Synthesis of Substituted 1H,5-Dihydroimidazolium Salts by Dehydrogenation of Imidazolidines

A study is presented on the scope of the method to obtain 1H-4,5-dihydroimidazolium salts 3 by dehydrogenation of 1,3-di and 1,2,3-trisubstituted imidazolidines 2. Of the dehydrogenating agents used, N-bromoacetamide leads to the best results, providing a simple and general method to prepare salts 3.     

Dynamics of H2 dissociation on the 1/2 ML c(2 × 2)-Ti/Al(100) surface

The dissociation of H(2) on Ti-covered Al surfaces is relevant to the rehydrogenation and dehydrogenation of the NaAlH(4) hydrogen storage material. The energetically most stable structure for a 1/2 monolayer of Ti deposited on the Al(100) surface has the Ti atoms in the second layer with a c(2 × 2) structure, as has been confirmed by both low-energy electron diffraction and low-energy ion scattering experiments and density functional theory studies. In this work, we investigate the dynamics of H(2) dissociation on a slab model of this Ti/Al(100) surface. Two six-dimensional potential energy surfaces (PESs) have been built for this H(2) + Ti/Al(100) system, based on the density functional theory PW91 and RPBE exchange-correlation functionals. In the PW91 (RPBE) PES, the lowest H(2) dissociation barrier is found to be 0.65 (0.84) eV, with the minimum energy path occurring for H(2) dissociating above the bridge to top sites. Using both PESs, H(2) dissociation probabilities are calculated using the classical trajectory (CT), the quasi-classical trajectory (QCT), and the time-dependent wave-packet methods. We find that the QCT H(2) dissociation probabilities are in good agreement with the quantum dynamics results in the collision energy range studied up to 1.0 eV. We have also performed molecular beam simulations and present predictions for molecular beam experiments. Our molecular beam simulations show that H(2) dissociation on the 1/2 ML Ti/Al(100) surface is an activated process, and the reaction probability is found to be 6.9% for the PW91 functional and 1.8% for the RPBE at a nozzle temperature of 1700 K. Finally, we have also calculated H(2) dissociation rate constants by applying transition state theory and the QCT method, which could be relevant to modeling Ti-catalyzed rehydrogenation and dehydrogenation of NaAlH(4).     

Mathematical Modeling of Arborescent Polyisobutylene Production in Batch Reactors

A novel model describes copolymerization of isobutylene and inimer (initiator‐monomer) via living carbocationic polymerization. Six different propagation rate constants and two types of equilibrium reactions are considered. Simplifying assumptions are made to enable implementation in PREDICI, so that the molecular weight distribution (MWD) could be predicted for molecules with different branching levels. Four apparent rate constants were estimated from experimental data with <5 branches per molecule. Model predictions provide a good fit to data, and simulation results show that polymers with high‐branching levels and ≥15 inimer units contribute significantly to the MWD, even though their concentrations are very low.

     

Morphology and mechanical response of S–B star block copolymer – Layered silicate nanocomposites

The influence on the mechanical response by incorporation of oligostyrene-modified montmorillonite (MMT) and oligostyrene-modified bentonite (BET) into star shaped styrene–butadiene block copolymer has been investigated. The modified silicates are highly intercalated with a gallery distance of more than 9 nm and partly exfoliated. The array of tactoids consisting of 1–8 layers showing uniform state of distribution as revealed from TEM. The layers of the modified MMT and BET are observed to undergo nano-confinement, i.e. restricted to the PS-domains of the styrene–butadiene star block copolymer with impinging/bridging effects of silicate layers through the SB soft phase. DMA studies have showed an appreciable shift of the glass transition temperatures of PB- and PS-rich phases towards higher temperatures in addition to an increase of the storage modulus due to nanoclay reinforcement. Generally the Young’s modulus and yield stress was strongly increased with the incorporation of modified nanoparticles whereas at the same time the strain at break reduces slightly. The elastic–plastic hysteresis–stress and the hysteresis–work are largely increased due to effective interfacial effect; an effect that is largely attributed to the presence of highly intercalated and partially exfoliated silicate layers. The extent of increase was more in the modified MMT than in the modified BET based nanocomposites. The stress-decay and the strain-recovery aspects have also been critically analyzed in relation to their micro-structural attributes. Our study fundamentally demonstrates two critical aspects related to mechanical properties and particularly with regard to elastic–plastic hysteresis response. Firstly, partial confinement of the silicate layers is promoted by PS-aided surface modification facilitating enhancement in mechanical properties and secondly, the nano-confinement of modified MMT seems to be more effective in improving the hysteresis performance when compared to BET with higher charge density.     

Congruence of selfadjoint operators

Given a bounded selfadjoint operator a in a Hilbert space , the aim of this paper is to study the orbit of a, i.e., the set of operators which are congruent to a. We establish some necessary and sufficient conditions for an operator to be in the orbit of a. Also, the orbit of a selfadjoint operator with closed range is provided with a structure of differential manifold.      

Reaction of poly‐L‐lysine with aryl acetates and aryl methyl carbonates. A mechanistic study

The pH profile (log k vs. pH) of the reactions of poly‐L ‐lysine (PL) with a series of aryl acetates and aryl methyl carbonates in aqueous solution show the same conformational changes as those determined by potentiometric titrations. When PL is a random coil, the most probable mechanism for the reactions studied is through the formation of a tetrahedral intermediate and its breakdown to products as the rate‐determining step. The tetrahedral intermediate is stabilized by a hydrogen bond interaction between the nitro groups in the substrate and the NH group of the principal chain or some NH₂ groups of the lateral chains. Copyright © 2007 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates in aqueous ethanol

The reactions of anilines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates (MPDNPC and ClPDNPC, respectively) and the latter substrate with secondary alicyclic (SA) amines are subjected to a kinetic study in 44 wt % ethanol-water solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of 2,4-dinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear and pH-independent, with slope k(N). The Br?nsted-type plots (log k(N) vs pK(a) of aminium ions) are linear, with slopes beta = 0.68 and 0.66 for the reactions of anilines with MPDNPC and ClPDNPC, respectively, and beta = 0.44 for the reactions of SA amines with ClPDNPC. The magnitude of the slope for the latter reaction indicates that its mechanism is concerted. The slope values for the reactions of anilines are in the borderline between stepwise and concerted mechanisms. The sensitivity of logk(N) to the basicity of the nonleaving group (beta(nlg)) is ca. -0.7 for the reactions of anilines, in agreement with that found for the SA reactions (beta(nlg) ca. -0.6). These results suggest that the reactions of anilines are concerted, although it is also possible that both mechanisms (stepwise and concerted) operate simultaneously. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions can be drawn: (i) ClPDNPC is more reactive than MPDNPC toward the two amine series. (ii) The change of water to aqueous ethanol destabilizes a zwitterionic tetrahedral intermediate. (iii) The change of the nonleaving group from MeO to 4-methylphenoxy or 4-chlorophenoxy also destabilizes this intermediate.     

Efficient and Reliable Algorithms for the Computation of Non-Twist Invariant Circles

Foundations of Computational Mathematics - This paper presents a methodology to study non-twist invariant circles and their bifurcations for area preserving maps, which is supported on the...     

(1)H- and (13)C-NMR analysis of a series of 1,2-diaryl-1H-4,5-dihydroimidazoles

An analysis of the (1)H- and (13)C-NMR spectra of a series of 1,2-diaryl-1H-4,5-dihydroimidazoles and comparisons with 4,5-dihydroimidazoles having different substitution patterns are presented. The influence of different 1-aryl and 2-aryl group substituents on spectroscopic parameters of the heterocyclic ring and on the contributions of possible mesomeric structures in the system was determined. Spectroscopic features are coherent with the presence of two conjugated systems (Ar(1)-N and Ar(2)-C=N) which compete with the delocalization characteristics of the amidine system.     

Effect of the crosslinking density and the method of sample preparation on the observed microstructure of macroporous and conventional poly(N,N-dimethylacrylamide) hydrogels

SEM micrographs of macroporous and conventional poly(N,N-dimethylacrylamide) hydrogels were obtained for specimens synthesized in different conditions and prepared for microscopy by different methods (freeze drying of different solvents and critical point drying). The crosslinking density of both types of samples was determined through T _g measurements. Open structures (honeycomb-like, fibrillar networks) were more frequently observed in specimens prepared by freeze drying of benzene, which was attributed to its large pressure and temperature at the triple point. In spite of the different structure in the millimeter scale, there is no significant difference in the mesh size of fibrillar networks observed for macroporous and conventional samples, and in both cases it decreases with increasing crosslinking density. Other effects of the crosslinking density are that only incomplete honeycomb-like structures were formed in low-crosslinking samples and that collapsed structures were developed by phase separation throughout polymerization in highly crosslinked samples. Fibrillar networks of 1-μm mesh size were observed for the uncrosslinked polymer.