Stereochemistry of seven-membered heterocycles: XLII. A theoretical study of stereochemistry of H complexes formed by conformationally nonuniform 2-R-1,3-dioxacyclohept-5-enes with some proton donors

According to semiempirical AM1 calculations, the stability of the boat and twist forms of 2-R-1,3-dioxacyclohept-5-enes depends on the size of substituents at the acetal carbon atom. The twist form gives diastereomeric H complexes with chloroform and methanol of the cis and trans structure, containing monocentered hydrogen bonds, whereas the chair conformation preferably forms complexes with a two-centered hydrogen bond. Based on theoretical data (ΓOH, ΔH, geometry of complexes), the specific features of H complexes of the conformers in electrophilic addition and cycloaddition were revealed. Considerable preferableness of the exo attack of the diastereotopic double bond in the H complex having the chair form is due to the steric accessibility of the exo side, whereas in the complexes of the twist form the facial selectivity is appreciably lower

Stereochemistry of seven-membered heterocycles: XLII. A theoretical study of stereochemistry of H complexes formed by conformationally nonuniform 2-R-1,3-dioxacyclohept-5-enes with some proton donors

According to semiempirical AM1 calculations, the stability of the boat and twist forms of 2-R-1,3-dioxacyclohept-5-enes depends on the size of substituents at the acetal carbon atom. The twist form gives diastereomeric H complexes with chloroform and methanol of the cis and trans structure, containing monocentered hydrogen bonds, whereas the chair conformation preferably forms complexes with a two-centered hydrogen bond. Based on theoretical data (ΓOH, ΔH, geometry of complexes), the specific features of H complexes of the conformers in electrophilic addition and cycloaddition were revealed. Considerable preferableness of the exo attack of the diastereotopic double bond in the H complex having the chair form is due to the steric accessibility of the exo side, whereas in the complexes of the twist form the facial selectivity is appreciably lower

Methylviologen-mediated electrochemical synthesis of platinum nanoparticles in solution bulk

© 2017, Pleiades Publishing, Ltd.Platinum nanoparticles (PtNPs) are synthesized by methylviologen-mediated reduction of PtCl2 at the potentials of the MV2+/MV•+ redox couple in 40% aqueous DMF solution. In the absence of stabilizing agents and in the presence of a stabilizer in the form of spherical silica NPs or alkylamine-modified silica NPs (SiO2-NHR), a part of PtNPs (14–18%) are deposited on the electrode while the rest of particles remain in solution to form coarse aggregates which precipitate. In the latter case, PtNPs are also partly bound to form individual ultrafine NPs (3 ± 2 nm) on the SiO2-NHR surface. In the presence of polyvinylpyrrolidone (PVP), the generated PtNPs (18 ± 9 nm) neither aggregate nor deposit on the cathode but are completely stabilized in solution being encapsulated within the PVP matrix. The obtained PtNPs are characterized by the methods of dynamic light-scattering and electron microscopy

Atmospheric waves disturbances from the solar terminator according to the VLF radio stations data

The perturbations from the solar terminator in the range of acoustic-gravity waves (AGWs) periods from 5 min to 1 h were analysed with the use of measurements of VLF radio signals amplitudes on the European radio path GQD–A118 (Great Britain–France). These observations provide information on the propagation of waves at altitudes near the mesopause ( 90 km), where VLF radio waves are reflected. On the considered radio path a systematic increase in fluctuations in the amplitudes of radio waves was observed within a few hours after the passage of the evening terminator. For April, June, October 2020 and February 2021 events, the distribution of the number of wave perturbations with large amplitudes over AGWs time periods has been studied. Our results show that the evening terminator for different seasons is dominated by waves in the range of periods of 15–20 min. The amplitudes of the AGWs from the terminator at the heights of the mesosphere (fluctuations in the concentration of neutral particles, velocity components and vertical displacement of the volume element) are approximately determined by the fluctuations of the amplitudes of the VLF radio signals. The amplitudes of the AGWs on the terminator are 12–14% in relative concentration fluctuations, which correspond to the vertical displacement of the atmospheric gas volume of 1.1–1.3 km. Based on the analysis of the AGW energy balance equation, it was concluded that the waves predominantly propagate in a quasi-horizontal direction at the terminator. The possibility of studying the long-term changes in the mesosphere parameters using fluctuations in the amplitudes of VLF radio waves at the terminator is shown

Universal energy distribution for interfaces in a random field environment

We study the energy distribution function $\rho (E)$ for interfaces in a random field environment at zero temperature by summing the leading terms in the perturbation expansion of $\rho (E)$ in powers of the disorder strength, and by taking into account the non perturbational effects of the disorder using the functional renormalization group. We have found that the average and the variance of the energy for one-dimensional interface of length $L$ behave as, $_{R}\propto L\ln L$, $\Delta E_{R}\propto L$, while the distribution function of the energy tends for large $L$ to the Gumbel distribution of the extreme value statistics.Comment: 4 pages, 2 figures, revtex4; the distribution function of the total and the disorder energy is include

Methylviologen mediated electrosynthesis of gold nanoparticles in the solution bulk

© 2015 The Royal Society of Chemistry. Electrosynthesis of gold nanoparticles (AuNp) was carried out by methylviologen mediated reduction of Au(i) at potentials of the MV2+/MV+ redox couple in water/0.1 M NaCl medium, in the absence and in the presence of stabilizers. In all the cases, AuNp are formed in the solution bulk and are not deposited on the cathode. In the absence of stabilizers, AuNp (14-100 nm) coalesce to give aggregates of various shapes that eventually form a deposit. Sonication reversibly destructs the deposit into nanoparticles. In the presence of alkylamino-modified silicate nanoparticles (SiO2-NHR, 120-160 nm), spherical AuNp (≤20 nm) are bound as inclusions in the SiO2-NHR surface layer. Polyvinylpyrrolidone (40 000 D) stabilizes spherical AuNp with a mean diameter of 5-14 nm. All the particles were characterized by electron microscopy methods (SEM, STEM) and X-ray powder diffraction (XRPD)