The effect of substituents in macrocyclic polyethers containing aromatic rings on ion-selective electrode properties

Ion-selective electrodes, containing derivatives of crown ethers with aromatic rings, have been investigated. The electrodes were characterized by determining their detection limits and selectivity coefficients. Thet-butyl derivative of naphtho-15-crown-5 was found to exhibit the best properties (L _d =1×10^–5,K _Na/K=6×10^–4).     

Theoretical study of homogeneous electron self-exchange in polar aromatic molecules: the role of the reactant structure

The solvent reorganization energies λ₀ of the electron self-exchange reaction between neutral molecules (nitrobenzene, its five para-substituted derivatives and benzonitrile) and the corresponding radical anions are discussed in terms of the Kirkwood approach combined with a semi-empirical calculation of the reactant structure (AM1 and intermediate neglect of differential overlap (INDO) methods). The theoretical values of λ₀ are compared with the experimental reorganization energies λ calculated from the Marcus equation and the homogeneous rate constants k_ex. It is found that the values of λ₀ obtained using the INDO method are approximately equal to the experimental λ, whereas the AM1 method gives poor results for para-substituted nitrobenzenes. The linear correlation of log k_ex with a_N² where a_N is a nitrogen coupling constant, established for para-substituted nitrobenzenes is interpreted. A simple criterion for slow electron transfer of an arbitrary flat polar arene is formulated in terms of the reactant structure and the model of the “specifying group” suggested earlier by some of the authors.     

Multiple-scattering approach to Sn L3-edge X-ray absorption near-edge structure (XANES) analyses for organic tin compounds

We apply multiple-scattering calculations to the analyses of Sn L₃-edge X-ray absorption near-edge structure (XANES) spectra for environmental organotin compounds such as SnCl_4−nMe_n, SnCl_4−nBt_n, and SnCl_4−nPh_n (n = 0–4) where Me = CH₃, Bt = C₄H₉, and Ph = C₆H₅. The XANES peak at 3960 eV has rich information on the local structure. Referring to the optimized structures by density functional theory (DFT) calculations, multiple-scattering calculations well explain the observed spectral changes for different “organic extents”. The present study also supports the widely-used semiempirical rule called ‘Natoli’s rule’ for these environmental compounds, which will be useful to use XANES spectra for the practical analytical tools.     

Effect of solvent on the methanol JHCOH value

³J_HCOH, which depends on solvent dielectric constant (ε), has been calculated by the INDO MO finite perturbation method incorporated with “Solvaton Theory”. The calculation results agree with the experimental finding that ³J_HCOH decreases with increase of ε.The respective contributions of conformational change and dielectric constant variation to the effect of solvent on experimental values of ³J_HCOH are discussed separately because ³J_HCOH, the vicinal coupling constant, is known to be sensitive to conformational change. It is concluded that ε contributes more to the change of the experimental value of ³J_HCOH by solvents.     

Investigation of the interactions between ginsenosides and amino acids by mass spectrometry and theoretical chemistry

In order to evaluate the essence of the interactions of ginsenosides and proteins which are composed by α-amino acids, electrospray ionization mass spectrometry was employed to study the noncovalent interactions between ginsenosides (Rb₂, Rb₃, Re, Rg₁ and Rh₁) and 18 kinds of α-amino acids (Asp, Glu, Asn, Phe, Gln, Thr, Ser, Met, Trp, Val, Gly, Ile, Ala, Leu, Pro, His, Lys and Arg). The 1:1 and 2:1 noncovalent complexes of ginsenosides and amino acids were observed in the mass spectra. The dissociation constants for the noncovalent complexes were directly calculated based on peak intensities of ginsenosides and the noncovalent complexes in the mass spectra. Based on the dissociation constants, it can be concluded that the acidic and the basic amino acids, Asp, Glu, Lys and Arg, bound to ginsenosides more strongly than other amino acids. The experimental results were verified by theoretical calculations of parameters of noncovalent interaction between ginsenoside Re and Arg which served as a representative example. Two kinds of binding forms, “head–tail” (“H–T”) and “head–head” (“H–H”), were proposed to explain the interaction between ginsenosides and amino acids. And the interaction in “H–T” form was stronger than that in “H–H” form.     

Effect of Stray Light on Characteristic Mass in Zeeman Graphite Furnace Atomic Absorption Spectrometry

The characteristic mass (m₀), the effective stray light (α), the Zeeman rollover absorbance (A_r), and the Zeeman sensitivity ratio (R) were studied systematically as a function of lamp current and slit width. The effective stray light, α, was considered to be the sum of all effects that either are different sources of stray light or behave like stray light. Mathematical expressions are presented to describe the relationships between m₀, R, α, and A_r. The characteristic mass, m₀ is directly related to the effective stray light and the Zeeman sensitivity ratio. R. The Zeeman rollover absorbance, A_r, is a quantitative measure of the effective stray light. By taking α and R into account, a stable characteristic mass value, ^cm₀ was obtained for copper, silver, thallium, and chromium for different lamp currents and slit widths. This gave rise to a new concept "the corrected characteristic mass." Without correction for these effects, the relative standard deviations (RSDs) of measured characteristic mass values, m₀, were 8% for Cr, 19% for Tl, 20% for Cu, and 26% for Ag, for a wide range of currents and/or slit widths. After correction for the effects of rs and R, the corrected characteristic masses, ^cm₀, were stable with an RSD of only 4% for Cu, Cr, and TI, and 7% for Ag.     

A rapid and sensitive LC–MS/MS method for analysis of iguratimod in human plasma: Application to a pharmacokinetic study in Chinese healthy volunteers

A rapid, sensitive and reproducible LC–MS/MS method was developed and validated to determine iguratimod in human plasma. Sample preparation was achieved by protein precipitation with acetonitrile. Chromatographic separation was operated on an Ultimate® XB‐C₁₈ column (2.1 × 50 mm, 3.5 μm, Welch) with a flow rate of 0.400 mL/min, using a gradient elution with acetonitrile and water which contained 2 mm ammonium acetate and 0.1% formic acid as the mobile phase. The detection was performed on a Triple Quad™ 5500 mass spectrometer coupled with an electrospray ionization interface under positive‐ion multiple reaction monitoring mode with the transition ion pairs of m/z 375.2 → 347.1 for iguratimod and m/z 244.3 → 185.0 for agomelatine (the internal standard), respectively. The method was linear over the range of 5.00–1500 ng/mL with correlation coefficients ≥0.9978. The accuracy and precision of intra‐ and inter‐day, dilution accuracy, recovery and stability of the method were all within the acceptable limits and no matrix effect or carryover was observed. As a result, the main pharmacokinetic parameters of iguratimod were as follows: C_max, 1074 ± 373 ng/mL; AUC_0–72, 13591 ± 4557 ng h/mL; AUC_0–∞, 13,712 ± 4613 ng h/mL; T_max, 3.29 ± 1.23 h; and t_1/2, 8.89 ± 1.23 h.     

Azasteroids. Synthesis by Diels-Alder Reaction between Maleimides, Citraconimide, and Triazolindiones and 1-(1-Trialkylsiloxyvinyl)-3,4-dihydronaphthalene Derivatives

Summary. 18-Nor-16-azaestrane derivatives with 8β, 13β, and 14β orientation were isolated from Diels-Alder reactions between maleimides or citraconimide and 1-(1-siloxyvinyl)naphthalene derivatives. (8RS)-13,14,16-Triazaestrane derivatives were synthesized from 1,2,4-triazolin-3,5-diones. The parent 11-oxo derivatives were obtained by desilylation, and they were transformed into 11-hydroxyimino derivatives. 3-Hydroxy derivatives, finally were synthesized by cleavage of the 3-methoxy group with BBr₃. During these transformations the stereochemistry of the steroidal skeleton was not changed. The stereochemistry of these “unnatural” steroids was elucidated by spectroscopic methods, and compared with results from calculations, and with the configuration of natural estrane derivatives. Finally, an improved method for the synthesis of the starting material, 6-methoxy-1-[(1-trialkylsiloxy)-vinyl]-3,4-dihydronaphthalene was developed.     

Kinetics and mechanism of “reduced CO2” electrooxidation on electrodispersed platinum in different acid solutions

The electrooxidation of “reduced CO₂” electroadsorbates on electrodispersed platinum electrodes has been investigated in 0.05 M HClO₄, 1 M HClO₄, 0.5 M H₂SO₄ and 1 M H₃PO₄ at 25° C through voltammetry and potential step techniques. The overall reaction comprises three distinguishable processes, namely a first order triggering process, and two second order surface processes. The latter are influenced remarkably by the solution composition (anions). The second order reaction mechanism involves two distinguishable “ reduced CO₂” electroadsorbates which react independently with the OH species formed from H₂O electrooxidation on the bare platinum sites as the reaction proceeds. An interaction term has to be included in the rate equations to account for the experimental results. The mechanistic interpretation accounts for the values of the number of electrons per site ranging from 1 to 2.     

Comparison between Theoretical and Experimental Deformation Density of 1,3,5,7-Tetra-t-Butyl-S-Indacene

1,3,5,7-Tetra-t-butyl-s-indacene is a twelve-membered fused-ring compound; the stabilization of the compound by bulky t-butyl groups is discussed. The distribution of bonding electron density is depicted in terms of deformation density using the experimental X-X method, a multipole expansion model and a calculation according to the extended Hückel molecular orbital method (EHMO). The molecules crystallize according to the space group P2₁/n with cell parameters a = 9.700(3), b = 11.746(3), c = 10.858(2)Å, β = 107.67(2)°, Z=2 at 100K. The molecule has a center of inversion( ) and belongs to the symmetry point group C_i. The packing of two unique t-butyl groups in the asymmetric unit appears significantly different. However, the map of deformation density in the plane of the ring shows pseudo D_2h symmetry. The accumulation of density at the midpoint of the bonds is observed as expected. The theoretical deformation densities agree with the experimental ones. Analyses of the molecular-orbital wavefunctions provide a good illustration of the aromatic π-electron system. A theoretical study of a series of substituents (-H, - CH_3, -t-butyl ) on s-indacene illustrates the stabilization effect of the t-butyl group on the 12-π-electron ring system.     

Polarographic Determination of Famotidine through Complexation with Nickel(II) Chloride

The present investigation describes the development of a sensitive, rapid polarographic method for the determination of famotidine in pure form and in certain dosage forms. The proposed method depends upon studying the polarographic activity of Nickel(II)‐famotidine complex in Britton Robinson buffer over the pH range 4–8 and its usefulness in the analysis of famotidine using direct current (DC_t), differential pulse (DPP), and alternating current (AC_t) polarography. The different experimental parameters affecting the cathodic waves were carefully investigated and optimized. Moreover, to check the validity of the proposed method, the standard addition method was applied by adding famotidine to the previously analyzed tablets. The recovery of the drug was calculated by comparing the concentration obtained from the spiked mixtures with those of the pure drug. The results of analysis of commercial tablets and the recovery study suggested that there is no interference from any excipients, which are present in tablets. Statistical comparison of the results was performed with regard to accuracy and precision using student's t‐test and F‐ratio at 95% confidence level. There is no significant difference between the comparison and proposed method with regard to accuracy and precision.     

A thermodynamic interpretation of the behavior of higher fatty alcohols and acids upon the chromatographic paper

An idea was presented of treating the chromatographed substance as a “solute,” and the chromatographic system, composed of the stationary and the mobile phase as a “solvent.” Moreover the concept of “local equilibrium” was introduced, allowing to regard a given chromatographic spot as a “binary solution.” Thus a possibility arose to apply the classical thermodynamic approach, normally used for binary solutions, and namely: μ_i = μ_i + RT ln x_iƒ_i, where μ_i—chemical potential of the “i”-th compound in the solution, μ_i—chemical potential of the pure “i”-th compound, x_i-molar fraction of the “i”-th compound, ƒ_i—its activity coefficient, in a modified form, suitable for the chromatographic purpose.     

Estimating the segregation strength of microphase‐separated diblock copolymers from the interfacial width

The ever‐growing catalog of monomers being incorporated into block polymers affords exceptional control over phase behavior and nanoscale structure. The segregation strength, χN, is the fundamental link between the molecular‐level detail and the thermodynamics. However, predicting phase behavior mandates at least one experimental measurement of χN for each pair of blocks. This typically requires access to the disordered state. We describe a method for estimating χN from small‐angle X‐ray scattering measurements of the interfacial width between lamellar microdomains, t_x, in the microphase‐separated melt. The segregation strength is determined by comparing t_x to self‐consistent field theory calculations of the intrinsic interfacial width, t_i, as a function of the mean‐field χN. The method is validated using a series of independent experimental measurements of t_x and χN, measured via the order–disorder transition temperature, T_ODT. The average absolute relative difference between χN calculated from t_x and the value calculated from T_ODT is a modest 11%. Corrections for nonplanarity of the interfaces are investigated but do not improve the agreement between the experiments and theory. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. J. Polym. Sci., Part B: Polym. Phys. 2019 © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 932–940     

N—H bond dissociation energies in <Emphasis Type="Italic">N,N</Emphasis>′-diphenyl-1,4-phenylenediamine and its aminyl radical

The N—H bond dissociation energy (D _NH) in the 4-anilinodiphenylaminyl radical formed from N,N-diphenyl-1,4-phenylenediamine was experimentally determined and calculated by the quantum-chemical method. The experimental D _NH value was found from the enthalpy of the reaction of N,N-diphenyl-1,4-benzoquinonediimine with 4-hydroxydiphenylamine taking into account the bond dissociation energies in 4-hydroxydiphenylamine and its aminyl and phenoxyl radicals, which were determined by the intersecting parabolas method from the kinetic data. The quantum-chemical calculations of D _NH used several semiempirical methods by the MOPAC program and the ab initio and DFT methods by the GAUSSIAN 94/98 program. The D _NH values, which were closest to the experimental values, were obtained by the B3LYP/6-31+G* method. The results of quantum-chemical calculations of the N—H and O—H bond dissociation energies in 4-hydroxydiphenylamine and its radicals are presented.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1549–1554, August, 2004.     

Squeeze-out from mixed monolayers of dipalmitoylphosphatidylcholine and egg phosphatidylglycerol

Plots of surface pressuer (π) vs surface area (A) are taken from mixed monolayers of dipalmitoylphosphatidylcholine (DPPC) and “egg” phosphatidylglycerol (egg PG) with the concentration of egg PG (X) ranging from 0 to 100%, the temperatures (T) from 37 to 41°C, and the compression rate (dA/dt) from −13.6 to −688 mm²s⁻, and at a relative humidity of over 90%. Between limiting values of X, T, and dA/dt the π-A plots of compression show regions of nearly constant π (plateaus) which start at values of π (π_pb) of 48 ± 2 mN m⁻¹, and which can be followed by an increase in π until collapse occurs at 70 mN m⁻¹. π_pb is independent of X, T, and dA/dt. The plateau is correlated with a loss of molecules from the monolayer, which increases strongly with X and T and decreases with dA/dt. The quantitative results are not in agreement with separate collapse of the components. A model is presented stating that plateaus occur when the transition from the liquid-expanded (LE) to the liquid-condensed phase (LC) is incomplete at π_pb. During plateau formation a mixed LE phase is squeezed out at its collapse pressure. The remaining LC phase can be compressed to 70 mN m⁻¹. Results of calculations based on this model are in agreement with the experimental results and predict slight enrichment of the squeezed-out phase with respect to egg PG.     

Unusual Disordering of Potassium Ions in the Structures of Cluster Rhenium Thiohalides K3[Re6S7Br7] and K4[Re6S8Cl6]

New cluster compounds — rhenium and potassium thiohalides K₃Re₆S₇Br₇ (I) and K₄Re₆S₈Cl₆ (II) — have been synthesized. Their crystal structures have been determined by single crystal X-ray diffraction. The compounds are monoclinic; (I): space group P2₁/c, a = 9.32(1) Å, b = 13.528 Å, c = 12.413 Å, β = 110.21°, Z = 2, R = 0.038; (II): space group C2/m, a = 10.614 Å, b = 17.268 Å, c = 10.448 Å, β = 110.755°, Z = 2, R = 0.042. In both structures, the potassium ions are considerably distorted. The occupancies of the potassium sites are 0.17-0.34 (I) and 0. 01-0.26 (II), correlating well with the coordination numbers (c.n. 7-10 and 2-7 for I and II, respectively). In I, adjacent positions of potassium atoms are aggregated into discrete tetrahedral and angular clusters; in II, the individual (four-and six-membered) cyclic clusters of potassium sites are present along with bent chains of vertex-and edge-sharing tetrahedral “potassium clusters.” The shortest K-K distances in these “clusters” vary from 1.31 Å to 1.54 Å (I) and from 0.66 Å to 1.65 Å (II). The “instability” of the potassium site suggests that I and II are ion conductors.Original Russian Text Copyright © 2004 by S. F. Solodovnikov, S. S. Yarovoi, Yu. V. Mironov, A. V. Virovets, and V. E. Fedorov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 909–917, September–October, 2004.     

Stability-Indicating Chromatographic Methods for Simultaneous Determination of Mosapride and Pantoprazole in Pharmaceutical Dosage Form and Plasma Samples

Simple, sensitive, selective, precise, and stability-indicating thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) methods for the determination of mosapride and pantoprazole in pharmaceutical tablets were developed and validated as per the International Conference on Harmonization guidelines. The TLC method employs aluminum TLC plates precoated with silica gel 60F₂₅₄ as the stationary phase and ethyl acetate/methanol/toluene (4:1:2, v/v/v) as the mobile phase to give compact spots for mosapride (R _f 0.73) and pantoprazole (R _f 0.45) separated from their degradation products; the chromatogram was scanned at 276 nm. The HPLC method utilizes a C18 column and a mobile phase consisting of acetonitrile/methanol/20 mM ammonium acetate (4:2:4, v/v/v) at a flow rate of 1.0 mL min⁻¹ for the separation of mosapride (t _R 11.4) and pantoprazole (t _R 4.4) from their degradation products. Quantitation was achieved with UV detection at 280 nm. The same HPLC method was successfully used in performing calibrations in lower concentration ranges for both drugs in human plasma using ezetimibe as internal standard. The methods were validated in terms of accuracy, precision, linearity, limits of detection, and limits of quantification. Mosapride and pantoprazole were exposed to acid hydrolysis and then analyzed by the proposed methods. As the methods could effectively separate the drugs from their degradation products, these techniques can be employed as stability-indicating methods that have been successively applied to pharmaceutical formulations without interference from the excipients. Moreover the HPLC method was successfully used in the determination of both drugs in spiked human plasma.

     

Joint X-ray spectroscopic and quantum-chemical study of the electronic structure of pentafluorophenylalkyl ethers

The high resolution X-ray emission O-Kα spectra of pentafluorophenylalkyl ethers C₆F₅OR (R=Et, Prⁱ, and Bu^t) exhibit differences related to a change in the electronic structure of the compounds as R is varied. The search for stable conformers was performed by the semiempirical PM3 method. The most probable structures of C₆F₅OR were determined by the comparison of the experimental and theoretical X-ray spectra plotted for each conformer usingab initio calculations in the 6–31 G basis set. Substituent R in pentafluorophenylalkyl ethers is situated outside of the ring plane. The fluorination of the benzene ring changes the energy level of the lone electron pair of oxygen relative to the levels of orbitals of the ring and substituent R and leads to an increase in the efficiency of interactions in the σ-system. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1998.     

Original close-packed structure and magnetic properties of the Pb4Mn9O20 manganite

The crystal structure of the Pb₄Mn₉O₂₀ compound (previously known as “Pb_0.43MnO_2.18”) was solved from powder X-ray diffraction, electron diffraction, and high resolution electron microscopy data (S.G. Pnma, a=13.8888(2) Å, b=11.2665(2) Å, c=9.9867(1) Å, R_I=0.016, R_P=0.047). The structure is based on a 6H (cch)₂ close packing of pure oxygen “h”-type (O₁₆) layers alternating with mixed “c”-type (Pb₄O₁₂) layers. The Mn atoms occupy octahedral interstices formed by the oxygen atoms of the close-packed layers. The MnO₆ octahedra share edges within the layers, whereas the octahedra in neighboring layers are linked through corner sharing. The relationship with the closely related Pb₃Mn₇O₁₅ structure is discussed. Magnetization measurements reveal a peculiar magnetic behavior with a phase transition at 52 K, a small net magnetization below the transition temperature, and a tendency towards spin freezing.