Geometrical features of hydrogen bonded complexes involving sterically hindered pyridines

The ability of strongly sterically hindered pyridines to form hydrogen bonded complexes was inspected using low-temperature 1H and 15N NMR spectroscopy in a liquefied Freon mixture. The proton acceptors were 2,6-di(tert-butyl)-4-methyl- and 2,6-di(tert-butyl)-4-diethylaminopyridine; the proton donors were hydrogen tetrafluoroborate, hydrogen chloride, and hydrogen fluoride. The presence of the tert-butyl groups in the ortho positions dramatically perturbed the geometry of the forming hydrogen bonds. As revealed by experiment, the studied crowded pyridines could form hydrogen bonded complexes with proton donors exclusively through their protonation. Even the strongest small proton acceptor, anion F-, could not be received by the protonated base. Instead, the simplest hydrogen bonded complex involved the [FHF]- anion. This complex was characterized by the shortest possible N...F distance of about 2.8 A. Because the ortho tert-butyl groups did not prevent the hydrogen bond interaction between the protonated center and the anion completely, an increase of the pyridine basicity caused a further shortening of the N-H distance and a weakening of the hydrogen bond to the counterion.     

Reduction of carbon monoxide by molybdenum-containing hydroxide systems

The reduction of CO by the nitrogen-fixing systems Ti(OH)₃−Mo(OH)₃ and MgTi₂O₄−Mo(OH)₃ was studied in aqueous and water-methanol media; in the latter,¹⁴CO was used as the reagent. The main reaction product is methanol, whose yield in the H₂O−MeOH−KOH mixture is almost an, order of magnitude higher than that in an aqueous alkaline solution. The data obtained were compared to those for the reduction of N₂ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10 pp. 2081–2083, October, 1998.     

An important role of intramolecular free radical reactions in antimalarial activity of artemisinin and its analogs

The kinetic schemes of intramolecular reactions of five analogs of artemisinin were built. The method of intersecting parabolas was used for the calculation of activation energies and rate constants of each elementary step of these schemes. The competition between monomolecular and bimolecular free radicals was taken into account. It was evidenced that the intramolecular oxidation of these compounds proceeds as a cascade of consecutive free radical reactions with the formation of hydroperoxide groups. The latter decompose via reactions with the Fe(II) complexes generating free radicals. Among the radicals formed, the hydroxyl radical was proved to play the key role. A correlation between the yield of hydroxyl radicals n(OH) and antimalarial activity of compounds (IC(50)) was observed. The dependence of index IC(50) on n(OH) is linear in the logarithmic coordinates: ln[IC(50)(Artemisinin)/IC(50)(Compound)] = -14.10 + 3.85 ×n(OH). The proposed scheme explains and demonstrates a strong dependence of the antimalarial effectiveness of a drug on the chemical structure.     

Synthesis and investigation of the heat capacity of Sm<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> in the 346–1050 K range

Optimal conditions for the synthesis of Sm₂Sn₂O₇ with pyrochlore crystal structure by solid-phase reactions were determined. The effect of temperature (346–1050 K) on the molar heat capacity of samarium stannate was studied by differential scanning calorimetry. Thermodynamic properties of Sm₂Sn₂O₇ in the temperature range under study were determined using the experimental data.     

Synthesis and study of high-temperature heat capacity of erbium orthovanadate

Orthovanadate ErVO₄ has been prepared by solid-phase synthesis from a stoichiometric mixture of high pure V₂O₅ and chemically pure Er₂O₃ by multistage calcination in air in the temperature range 873–1273 K. The effect of temperature (380–1000 K) on the heat capacity of orthovanadate ErVO₄ was studied by hightemperature calorimetry. Thermodynamic properties of erbium orthovanadate (enthalpy change H°(T)–H°(380 K), entropy change S°(T)–S°(380 K), and reduced Gibbs energy Φ°(T)) have been calculated from the experimental C_p = f(T) data. It has been shown that the specific heat varies in a row of oxides and orthovanadates of Gd-Lu naturally depending on the radius of the R³⁺ ion within the third and fourth tetrads.     

The ν(OH/OD) band shape of strong hydrogen bonded dimers of phosphinic acids. Phenomenology and formation models

The infrared spectra of phosphinic acid R₂POOH dimers (R=CH₃, CH₂Cl, C₆H₅) have been studied in CCl₄ and CH₂Cl₂ solutions (T=300 K). The infrared spectra of deuterated R₂POOD dimers (R=CH₃, CH₂Cl) were also studied in the gas phase (T=400–550 K) and solid state (T=100–300 K). They are compared with previously studied spectra of the light (non-deuterated) dimers in the gas phase, in the solid state and in low-temperature argon matrices (T=12–30 K) in the 4000–400 cm⁻¹ spectral region. It is found that the strong and broad ν(OH) dimer bands have similar shapes, nearly equal values of bandwidth and low-frequency shift, and possess the Hadzi ABC structure irrespective of the type of acid, significant differences of dimerization enthalpies, influence of solvent, the type of H-bonded complexes (cyclic dimers in the gas phase, in solutions, and in inert matrices, and infinite chains in the solid state), and temperature in the range 12–600 K. Isotopic ratio of the first moments of light and deuterated acid bands has been measured. Analysis of the ν(OH/OD) band of hydrogen bonded dimers of phosphinic acids shows that the interaction between the two intermolecular bonds O–HOP in a cyclic complex plays virtually no role in the mechanism of the ν(OH/OD) band formation; the shape of ν(OH/OD) band is controlled mainly by the POOH(D)O fragment; and the band shape of strong hydrogen bonded complexes is formed by a number of vibrational transitions from the ground state to different combination levels in the region 3500–1500 cm⁻¹.     

Schrödinger operators and associated hyperbolic pencils

For a large class of Schrödinger operators, we introduce the hyperbolic quadratic pencils by making the coupling constant dependent on the energy in the very special way. For these pencils, many problems of scattering theory are significantly easier to study. Then, we give some applications to the original Schrödinger operators including one-dimensional Schrödinger operators with L²-operator-valued potentials, multidimensional Schrödinger operators with slowly decaying potentials.     

RF Analysis of ITER Remote Steering Antenna for Electron-Cyclotron Plasma Heating

An application of remote RF beam steering concept, based on image multiplication phenomena in a corrugated square waveguide, to electron-cyclotron plasma heating and current drive for ITER has been considered. In the present paper high attention is paid to studies of reliability of the remote steering capability in realistic waveguide condition. Different factors, could limit efficiency (RF breakdown, random technological spread of parameters, and heating of system), are simulated. Polarization properties of the remote steering antenna, arising due to non-ideal corrugation, are investigated. It is suggested a new configuration of antenna to be not sensible to corrugation parameters. RF analysis of the system, supplemented by dog-legs and a vacuum window, is also presented. A new way to use beating wave transmission in the remote steering waveguide is suggested, in order to reduce losses caused by mitre bends.     

Verifiable post-Maxwellian effect of the nonlinear electrodynamics in vacuum

The lasing frequencies of a ring laser with counterpropagating electromagnetic waves are calculated for the case when a part of the laser circuit is subjected to a constant electromagnetic field. It is demonstrated that, according to equations of the parametrized post-Maxwellian electrodynamics of vacuum, two waves with different planes of polarization and frequencies should propagate in each direction of the ring laser. The analysis has shown that, with the help of advanced ring lasers and external electromagnetic fields attainable in the laboratory, the post-Maxwellian parameters can be measured with a precision that would be sufficient to verify the most popular theoretical models of the nonlinear electrodynamics of vacuum.