Ring-opening of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane in the presence of transition metal catalysts

Ring opening of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane in the presence of various transition metal catalysts has been studied. Pd and Pt compounds are most active. Compounds with bulky ligands [e.g., Pd(PPh₃)₄] lead to the formation of “dimeric” cyclooctasilanes. Compounds with less bulky groups convert monomer to dimers and mixture of oligomers and polymers. Reactions with the all-trans monomer, tttt-(MePhSi)₄, result in the formation of stereoregular dimer, presumably all-trans, tttttttt-(MePhSi)₈, in high yields. Reactions in the presence of oxygen lead to the formation of polymers with inserted oxygen atoms in the main chain. © 1996 John Wiley & Sons, Inc.     

Solvent effects on free-radical polymerization, 4. Modelling of hompolymerization

A new so-called reactant-solvent complex model is proposed to describe the effect of solvent on chain propagation in homopolymerization. It takes into account complex formation of both monomer and radical with solvent by equilibria. Evaluation methods presented permit to estimate the complex equilibrium constant K which is assumed to be nearly the same for both monomer and radical complexation and the relative reactivity ratio r₁₁, for complexed monomer. Measured reaction rates as function of monomer concentration are needed for calculations.     

Exchange reactions in the living cationic polymerization of alkenes

Living cationic polymerization of alkenes is discussed in terms of rapid dynamic equilibration between carbenium ions and either covalent species or onium ions. Rates of exchange between these species have strong effect on polydispersities of the synthesized polymers. In the discussed living systems, rates of exchange are faster than rates of propagation. Propagation proceeds via classic carbocationic mechanism.     

Carbenium ions,onium ions,and covalent species in the cationic polymerization of alkenes

The importance of exchange reactions between various types of active species present in the cationic polymerization of alkenes is addressed. Reactivities of carbocations, corresponding ion pairs, onium ions, and covalent species are discussed for model systems and in real polymerization. The influence of various parameters on the equilibria between carbenium ions, onium ions, and covalent species is described. Rate constants of the isomerization of active sites are estimated and correlated with polydispersities. General ways to improve control of cationic polymerization of alkenes are discussed.     

Antimicrobial Activities of Plant Extracts against Solanum tuberosum L. Phytopathogens

The purpose of the study was to select an environmentally friendly plant biopesticide to protect seed potatoes against phytopathogens. The scope included the evaluation of the antimicrobial activities of 22 plant water extracts, 22 water-glycol extracts, and 3 subcritical carbon dioxide extracts using the agar diffusion method against 10 potato phytopathogens. For the most effective extracts, minimal inhibitory concentration (MIC), chemical composition analysis by gas chromatography–mass spectrometry and in situ assays on seed potatoes were performed. Garlic water extract was finally selected as the most effective in phytopathogen growth inhibition, both in vitro and in situ, with MIC values ranging between 6.3–25 mg/mL. 5-Hydroxymethylfurfural was determined to be the main component of this extract (33.24%). Garlic water extract was proposed as a potential biopesticide against potato phytopathogens.     

First Experimental Quantitative Charge Density Studies of Advanced Intermediate of Vitamin D Analogues

Vitamins D are a group of fat-soluble secosteroids which play a regulatory role in the functioning of most cells. Rational design of new vitamin D analogs, of increased therapeutic potency and lowered calcemic side effects, requires high-resolution initial structures and a deep understanding of interactions with the molecular targets. In this paper, using quantum crystallography, we present the first determination of the experimental quantitative charge density of an advanced intermediate of vitamin D analogues as well as a reconstruction of the theoretical electron density of final vitamin D analogues. Application of these methods allows for topological and electrostatic interaction energy analysis. We showed that the A-ring chair conformation has a significant influence on the topological properties of vitamin D compounds. Moreover, the interactions between the CD-ring and side-chain additionally stabilize the crystal structure. These results are supported by our theoretical calculations and previous biological studies.     

How random is your heart beat?

We measure the content of random uncorrelated noise in heart rate variability using a general method of noise level estimation using a coarse-grained entropy. We show that usually, except for atrial fibrillation, the level of such noise is within 5-15% of the variance of the data and that the variability due to the linearly correlated processes is dominant in all cases analyzed but atrial fibrillation. The nonlinear deterministic content of heart rate variability remains significant and may not be ignored.     

A Fleming–Viot Particle Representation¶of the Dirichlet Laplacian

We consider a model with a large number N of particles which move according to independent Brownian motions. A particle which leaves a domain D is killed; at the same time, a different particle splits into two particles. For large N, the particle distribution density converges to the normalized heat equation solution in D with Dirichlet boundary conditions. The stationary distributions converge as N→∞ to the first eigenfunction of the Laplacian in D with the same boundary conditions. Received: 11 November 1999 / Accepted: 19 May 2000     

Concepts of lower semicontinuity and continuous selections for convex valued multifunctions

We discuss several concepts of continuity, weaker than lower semicontinuity, but still implying the existence of a continuous selection for a closed convex valued multifunction from a paracompact Hausdorff topological space into a Banach space. In this way, an extension of Michael's celebrated selection theorem is given. The behavior of -envelop approximations, as well as the localization of continuous selections, is also discussed.     

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.