Synthesis,physico-chemical,structure and supramolecular properties of pinacolophanes: versatile synthetic precursors to stilbenophanes

Novel pinacolophanes tethered by methyl and ethyl chains were synthesised and characterised. The stereochemistry of these small-sized cyclophanes was determined by NMR spectral data as well as by single-crystal X-ray analysis. Intramolecular hydrogen bondings in the crystal structure of the molecules play an important role in holding their configuration, while the intermolecular hydrogen bondings lead to a designed arrangement of the molecules. The stereoselective transformation of pinacolophanes to stilbenophanes in improved yield is also reported.     

LC Determination of Trace Amounts of Phenoxyacetic Acid Herbicides in Water after Dispersive Liquid–Liquid Microextraction

Dispersive liquid–liquid microextraction (DLLME) for extraction and preconcentration of phenoxyacetic acid herbicides in water samples is described. After adjusting the pH to 1.5, the sample was extracted in the presence of 10% w/v sodium chloride by injecting 1 mL acetone as disperser solvent containing 25 μL of chlorobenzene as extraction solvent. The effect of parameters, such as the nature and amount of extraction and disperser solvents, ionic strength of the sample, pH, temperature and extraction time were optimized. DLLME was followed by LC for the determination of 2,4-dichlorophenoxyacetic acid and 4-chloro-2-methyl phenoxyacetic acid. The method had good linearity and a wide linear dynamic range (0.5–750 μg L^?1) with a detection limit of 0.16 μg L^?1 for both the PAAs, making it suitable for their determination in water samples.     

The Complexity of Upward Drawings on Spheres

Although there is a linear time algorithm to decide whether an ordered set has an upward drawing on a surface topologically equivalence to a sphere, we shall prove that the decision problem whether an ordered set has an upward drawing on a sphere is NP-complete. The proof involves the investigation of the surface topology of ordered sets highlighting especially their saddle points. It echoes the recent, important result due to A. Garg and R. Tamassia (1995) that upward planarity testing is NP-complete, for which we give a new proof.     

An overview of some classical models and discussion of the signature-based models of preventive maintenance

In reliability engineering literature, a large number of research papers on optimal preventive maintenance (PM) of technical systems (networks) have appeared based on preliminary many different approaches. According to the existing literature on PM strategies, the authors have considered two scenarios for the component failures of the system. The first scenario assumes that the components of the system fail due to aging, while the second scenario assumes the system fails according to the fatal shocks arriving at the system from external or internal sources. This article reviews different approaches on the optimal strategies proposed in the literature on the optimal maintenance of multi-component coherent systems. The emphasis of the article is on PM models given in the literature whose optimization criteria (cost function and stationary availability) are developed by using the signature-based (survival signature-based) reliability of the system lifetime. The notions of signature and survival signature, defined for systems consisting of one type or multiple types of components, respectively, are powerful tools assessing the reliability and stochastic properties of coherent systems. After giving an overview of the research works on age-based PM models of one-unit systems and $k$-out-of-$n$ systems, we provide a more detailed review of recent results on the signature-based and survival signature-based PM models of complex systems. In order to illustrate the theoretical results on different proposed PM models, we examine two real examples of coherent systems both numerically and graphically.