Allyl ether of aralkyl phenolic resin with low melt viscosity and its Alder‐ene blends with bismaleimide: synthesis,curing, and laminate properties

This paper outlines the synthesis and characterization of O‐allyl aralkyl phenolic (O‐allyl Xylok, OAX) resins having low melt viscosity and its Alder‐ene blends with 2, 2′‐bis 4‐[(4′‐maleimido phenoxy) phenyl] propane. The blends manifested a three‐stage curing pattern that converged to a two‐stage pattern on enhancing the maleimide content. The polymerization kinetics of typical allyl and maleimide rich resin systems showed apparent activation energy increasing and pre‐exponential factor decreasing from ene to the Diels–Alder step. Increased allyl content improved mechanical and impact properties of the composites at ambient temperature, although it diminished the retention of interlaminar shear strength at elevated temperature. Increased maleimide content of the resin was conducive for the higher rigidity for the composite and its retention at elevated temperature. A substantial increase in T_g (from 153°C to 280°C) and thermal stability was observed with an increase in maleimide content. High allyl content resulted in improved mechanical properties thanks to better resin–reinforcement interaction as revealed from morphological analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Peptide Dimethylation: Fragmentation Control via Distancing the Dimethylamino Group

Direct reductive methylation of peptides is a common method for quantitative proteomics. It is an active derivatization technique; with participation of the dimethylamino group, the derivatized peptides preferentially release intense a₁ ions. The advantageous generation of a₁ ions for quantitative proteomic profiling, however, is not desirable for targeted proteomic quantitation using multiple reaction monitoring mass spectrometry; this mass spectrometric method prefers the derivatizing group to stay with the intact peptide ions and multiple fragments as passive mass tags. This work investigated collisional fragmentation of peptides whose amine groups were derivatized with five linear ω-dimethylamino acids, from 2-(dimethylamino)-acetic acid to 6-(dimethylamino)-hexanoic acid. Tandem mass spectra of the derivatized tryptic peptides revealed different preferential breakdown pathways. Together with energy resolved mass spectrometry, it was found that shutting down the active participation of the terminal dimethylamino group in fragmentation of derivatized peptides is possible. However, it took a separation of five methylene groups between the terminal dimethylamino group and the amide formed upon peptide derivatization. For the first time, the gas-phase fragmentation of peptides derivatized with linear ω-dimethylamino acids of systematically increasing alkyl chain lengths is reported. Figure

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Ni0.1Co0.9Fe2O4-based electrochemical sensor for the detection of paracetamol

A novel electrochemical sensor based on nickel-doped cobalt ferrite nanoparticles (Ni_0.1Co_0.9Fe₂O₄)-modified glassy carbon electrode (NCF/GCE) was presented for the sensitive detection of paracetamol. Experimental conditions such as pH, applied potentials and concentration were investigated using cyclic voltammetric and chronoamperometric techniques. The modified electrode exhibited excellent catalytic response towards the oxidation of paracetamol with good reproducibility. The overpotential for oxidation of paracetamol is decreased, and the current response enhanced significantly on the modified electrode in comparison with that of bare electrode. Linear calibration curve is obtained over the range 2 μM to 8,000 μM having a detection limit of 11 nM. The modified electrode facilitated the simultaneous detection of paracetamol, ascorbic acid, and dopamine with good reproducibility.     

Following Solid‐Acid‐Catalyzed Reactions by MAS NMR Spectroscopy in Liquid Phase—Zeolite‐Catalyzed Conversion of Cyclohexanol in Water

A microautoclave magic angle spinning NMR rotor is developed enabling in situ monitoring of solid–liquid–gas reactions at high temperatures and pressures. It is used in a kinetic and mechanistic study of the reactions of cyclohexanol on zeolite HBEA in 130 °C water. The ¹³C spectra show that dehydration of 1‐¹³C‐cyclohexanol occurs with significant migration of the hydroxy group in cyclohexanol and the double bond in cyclohexene with respect to the ¹³C label. A simplified kinetic model shows the E1‐type elimination fully accounts for the initial rates of 1‐¹³C‐cyclohexanol disappearance and the appearance of the differently labeled products, thus suggesting that the cyclohexyl cation undergoes a 1,2‐hydride shift competitive with rehydration and deprotonation. Concurrent with the dehydration, trace amounts of dicyclohexyl ether are observed, and in approaching equilibrium, a secondary product, cyclohexyl‐1‐cyclohexene is formed. Compared to phosphoric acid, HBEA is shown to be a more active catalyst exhibiting a dehydration rate that is 100‐fold faster per proton.     

Heterobimetallic s‐Block Hydrides by σ‐Bond Metathesis

Reactions between PhSiH₃ and alkali‐metal diamidoalkylmagnesiates ([M{N(SiMe₃)₂}₂MgBu], M=Li, Na, K) provide either selective alkyl metathesis or the formation of polyhydride aggregates contingent upon the identity of the Group 1 metal. In the case of [M{N(SiMe₃)₂}₂MgBu], this reactivity results in a structurally unprecedented dodecametallic decahydride cluster species.     

One‐Step Multicomponent Self‐Assembly of a First‐Generation Sierpiński Triangle: From Fractal Design to Chemical Reality

A novel terpyridine‐based architecture that mimics a first‐generation Sierpiński triangle has been synthesized by multicomponent assembly and features tpy? Cd^II? tpy connectivity (tpy=terpyridine). The key terpyridine ligands were synthesized by the Suzuki cross‐coupling reaction. Mixing two different terpyridine‐based ligands and Cd^II in a precise stoichiometric ratio (1:1:3) produced the desired fractal architecture in near‐quantitative yield. Characterization was accomplished by NMR spectroscopy, mass spectrometry, and transmission electron microscopy.     

Fabrication and characterization of TEOS-based silica aerogels prepared using rapid supercritical extraction

Silica aerogels were prepared using the precursor tetraethylorthosilicate (TEOS) via a rapid supercritical extraction (RSCE) method. Multiple consistent batches of monolithic TEOS-based aerogels were fabricated via an 8-h RSCE process. Fabricating TEOS-based aerogels with an RSCE method offers some distinct advantages. One advantage is the relative simplicity of the RSCE approach: liquid precursors are mixed and poured into a metal mold in a hydraulic hot-press, where gelation, aging and extraction of liquid from the pores occur. The precursor recipe employs TEOS, ethanol, water, oxalic acid to catalyze hydrolysis, and ammonia to catalyze the subsequent polycondensation reactions. Another advantage is that reaction of TEOS to form sol gels yields ethanol as a byproduct. A process that releases ethanol, rather than methanol (as in tetramethylorthosilicate (TMOS)-based aerogels) may be more appealing for commercial applications, involving scale-up of the process. The significantly lower cost of TEOS, compared to TMOS, is a considerable advantage. The TEOS-based RSCE aerogels are mesoporous and optically translucent, have bulk densities of 0.099(±0.003) g/cm³ and surface areas of 460(±10) m²/g. Signals observed in infrared and Raman spectra of the aerogels are consistent with Si–O framework bonds. Using scanning electron microscopy imaging, the surface morphology of the aerogel samples was imaged at magnifications up to 150 kX.     

Reliable radical stabilization energies from diffusion Monte Carlo calculations

We assess the performance of variational (VMC) and diffusion (DMC) quantum Monte Carlo methods for calculating the radical stabilization energies of a set of 43 carbon-centered radical species. Even using simple single-determinant trial wavefunctions, both methods perform exceptionally well, with mean absolute deviations from reference values well under the chemical accuracy standard of 1 kcal/mol. In addition, the use of DMC results in a highly concentrated spread of errors, with all 43 results within chemical accuracy at the 95% confidence level. These results indicate that DMC is an extremely reliable method for calculating radical stabilization energies and could be used as a benchmark method for larger systems in future.     

Electrochemical potentials from first principles

The electrochemical potential is the fundamental parameter in the theory of electrochemistry. Not only does it determine the position of electrochemical equilibria but also it acts as the driving force for electron transfer reactions, diffusion-migration phenomena, and phase transformations of all kinds. In the present work, the electrochemical potential is defined as the total work done in transferring a single particle of a substance from a universal reference state to a specified location, at constant temperature and pressure. It is the sum of two scalar fields: the electrostatic potential energy and the chemical potential energy. The electrochemical potential is widely underutilized within the fields of solid-state science and electrochemical engineering. For historical reasons, many authors prefer to analyze driving forces in terms of electrode potentials, concentration gradients, or Gibbs free energies. In this paper, the author provides a short introduction to the electrochemical potential and then shows how some of the major branches of electrochemistry can benefit from using it. Topics examined include the Volta potential difference, the membrane potential difference, the scanning Kelvin probe microscope, the electromotive force, the proton motive force, and the activation of electron transfer.

     

A Convergent Total Synthesis of the Death Cap Toxin α‐Amanitin

The toxic bicyclic octapeptide α‐amanitin is mostly found in different species of the mushroom genus Amanita, with the death cap (Amanita phalloides) as one of the most prominent members. Due to its high selective inhibition of RNA polymerase II, which is directly linked to its high toxicity, particularly to hepatocytes, α‐amanitin received an increased attention as a toxin‐component of antibody‐drug conjugates (ADC) in cancer research. Furthermore, the isolation of α‐amanitin from mushrooms as the sole source severely restricts compound supply as well as further investigations, as structure–activity relationship (SAR) studies. Based on a straightforward access to the non‐proteinogenic amino acid dihydroxyisoleucine, we herein present a robust total synthesis of α‐amanitin providing options for production at larger scale as well as future structural diversifications.