Effect of the structure of nitroxyl radicals on the kinetics of their acid‐catalyzed disproportionation

Acid‐catalyzed disproportionation of cyclic nitroxyl radicals R₂NO^? includes the half‐reactions of their oxidation to oxoammonium cations R₂NO⁺ and reduction to hydroxylamines R₂NOH. For many nitroxyl radicals, this reaction is characterized by its ~100% reversibility. Quantitative characteristics of acid–base and redox properties of the whole redox triad may be obtained from research of kinetics and equilibrium of this reaction. Here, we have examined the kinetics for the disproportionation of twenty piperidine‐, pyrroline‐, pyrrolidine‐, and imidazoline nitroxyl radicals in aqueous H₂SO₄, and interpreted it in terms of the excess acidity function X. The rate‐limiting step of this reaction is R₂NO^? oxidation by its protonated counterpart R₂NOH^+?. Kinetic stability of R₂NO^? in acidic media depends on the basicity of nitroxyl group. This basicity is influenced predominantly by protonation of another, more basic group in radical structure, and its proximity to nitroxyl group. The discovered estimates of pK values for radical cations R₂NOH^+? (from ?5.8 to ?12.0) indicate a very low basicity of nitroxyl groups in all commonly used R₂NO^?. For the first time, a linear correlation is obtained between the one‐electron reduction potentials of oxoammonium cations and the basicity of nitroxyl groups of related radicals. Copyright © 2013 John Wiley & Sons, Ltd.     

Use of Huygens’ principle for analysis and synthesis of the fields in oversized waveguides

We develop a method based on Huygens’ principle for calculation of the traveling-wave field in a cylindrical oversized waveguide with smooth and shallow wall corrugations with allowance for diffraction by the nonsymmetric end of the waveguide. Algorithms for synthesizing the specified field distribution on the surface of such a waveguide are proposed. The performed experiments confirm the adequacy of this approach for calculation of oversized electrodynamic systems. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 5, pp. 381–390, May 2006.     

First observation of the point spread function of optical transition radiation

We represent the first experimental observation of the point spread function (PSF) of optical transition radiation (OTR) performed at KEK-Accelerator Test Facility extraction line. We have demonstrated that the PSF vertical polarization component has a central minimum with a two lobe distribution. However, the distribution width varied significantly with wavelength. We assume that we observed a severe effect from spherical or chromatic aberrations which are not taken into account in any existing theoretical model. We believe that the result of this work will encourage theoreticians to continue developing the theory as it is important for various transition radiation applications. Nonuniform distribution of the OTR PSF creates an opportunity for developing a submicrometer transverse beam size monitor.     

Continuous-wave simultaneous dual-wavelength operation at 912 nm and 1063 nm in Nd:GdVO<Subscript>4</Subscript>

A continuous-wave, diode-pumped Nd:GdVO₄ thin disk laser with simultaneous dual-wavelength emission at the 912 nm ⁴ F _3/2→⁴ I _9/2 quasi-three-level transition and the 1063 nm ⁴ F _3/2→⁴ I _11/2 four-level transition is demonstrated and analyzed. Output powers of 1.7 W at 912 nm and of 1.6 W at 1063 nm were achieved simultaneously from a 0.3-at.%, 300-μm thick Nd:GdVO₄ crystal that was multi-pass excited with 26.8 W of available diode pump power. Second harmonic generation to 456 nm with LiB₃O₅ yielded 0.96 W in 912 nm single-wavelength operation and 0.73 W in 912 nm/1063 nm dual-wavelength operation. PACS 42.55.Rz; 42.60.By; 42.65.Ky     

Energy-level statistics of model quantum systems: Universality and scaling in a lattice-point problem

We investigate the statistics of the numberN(R, S) of lattice pointsnZ ², in an annular domain (R, w)=(R+w)A\RA, whereR, w>0. HereA is a fixed convex set with smooth boundary andw is chosen so that the area of (R, w) isS. The statistics comes fromR being taken as random (with a smooth density) in some interval [c ₁ T,c ₂,T],c ₂>c ₁>0. We find that in the limitT the variance and distribution of N=N(R; S)–S depend strongly on howS grows withT. There is a saturation regimeS/T, asT, in which the fluctuations in N coming from the two boundaries of are independent. Then there is a scaling regime,S/Tz, 0<z<, in which the distribution depends onz in an almost periodic way going to a Gaussian asz0. The variance in this limit approachesz for genericA, but can be larger for degenerate cases. The former behavior is what one would expect from the Poisson limit of a distribution for annuli of finite area.     

Ab-initio calculation of electronic structure of partially inverted manganese ferrite

Electronic structure of manganese ferrite is calculated using the density functional theory. Partially inverted structure with the inversion parameter y=0.0625 is considered, using four different supercells (Mn₁₅Fe)_A[Fe₃₁Mn]_BO₆₄ that differ in relative position of Fe_A and Mn_B. The crystal structure was optimized by minimizing the forces acting on the atoms. The spin magnetic moments m of Mn and Fe ions depend to some extent on their position in the supercell, but we found that in all cases m(Mn_B) is considerably smaller (by approx. one Bohr magneton) than m(Mn_A). This indicates strongly that the formal valence of Mn_B is 3+. Trivalent manganese in an octahedral position is expected to exhibit a Jahn–Teller effect and corresponding analysis was performed. No pronounced localization of the extra electron in the octahedral sublattice that would represent the Fe²⁺ ion was found.     

Dynamics of an RF Discharge in a Magnetic Field under Strong Initial Preionization

We demonstrate experimentally the possibility to create and sustain a strongly ionized plasma at low gas pressures by the electromagnetic radiation of the intensity which is below the conventional breakdown level. The conditions of ignition and sustaining the plasma are found, and the plasma parameters are measured. The necessary condition for the discharge formation is strong preliminary ionization of an initial gas. Significant electron heating in the RF field, which ensures the effective gas ionization takes place due to Coulomb collisions with protoplasma ions. Such collisions are much more frequent than the electron-neutral collisions. We propose a qualitative analytical model of the discharge evolution, which describes the effects observed.     

Unexpected formation of dinaphthoaza-17-crown-5 ether containing γ-aminopiperidine subunit

1. Download : Download high-res image (90KB)
2. Download : Download full-size image

     

Influence of pyridine-based ligands on photostability of MAPbI3 thin films

1. Download : Download high-res image (102KB)
2. Download : Download full-size image