"Squaring the clusters": a Mn(III)4Ni(II)4 molecular square from nickel(II)-induced structural transformation of a Mn(II/III/IV)12 cage

A Mn(III)(4)Ni(II)(4) molecular square exhibiting slow magnetization relaxation has been prepared from the reaction of a Mn(II)(4)Mn(III)(6)Mn(IV)(2) cluster and a simple Ni(II) source.     

Preparation of multicompartment lipid-based systems based on vesicle interactions

Various strategies for constructing artificial multicompartment vesicular systems, which primitively mimic the structure of eukaryotic cells, are presented. These model systems are appropriate for addressing several issues such as the understanding of cell processes, the development of nanoreactors and novel multicompartment delivery systems for specific drug applications, the transport through bilayer membranes, and also hypothesizing on the evolution of eukaryotic cells as originating from the symbiotic association of prokaryotes.     

Inclusion Compounds of Plant Growth Regulators in Cyclodextrins. Part VI. Study of the Inclusion Compound of MCPA in β-Cyclodextrin by X-Ray Crystallography

Abstract  

The structure of the 4-chloro-2-methylphenoxyacetic acid (MCPA) inclusion compound in β-Cyclodextrin (β-CD) has been investigated by X-ray crystallography. The inclusion complex crystallizes in the P2₁ space group with unit cell dimensions a = 15.860(2) ?, b = 24.275 (5) ?, c = 19.334(6) ? and β = 108.80(3)°. Its asymmetric unit contains two β-CD molecules which host two disordered MCPA guest molecules (2:2 host:guest stoichiometry) occupying overall five sites. It also contains 16.13 water molecules distributed over 36 sites. One water molecule is entrapped inside the dimeric cavity, forming a hydrogen bond with the chlorine atom of a guest. The two complexes of the asymmetric unit are located alongside and they form head-to-head dimers with the symmetry related complexes by the 2-fold screw axis (b-axis). The dimers form channels developed along the a-axis.     

Embedded birefringent computer-generated holograms fabricated by femtosecond laser pulses

Birefringent computer-generated holograms are fabricated in bulk fused silica by tight focusing of infrared femtosecond laser pulses. The polarization properties of the elliptically polarized diffracted light are in excellent agreement with the theoretical model. We experimentally demonstrate that for such birefringent structures the signal-to-noise ratio increases by approximately 9 dB when polarization filtering is used to suppress the undiffracted beam.     

Adsorption and radical stabilization of humic-Acid analogues and Pb2+ on restricted phyllomorphous clay

Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.     

Magnetic and luminescent properties of manganese-doped ZnSe crystals

Magnetic and photoluminescent properties of manganese-doped ZnSe crystals with different impurity concentrations were investigated. The concentration of Mn²⁺ ions in ZnSe crystals has been varied from 0.01 to 0.3 at%. Magnetic and photoluminescent studies have confirmed the introduction of Mn in ZnSe crystals. It was established that Mn²⁺ ions are responsible for the emission bands with maximum at 616 nm and 633 nm, which correspond to ⁴T₂→⁶A₁ and ⁴T₁→⁶A₁ intracentre transitions of Mn²⁺ ions respectively. It was found that the concentration quenching of the photoluminescent bands is associated with Mn²⁺ ions, which are due to the formation of Mn–Mn clusters. Magnetic properties studies have shown that at high doping levels the manganese atoms form Mn–Mn clusters in ZnSe. From the temperature dependence of magnetic susceptibility of ZnSe:Mn crystals that follows the Curie–Weiss law, it was possible to estimate the Curie–Weiss temperature Θ(x) and the effective Mn–Mn antiferromagnetic exchange constant (J₁).     

Large deviations analysis of the generalized processor sharing policy

In this paper we consider a stochastic server (modeling a multiclass communication switch) fed by a set of parallel buffers. The dynamics of the system evolve in discrete-time and the generalized processor sharing (GPS) scheduling policy of [25] is implemented. The arrival process in each buffer is an arbitrary, and possibly autocorrelated, stochastic process. We obtain a large deviations asymptotic for the buffer overflow probability at each buffer. In the standard large deviations methodology, we provide a lower and a matching (up to first degree in the exponent) upper bound on the buffer overflow probabilities. We view the problem of finding a most likely sample path that leads to an overflow as an optimal control problem. Using ideas from convex optimization we analytically solve the control problem to obtain both the asymptotic exponent of the overflow probability and a characterization of most likely modes of overflow. These results have important implications for traffic management of high-speed networks. They extend the deterministic, worst-case analysis of [25] to the case where a detailed statistical model of the input traffic is available and can be used as a basis for an admission control mechanism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Multidimensional scaling and visualization of large molecular similarity tables

Multidimensional scaling (MDS) is a collection of statistical techniques that attempt to embed a set of patterns described by means of a dissimilarity matrix into a low‐dimensional display plane in a way that preserves their original pairwise interrelationships as closely as possible. Unfortunately, current MDS algorithms are notoriously slow, and their use is limited to small data sets. In this article, we present a family of algorithms that combine nonlinear mapping techniques with neural networks, and make possible the scaling of very large data sets that are intractable with conventional methodologies. The method employs a nonlinear mapping algorithm to project a small random sample, and then “learns” the underlying transform using one or more multilayer perceptrons. The distinct advantage of this approach is that it captures the nonlinear mapping relationship in an explicit function, and allows the scaling of additional patterns as they become available, without the need to reconstruct the entire map. A novel encoding scheme is described, allowing this methodology to be used with a wide variety of input data representations and similarity functions. The potential of the algorithm is illustrated in the analysis of two combinatorial libraries and an ensemble of molecular conformations. The method is particularly useful for extracting low‐dimensional Cartesian coordinate vectors from large binary spaces, such as those encountered in the analysis of large chemical data sets. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 488–500, 2001     

Accelerated Electrophotocatalytic C(sp3)−H Heteroarylation Enabled by an Efficient Continuous-Flow Reactor**

Electrophotocatalytic transformations are garnering attention in organic synthesis, particularly for accessing reactive intermediates under mild conditions. Moving these methodologies to continuous-flow systems, or flow ElectroPhotoCatalysis (f-EPC), showcases potential for scalable processes due to enhanced irradiation, increased electrode surface, and improved mixing of the reaction mixture. Traditional methods sequentially link photochemical and electrochemical reactions, using flow reactors connected in series, yet struggle to accommodate reactive transient species. In this study, we introduce a new flow reactor concept for electrophotocatalysis (EPC) that simultaneously utilizes photons and electrons. The reactor is designed with a transparent electrode and employs cost-effective materials. We used this technology to develop an efficient process for electrophotocatalytic heteroarylation of C(sp³)−H bonds. Importantly, the same setup can also facilitate purely electrochemical and photochemical transformations. This reactor represents a significant advancement in electrophotocatalysis, providing a framework for its application in flow for complex synthetic transformations.     

Robust convex optimization: A new perspective that unifies and extends

Mathematical Programming - Robust convex constraints are difficult to handle, since finding the worst-case scenario is equivalent to maximizing a convex function. In this paper, we propose a new...