Nucleophilic substitution reactions of pyranose polytosylates

The 2,3,4-tri-toluenesulfonate ester derivatives of the methyl pyranosides of l-arabinose, d-ribose, d-lyxose, and d-xylose have been prepared, and their substitution reactions with various nucleophiles have been examined. For arabinose, xylose, and ribose, highly regioselective monosubstitutions were observed with benzoate, nitrite, and azide anions. These reactions have led to short and simple routes from d-xylose to l-arabinose derivatives, from l-arabinose to d-xylose derivatives, and from d-ribose to l-lyxose derivatives. The tritosylate derived from methyl alpha-d-lyxopyranoside was unreactive toward nucleophilic substitution reactions, giving instead a dihydropyran product arising from an initial E2 elimination reaction of the 2-tosylate.     

Computational chemistry predictions of reaction processes in organometallic vapor phase epitaxy

Quantitative understanding of reaction mechanisms in organometallic vapor phase epitaxy (OMVPE) is critical for selection of precursors, design of equipment, and optimization of process conditions. Progress has been made in the simulation of fluid flow as well as heat and mass transfer, but predictions of growth rates, alloy composition, and dopant incorporation are limited by the availability of thermodynamic and kinetic data for OMVPE precursors. Chemical kinetic experiments are expensive and difficult to perform, and the organometallic compounds being toxic and/or pyrophoric further complicates the situation. It is therefore desirable to study OMVPE reactions from first principles, quantum chemistry computations. We describe current quantum chemistry methods, Hartree-Fock and post-Hartree-Fock ab initio molecular orbital techniques and density functional theory (DFT) methods, with emphasis on issues related to OMVPE applications. The primary examples in this review are drawn from OMVPE applications, but studies on silicon chemistry are also included to illustrate important elements in simulation of vapor phase growth processes. Molecular structure and energy are reported for trialkyl group III species and group V hydrides by ab initio molecular orbital and density functional theory. The results are evaluated against experimental data. Vibrational frequencies needed for calculation of thermochemical properties (e.g., ΔH and ΔS) at process temperatures are also computed and compared to experimental data. The bimolecular reaction of methyl with arsine exemplifies the combined use of quantum chemistry and transition state theory to predict a reaction rate. A reaction mechanism for thermal decomposition of phosphine further demonstrates the use of these techniques. Lewis-acid-base adduct reactions of group III and V precursors exemplifies the use of quantum chemistry to evaluate adduct bond strengths and potential alkane elimination reaction pathways. A study of thermochemical properties of bridging organometallic aluminum compounds serves to illustrate variations in accuracy among different first principle methods. Overall, the selected examples demonstrate that computational chemistry techniques can provide useful insight into OMVPE chemical processes, but also that additional investigations are needed to establish which methods would be best for particular subsets of OMVPE chemistry.     

Variations on ω-boundedness

Let be a property (or, equivalently, a class) of topological spaces. A space X is called -bounded if every subspace of X with (or in) has compact closure. Thus, countable-bounded has been known as ω-bounded and (σ-compact)-bounded as strongly ω-bounded. In this paper we present a systematic study of the interrelations of these two known “boundedness” concepts with -boundedness where is one of the further countability properties weakly Lindelöf, Lindelöf, hereditarily Lindelöf, and ccc.     

Paradigms and paradoxes: decoding the “genetic code”

We propose to keep the term “genetic code” to describe the nucleotide sequence in DNA and RNA and use the term “genetic cipher” to describe the key for decoding the genetic codes of DNA and RNA into the amino acid sequences of proteins.

     

The dosage of small volumes for chromatographic quantifications using a drop-on-demand dispenser system

A commercially available piezo-driven drop-on-demand dispenser was tested for its suitability for the preparation of analytical calibration standards and in a standard addition approach prior to quantitative ultra performance liquid chromatography (UPLC) analysis of homoserines. The reproducibility of the drop-on-demand dosing system was tested and the verification of the droplet volume was performed by preparing a series of 1.0 mg/L caffeine standard solutions from a 1,000.0 mg/L stock solution and analysis of the concentrations obtained by UPLC. The reproducibility was better than 1% relative standard deviation from measurement to measurement and the highest was 1.6% from day to day. The results were compared with the conventional way of generating standard solutions (pipetting). A gravimetric method and a photography-based method for the determination of the average single droplet volume were compared and found to be in very good agreement. The system was employed for the quantification of N-decanoyl homoserine by standard addition in bacterial culture supernatants containing this analyte. The agreement with conventional quantification techniques was high. The paper shows the feasibility of the approach with advantages in low sample and solvent volume consumption and very good reproducibility and reliability combined with easy usage. Figure Ejected droplet, 60 μs after application of the pulse     

Preparation of Polymer‐Metal Composites via In Situ Decomposition of Metal Salts by Frontal Polymerization

Composites containing powdered zinc, and zinc/lead acetate were prepared via frontal polymerization. In the case of the acetates, elemental metal was formed in an in situ decomposition process. The local area function was used to demonstrate the distribution of fillers, and the uniformity of the area fraction for the quantitative characterization of the distribution. With the use of metal acetates, composites of uniform metal distribution can be produced, unlike in systems with metal powder, where the metal particles are enriched at the margin of the sample. It can be established that the specific direct‐current resistance significantly decreases in AA‐TGDMA composites by the addition of zinc acetate, compared to that of the initial monomer mixture. On heating, the unreacted zinc acetate decomposes further, which results in the further decrease in electrical resistance.

     

Quantitative determination of lipase activity by liquid chromatography-mass spectrometry

We developed a novel in vitro lipase assay based on the quantitation of fatty acids by liquid chromatography-mass spectrometry. Oleic acids enzymatically released from triolein substrates were isolated from the reaction mixture by reverse-phase chromatography, ionized in negative mode electrospray mass spectrometry and quantitated with the aid of [(13)C]-oleic acid internal standard. The enzymatic activity was measured by monitoring oleic acid productions at multiple time points. This method overcomes the substrate and pH limitations of conventional techniques and thus serves as a generic lipase activity assay.     

Complexation of phenols by calix[4]arene Diethers in a low-permittivity solvent. Self-switched complexation by 25,27-Dibenzyloxycalix[4]arene

To improve the selectivity of sensing, the thermodynamics of the complex formation of some calix[4]arene hosts with neutral phenol guests was studied in carbon tetrachloride as nonpolar solvent. The molecular shape of calixarenes was varied by the selective functionalization with tBu and O-CH2-Ph (O-benzyl) or OPr groups at the upper and lower rim, respectively. To vary the electron density on the guest's aromatic rings, the parent phenol was functionalized in the para position with electron-withdrawing Cl, as well as H, and electron-releasing CH3 and tBu groups. To study the interaction between calixarene and the guests, PL and quantum-chemical methods were applied. The results revealed an overall 1:1 complex stoichiometry except for the parent dibenzyloxycalix[4]arene, where 1:2 host-guest stoichiometries were observed irrespective of the quality of phenol. In the latter case, the complex formation shows a self-switched character: the first phenol molecule is included in the calixarene cavity, and only afterward, a second guest molecule is bound by the two benzyloxy aromatics. Although the enthalpy change predicts strong interaction between the host and the guest, the Gibbs free energy change of the complex formation is small, resulting in a relatively low complex stability. The solvent-relaxation measurements support that the unexpected entropy change could be the consequence of the reorientation of solvent molecules around the calixarene building block. The reorientation is assisted by dispersive forces between solute and solvent molecules. IR and RAMAN analysis of the complexes exclude a considerable participation of the phenolic OH group in the stabilization of the complex. This result is in agreement with earlier findings where deterministic role of pi-pi interaction in the complex stability was assumed.