Stability of silicon and titanium carbide suspensions in electrolyte, poly(ethylene oxide), and PEO-surfactant solutions

It has been shown that the coagulation values of counterions for SiC and TiC suspensions with particle radius from 0.5 to 5 microm obey a z(2.5-3.5) law and there is an insufficient change in the critical concentration of 1-1 electrolytes (CCE) when the surface potential of particles increases more than two times. Also, the CCE values hardly depend on the position of counterions in the lyotropic sequence. This is explained by aggregation of SiC and TiC particles at a secondary minimum, which is proved by calculations of the potential curves of interparticle interactions using the DLVO theory. The adsorption of poly(ethylene oxide) on the surfaces studied does not cause--in contradiction to dispersions with smaller particles--an unlimited growth in the stability of suspensions. This is due to the aggregation of large particles with adsorbed PEO, as in polymer-free dispersions, under barrierless conditions in which the coordinates of the secondary minimum are determined by superposition of molecular attractive forces and steric repulsive forces of adsorbed polymeric chains, without a contribution from the electric repulsion term. PEO-anionic surfactant complexes possess higher stabilizing capacity compared to the individual components of the mixture. Our results show that the adsorbed polymer layers may hinder the aggregation both in the primary and in the secondary minimum for not very large particles only, the critical size of which depends on the dispersed phase nature and the molecular mass of the polymer.     

On the models for deuterium long-range isotope effects in13C NMR spectroscopy

The long-range deuterium isotope effects on¹³C nuclear shielding are physically not yet completely understood. Two existing models for explaining these effects, vibrational and substituent, are compared here. The vibrational model is based on the Born-Oppenheimer approximation, but it can explain only one-bond deuterium effects. To the contrary, the substituent model may explain many long-range isotope effects, but it is controversial due to the assumption of some distinct electronic properties of isotopes. We explain how long-range deuterium isotope effects may be rationalized by the subtle electronic changes induced by isotope substitution, which does not violate the Born-Oppenheimer approximation.     

A study of the nonlinear refractive index in amorphous Ge–As–S layers and nanocomposites with gold nanoparticles

Nonlinear refractive indices of simple and composite chalcogenide glasses with gold nanoparticles are measured by the Z-scan method using a femtosecond Ti:sapphire laser with a central wavelength of 800 nm and a pulse duration of 40 fs. It is shown that introduction of nanoparticles into thin layers of amorphous GeS₂, As₃₀Ge₂₀S₅₀, and As₃Ge₃₅S₆₂ leads to a decrease in the total nonlinear refractive index due to additional absorption of femtosecond laser radiation without efficient excitation of localized plasmons outside the resonance region. For example, the nonlinear refractive index decreases with addition of gold nanoparticles from 16.1 × 10^–12 to 13.0 × 10^–12 cm²/W for GeS₂, from 3.9 × 10^–12 to 3.2 × 10^–12 cm²/W for As₃₀Ge₂₀S₅₀, and from 5.8 × 10^–12 to 4.7 × 10^–12 cm²/W for As₃Ge₃₅S₆₂.     

Vesicle Origami: Cuboid Phospholipid Vesicles Formed by Template-Free Self-Assembly

Phospholipid liposomes are archetypical self-assembled structures. To minimize the surface tension, the vesicles typically are spherical. Deciphering the bilayer code, the basic physical interactions between phospholipids would allow these molecules to be utilized as building blocks for novel, non-spherical structures. A 1,2-diamidophospholipid is presented that self-assembles into a cuboid structure. Owing to intermolecular hydrogen bonding, the bilayer membranes form an exceptionally tight subgel packing, leading to a maximization of flat structural elements and a minimization of any edges. These conditions are optimized in the geometrical structure of a cube. Surprisingly, the lateral surface pressure in the membrane is only one third of the value typically assumed for a bilayer membrane, questioning a long-standing rule-of-thumb.     

Long-Lived Photocharges in Supramolecular Polymers of Low-Band-Gap Chromophores

Photoinduced charge separation in supramolecular aggregates of π-conjugated molecules is a fundamental photophysical process and a key criterion for the development of advanced organic electronics materials. Herein, the self-assembly of low-band-gap chromophores into helical one-dimensional aggregates, due to intermolecular hydrogen bonding, is reported. Chromophores confined in these supramolecular polymers show strong excitonic coupling interactions and give rise to charge-separated states with unusually long lifetimes of several hours and charge densities of up to 5 mol % after illumination with white light. Two-contact devices exhibit increased photoconductivity and can even show Ohmic behavior. These findings demonstrate that the confinement of organic semiconductors into one-dimensional aggregates results in a considerable stabilization of charge carriers for a variety of π-conjugated systems, which may have implications for the design of future organic electronic materials.     

Adenosine Cyclic 3′, 5′-Phosphoramidate and N,N-Dimethylphosphoramidate: Synthesis Via Symmetrical and Mixed Anhydrides and Hydrolysis

Abstract

The title compounds were synthesized by converting cyclic AMP with a sulfonyl chloride to the symmetrical anhydride or with diphenyl phosphorochloridate to the mixed anhydride, then aminolyzing the anhydrides without isolation. The synthesis preferentially gave Sp-amides. Characteristic differences were observed in the behavior of the unsubstituted amide and the dimethylamide under hydrolysis conditions.     

Scrambling of sequence information in collision-induced dissociation of peptides

Collision-induced dissociation (CID) of protonated YAGFL-NH2 leads to nondirect sequence fragment ions that cannot directly be derived from the primary peptide structure. Experimental and theoretical evidence indicate that primary fragmentation of the intact peptide leads to the linear YAGFLoxa b5 ion with a C-terminal oxazolone ring that is attacked by the N-terminal amino group to induce formation of a cyclic peptide b5 isomer. The latter can undergo various proton transfer reactions and opens up to form something other than the YAGFLoxa linear b5 isomer, leading to scrambling of sequence information in the CID of protonated YAGFL-NH2.     

On the formation of water-soluble buckminsterfullerene-γ-cyclodextrin complexes

Abstract

γ-Cyclodextrin appears to catalyze the reaction of C₆₀ with water during reflux and in addition to the water soluble 1:1 and 2:1 complexes (whose stability constants could be estimated as ≥ 4 × 10² and ≥ 4 × 10⁴, respectively) some (complexed) fullerene derivatives are also formed.