Types of rotary crystal state of solid solutions of paraffins

Thermal deformations and polymorphous transformations of solid solutions of paraffins in C₁₇–C₁₉, C₁₉–C₂₁, C₂₁–C₂₃, C₂₂–C₂₄, and C₂₃–C₂₄ systems are investigated by thermal X-ray diffractometry using a temperature step of several tenths of a degree. It is examined how the length of a molecular chain of a homolog (n) and the difference in length (Δn) between the chains as well as the molecular composition of a solid solution affect these transitions, and the data are compared with those for the individual homologs of paraffins. St. Petersburg University. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 3, pp. 380–394, May–June, 1998. This work was supported by RFFR grant No. 97-05-65534 and ISSEP grants No. 156p and a97-2633.     

Adsorption of Kryptate Complexes of Alkali Metals in Electrolytes of Different Compositions and in the Presence of 1-Adamantanol

Data on the adsorption of kryptates of alkali metals on a mercury electrode are obtained for the first time by a method of the measurement of the differential capacitance as a function of potential and are compared to one another. Shown is that there takes place a deep mutual relationship between the structure of a complex and the regularities of its adsorption. An assumption is put forth about the possibility of adsorption of a free ligand in solutions of lithium kryptate, which possesses relatively low strength. The co-adsorption of sodium kryptate with 1-adamantanol, which forms a two-dimensional condensed layer, is investigated. Established is that the C vs. E curves, obtained in 1 M Na₂SO₄ at a constant activity of 1-adamantanol and a varying concentration of Kryptofix^R 222, exhibit two distinctly pronounced portions, each of which corresponds to predominant adsorption of one of the surface-active components. The region of predominant adsorption of Kryptofix^R 222 expands with its concentration.     

Determination of carbonate in the presence of hydroxide : Part I. Analysis of first-derivative potentiometric curves

Mathematical consideration of the course of neutralization of a solution containing a moles of a strong base (e.g. sodium hydroxide) and a moles of a salt of a weak dibasic acid (e.g. sodium carbonate) with a strong acid (e.g. hydrochloric acid) shows that the first derivative of the titration curve should contain three maxima. The first, corresponding to neutralization of most of the hydroxide, is small (with height proportional to a^{$\text{1}\text{2}$}), displaced slightly from the equivalence point, and disappears for a less than 0.01 mol l^-1. The second corresponds to the conversion of most of the CO₃^2- to HCO₃^- and its height is almost independent of a. The third corresponds to the conversion of HCO₃^- to H₂CO₃, with height proportional to a^{$\text{1}\text{2}$}. The two minima are independent of a and of the dissociation constants of the weak acid. These conclusions were examined experimentally and were extended to the titration of hydroxide contaminated with a small amount of carbonate.     

Flash photolysis of solutions of sodium in ethers

Flash photolysis of ethereal solutions of sodium results in the almost complete disappearance of the original absorption band at 15000 cm⁻¹ and in the formation of solvated electrons. After the flash an intermediate with an absorption maximum at 12000 cm⁻¹ is formed. This absorption is assigned to a species which has the stoichiometric composition of a sodium atom. Although it may be described as a metal “monomer” we propose as an alternative model the concept of an “ion pair” of a solvated electron and a sodium cation. In analogy to the 12000 cm⁻¹ transient we propose that the blue species can be described as an associate of a sodium cation with two solvated electrons.     

Unit cell dimensions of some oxofluorometallates of transition metals

Unit cell dimensions of seven oxofluorometallates of transition metals were investigated by powder X-ray diffraction method. The compounds K₃NbO₂F₄ and K₃TaO₂F₄ were found to be isomorphous with cubic (FCC) structure and having lattice parameters 8.885 and 8.942 Å respectively. Similarly, the compounds K₂NbOF₅ (a = 8.367 Å, c = 13.038 Å) and K₂TaOF₅ (a = 8.463 Å, c = 13.139 Å) were also found to be isomorphous with a tetragonal structure. The compound K₃Zr₂O₂F₇ (a = 9.367 Å) was found to possess a cubic (FCC) structure. Both K₂V₂O₅F₂ (a = 6.739 Å, c = 10.635 Å) and K₂VO₃F (a = 5.984 Å, c = 10.914 Å) have a hexagonal structure.     

Assignment of absolute configuration at phosphorus of P-chiral diastereomers of deoxyribonucleoside methanephosphonamidates by means of NMR spectroscopy

Recently, we have prepared a novel class of DNA analogues containing the [3′-NH-P(CH₃)(O)-O-5′] methanephosphonamidate linkage. Synthesis of such analogues requires preparation of the dinucleoside methanephosphonamidates N×N, where N is a 2′-deoxyribonucleoside moiety and × is the methanephosphonamidate linkage. Dimers T×T and C×T were obtained in a non-stereospecific manner giving rise to a pair of P-chiral diastereomers. Such diastereomers were effectively separated into fast and slow migrating ones by means of chromatographic methods (TLC). As described in our previous work (Nawrot et al. Nucleic Acids Res.1998, 26, 2650), the stereochemistry of the phosphorus chiral center of T×T fast migrating diastereomer is R_P and of T×T slow migrating diastereomer is S_P, as established by means of 2D ROESY experiments. Here we describe assignment of the absolute configuration at the phosphorus center of fast and slow migrating diastereomers of C×T dimer. The 2D ROESY sequence with phosphorus decoupling during acquisition used in these measurements allowed observation of the P-Me group as a singlet instead of a ¹H-³¹P-coupled doublet. The apparent advantage of this approach was a much better signal to noise ratio and improved resolution in the F1 dimension. For the fast migrating C×T diastereomer an R_P and for slow migrating C×T diastereomer an S_P configuration was assigned. Conformational analysis of both pairs of diastereomers T×T and C×T indicates significant differences in sugar ring puckering, which strongly depend on the nature of the nucleobase at the 5′-terminus of the dimer. The ribose rings of the 3′-amino-2′,3′-dideoxycytidine moiety of both diastereomers of C×T adopt predominantly a C3′-endo (North) conformation, while thymine-substituted ribofuranoses originating either from C×T or T×T dimers prefer a C2′-endo (South) conformation.     

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.     

The drag of the row of parallel chains of spherical particles in the Stokes flow

The drag of thin-layered porous deposit consisting of dendrites of identical spherical particles with respect to the flow of viscous incompressible liquid is calculated. The deposit is approximated by a model system, a row of parallel chains of particles oriented perpendicular to a flow direction. The expression is derived for the dimensionless drag force acting on the unit chain length as a function of the ratio of a particle radius to a half-distance between chain axes, a/h. It is shown that, at a/h < 0.5, the hydrodynamic equivalent of the chains is the smooth cylinder whose radius is 1.16 times smaller than the particle radius that agrees with the experiment. It is also shown that, at a/h = 1, the drag force of a particle contacting with four adjacent particles in the layer with square packing is equal to F = 44F _St, where F _St is the Stokes drag force of a spherical particle. The pressure drop in this single layer is by 3.5% higher than in the layer of spherical particles with cubic packing. At a/h = 2/√3, drag force F of the particle contacting with six particles in a single layer with hexagonal packing is equal to 340F _St.     

The initial stage of salt passivation of metals

A theoretical model of the nucleation of a passivating salt layer on the surface S of an anode-dissolving metal was developed. The layer is considered a new phase formed by the mechanism of heterogeneous nucleation, which serves as a related substrate for subsequent growth of the passivating layer. According to experiments, the surface S of real metals is inhomogeneous, that is, anode current density fluctuates on S. It is assumed that, at large current fluctuations, localized regions with volume ΔV _ϕ appear, in which the C _M and C _A concentrations of the M ^z+ and A ⁿ⁻ ions are increased. At fairly large current fluctuations, these concentrations can reach saturation. For this reason, the ΔV _ϕ regions are treated as mother phases, in which A _a M _m salt nuclei are formed with a certain probability. The formation of such ΔV _ϕ volumes and the kinetics of formation of nuclei in them, from which a passivating layer with a finite thickness is formed, are considered.     

Effect of Solubilization in Bitemplate Synthesis of a Homologous Series of Organosilica Mesophases and Mesoporous Molecular Sieves

Peculiarities of bitemplate synthesis of a homologous series of organosilica mesophases (precursors of mesoporous molecular sieves of MCM-41 type) were studied, and an influence of a nature of templating substances and a solubilization on a perfection of their spatial structure was found. n-alkyl(C₈–C₁₈)pyridinium halides as solubilizers (micelle-forming surfactants) and monoethanolamides of saturated n-(C₁₀–C₁₆)aliphatic acids as solubilizate were used. Some physicochemical factors promoting a spatial ordering of organosilica mesophases and mesoporous molecular sieves were considered.     

Fluorescence quantification of tetracycline in the presence of quenching matrix effect by means of a four-way model

In this work, a four-way tensor is used to model the quenching effect in fluorescent measurements. By means of the analysis of excitation-emission matrices obtained in the determination of tetracycline in tea, which acts as quencher, it is shown as the impossibility to use a calibration, or an addition standard based on a three-way model. It is analysed the quencher multiplicative effect made on the tetracycline signal by means of an ANOVA. However, by arranging the experimental data in a four-way tensor, it is viable to perform a calibration based on the parallel factor analysis, PARAFAC, decomposition and a four-way partial least squares, 4-PLS, regression to quantify the tetracycline in the presence of the matrix quencher effect. 4-PLS calibration provides better results. In the range from 40 to 220 μg L⁻¹ gives an average of relative errors in absolute value equal to 8.02% in prediction (3.40% in calibration). The repeatability as standard deviation in this range is 5.08 μg L⁻¹ and the method is accurate, slope and intercept being statistically equal to 1 and 0, respectively when a regression calculated versus true concentration is performed. Moreover, it has a decision limit (CCα) of 13.87 μg L⁻¹ for a probability of false positive, α, equal to 0.05 and a capability of detection (CCβ) of 26.63 μg L⁻¹ (for probabilities of false positive, α, false negative, β, equals to 0.05).     

Ketals of -rhamnoheptonolactones: Potential mimics of -rhamnose

Isopropylidene and cyclohexylidene ketals of γ- and δ-heptonolactones derived from Kiliani ascensions of protected L-rhamnose may constitute suitable intermediates for the preparation of mimics of L-rhamnose. The X-ray crystal structure of 7-deoxy-3,4-O-isopropylidene-L-glycero-L-galacto-heptono-1,5-lactone provides an example of a δ-lactone in a boat conformation with a substituent in a flagpole position. The size of ring in the lactones is completely consistent with Hudson's classical lactone rotation rule of 1910.     

Design of an electroactive peptide probe for sensing of a protein

We designed a new electroactive peptide probe that has a molecular recognition function for the sensing of a protein. Ovalbumin (OVA) was the model protein, and when RNRCKGTDVQAW interacted with OVA, it conjugated with a tyrosine-rich peptide (Y₄C). This peptide is electroactive, has a high degree of biocompatibility, and offers the possibility of gene expression. To measure the effect of a number of the tyrosine residues, voltammetric measurements were conducted using a series of tyrosine-rich peptides (Y_nC, n = 3–7) with sensitivities that ranged from 10⁻⁹ to 10⁻⁸ M. The electrode response of Y₅C was the maximum value in the series. However, the peak current did not increase when the number of tyrosine residues was increased in a linear fashion. This may have been due to the micelles that are formed by a tyrosine-rich surfactant peptide. Thus, Y₄C was suitable as an electroactive label for the construction of the peptide probe. The electrode response of Y₄CRNRCKGTDVQAW obtained by a glassy carbon electrode was 100-fold that of tyrosine alone. The measurement of OVA via the peptide probe resulted in a detection on the order of 10⁻¹² M. In contrast, the sensitivity of OVA using RCKGTDVQAWY₄C probe was at the 10⁻¹¹ M level, because the hydrophobic moiety gave it a molecular recognition function. The recoveries of the OVA using Y₄CRNRCKGTDVQAW in a solution containing fetal bovine serum ranged between 98 and 101%. Consequently, the combination of a specific peptide and an electroactive element could be a powerful probe for the sensing of proteins.     

Kinetics of Ergothioneine Inhibition of Mushroom Tyrosinase

The native amino acid ergothioneine, a thiourea derivative of histidine, inhibits mushroom tyrosinase activity in a dose-dependent manner, with an IC₅₀ value of 1.025 mg/ml (4.47 mM). By contrast, histidine exhibited no inhibitory effect on mushroom tyrosinase activity. We characterized ergothioneine as a noncompetitive tyrosinase inhibitor using a Lineweaver–Burk plot of experimental kinetic data. The IC₅₀ value for ergothioneine scavenging of 2,2-diphenyl-1-picrylhydrazyl was 6.110 ± 0.305 mg/ml, much higher than the IC₅₀ for inhibition of tyrosinase activity which indicating ergothioneine on tyrosinase shows a weak correlation to its antioxidative activity. The results demonstrated that ergothioneine has a potent inhibition effect on tyrosinase enzyme activity, resulting from the presence of the sulfur substituted imidazole ring in ergothioneine.     

The structure and magnetic properties of cobalt ferrite nanoparticles synthesized in a system of direct micelles of amphiphiles by means of ion flotoextraction

A method for obtaining nanoparticles in a system of direct micelles of amphiphiles using ion flotoextraction for concentrating ions from diluted water solutions is developed. Nanoparticles of the composition CoFe₂O₄ with a structure close to spinel are obtained. It is established that the particles consist of two components: well-formed phases of cobalt ferrite CoFe₂O₄ and its amorphous analog. It is shown that the nanoparticles have a maximum size distribution ranging from 4 to 6 nm; on a carbon substrate, they self-organize into a periodic colloidal structure. It is concluded that nanosized CoFe₂O₄ powder has a blocking temperature of about 45 K, a magnetic moment of about 0.85–0.9μ_B at a temperature of 0 K, and the Neöl temperature lies in the range of 110 to 250 K.     

Conformational changes of a polyelectrolyte in mixtures of water and acetone

Samples of a polyelectrolyte poly(methacryloylethyl trimethylammonium methylsulfate), PMETMMS, with molar masses M_w = 22−25 × 10⁶ were examined with viscosity, static light scattering, and conductivity measurements in a water–acetone solvent. Because acetone is a nonsolvent for this polymer the measurements were performed to determine the influence of the solvent composition, the polymer concentration, and the presence of added ions on the conformation of the polyelectrolyte in mixed solvents. The possible influence of a hydrodynamic field on the polymer conformation was also studied. The viscosity of the polymer solutions as a function of polymer concentration, as well as of the solvent composition, was studied using a broad range of shear rates. When the mass fraction of acetone in the solvent, γ, is below 0.5, the solutions show a usual polyelectrolyte behavior. When γ ≥ 0.80, the polymer adopts a compact conformation. This is observed as a decrease of the radius of gyration, R_g, second virial coefficient, A₂, the viscosity, and also as a change in the conductivity of the solution. The change in the polymer conformation may be induced also by dilution. When 0.60 ≤ γ < 0.80, a gradual decrease in the polymer concentration leads to a sudden decrease of the reduced viscosity, which indicates a decrease in the particle size. The values of M_w measured by static light scattering were constant in all experiments. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1107–1114, 1998     

Measurement of production date (age) of nanogram amount of uranium

JRC-Karlsruhe obtained a swipe sample from a highly enriched uranium seizure, which had taken place in 2011. Due to the very low amount of uranium (nanograms) a new method needed to be developed to determine the U production date (age). The particles on the swipe were collected on a pyrolytic graphite planchet using a vacuum impactor and they were subsequently leached with ccHNO₃. The “bulk” U isotopic composition (²³⁵U: 72.51?±?0.03 wt%) and the production date (December 1992?±?1 year) determined by MC-ICP-MS indicated that the material showed similarity with two other HEU cases seized earlier in Europe.

     

Dense-gas fractionation of mixtures of petroleum macromolecules

Dense-gas extraction (DGE) was used to fractionate a petroleum pitch feedstock of broad molecular weight distribution (MWD) into selected oligomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (T_c = 318.6 °C, P_c = 41.1 bar) and a pitch charge of 10–15 g. A packed column with reflux produced the equivalent of several equilibrium stages. The apparatus was operated at near and supercritical temperatures, with the stillpot at 320 °C and the column temperature increasing linearly up to the reflux condenser temperature of 360 °C. Operating pressures were determined by the conditions required to extract a given oligomer and varied from 44 to 84 bar. Using a monomer-rich petroleum pitch feed (MW_n = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MW_n = 521, PDI = 1.07) was obtained, with the remaining residue (MW_n = 883, PDI = 1.24) consisting of 95% trimer and higher oligomers. MALDI-TOF mass spectrometry was used to determine the MWD of the feed pitch and pitch fractions, with a unique matrix being used to enhance the response of higher MW species. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels.     

Effect of fluorination: Conformation of 2,6-difluoroanisole

The geometric structure and conformation of 2,6-difluoroanisole, 2,6-C₆H₃F₂OCH₃, has been determined with gas electron diffraction (GED) and quantum chemical calculations (MP2 and B3LYP with 6-311++G(2df,pd) basis sets). The GED intensities have been analyzed with a static as well as with a dynamic model. The static model resulted in a structure with near-perpendicular orientation of the O-CH₃ bond with a thermal average dihedral angle τ(C2C1-OC) = 70 ± 3°. With the dynamic model a perpendicular equilibrium structure and a potential function for internal rotation around the C-O bond with a very flat minimum at τ(C2C1-OC) = 90° and with a barrier of 1.8 ± 0.6 kcal/mol at τ(C2C1-OC) = 0° was derived. Quantum chemical calculations predict double-minimum potential functions for the electronic energies, which become single-minimum potentials upon addition of zero-point vibrational energies. The change of conformation from planar in anisole to perpendicular in 2,6-difluoroanisole has been rationalized by orbital interactions between the oxygen electron lone pairs and benzene ring orbitals and by steric repulsion.