Asynchronous parametric excitation: validation of theoretical results by electronic circuit simulation

Nonlinear Dynamics - A validation of recent theoretical results on the stability effects of asynchronous parametric excitation is presented. In particular, the coexistence of both resonance and...     

Synthesis of Ultra-High Molecular Weight Polyolefins of Regular Structure in Octafluorobutane Medium

Russian Journal of Applied Chemistry - The suspension polymerization in octafluorobutane, initiated by Ziegler–Natta catalyst system, has been used to synthesize ultra-high molecular weight...     

Sol-gel made titanium dioxide nanostructured thin films as gas-sensing materials for the detection of oxygen

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Interplay between test sets and statistical procedures in ranking DFT methods: The case of electron density studies

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Three-dimensional structure of the Arg32His mutant of the human tumor necrosis factor determined at 2.5 Å resolution from X-ray data for a twin crystal

The three-dimensional structure of the Arg32His mutant of the human tumor necrosis factor (TNF-α) was established at 2.5 Å resolution by the molecular replacement method. The crystals of the mutant belong to sp. gr. R3. The specimen has a hemihedral twinning fraction of approximately one half with the twin law corresponding to an additional twofold axis along the a-or b-axis of the crystal lattice. The model analysis of interactions between functionally important loop 29–36 of the mutant and the receptors p55 and p75 was performed.     

Ultrafast charge photogeneration dynamics in ground-state charge-transfer complexes based on conjugated polymers

The charge photogeneration and early recombination in MEH-PPV-based charge-transfer complexes (CTCs) and in MEH-PPV/PCBM blend as a reference are studied by ultrafast visible-pump-IR-probe spectroscopy. After excitation of the CTC band, an immediate (<100 fs) electron transfer is observed from the polymer chain to the acceptor with the same yield as in the MEH-PPV/PCBM blend. The forward charge transfer in the CTCs is followed by an efficient (approximately 95%) and fast (<30 ps) geminate recombination. For comparison, the recombination efficiency obtained in the MEH-PPV/PCBM blend does not exceed a mere 50%. Polarization-sensitive experiments demonstrate high (approximately 0.3) values of transient anisotropy for the CTCs polaron band. In contrast, in the MEH-PPV/PCBM blend the dipole moment orientation of the charge-induced transition is less correlated with the polarization of the excitation photon. According to these data, photogeneration and recombination of charges in the CTCs take place locally (i.e., within a single pair of a polymer conjugation segment and an acceptor) while in the MEH-PPV/PCBM blend exciton migration precedes the separation of charges. Results of the ultrafast experiments are supported by photocurrent measurements on the corresponding MEH-PPV/acceptor photodiodes.     

Structure and dynamics of pyrimidine-based macrocycles in solution

The conformational structure of macrocycles obtained from two thiopyrimidine and uracil nucleic acids linked by polymethylene spacers is determined by the length of the spacers, intramolecular NH bonding, pH and solvent. In CDCl₃, NH-OC hydrogen bonding can impact the overall stabilization of the folded conformation, however spatial preorganization to such hydrogen bonding is a prerequisite. Protonation leads to disruption of intramolecular hydrogen bonds, destabilization of the folded conformation and to strong counterion assisted self-aggregation of macrocyles which can be destroyed in polar solvents.     

Lewis acid stabilized group 13–15 element analogs of ethylene

Stabilization of hydrogen-substituted group 13–15 compounds H₂EE′H₂ (E = B, Al, Ga; E′ = P, As, Sb) by Lewis acids is considered at B3LYP/def2-TZVP, B3LYP-D3/def2-TZVP and M06-2X/def2-TZVP levels of theory. It is shown, that for many Lewis acids additional reactivity beyond the DA complex formation with H₂EE′H₂ monomer is expected. In case of complexation with E(C₆F₅)₃, F/H exchange reactions with group 13 bound hydrides are predicted to be exothermic and accompanied by the activation energies which are smaller than dissociation of the complex into components. In case of complex formation with transition metal (TM) carbonyls, additional O → Al, TM–C → Al interactions are observed, which in several cases lead to cyclic structures. The most promising candidates for the experimental studies have been identified. Synthetic approaches to the most promising LA-only stabilized compounds are recommended.