.
Geometrical isomerism in uranyl chromates I. Crystal structures of (UO2)(CrO4)(H2O)2, [(UO2)(CrO4)(H2O)2](H2O) and [(UO2)(CrO4)(H2O)2]4(H2O)9
Authors: Krivovichev Sergey V. , Burns Peter C. 
Published: December 2003

Abstract
Three uranyl chromate hydrates, (UO2)(CrO4)(H2O)2, [(UO2)(CrO4)(H2O)2](H2O) and [(UO2)(CrO4)(H2O)2]4(H2O)9, have been prepared by evaporation from aqueous solutions. The crystal structure of (UO2)(CrO4)(H2O)2 (monoclinic, C2/m, Z = 16, a = 16.786(2), b = 22.731(2), c = 6.9969(7) Å, β = 90.051(3)°, V = 2669.8(5) Å3) has been solved by direct methods and refined to R1 = 0.065, calculated for 6702 unique observed reflections (|F
o| ≥ 4σF
). The crystal structure of [(UO2)(CrO4)(H2O)2](H2O) (monoclinic, P21, Z = 4, a = 9.7206(14), b = 7.1617(10), c = 11.0909(16) Å, β = 92.388(3)°, V = 771.43(19) Å3) has been solved by direct methods and refined to R1 = 0.052, calculated for 4879 unique observed reflections (|F
o| ≥ 4σF
). The crystal structure of [(UO2)(CrO4)(H2O)2]4(H2O)9 (monoclinic, P21/c, Z = 4, a = 31.397(1), b = 7.1701(3), c = 16.2480(7) Å, β = 97.515(1)°, V = 3626.3(3) Å3) has been solved by direct methods and refined to R1 = 0.051, calculated for 8164 unique observed reflections (|F
o| ≥ 4σF
). Each structure contains chains of composition [(UO2)(CrO4)(H2O)2]. Within the chains, (UO2)O3(H2O)2 pentagonal bipyramids share three equatorial O atoms with CrO4 tetrahedra. The CrO4 tetrahedra in the [(UO2)(CrO4)(H2O)2] chains each share three corners with uranyl polyhedra, and contain one non-shared (terminal) ligand pointing either up or down relative to the plane of the chain. This results in a geometrical isomerism of the uranyl chromate chains that can be described as a sequence of tetrahedra orientations along the chain extension, e.g. ... up-down-up-down ... or ... ududud .... [or (ud)∞]. The structures of (UO2)(CrO4)(H2O)2 and [(UO2)(CrO4)(H2O)2](H2O) are based upon the chains of the (ud)∞ type, whereas [(UO2)(CrO4)(H2O)2]4(H2O)9 contains two types of chains: (ud)∞ and (u)∞. The [(UO2)(CrO4)(H2O)2] chains extend in the same direction in each structure and are linked via hydrogen bonds involving H2O molecules.
Pressure Perturbation Calorimetic Studies of the Solvation Properties and the Thermal Unfolding of Proteins in Solution
Authors: Ravindra R. , Winter Roland 
Published: December 2003

Abstract
In this study, pressure perturbation calorimetry (PPC), a relatively new and efficient technique, was used to study the solvation and volumetric properties of the enzyme ribonuclease A (RNase A) in its native and unfolded state. In PPC, the coefficient of thermal expansion of the partial volume of the protein is deduced from the heat consumed or produced after small isothermal pressure jumps (±5bar), which strongly depends on the interaction of the protein with the solvent or co-solvent at the protein-solvent interface. The effects of various chaotropic and kosmotropic co-solvents (glycerol, sorbitol, urea, guanidinium hydrochloride, K2SO4) on the solvation and unfolding behaviour of RNase A was investigated as well, and the observed volume and expansivity changes are correlated with further thermodynamic properties obtained from differential scanning calorimetry (DSC). Depending on the type of co-solvent and its concentration, specific differences are found for the solvation properties of the protein, and the volume change upon unfolding may even change sign. Taken together, the data obtained lead to a deeper understanding of the solvation process of proteins in different co-solvents in their native and unfolded states.
From Ostwald′s Times to Solid State Ionics: Migration and Localised Hopping of Silver Ions in Crystalline Rubidium Silver Iodide
Authors: Funke K. , Banhatti R.D. , Ross I. , Wilmer D. 
Published: December 2003

AbstractSince Ostwald′s times, the concepts of ionic motion in condensed matter have been extended to comprise systems with increasingly complex dynamic properties. Today, one of the central problems in the field of SOLID STATE IONICS consists in finding simple, yet relevant rules for the correlated hopping motion of the mobile ions in structurally disordered ionic materials. Rubidium silver iodide, RbAg4I5, belongs to this class of materials. To analyse the hopping dynamics of the mobile silver ions in the three phases of RbAg4I5, we have taken conductivity spectra at frequencies up to the far infrared. Also, a simple set of rules for the development of the ion dynamics with time has been provided by the MIGRATION concept, the acronym standing for MIsmatch Generated Relaxation for the Accommodation and Transport of IONs. In the phasesαandβ, an increasing tendency for correlated forward-backward hopping is observed as the temperature is decreased from 298K to 129K. At the first-orderβ-γphase transition at 121.8K, the number of translationally mobile silver ions is found to be markedly reduced. The conductivity spectra of the different phases of RbAg4I5are well explained by the MIGRATION concept as long as the angular frequency does not exceed the elementary hopping rate of the mobile silver ions. At higher frequencies, an additional dynamic feature is encountered, which is superimposed onto the MIGRATION-type conductivity. This feature, which shows a nearly constant loss (NCL) behaviour, becomes increasingly pronounced as the temperature is lowered. It is caused by a strictly localised motion of interacting silver ions. In our model treatment for the MIGRATION and NCL parts of the dynamic conductivity, the potentially translational and the strictly localised ionic hopping motion are best described with a single elementary hopping rate. Consequently, this rate marks the beginning of the NCL behaviour on the angular frequency scale. This observation is in agreement with earlier results obtained by Leon et al. on solid lithium-ion conductors.
On the Equilibration of Solid Phases. Some Thoughts on Ostwalds Contributions
Authors: Schmalzried Hermann 
Published: December 2003

AbstractThis paper discusses two of Ostwalds experimental observations which the literature connects with his name. The generalized conclusions Ostwald inferred are presented and discussed. His discoveries deal essentially with the formation of solid phases. Later both played important roles in the field of solid state reactions. The two items are: 1. Ostwalds rule of successive reaction steps (ORS-rule) and 2. the Ostwald-ripening process (ORP-process). Since material scientists and engineers try to obtain special properties of a solid by establishing its microstructure, Ostwalds discoveries and ideas do influence the field of materials science till nowadays.In what follows his farsightedness and his limitations are placed in the context of solid state physical chemistry and its history.
The Struggle for Precise Rate Constants in Gas Phase Reaction Kinetics: The Reaction H + O2⇔ HO + O
Authors: Troe Jürgen 
Published: December 2003

AbstractA short sketch of the history over the last 100 years of the H2–O2-system in general and of the H + O2⇔ HO + O reaction in particular is given. Only after revision of the enthalpy of formation of OH in the year 2000, experimental and theoretical rate constant determinations approach an agreement within about 10 percent. Further improvements of the modeling depend on more precise thermochemical data for the reaction and on a detailed analysis of the contributions from all electronic states arising from the four open electronic shell species involved. Vibrational zeropoint energy problems of OH in classical trajectory calculations and detailed angular momentum couplings in adiabatic channel treatments have also to be mastered.
The Chemisorption of 2,3-Dimercapto-n-Propane Sulfonate at the Au(111)-Electrode
Authors: Donner C. , Valet O. , Yang Y. , Baumgärtel H. 
Published: December 2003

AbstractIn this paper we report on the adsorption behaviour of 2,3-dimercapto-n-propan-sulfonate (DMPS) on the Au(111) electrode. DMPS is a water-soluble dithiol which is unstable in alkaline solution. The absorption decreases by 50 within two hours.The electrochemical behaviour is in accordance to that of other thiols, including monothiols. The interaction between sulfur and the Au(111) surface leads to a strong chemisorption in the double layer region which is limited by the reductive and oxidative desorption, respectively. The oxidative behavior (OD) of DMPS takes place simultaneously with the oxidation of the gold surface itself.We propose a mechanism for the OD which, in contrast to the well-known mechanism of the sulfur oxidation itself, includes the formation of a goldsulfur complex. This OD mechanism acts like a corrosion and causes an increased roughness of the surface. Therefore, the charge consumed during the OD is exclusively due to the oxidation of Au0to Au3+in form of the complex and in form of the well-known Au2O3oxide.
Ion–Solvent Interactions and Determination of Single-Ion Gibbs Energies of Transfer from the Studies of the Solubility and Dissociation Constants of Oxalic Acid in Aquo + Methanolic Mixtures
Authors: Gumtya S. K. , Lahiri S. C. 
Published: December 2003

Abstract
The solvent effects on the dissociation of the simplest dibasic organic acid were examined from the determination of the first and second dissociation constants of oxalic acid conductometrically and pH-metrically in water + methanol mixtures (0–87 wt% of methanol). The solubility values of oxalic acid and potassium hydrogen oxalate were also determined in water + methanol mixtures. The dissociation constant and the solubility values were used to calculate the Gibbs energies of transfer ΔG
0

t
of oxalic acid (H2OX), bioxalate (HOX−), oxalate (OX2−) and potassium (K+) ions from water to aquo-methanolic mixtures. These represent the quantitative measures of solute or ionsolvent interactions of the respective species in going from water to methanol + water mixtures. pK
1, pK
2 and the solubility values of oxalic acid increase continuously with increase in organic co-solvent. ΔG
0

t
(H2OX) and ΔG
0

t
(H+) are negative i.e. favourable but ΔG
0

t
(H2OX), ΔG
0

t
(OX2−) and ΔG
0

t
(K+) are in general positive and hence unfavourable for the dissociation processes. The increase in basicity fecilitated the dissociation processes but decrease in dielectric constant and H-bonding capability enhance the association processes as the organic co-solvent increases. Attempts have been made to understand the solvation processes in terms of the structure of the aquo + organic mixtures and Walden products. The pK values can be suitably utilised to prepare buffer solutions in methanol + water mixtures. The solvent effect and solvation phenomena can be better understood from the collection of single-ion values of transfer. These may be utilised to calculate the solubilities of electrolytes in methanol + water mixtures.
Formation of Nanoparticles by Photolysis from Metal and Carbon Bearing Molecules
Authors: Emelianov A. , Eremin A. , Jander H. , Wagner H. Gg. 
Published: December 2003

Abstract
The formation of particles following the photolysis of C3O2, Fe(CO)5 and Mo(CO)6, diluted with Ar or He was registered at room temperature. Particle growth was followed by taking light extinction profiles at 633nm and at 220nm for various mixture compositions and pressures. The particles obtained at different conditions were analyzed using transition electron microscope (TEM) technique. It was found that in the pure undiluted gases at the partial pressures shown in the pictures no light absorption and no particle formation could be observed. Light absorption started for partial pressures of the diluent gas > 10mbar. A comparison of particle size measured here at room temperature with data obtained at elevated temperature shows that the data obtained here fit well to the elevated temperature data.
Electrochemical Behavior and Enhanced Stability of a Thin Film of Prussian Blue Deposited under Magnetic Field
Authors: Eftekhari Ali 
Published: December 2003

Abstract
Magnetic field was employed as a useful tool to improve stability of electroactive films deposited on substrate surfaces. To this aim, electrochemical preparation/deposition of Prussian blue (PB) was carried out in the presence of an external magnetic field to form a thin film onto a Pt substrate electrode. The results were compared with a conventional PB modified electrode throughout the research. For the electrode prepared in the presence of the magnetic field, a stronger voltammetric behavior with higher peak current was observed. It was accompanied by higher chemical and electrochemical stabilities of the deposited electroactive film. Spectroscopic measurements were also performed to investigate the influence of the applied magnetic film on the properties of the electroactive film deposited. The results were indicative of the fact that deposition under magnetic filed is accompanied by stronger chemical bonds and different elemental composition of the electroactive material formed on the electrode surface. Based on the experimental results, it was concluded that both Lorentz and magnetic forces are responsible for the behavior observed.
Influence of Iron Content on the Electrical and Dielectric Properties of La0.7Ba0.3Co1−

y
Fe
y
O3
Authors: Khalil M. Sh. 
Published: December 2003

Abstract
Powders of La0.7Ba0.3Co1−

y
Fe
y
O3, LBCF, y=0.0 to 1.0, were prepared by thermal decomposition of metal polymerized complex solutions. Thermal and chemical analyses of cation concentrations were used to confirm the desired composition sintered at 900°C. Powder X-ray diffraction data indicate that all compositions possess the perovskite structure. Cubic unit cell parameters increase with increasing iron content except for y=0.3, which was explained in terms of the average valencies and charge neutrality of Fe and Co ions and oxygen ion deficiency. The electrical properties such as conductivity, dielectric constant and dielectric loss factor of the LBCF samples were studied as a function of frequency (100Hz–1MHz) and temperature (293–625K). Iron substituted samples exhibit semiconductor behavior with decreased electrical conductivity values compared to that of the free Fe sample. The possible conduction mechanisms were discussed. Dielectric studies show apparent high dielectric behavior, which was explained by the double layer capacitor model.
Establishment of Micellar Catalytic Field Using the p-Nitro-o-Carboxylbenzeneazo-8-Amino-quinoline – Cetyltrimethylammonium Bromide – Co(II) Ternary Complexation
Authors: Gao Hong-Wen , Yang Jia-Xiang , Zhao Jian-Fu 
Published: December 2003

Abstract
The micellar catalytic field was proposed in surfactant medium by investigating the complexation between p-nitro-o-carboxylbenzeneazo-8-aminoquinoline (NCBAAQ) and Co(II) at pH 8.8. The cationic surfactant, cetyl trimethylammonium bromide (CTAB) was added and the aggregation of NCBAAQ on CTAB obeys the Langmuir isothermal adsorption. The enrichment of NCBAAQ on CTAB micelle resulted in a sensitive and rapid complexation of NCBAAQ with Co(II). The ternary complexation was characterized by the microsurface adsorption – spectral correction (MSASC) technique. Results showed that the complex Co3NCBAAQ12CTAB78 is formed.
Heteroassociation of Selected Diols with some Tertiary Amines
Authors: Kolbe Alfred , Plass Monika 
Published: December 2003

AbstractDiols exhibiting the structure (HO)CH2–(CH2)n–CH2(OH) withn=1 up ton=4, (HO)CH2–CH(OH)–CH3and cyclohexan-1,2-diols as well as cyclohexan-1,4-diol, which may establish different intramolecular interactions, are used as model substances to describe the external hydrogen bonding behaviour of multivalent hydrogen bond donors in the presence of hydrogen bond acceptor molecules. In this study, hydrogen bonds formed by the diols with tertiary aromatic amines have been investigated. In solution, different associate formation between the diols and the acceptor molecules as sketched in Fig.1 may occur. Besides 1:1 associates formed by the interaction of one diol molecule with one amine molecule, 1:2 associates may be observed where each OH function interacts with one amine molecule. The equilibrium constants of the associates of those interactions have been studied by FTIR spectroscopy. The results allow a classification of the used diols in three different groups based on the position of the OH groups in the donor molecule. For diols with proton donating OH where no intramolecular hydrogen bond can be formed, the intermolecular hydrogen bonds for a 1:1 system may be described by thermodynamic parameters which are nearly twice the value of the corresponding equilibrium constants of monovalent alcohol systems due to the statistical weight of the OH groups. Secondly, when intramolecular hydrogen bond exists in the diols, the equilibrium constants in the interaction with the amine raise up by a factor ofca.2–3 due to the cooperativity effect. Thirdly, if the OH groups are arranged in 1,2 positions, both OH groups may be described as independent of each other in their intermolecular interaction with the amine.
Bursting Oscillations in the Revised Mechanism of the Hemin – Hydrogen Peroxide – Sulfite Oscillator
Authors: Straube Ronny , Müller Stefan C. , Hauser Marcus J. B. 
Published: December 2003

Abstract
The reaction mechanism for the hemin-mediated oxidation of sulfite by hydrogen peroxide was reinvestigated. This has become necessary due to an erroneous stoichiometric factor used during a previous numerical analysis. A simple correction of the previous model failed to reproduce the experimentally observed bursting oscillations of the pH value. Therefore, we propose an extended mechanism which includes not only the autocatalytic production of H+ and pH-dependent equilibria of hemin species, but also the decomposition of hemin in media of high H2O2 concentration. The dynamic properties of this 7-variable reaction mechanism are in good agreement with previous experimental and numerical observations. A closer analysis reveals that the pH-dependent dimerisation equilibrium of hemin provides only a minor contribution to the onset of oscillations. Therefore, this equilibrium can be neglected yielding a 6-variable version of the revised 7-variable model, while maintaining the local bifurcation structure. The revised mechanisms are considered realistic models that describe the observed dynamical features of the hemin-containing pH oscillator.
Hydrogen Bond Mediated Association of Dinucleotide Analogs
Authors: Basílio Janke Eline M. , Weisz Klaus 
Published: December 2003

Abstract
Several dinucleotide analogs, in which two of the nucleobases adenine, thymine and uracil are connected by a hexadecamethylene linker have been synthesized and studied for their base-base interaction in a chloroform solution. Using 1H NMR spectroscopic techniques intramolecular base pair formation through hydrogen bonding is observed at ambient temperatures but found to strongly depend on the identity of paired bases. Thus, whereas cyclization through intramolecular base-base interactions predominate for adenine–(CH2)16–thymine and adenine–(CH2)16–uracil, no intramolecular adenine-adenine pairing was observed. Upon addition of acetic acid to adenine–(CH2)16–thymine, strong adenine-acetic acid interactions result in the disruption of preformed intramolecular adenine-thymine base pairs.
Revealing CSA Tensors and Hydrogen Bonding in Methoxycarbonyl Urea: A combined 13C, 15N and 13C14N2 Dipolar Chemical Shift NMR and DFT Study
Authors: Macholl Sven , Börner Frank , Buntkowsky Gerd 
Published: December 2003

Abstract
Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by 13C and 15N dipolar chemical shift NMR spectroscopy and ab initio calculations. Employing a combination of dipolar chemical shift NMR, selective isotope labeling and ab initio gas phase calculations, possible molecular structures and chemical shielding tensors of all 15N nuclei and of two out of the three 13C nuclei were revealed. Four possible stable configurations of the molecule with different energies were found in the calculations. The CSA tensors were calculated for these configurations. While the calculated 13C(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tensor orientation, there are pronounced differences between calculated and experimental tensor eigenvalues. These differences are a clear indication of the presence of intermolecular hydrogen bonds in the experimental sample, which are neglected in the gas phase calculations. Four different possible orientations of the experimental 13C(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the 13C CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where δ11 points approximately in direction of N(imide), δ22 approximately in direction of the C=O bond, and δ33 is oriented perpendicular to the molecular frame, is adopted in the molecule.