Theoretical studies of alkyl radicals in the NaY and HY zeolites

Interplay of quantum mechanical calculations and experimental data on hyperfine coupling constants of ethyl radical in zeolites at several temperatures was engaged to study the geometries and binding energies and to predict the temperature dependence of hyperfine splitting of a series of alkyl radicals in zeolites for the first time. The main focus is on the hyperfine interaction of alkyl radicals in the NaY and HY zeolites. The hyperfine splitting for neutral free radicals and free radical cations is predicted for different zeolite environments. This information can be used to establish the nature of the muoniated alkyl radicals in the NaY and HY zeolites via muSR experiments. The muon hyperfine coupling constants of the ethane radical cation in these zeolites are very large with relatively little dependence on temperature. It was found that the intramolecular dynamics of alkyl free radicals are only weakly affected by their strong binding to zeolites. In contrast, the substrate binding has a significant effect on their intermolecular dynamics.     

Organic free radicals in superheated water studied by muon spin spectroscopy

There is a pressing need to identify and monitor reaction intermediates in water at high temperatures and pressures, but conventional techniques have limited capability for studying transient free radicals under such challenging conditions. Apparatus has now been developed to permit muon avoided-level crossing spectroscopy (muLCR) of organic free radicals in superheated water. The combination of muLCR with transverse-field muon spin rotation (TF-muSR) provides the means to identify and characterize free radicals via their nuclear hyperfine coupling constants. Because the radicals are derived from the addition of muonium (Mu = mu+ e-) to unsaturated compounds, the ensuing muoniated free radicals correspond to conventional organic free radicals but with a muon spin label substituted for one of the protons. Muon spin spectroscopy is the only technique presently being used to characterize transient free radicals under hydrothermal conditions in an unambiguous manner, free from interference from other reaction intermediates. This paper demonstrates how muoniated radicals can be used to monitor the species present in hydrothermal systems, and examples are presented from two classes of reaction: dehydration of alcohols and enolization of ketones. Spectra are displayed and hyperfine constants reported for muoniated forms of the following free radicals in superheated water (typically 350 degrees C at 250 bar): 2-propyl, 2-methyl-2-propyl (tert-butyl), and 2-hydroxy-2-propyl. The latter radical is the product of muonium addition to both the keto and the enol forms of acetone, but different isotopomers are produced according to which reaction channel is dominant. This should prove invaluable in future studies of the role of enols in combustion.     

Nuclear magnetic resonance spectroscopic studies of the trihexyl (tetradecyl) phosphonium chloride ionic liquid mixtures with water

Tetra-alkyl Phosphonium ionic liquids are phosphonium salts with melting points near room temperature. We report the NMR studies of water-trihexyl (tetradecyl) phosphonium chloride ionic liquid mixtures. The proton chemical shifts were used to investigate the intermolecular interactions in mixtures of ionic liquids and water. The OH chemical shifts were found to decrease as the water concentration in the ionic liquid increased, and their rate of change with temperature decreased with water concentration. The CH₂ and CH₃ chemical shifts were found to move downfield with the increase in temperature, and moved further downfield as water concentration was decreased. The interface of experimental data and the results of quantum calculations suggest a significant binding of phosphonium cations to chloride anion and water molecules. As well, the analysis of the data suggests a possible transformation from cationchloride-water configuration at low water concentrations to cation-water-water at higher water concentrations. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Determination of complex modes in photonic crystal waveguides using the phase variation in characteristic coefficients

An efficient frequency-domain method, the phase variation monitoring (PVM) method, is proposed to determine the electromagnetic eigenmodes in two-dimensional photonic crystal waveguides. The proposed method is based on monitoring the reflection and transmission coefficients of incident plane waves. It is successfully applied to an illustrative line-defect photonic crystal waveguide and proved to be capable of calculating the in-plane leakage through the finite-size photonic crystal surrounding the line-defect. Calculation of the leakage loss is not only important for proper understanding of wave propagation within the defect but also for its significant role in applications of photonic structures.     

Dynamical Aspects of Mean Field Plane Rotators and the Kuramoto Model

The Kuramoto model has been introduced in order to describe synchronization phenomena observed in groups of cells, individuals, circuits, etc. We look at the Kuramoto model with white noise forces: in mathematical terms it is a set of N oscillators, each driven by an independent Brownian motion with a constant drift, that is each oscillator has its own frequency, which, in general, changes from one oscillator to another (these frequencies are usually taken to be random and they may be viewed as a quenched disorder). The interactions between oscillators are of long range type (mean field). We review some results on the Kuramoto model from a statistical mechanics standpoint: we give in particular necessary and sufficient conditions for reversibility and we point out a formal analogy, in the N→∞ limit, with local mean field models with conservative dynamics (an analogy that is exploited to identify in particular a Lyapunov functional in the reversible set-up). We then focus on the reversible Kuramoto model with sinusoidal interactions in the N→∞ limit and analyze the stability of the non-trivial stationary profiles arising when the interaction parameter K is larger than its critical value K _c . We provide an analysis of the linear operator describing the time evolution in a neighborhood of the synchronized profile: we exhibit a Hilbert space in which this operator has a self-adjoint extension and we establish, as our main result, a spectral gap inequality for every K>K _c .     

Stability of W/O Emulsions Encapsulating Polysaccharides

In the frame of formulation of W/O emulsions entrapping polysaccharides devoted to agricultural applications, the aim of this work was to study the stability over time of these emulsions, stabilized with either soybean lecithin or polyglycerol polyricinoleate (PGPR) as emulsifiers. Emulsifiers were dissolved in oil phase, and polysaccharides (carboxymethycellulose (CMC), guar, xanthan) in ultrapure water. Emulsions stability was studied through natural aging tests and accelerated aging tests, using bottle tests, microscopy and calorimetry. Experiments showed that PGPR was more efficient than lecithin to stabilize emulsions containing the polysaccharides studied, and that emulsions prepared with CMC showed the best stability.     

Top-down approach to fiber-top cantilevers

Taking inspiration from conventional top-down micromachining techniques, we have fabricated a low mass gold fiber-top cantilever via align-and-shine photolithography. The cantilever is characterized by measuring its resonance frequency and mechanical quality factor. Our results show that the device grants mass sensitivity comparable to that reported for similar standard cantilevers. This proof-of-concept paves the way to series production of highly sensitive fiber-top devices for remote detection of biochemical substances.     

Study of the numerical artifacts in differential analysis of highly conducting gratings

Li's Fourier factorization rule [J. Opt. Soc. Am. A13, 1870 (1996)] was recently shown to be problematic to apply to highly conducting metallic gratings. We provide further information about the applicability of different differential methods and are concerned with the relation of observed numerical artifacts, the total number of retained space harmonics, the presence of both positive and negative permittivity inside the groove region, and the validity of Li's inverse rule. Two different cases corresponding to lossless and low-loss binary metallic gratings are considered, and it is shown that an increase in the number of retained space harmonics can relieve the presence of numerical artifacts.     

Electrochemical and mechanical characteristics of Al-(x)SiC composite fabricated by high energy compaction

Simultaneous compaction and sintering using the energy generated by the underwater explosion is a rapid method that does not have the negative consequences of normal sintering, such as unwanted reactions between components and grain growth. For this reason, this study used the energy from this process to produce Al/SiC composites with different amounts of SiC. As a result, 10, 20, and 30% of SiC powder were added to the Al powder, and then the samples were produced by compression under the influence of an underwater explosion. The results showed that as the amount of SiC increased, the samples' density and the amount of porosity for further agglomeration increased. Microstructural examination by Scanning Electron Microscopy (SEM) confirmed the increase in agglomeration with increasing SiC. Besides, Samples experienced an increase in hardness as the amount of SiC raised. Adding 10% of SiC increased the flexural strength while adding more (20 and 30%) increased the agglomeration resulting in a decrease in strength and elongation. Polarization and impedance spectroscopy (EIS) methods were used to study the samples' electrochemical behavior in a 3.5% NaCl solution. Results showed that the addition of SiC particles up to 10% (wt%) reduces the corrosion density from 9.7 (μA/cm²) to 2.06 (μA/cm²). The Al–10%SiC composite had the lowest corrosion current density compared to 20% and 30% SiC composites. In addition, the polarization resistance of the Nyquist plot for pure Al and Al composite containing 10% SiC was 3965 and 7862, respectively. Therefore, these results showed the beneficial effect of 10% SiC as reinforcing particles on the corrosion behavior of Al composites and thus improved corrosion resistance.     

Hyperfine coupling in methyl radical isotopomers

The hyperfine coupling constants (hfcs) of two methyl radical isotopomers, CH2Mu and CD2Mu, have been measured over a wide range of temperature in ketene and ketene-d2, from which the radicals were generated. The magnitudes of the hfcs of these muoniated methyl radical isotopomers are larger than those of CH3 and CD3 due to larger zero-point energy in the out-of-plane bending mode. In contrast to CH3 and CD3, where the coupling constants become smaller with increasing temperature, the negative hfcs of the muoniated radicals were found to increase in magnitude (become more negative) with temperature, passing through a maximum near the boiling point of ketene. This behavior is attributed to a solvent-induced change in the force constant of the out-of-plane bending mode. The opposite temperature effect known for CH3 and CD3 is explained by excitation of the low frequency out-of-plane bending mode. This effect is much smaller in the muoniated radicals, where the vibrational frequency is significantly higher due to the light mass of muonium; consequently, the solvent effect dominates at low temperatures.