Quantum mechanical calculations of conformationally relevant 1H and 13C NMR chemical shifts of calixarene systems

[graphs: see text] QM GIAO calculations of 13C and 1H chemical shift values of the ArCH2Ar group have been performed, using the hybrid DFT functional MPW1PW91 and the 6-31G(d,p) basis set, on some representative calixarenes and on a series of simplified calixarene models allowing derivation of chemical shift surfaces versus phi and chi dihedral angles. A good reproduction of experimental data was obtained. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated.     

Uncharged water-soluble Co(II)-porphyrin: a receptor for aromatic alpha-amino acids

Changes in the UV-vis spectra and induced circular dichroism (ICD) signals observed, in correspondence with the porphyrin Soret region, for aqueous solutions of achiral 5,10,15,20-tetrakis{p-[omega-methoxy poly(oxy-ethylene)]phenyl}porphyrin cobalt (II) (Co-P) and aromatic alpha-L-amino acids (Trp and Phe) give direct evidence for the coordination between the Co-P and amino acids. Considering that Co-P, besides the Co atom (one-fixation-point system), does not contain in the molecule active ligand groups and that no ICD signals have been observed in the case of Co-P/Ala, it has been concluded that hydrophobic interactions or stacking interactions between the aromatic rings of the porphyrin and those of Trp or Phe, acting as further amino acid (AA) fixation points, can strongly reduce the mobility of the chiral guest, thus permitting the generation of ICD signals. The effects of changes of both pH (in the range 2-9) and amino acid structure on the ICD phenomenon have also been investigated. In particular, the following have been observed: (i) strong ICD signals for all of the Co-P/N-acetyl amino acid aqueous solutions at pH 7, (ii) an unexpected ICD band with a bisignate form for the Co-P/Ala solution at pH 9 after long aging, and (iii) an opposite ICD signal when alpha-D-Phe and alpha-D-Trp enantiomers have been used. The data reported in this paper show how the binding mechanism between receptor and AAs changes by modulating properly the pH or the molecular structures and indicate that in these aqueous solutions the coordination Co-N is not the fundamental mechanism giving rise to the formation of the complexes and that the binding can be driven by hydrophobic interactions. These occurrences, through the analysis of the spectroscopic response (and, in particular, the form of the ICD band), can allow the recognition of AAs.     

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl₄ in mixtures with H₂ and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H₂ ratio strongly affects the plasma composition as well as the deposition (r _D) and etch (r _E) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiCl_x species, which are the precursors of the film growth. Chemisorption of SiCl_x and the reactive surface reaction SiCl_x+H–SiCl_(x–1)0+HCl are important steps in the deposition process. The suggested deposition model givesr _D [SiCl_x][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H₂ ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.     

Atropisomerism in 1,5-bridged calix[8]arenes

[structure: see text] The first example of two discrete calix[8]arene conformational isomers, 2 and 2a, has been obtained by exhaustive benzylation of 1,5-tetramethylene-bridged calix[8]arene 1. It is demonstrated, with the aid of X-ray crystallography, that these atropisomers have two 3/4-cone halves oriented syn or anti with respect to the bridge/bridgeheads moiety. VT NMR studies indicate that the tert-butyl-through-the-annulus inversion is inhibited in 1, while groups larger than n-hexyl or benzyl are required for curtailing the O-through-the-annulus route.     

Effect of delay time and grid voltage changes on the average molecular mass of polydisperse polymers and polymeric blends determined by delayed extraction matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The dependence of the calculated average molecular mass of a polyethylene glycol with a large polydispersity on the instrumental parameters adopted in the acquisition of mass spectra using delayed extraction matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (DE MALDI-TOFMS) was investigated. It has been shown that a combined effect of delay times and potential gradients can act on the ion cloud in the source chamber affecting both mass resolution and average molecular mass value of the analyzed polymeric sample. Also examined was a blend of two different polymers (a PEG and a PMMA commercial sample having a similar average molecular mass), which presents an additional problem concerning the discrimination among the different polymer species as a function of the experimental conditions. In this work, the best instrumental conditions to obtain both good resolution and a correct average molecular mass for the examined polydisperse sample are reported.     

Light propagation in 1D photonic crystals with dissipation

Light propagation in one-dimensional (1D) photonic crystals with dissipation (including metal layers) is studied analytically. The proper effective refractive indices and extinctions of dissipative layers in case of oblique incidence are introduced. Then, the formalism available for nondissipative photonic crystals is modified by replacing the conventional refractive indices with effective ones. Propagation in a bilayer dielectric-metallic crystal is computed as an example.     

Oxygenation of calixarene phenol rings

[structure: see text] The first examples of epoxy-p-quinol and diepoxy-p-quinol calixarene derivatives have been obtained by base-promoted direct addition of O(2)(oxygenation) to calixarene phenol rings. The regio- and stereochemistry of these derivatives was determined by 2D NMR studies, in conjunction with MM3 calculations, and X-ray crystallography. Both the oxygenation and the subsequent carbonyl reduction occur with a preferential attack to the less hindered exo face of the calixarene rings.     

The influence of sulfonation degree on the thermal behaviour of sulfonated poly(arylene ethersulfone)s

Initial decomposition temperature (T_i), apparent activation energy of degradation (E_a) and glass transition temperature (T_g) of some low molar mass (M_n ≈ 8000 g mol⁻¹) sulfonated poly(arylene ethersulfone)s s-(PAES)s were determined to check their dependence on sulfonation degree (SD). The results obtained were compared with those for unsulfonated poly(arylene ethersulfone) PAES. In order to have an accurate control of the chemical structure, a pre-sulfonation route was followed for the preparation of sulfonated compounds. The thermal behaviour of the investigated s-(PAES)s as well as that of PAES appears not to be influenced by the environment (flowing nitrogen or static air atmosphere) of degradation. Both T_i and T_g values of s-(PAES)s were higher than those of PAES and increased quite linearly as a function of sulfonation degree. An analogous linear trend was observed for the apparent degradation energy of s-(PAES)s, but the values found were largely lower than those of unsulfonated homopolymer. The results are discussed and interpreted.     

Synthesis and characterization of sulfonated copolyethersulfones

The present article deals with the synthesis and characterization of some sulfonated copolyethersulfones. The synthetic approach differs from the post sulfonation approach traditionally reported in the literature. The synthetic procedure is based on the use of sulfonated monomers which are then reacted with previously synthesized telechelic hydoxy‐ended poly (ether sulpnone)s. Combining the MALDI‐TOF MS and ¹H NMR analyses, with SEC‐Viscometry and TGA measurements, we demonstrate a powerful tool for characterizing the chemical composition, end chains, degree of sulfonation (DS) and molecular mass distribution (MMD) of disulfonated poly(arylene ether‐sulfone) copolymers. The characterization techniques allowed to determine the exact nature of the copolymers synthesized and to reveal some interesting features about the reaction. DMA data show that the glass transition temperature of sulfonated copolymers with similar DS increase as raise their MMD. Copolymers with a DS of 10–11 mol % reach a T_g of 244–246 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3010–3023, 2010     

Weighted residuals as a basis of a general solution method in elasticity

Summary It is shown that the general integral form of the elastic equilibrium equations obtainable through the weighted residuals agrees with the variational formulation given by the extremum conditions of the Washizu functional allowing a complete relaxation of the interelement continuity requirements.

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Sommario Si mostra che la forma generate integrale del sistema di equazioni dell'equilibrio elastico a cui conduce il metodo dei residui con fattori ponderali concorda con la formulazione variazionale fornita dalle condizioni di estremo del funzionale di Washizu che deriva da quello di Hellinger-Reissner mediante sblocco completo dei vincoli di continuità fra regioni contigue.