On the exceedance point process for a stationary sequence

Summary It is known that the exceedance points of a high level by a stationary sequence are asymptotically Poisson as the level increases, under appropriate long range and local dependence conditions. When the local dependence conditions are relaxed, clustering of exceedances may occur, based on Poisson positions for the clusters. In this paper a detailed analysis of the exceedance point process is given, and shows that, under wide conditions, any limiting point process for exceedances is necessarily compound Poisson. More generally the possible random measure limits for normalized exceedance point processes are characterized. Sufficient conditions are also given for the existence of a point process limit. The limiting distributions of extreme order statistics are derived as corollaries.This research has been supported by the Air Force Office of Scientific Research Grant No. F 49620 85 C 0144 and the Katholieke Universiteit Leuven     

3-aminoacyl-tetrahydrothiazole-2-thione as an active amide for peptide synthesis (I)

3-Aminoacyl-tetrahydrothiazole-2-thione can be used as an active amide for peptide synthesis. A series of peptides have been synthesized with satisfactory yields by this methods.     

Amphiphilic poly(D,L-lactic acid)/poly(ethylene glycol)/poly(D,L-lactic acid) nanogels for controlled release of hydrophobic drugs

Photocrosslinked nanogels with a hydrophobic core and hydrophilic shell are successfully fabricated with the goal of obtaining a biocompatible and biodegradable drug carrier for hydrophobic anticancer drugs. These nanogels are composed of amphiphilic triblock copolymers, poly(D,L-lactic acid)/poly(ethylene glycol)/poly(D,L-lactic acid) (PLA-PEG-PLA), with acrylated groups at the end of the PLA segments. The copolymers are synthesized by ring-opening polymerization and possess a low CMC (49.6 mg x L(-1)), which easily helps to form micelles by self-assembly. The acrylated end groups allow the micelles to be photocrosslinked by ultraviolet irradiation, which turn the micelles into nanogels. These nanogels exhibit excellent stability as a suspension in aqueous media at ambient temperature as compared to the micelles. Moreover, the size of the nanogels is easily manipulated in a range of 150 to 250 nm by changing the concentration of crosslinkers, e.g., ethylene glycol dimethacrylate, and ultraviolet light irradiation time. The nanogels achieve a high encapsulation efficiency and offer a steady and long-term release mechanism for the hydrophobic anticancer drug, CPT. It shows that these nanogels are useful for a hydrophobic anticancer drug-carrier system. [pictures: see text] Formation of the PLA-PEG-PLA nanogels.     

Solvothermal Syntheses and Magnetic Properties of Two New Coordination Polymers with Mixed Ligands

Two new coordination polymers, [Ni(H₂O)(BDC)(bpp)]_n ( 1 ) and [Co(mBDC)(bpp)]_n ( 2 ) (BDC = 1,4‐benzene dicarboxylate anion; bpp = 4,4′‐trimethylene dipyridine; mBDC = 1,3‐benzene dicarboxylate anion), were synthesized by solvothermal reaction and structurally characterized. Single crystal X‐ray diffraction showed that 1 consists of an inclined interpenetration of 1D nets generating an overall 3D entanglement and compound 2 of 1D chains. The luminescent and magnetic properties of both compounds in the solid state were also investigated. Crystal data: 1 : orthorhombic, Pnna; a = 18.3648(6), b = 13.6854(4), c = 16.2307(5) Å, V = 4079.3(2) ų, and Z = 8. Crystal data: 2 : triclinic, ; a = 8.6966(4), b = 10.1093(4), c = 11.8334(4) Å, α = 68.263(1)°, β = 82.895(1)°, γ = 78.370(1)°, V = 945.16(7) ų, and Z = 2.     

Separation and Retention Behavior of Aromatic Carboxylic Acid Isomers by High‐Performance‐Liquid‐Chromatography Using β‐Cyclodextrin Bonded Phase with Diamine‐s‐Triazine Moiety

A β‐cyclodextrin (β‐CD) bonded phase with diamine‐s‐triazine moiety was prepared. The separation and retention behavior of the isomers of five aromatic carboxylic acids, including toluic acid, aminobenzoic acid, nitrobenzoic acid, hydroxybenzoic acid, and naphthoic acid were investigated by a high‐performance liquid chromatography (HPLC) using the β‐CD bonded phase prepared. The influence of mobile phase pH in the range of 2.7‐3.6 on the retention of these analytes was examined. The isomers of the aromatic carboxylic acids, with the exception of nitrobenzoic acid, were optimally and effectively separated at pH 2.7, while the three isomers of nitrobenzoic acid could be well separated at pH 3.3. Compared with the chromatographic results obtained previously on the amine‐s‐triazine‐β‐CD bonded phase, the retention factors of the isomers of aromatic carboxylic acid on the diamine‐s‐triazine‐β‐CD bonded phase increase to a relatively much greater extent. Thus, the functionality of the spacer arm of the bonded phase playing an important role in the retention of aromatic carboxylic acid isomers is demonstrated. The results also imply that the hydrogen‐bonding interaction and the mechanism of anion exchange sorption as well may contribute significantly to the retention mechanisms.     

In Situ Raman Monitoring of Silver(I)‐Aided Laser‐Driven Cleavage Reaction of Cyclobutane

The cyclobutane cleavage reaction is an important process and has received continuous interest. Herein, we demonstrate the visible laser‐driven cleavage reaction of cyclobutane in crystal form by using in situ Raman spectroscopy. Silver(I) coordination‐induced strain and thermal effects from the laser irradiation are the two main driving forces for the cleavage of cyclobutane crystals. This work may open up a new avenue for studying cyclobutane cleavage reactions, as compared to the conventional routes using ex situ techniques.     

Effect of processing conditions on the properties of high molecular weight conductive polyaniline fiber

Polyaniline–emeraldine base (EB) fiber with excellent mechanical and electrical properties have been spun from highly concentrated (20% w/w), EB/N‐methyl‐2‐pyrrolidinone (NMP)/2‐methylaziridine (2 MA) solution. These solutions had gelation times, which varied from hours to days depending on the molar ratio of 2 MA to EB tetramer repeating unit in the N‐methyl‐2‐pyrrolidinone (NMP) solvent. To better compare the mechanical and electrical properties, dense films were also prepared by thermal evaporation of less concentrated solution (1% w/w). Both fibers and films were amenable to thermal stretching with maximum draw ratios of 4 : 1 and these stretched samples exhibited the greatest tensile strength overall. Wide‐angle X‐ray diffraction (WAXD) of as‐spun and 4‐times stretched fiber showed a completely amorphous structure. Fiber subjected to heat treatment at 250 °C under N₂ flux for 2 h displayed further improvements in mechanical properties because of crosslinking between the polymer chains. Fibers and films were later doped by immersion in a variety of aqueous acid solutions. Room temperature DC conductivities for the doped samples ranged from 6 × 10⁻⁴ to 45 S/cm depending on the specific choice of acid. Scanning electron microscopy of fiber samples shows the presence of macrovoid formation during fiber spinning. Continued refinement of the processing parameters and fiber post‐treatment, to enhance chain alignment and increase fiber density, will likely lead to additional improvements in the fiber mechanical and electrical properties. Characterization of emeraldine base (EB) powder, solution, films, and fibers by UV‐Vis, DSC, TGA, and WAXD were also performed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 194–204, 2000     

Synthetic polyhydroxypolyamides from galactaric,xylaric, D‐glucaric,and D‐mannaric acids and alkylenediamine monomers—some comparisons

The condensation polymerization in a methanol solution of four different esterified aldaric acids (D ‐glucaric, meso‐xylaric, meso‐galactaric, and D ‐mannaric) with even‐numbered alkylenediamines (C₂–C₁₂) gave polyhydroxypolyamides whose water solubilities and melting points were compared. In general, an increase in the alkylenediamine monomer length resulted in decreased polyamide water solubility. Differences in the polymer melting points and water solubilities were linked primarily to conformational differences of the monomer aldaryl units; for example, polyamides from meso‐galactaric acid with an extended zigzag conformation aldaryl monomer unit had higher melting points and lower water solubilities than those from D ‐glucaric and meso‐xylaric acids. The latter acid monomer units tended toward bent conformations that served to diminish intermolecular attractive forces between polymer chains, affecting polymer solubility and melting characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 594–603, 2000     

Structure and stability of high‐spin Aun(n = 2–8) clusters

The structures and relative stability of the maximum‐spin ⁿ⁺¹Au_n and ⁿAu (n = 2–8) clusters have been determined by density‐functional theory. The structure optimizations and vibrational frequency analysis are performed with the gradient‐corrections of Perdew along with his 1981 local correlation functional, combined with SBKJC effective core potential, augmented in the valence basis set by a set of f functions. We predicted the existence of a number of previously unknown isomers. The energetic and electronic properties of the small high‐spin gold clusters are strongly dependent on sizes. The high‐spin clusters tend to holding three‐dimensional geometry rather than planar form preferred in low‐spin situations. In whole high‐spin Au_n (n = 2–8) neutral and cationic species, ⁵Au₄, ²Au, and ⁴Au are predicted to be of high stability, which can be explained by valence bond theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009