Mechanism of the oxidation of sulfides by dioxiranes. 1. Intermediacy of a 10-S-4 hypervalent sulfur adduct

Earlier studies established that dimethyldioxirane (1a) reacts with sulfides 2 in two consecutive concerted electrophilic oxygen-transfer steps to give first sulfoxides 3 and then sulfones 4. The same sequential electrophilic oxidation model was assumed for the reaction of sulfides 2 with the strongly electrophilic methyl(trifluoromethyl)dioxirane (1b). In this paper we report on a systematic and general study on the mechanism of the reaction of simple sulfides 2 with DMDO (1a) and TFDO (1b) which provides clear evidence for the involvement of hypervalent sulfur species in the oxidation process. In the oxidation of sulfides 2a-c, diphenyl sulfide (2d), para-substituted aryl methyl sulfides 2e-i, and phenothiazine 2k with 1b, the major product was the corresponding sulfone 4, even when a 10-fold excess of sulfide relative to 1b was used. The sulfone:sulfoxide 4:3 ratio depends among other factors on the dioxirane 1a or 1b used, the sulfide substitution pattern, the polar, protic, or aprotic character of the solvent, and the temperature. The influence of these factors and also deuterium and (18)O tracer experiments performed allow a general mechanism to be depicted for these oxidations in which the key step is the reversible cyclization of a zwitterionic intermediate, 6, to form a hypervalent sulfur species, 7. The classical sequential mechanism which establishes that sulfides are oxidized first to sulfides and then to sulfones can be enclosed in our general picture of the process and represents just those particular cases in which the zwitterionic intermediate 6 decomposes prior to undergoing ring closure to afford the hypervalent sulfurane intermediate 7.     

Evaluation of molecular weight distribution from dynamic moduli

A method to evaluate molecular weight distribution (MWD) from dynamic moduli is presented here. It relies on the least-square fitting of the dynamic data to a model whose parameters depend on the MWD. In particular, the analytical solution for the relaxation modulus previously obtained from the double reptation model, with the Tuminello step relaxation function and the Generalized Exponential Function (GEX) describing the MWD (Nobile and Cocchini 2000), has been used. A Finite Element Approximation (FEA) has been applied to calculate dynamic moduli from the relaxation modulus as a function of MWD. The sensitiveness of the GEX-double reptation dynamic moduli on the model parameters has also been investigated and the results show that large changes of the M_w/M_n ratio weakly affect the dynamic moduli, while small changes of the M_z/M_w ratio significantly deform the dynamic moduli curves. The use of rheological data to obtain MWD, by the model used in this paper, will, therefore, be able to give rather well defined M_z/M_w ratios, while more uncertainty will be presented in the M_w/M_n results. The so-called GEX-rheological model for the dynamic moduli was applied to fit the experimental data of different polymers in order to obtain the best-fit parameters of the MWD of these polymers, without the need for the inversion of the double reptation integral equation. The stability of the results has been confirmed through the evaluation of the 90% confidence intervals for the first molecular weight averages. Finally, concerning the M_w and M_z values, the predictions obtained from the dynamic moduli measurements differ by less than 10% from those obtained from GPC measurements while, as expected, more uncertainty is present in the M_n predictions. Received: 6 February 2000 Accepted: 22 August 2000     

Tunable Interfacial Properties in Silk Ionomer Microcapsules with Tailored Multilayer Interactions

Microencapsulation techniques represent a critical step in realizing highly controlled transport of functional materials in multiphase systems. The first demonstration of microcapsules prepared from minimally grafted silk ionomers (silk fibroin modified with cationic/anionic charge groups) are presented here. These tailored biomacromolecules have shown significantly increased biocompatibility over traditional polyelectrolytes and heavily grafted silk ionomers, but the low grafting density had previously limited attempts to fabricate stable microcapsules. In addition, the first microcapsules from polyethylene‐glycol‐grafted silk ionomers are fabricated and the corresponding impact on microcapsule behavior is demonstrated. The materials are shown to exhibit pH‐responsive properties, with the microcapsules demonstrating an approx. tenfold decrease in stiffness and an approx. threefold change in diffusion coefficient when moving from acidic to basic buffer. Finally, the effect of assembly conditions of the microcapsules are shown to play a large role in determining final properties, with microcapsules prepared in acidic buffers showing lower roughness, stiffness, and an inversion in transport behavior (i.e., permeability decreases at higher pH).     

Muonic atom X-ray spectroscopy for non-destructive analysis of archeological samples

Journal of Radioanalytical and Nuclear Chemistry - The implementation in the RIKEN-RAL negative muons facility of a new muon beamline monitoring and novel digital data acquisition system for gamma...     

The Cauchy problem for a class of Kovalevskian pseudo-differential operators

We prove the well-posedness of the forward Cauchy problem for a pseudo-differential operator of order with the Log-Lipschitz continuous symbol in the time variable. The characteristic roots of are distinct and satisfy the necessary Lax-Mizohata condition Im . The Log-Lipschitz regularity has been tested as the optimal one for well-posedness in the case of second-order hyperbolic operators. Our main aim is to present a simple proof which needs only a little of the basic calculus of standard pseudo-differential operators.

     

TiO2 thin films for spintronics application: a Raman study

We present the results of a systematic study performed by micro‐Raman spectroscopy on pure anatase, pure rutile and mixed anatase–rutile TiO₂ thin films, deposited by radio frequency magnetron sputtering on quartz substrates, with different thicknesses. The crystal structures of the as‐deposited films were unambiguously determined and a good crystalline homogeneity was revealed by a systematic mapping of the samples. In the mixed‐phase films, the relative amount of the two phases was monitored by a simple analysis of the components of the multi‐Lorentzian fitting curves. For the single‐phase films, the influence of the thickness and the effect of different thermal treatments, carried out to obtain series of thin films differing only for oxygen content, are discussed. The analysis of the scattered light has provided indication about the presence of an interface layer between the substrate and the film, which can play a role in driving the interesting magnetic properties exhibited by our samples, which are of potential usefulness for spintronics application. The results obtained from other techniques are briefly reported and discussed in relation to our systematic Raman characterization. This study points out how Raman investigation can provide suggestions toward the understanding of the complex physical phenomena leading to room‐temperature ferromagnetism in TiO₂ thin films. Copyright © 2009 John Wiley & Sons, Ltd.