Transacetalization with gaseous carboxonium and carbosulfonium ions

Primary carboxonium (H2C=O+-R) and carbosulfonium (H2C=S+-R) ions (R = CH3, C2H5, Ph) and the prototype five-membered cyclic carboxonium ion are found to react in the gas phase with cyclic acetals and ketals by transacetalization to form the respective O-alkyl-1,3-dioxolanium and S-alkyl-1,3-oxathiolanium ions. The reaction, which competes mainly with proton transfer and hydride abstraction, initiates by O-alkylation and proceeds by ring opening and recyclization via intramolecular displacement of the carbonyl compound previously protected in its ketal form. As indicated by product ion mass spectra, and confirmed by competitive reactions, carbosulfonium ions are, by transacetalization, much more reactive than carboxonium ions. For acyclic secondary and tertiary carboxonium ions bearing acidic alpha-hydrogens, little or no transacetalization occurs and proton transfer dominates. This structurally related reactivity distinguishes primary from both secondary and tertiary ions, as exemplified for the two structural isomers H2C=O+-C2H5 and CH3C(H)=O+-CH3. The prototype five- and six-membered cyclic carboxonium ions react mainly by proton transfer and adduct formation, but the five-membered ring ion also reacts by transacetalization to a medium extent. Upon CID, the transacetalization products of the primary ions often dissociate by loss of formaldehyde, and a +44 u neutral gain/-30 u neutral loss MS3 scan is shown to efficiently detect reactive carboxonium and carbosulfonium ions. Transacetalization with either carboxonium or carbosulfonium ions provides a route to 1,3-oxathiolanes and analogs alkylated selectively either at the sulfur or oxygen atom.     

Nanofibers composite vanadium oxide/polyaniline: synthesis and characterization of an electroactive anisotropic structure

This contribution describes the synthesis of a hybrid nanocomposite constituted by vanadium oxide/polyaniline (PAni) with an interesting fibrilar morphology. Nanofibers can be obtained as main reaction products of nanocomposite V₂O₅/PAni and hexadecylamine after hydrothermal treatment. In general, nanocomposite nanofibers present a typical length varying from 1 to 10 μm and a width varying from 15 to 400 nm. Electrochemical experiments have shown a specific capacity of about 150 A h kg⁻¹ during the 10 initials cycles, revealing a promissory material for utilization as cathode for ion-Li batteries.     

Ultra‐light carbon nanotube sponge as an efficient electromagnetic shielding material in the GHz range

CVD synthesised CNT flexible sponge with density lower than 0.02 g cm^–3 has been found to serve as high performance EMI shielding material without the aid of any polymer infiltration or impregnation. Due to its extreme lightweight, the specific SE of the CNT‐sponge was found to be as high as 1100 dB cm³ g^–1, having a total SE above 20 dB in the whole 1–18 GHz range, and being able to shield by absorption. The material is the best of our knowledge this specific SE value appears to be the highest reported hitherto. Improved EM absorbers should fulfil the synergic requirements of being low reflective and highly absorptive. In our CNT‐sponges this condition is not satisfied because, although their net absorption ability is strongly remarkable, their high electrical conductivity favours the wave to be reflected at the input interface. Therefore, this sponge material would have a great potential for microwave‐frequency applications that need negligible reflection and great absorption when combined in a multilayered structure that could prevent the wave to be reflected at the input interface. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Concentration-compactness principle for an inequality by D. Adams

This paper brings a generalization of the Lions concentration–compactness principle to the Sobolev space \(W_0^{m,p}(\Omega )\) when \(mp=n\) and \(\Omega \subset \mathbb {R}^n \, (n \ge 2)\) is a smooth domain with finite \(n\) -measure. Moreover, our result sharpens an inequality by D. Adams improving its best exponent. The proof is established using the decreasing rearrangement and comparison principle due to Talenti combined with the maximum principle. It is also shown how this concentration–compactness principle yields the existence of critical points of mountain-pass type for an energy functional associated with a class of polyharmonic problems.     

Correlation between the piezoelectric properties and the structure of lead‐free KNN‐modified ceramics,studied by Raman Spectroscopy

The structure of (Li,Na,K)(Nb,Ta,Sb)O₃ piezoelectric ceramics was thoroughly studied by X‐ray diffraction (XRD) and Raman spectroscopy for samples with different compositions and sintered for different times. A linear relationship is evidenced between the tetragonality ratio derived from XRD and the Raman shift. The Raman stretching modes are used to demonstrate the correlation of the structure and the piezoelectric properties of the materials, and to calculate the effective ionic displacement causing the piezoelectric polarization. The methodology proposed here offers invaluable insight into the nondestructive Raman analysis of perovskite structures based on the potassium sodium niobate systems, which are not amenable by studies of diffraction or other spectroscopic techniques. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization of a sealed Americium–Beryllium (AmBe) source by inductively coupled plasma mass spectrometry

Two Americium–Beryllium neutron sources were dismantled, sampled (sub-sampled) and analyzed via inductively coupled plasma mass spectrometry (ICP-MS). Characteristics such as “age” since purification, actinide content, trace metal content and inter and intra source composition were determined. The “age” since purification of the two sources was determined to be 25.0 and 25.4 years, respectively. The systematic uncertainties in the “age” determination were ±4% 2σ. The amount and isotopic composition of U and Pu varied substantially between the sub-samples of Source 2 (n = 8). This may be due to the physical means of sub-sampling or the way the source was manufactured. Source 1 was much more consistent in terms of content and isotopic composition (n = 3 sub-samples). The Be–Am ratio varied greatly between the two sources. Source 1 had an Am–Be ratio of 6.3 ± 52% (1σ). Source 2 had an Am–Be ratio of 9.81 ± 3.5% (1σ). In addition, the trace element content between the samples varied greatly. Significant differences were determined between Sources 1 and 2 for Sc, Sr, Y, Zr, Mo, Ba and W.     

Development of an electrochemical sensor for potassium ions based on KSr2Nb5O15 modified electrode

In the work described by this paper, we studied the development of a selective potassium ion sensor constituted of a carbon paste electrode modified (CPEM) with a novel KSr₂Nb₂O₁₅. The material KSr₂Nb₂O₁₅ is an oxide with the tetragonal tungsten bronze structure (TTB) type are in forefront both in the area of research as well as in industrial applications. The sensor response to potassium ions was linear in the concentration range 1.26 x 10^-5 at 1.62 x 10^-3 mol L^-1 (E (mV) = 32.7 + 51.1 log [K⁺]). The sensor based KSr₂Nb₂O₁₅, of the TTB-type presented very good potentiometric response, with a slope of 51.1 mV/dec (at 25 °C) and detection limit for the potassium ions of 7.27 x 10^-5 mol.L^-1.     

Dimerization of ionized 4‐(methyl mercapto)‐phenol during ESI,APCI and APPI mass spectrometry

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4‐(methyl mercapto)‐phenol followed by fast H^· loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI‐charged droplets and in the gas‐phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4‐(methyl mercapto)‐phenol. Copyright © 2009 John Wiley & Sons, Ltd.     

Convergent procedure for the synthesis of trifluoromethyl-containing N-(pyridinyl-triazolyl)pyrimidin-2-amines

This study describes a simple and efficient procedure to synthesize a novel series of fourteen 4-substituted N-(5-pyridinyl-1H-1,2,4-triazol-3-yl)-6-(trifluoromethyl)pyrimidin-2-amines, where the 4-substituents are H, CH₃, C₆H₅, 4-FC₆H₄, 4-CH₃C₆H₄, 4-CH₃OC₆H₄ and 2-Furyl; from the cyclocondensation reaction of N-[5-(pyridinyl)-1H-1,2,4-triazol-3-yl]guanidines with 4-alkoxy-4-alkyl(aryl/heteroaryl)-1,1,1-trifluoroalk-3-en-2-ones. The reactions were carried out in ethanol under reflux for 18 h and led to 40-68% yields. N-(Pyridyl-triazolyl)guanidine precursors were further obtained from reactions of cyanoguanidine with nicotinic or isonicotinic acid hydrazides and the halogenated enones from trifluoroacetylation of enolethers or acetals.