Gold-catalyzed cyclization in the synthesis of antimitotic 2,3-dihydrobenzo[b]oxepine derivatives of colchicine

Russian Chemical Bulletin - New allocolchicine derivatives bearing 2,3-dihydrobenzo[b]oxepine moiety were synthesized via gold-catalyzed cyclization as a key synthetic step. The obtained...     

Enhancement of band magnetism and features of the magnetically ordered state in the CeB<Subscript>6</Subscript> compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB₆ single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T ^?1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T ^?0.8 is obtained for magnetic susceptibility of CeB₆ in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB₆ and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB₆ is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB₆ matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at T _Q ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB₆, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.     

Doping of boron carbides with cobalt, using cobaltocene

The decomposition of cobaltocene under low energy electron irradiation appears facile, and evidence suggests that the decomposition products lead to an increase in the carrier concentration in semiconducting boron carbides. Using cobaltocene to introduce dopants, we fabricated a semiconducting boron carbide homojunction PACS 81.05.Je; 81.15.Gh; 61.72.Ww; 82.33.Ya     

Monte-Carlo generator for e+e- annihilation into lepton and hadron pairs with precise radiative corrections

Recently, various cross sections of e⁺e^- annihilation into hadrons were accurately measured in the energy range from 0.37 to 1.39 GeV with the CMD-2 detector at the VEPP-2M collider. In the π⁺π^- channel a systematic uncertainty of 0.6% has been achieved. A Monte-Carlo Generator Photon Jets (MCGPJ) was developed to simulate events of Bhabha scattering as well as production of two charged pions, kaons and muons. Based on the formalism of structure functions, the leading logarithmic contributions related to the emission of photon jets in the collinear region are incorporated into the MC generator. Radiative corrections (RC) in the first order of α are accounted for exactly. The theoretical precision of the cross sections with RC is estimated to be better than 0.2%. Numerous tests of the program as well as a comparison with other MC generators and CMD-2 experimental data are presented.     

Latest results from GRAAL collaboration

The GRAAL experimental set-up consists of a polarized and tagged photon beam that covers an energy range from a minimum of 600 MeV up to a maximum of 1500 MeV, of a liquid Hydrogen or Deuterium target and of the 4π Lagrange detector optimized for photon detection. It allows the study of pseudo-scalar and vector meson photoproduction on the nucleon in the energy range corresponding to the second and the third resonance regions. In the following, the Σ beam asymmetries in η and π~0 photoproduction on quasi-free nucleon are shown. Also single and double polarization observables in K~+A photoproduction on free proton are shown; they are important to confirm the role of new or poorly known resonances in the 1900 MeV mass region.     

New limit on the lepton-flavor-violating decay μ+→e+γ

We present a new result based on an analysis of the data collected by the MEG detector at the Paul Scherrer Institut in 2009 and 2010, in search of the lepton-flavor-violating decay μ(+)e(+)γ. The likelihood analysis of the combined data sample, which corresponds to a total of 1.8×10(14) muon decays, gives a 90% C.L. upper limit of 2.4×10(-12) on the branching ratio of the μ(+)→e(+)γ decay, constituting the most stringent limit on the existence of this decay to date.     

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.     

Fragmentation of16α, 17α-cyclobutenopregnane

Conclusions An unusual fragmentation was discovered and studied for 16, 17-cyclobutenopregnenolone leading to pregnenolone. A possible mechanism was discussed for this transformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2620–2622, November, 1984.     

Unexpected features of stretched Si-H...Mo beta-agostic interactions

Coupling of silanes with the imido group of (Ar'N)2Mo(PMe3)3 gives either the silanimine dimer (ArN-SiHCl)2 or Si-H agostic silylamido complexes which do not exhibit the commonly expected correlation between the nature of the substituents on silicon, the degree of Si-H addition and the value of the Si-H coupling constant.