Synthesis and conformational analysis of macrocyclic dihydroxystilbenes linked between the para-para positions

A new family of diphenylethanes has been synthesized as conformationally restricted analogues of antimitotic combretastatins. The two phenyl rings are linked between the para-phenolic positions through a 3-oxapentamethylene or hexamethylene chain. The key macrocyclization step was achieved in moderate yields by using an intramolecular McMurry pinacol coupling of linked aromatic dialdehydes, except for the nitro-substituted compounds. The relative stereochemistry of the isomeric pinacols was determined by a combination of spectroscopic, chemical derivatization, and molecular-modeling approaches. The NMR spectra of these compounds (with a polyoxygenated crownophane skeleton) indicate severe conformational restrictions relative to their parent combretastatins; the rotation of the phenyl rings is hampered by interactions of their substituents and the linker and the conformational restrictions imposed by the substituted bridge.     

Analysis of the form of ultrashort pulses by spectral nonlinear interferometry

A new method for recording the phase structure of ultrashort optical pulses is proposed that makes no use of a reference pulse. Using three-wave interactions in a nonlinear optical medium, a wave of spectral decomposition of an ultrashort pulse is transformed into a monochromatic wave. The spatial phase structure of this wave contains information on the phase structure of the pulse spectrum and on the pulse form. This structure is analyzed by shearing interferometry.

     

Synthesis of Polydisperse Boron Carbide and Synthesis of a Ceramic on Its Basis

Russian Journal of Applied Chemistry - Results obtained in a study of the process of synthesis of a polydisperse boron carbide powder (average particle size 2.10 µm) characterized by a broad...     

Atomistic simulations of a thermotropic biaxial liquid crystal

We have performed molecular dynamics simulations of a 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole mesogen (ODBP-Ph-C(7)) at a fully atomistic level for a range of temperatures within the region that has experimentally been assigned to a biaxial nematic phase. Analysis of the data shows that the simulated nematic phase is biaxial but that the degree of biaxiality is small. The simulations show also the formation of ferroelectric domains in the nematic where the molecular short axis is aligned with the oxadiazole dipoles parallel to each other. Removal of electrostatic interactions leads to destabilization of ferroelectric domains and destabilization of the biaxiality. An additional simulation shows the slow growth of a mesophase directly from the isotropic fluid over a period of approximately 50 ns. This is the first time this has been achieved within the framework of an all-atom model.     

Littlewood-Paley Formulas and Carleson Measures for Weighted Fock Spaces Induced by A ∞ $A_{\infty }$ -Type Weights

Potential Analysis - We obtain Littlewood-Paley formulas for Fock spaces ${\mathcal {F}}_{\beta ,\omega }^{q}$ induced by weights $\omega \in {A}_{\infty }^{restricted} = \cup _{1 \le p <...     

Charged colloid-polymer mixtures: a study on electrostatic depletion attraction

In this work, light scattering methods have been used to study the effect of adding charged polymer chains on the structural and dynamic properties of a charged colloidal system. The experimental measurements of the static structure factor S(cc)(q) show that as the polymer concentration increases, the main peak moves to higher q-values, which is interpreted in terms of the electrostatically enhanced depletion attraction induced by the polymer. Moreover, we found that the shift of the peak depends on the interplay between two relevant length scales, the polymer radius of gyration, R(g), and the Debye length, κ(-1). To reach these conclusions, the polymer reference interaction site model has been employed to explain the experimental results and to study how the effective depletion attraction depends on the polymer concentration, R(g) and κ(-1). Additionally, the measurements of the dynamic structure factor f(q, τ) indicate that the colloidal diffusion increases with the polymer concentration. Both static and dynamic analysis point out that the repulsion between colloids becomes weaker as the charged polymer is added.     

Breaking processes in nickel nanocontacts: a statistical description

In this work we perform a statistical study of favorable atomic configurations of nickel nanocontacts during their stretching at 4 K and 300 K. Nanowire breaking events are simulated using molecular dynamics (MD) where atomic interactions are represented with state-of-the-art embedded atom (EAM) interatomic potentials. The full determination of atomic positions during the contact evolution allows determination of the evolution of the minimum-cross section S_m during stretching. By accumulating many breaking traces, we built minimum cross-section histograms H(S_m). These simulated histograms reveal the presence of preferential geometrical arrangements during the nanocontact breaking, showing that no remarkable differences should appear between the low (4 K) and room temperature (300 K) situations. These results show that differences observed between low and room temperature experimental Ni conductance histograms, are not caused by the different structural evolution and, that therefore, other phenomena are involved. PACS  81.07.Lk; 68.65.-k; 73.63.Rt; 31.15.Qg