Extended resonant feedback technique for controlling unstable periodic orbits of chaotic system

An improvement of the recently described resonant chaos control method is suggested. Negative feedback loop containing a notch-rejection filter, tuned to the main harmonic of the unstable periodic orbit, is supplemented with a set of notch filters tuned to the higher harmonics. The extended method is applied to an electrical circuit representing the Duffing–Holmes type non-autonomous two-well chaotic oscillator. Stabilization of the period-1 orbit is achieved with very small control force. The residual control signal is about 1% compared to the main variable. Mathematical model based on a two-well piecewise parabolic potential is presented and numerical simulation is performed. Numerical results are confirmed by hardware experiments.     

DFt study of the regioselectivity of addition of sulfenylchloride to ethenes

The electrophilic addition reactions of methylsulfenyl chloride to the double bonds of functionalized ethenes have been studied theoretically. Density functional theory (DFT) calculations have been applied for starting species and ethene‐based sulfonium intermediates bearing substitutes at α‐carbon atom to study geometrical parameters and electronic states of plausible intermediate forms. The quantum chemical optimizations of intermediates indicate that the episulfonium ion is the most likely methyl‐ or carboxyl‐substituted ethane‐based intermediate. However, with phenyl substituents the intermediate is more like a carbonium than an episulfonium ion. The role of sulfur appears to be that of directing the stereochemistry of the addition reaction of chloride, forming the trans product upon nucleophilic attack on the C—C bond of the episulfonium ion. The regioselectivity features of the opening of the episulfonium ion by the chloride anion depend on the LUMO and LUMO+1 of the episulfonium ion and the approaching HOMO of chlorine. The results of the theoretical investigations are in agreement with experiment. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:695–703, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20378     

Control of forced self-sustained oscillations via a backward time controller

A weakly non-linear self-sustained oscillator can be synchronized by an external force only in a certain domain of parameters. We exploit unstable periodic orbits and extend this domain via a small control perturbation. The controller is constructed as a backward time replica of the original oscillator that has the same periodic orbits but with the opposite stability properties. The control is achieved by synchronizing the original oscillator with its backward time replica. We demonstrate these ideas both theoretically and experimentally.     

Features of charge carrier concentration and mobility in π-conjugated polymers

We show the possibilities of experimental investigation of charge carrier mobility and concentration features by extraction methods of equilibrium, photoexcited and injected charge carriers in π-conjugated polymers, where, due to relatively high conductivity, the classic time-of-flight method is inappropriate.     

Chaotic Colpitts Oscillator for the Ultrahigh Frequency Range

PSpice simulation and experimental results demonstrating chaotic performance of the Colpitts oscillator in the ultrahigh frequency (300–1000 MHz) range are presented. Various combinations of the resonance tank parameters are considered to achieve a fundamental frequency as high as possible. Simulations indicate that chaotic oscillations observed experimentally at higher frequencies, e.g., at about 1000 MHz are caused by parasites, like wiring inductances, loss resistance appearing due to skin effect, and collector-emitter capacitance of the transistor. Reliable and reproducible chaos can be generated at fundamental frequencies up to about 500 MHz with the single-stage Colpitts oscillator using the microwave 9 GHz bipolar junction transistors.     

Specificity of Dnmt1 for methylation of hemimethylated CpG sites resides in its catalytic domain

The maintenance methylation of hemimethylated CpG sites by the DNA methyltransferase Dnmt1 is the molecular basis of the inheritance of DNA methylation patterns. Based on structural data and kinetics obtained with a truncated form of Dnmt1, an autoinhibition model for the specificity of Dnmt1 was proposed in which unmethylated DNA binds to Dnmt1's CXXC domain, which prevents its methylation. We have prepared CXXC domain variants that lost DNA binding. Corresponding full-length Dnmt1 variants did not display a reduction in specificity, indicating that the autoinhibition model does not apply in full-length Dnmt1. Furthermore, we show that the Dnmt1 M1235S variant, which carries an exchange in the catalytic domain of the enzyme, has a marked reduction in specificity, indicating that the recognition of the hemimethylated state of target sites resides within the catalytic domain.     

Fluorescence and amplified spontaneous emission of glass forming compounds containing styryl-4H-pyran-4-ylidene fragment

Potential of glassy films of newly synthesised low molecular weight organic molecules for light amplification and lasing applications has been investigated by analysing fluorescence, transient differential absorption and amplified spontaneous emission properties. These non-symmetric and symmetric molecules contain styryl-4H-pyran-4-ylidene fragment with three different electron acceptor groups: dicyanomethylene, barbituric acid, indene-1,3-dione. Fluorescence quantum yields of the investigated compounds in solutions are between 0.32 and 0.54, while they drop down by an order of magnitude in thin solid films. Incorporation of bulky side groups reduced excitonic interactions enabling manifestation of amplified spontaneous emission in the neat films of the investigated derivatives.