Separating principles below

In this paper, we study the Ramsey‐type weak K?nig's Lemma, written , using a technique introduced by Lerman, Solomon, and the second author. This technique uses iterated forcing to construct an ω‐model satisfying one principle T₁ but not another T₂. The technique often allows one to translate a “one step” construction (building an instance of T₂ along with a collection of solutions to each computable instance of T₁) into an ω‐model separation (building a computable instance of T₂ together with a Turing ideal where T₁ holds but this instance has no solution). We illustrate this translation by separating from (reproving a result of Ambos‐Spies, Kjos‐Hanssen, Lempp, and Slaman), and then apply this technique to separate from (which has been shown separately by Bienvenu, Patey, and Shafer).     

Influence of a surfactant and electrolytes on adsorbed polymer layers

Solvent relaxation NMR and small-angle neutron scattering have been used to characterize adsorbed poly(ethylene oxide) (PEO) layers on silica at a range of surfactant and electrolyte concentrations. Below the critical aggregation concentration (cac), the results suggest that sodium dodecyl sulfate (SDS) interacts relatively weakly, perhaps analogously to a simple salt reducing the solvency of PEO. This is evidenced by a decrease in the adsorbed layer thickness combined with an increase in the bound fraction, although the total adsorbed amount is not greatly affected. The layer thickness goes through a minimum at the cac, after which further SDS addition results in the formation of PEO/SDS aggregates that repel each other and, hence, tend to desorb. The adsorbed amount therefore decreases, from 0.7 mg m(-2) initially to 0.2 mg m(-2) with 32 mM SDS. The aggregates that remain adsorbed also repel, and hence, there is an increase in the layer thickness and the persistence length, while the bound fraction is reduced. In comparison, the effects of electrolyte at the ionic strength studied are relatively minimal. There is, however, evidence that the repulsions between adsorbed PEO/SDS aggregates are partially screened, allowing them to approach each other more readily. This leads to a contraction of the adsorbed layer when the SDS concentration is sufficiently high.     

Direct equilibration of soil water for delta(18)O analysis and its application to tracer studies

Current methods for stable oxygen isotopic (delta (18)O) analysis of soil water rely on separation of water from the soil matrix before analysis. These separation procedures are not only time consuming and require relatively large samples of soil, but also have been shown to introduce a large potential source of error. Current research at Queen's University Belfast is focused on using direct equilibration of CO(2) with the pore water to eliminate this extraction step using the automated Multiprep system and a Micromass Prism III isotope ratio mass spectrometer (IRMS). The findings of this research indicate the method is less time consuming, more reliable, and reproducible to within accepted limits (+/-0.1% per thousand delta (18)O). In this study the direct equilibration method is used to analyse delta (18)O tracer profiles in the unsaturated zone of field soils, concurrently with chloride tracer profiles, which can be used to assess infiltration rates and mechanisms through the unsaturated zone. Copyright 1999 John Wiley & Sons, Ltd.     

Strong CH⋅⋅⋅Halide Hydrogen Bonds from 1,2,3‐Triazoles Quantified Using Pre‐Organized and Shape‐Persistent Triazolophanes

The structures associated with halide (F^?, Cl^?, Br^?) complexation inside CH hydrogen‐bonding macrocyclic receptors, called triazolophanes, are characterized using density functional theory (DFT). The associated binding energies in the gas and solution phases are evaluated. The ruffles in the empty triazolophane become smoothed‐out upon Cl^?‐ and Br^?‐ion binding directly into the middle of the cavity. The largely pre‐organized cavity morphs into an elliptical shape to facilitate shorter hydrogen bonds in the north and south regions and longer ones west and east. The smaller F^? ion sits in, and flattens‐out, only the north (or south) region. The 1,2,3‐triazoles show shorter CH???Cl^? contacts than for the phenylenes. Both Cl^? and Br^? show the same binding geometries but Cl^? has a larger binding energy consistent with its stronger Lewis basicity. Model triads were used to decompose the overall binding energy into those of its components. In the course of this triad analysis, anion polarization was identified and its contribution to the triad???Cl^? binding energy estimated. Consequently, the binding energies for the individual aryl units within the comparatively non‐polarized triazolophanes were estimated. The 1,2,3‐triazoles are twice as strong as the phenylenes thus contributing most of the interaction energy to Cl^?‐ion binding. Therefore, the 1,2,3‐triazoles appear to approach the hydrogen bond strengths of the NH donors of pyrrole units.     

Supramolecular self-assembly of dendronized polymers: reversible control of the polymer architectures through acid-base reactions

Acid-base switchable supramolecular dendronized polyacetylenes (DPAs) with increasing steric bulk on going from generation one [G1] to three [G3], were constructed using multiple self-assembly processes between Fréchet-type [G1]-[G3]-dendritic dialkylammonium salts and a dibenzo[24]crown-8-containing polymer. The formation of the supramolecular systems is acid-base switchable to either an ON (rodlike dendronized polymers) or an OFF (flexible polymers) state. Thus, by controlling the superstructures of the supramolecular polymers with the [G1]-[G3] dendrons, it is possible to induce conformational changes within the polymer backbones. The supramolecular dendronized polymers, as well as their threading-dethreading properties, were characterized by (1)H NMR and UV absorption spectroscopies, gel permeation chromatography (GPC) and light scattering (LS). Independent measures of molecular weight (GPC, LS) indicate that DPAs behave as increasingly rigid macromolecules with each generation in solution. Molecular dynamics simulations of each DPA suggest that the lengths of the polymer backbones increase accordingly. Atomic force microscopy of the [G3]-dendronized polystyrene (DPS), as well as the DPAs, reveal surface morphologies indicative of aggregated superstructures.     

Shape persistence delivers lock-and-key chloride binding in triazolophanes

Triazolophanes have recently surfaced as a new class of shape-persistent macrocycles that bind anions. Bearing only triazole- and phenyl-derived CH hydrogen bond donors, these receptors have shown extraordinary Cl(-) binding strengths (>10(6) M(-1), CH(2)Cl(2)). The attributes of the triazolophane that are responsible are presented herein alongside recent literature accounts that have utilized similar strategies in new and exciting supramolecular systems. This review describes how triazolophanes leverage the structural pre-organization characteristic of spherands to take advantage of non-traditional hydrogen bonds originating from extrinsic CH donors.     

帕隆藏布流域堆积体边坡的工程地质特征及稳定性评价

位于藏东南的帕隆藏布流域,处在东喜马拉雅构造结的北缘,是新构造强烈隆升区,断层、褶皱等地质构造发育。在青藏高原隆升及气候变化等内外动力作用下,研究区内堆积体分布广泛,具有独特的工程地质特征并成为该区内地质灾害的主要物源。由于河流的下切及川藏公路的修建,形成了许多高陡的堆积体边坡,以冰碛边坡最为典型。由于其特殊的物源特征及形成过程,冰碛物具有粒度分布范围广、粗粒含量多等与其他沉积物不同的粒度特征。冰碛物粒度分布范围广的特征,导致随着研究尺度的不同,其结构特征也不相同,具有明显的尺度效应。冰碛土的力学强度一般强于其他堆积体,但受不同粒径颗粒的含量及分布、形成时代的影响,其力学特征也有所差异。作者对川藏公路沿线部分典型的剖面进行了量测,所量测的剖面虽然只是整个斜坡的一部分,与经验数据相比,仍表现出高陡的特征,而且很多边坡在既高且陡的形态下仍能保持稳定。本文以极限平衡理论为基础,采用多种方法对这类边坡的稳定性进行计算分析,求出边坡的安全系数。考虑各种地震烈度下边坡的稳定性,得出这些边坡的安全系数与地震加速度具有严格的指数相关关系。     

Aggregation and the estimation of equation systems

Simulations based on two sets of data are used to assess the impact of combining equations on the accuracy of parameter estimates and their asymptotic standard errors.     

Leveraging the Knorr Pyrazole Synthesis for the Facile Generation of Thioester Surrogates for use in Native Chemical Ligation

Facile synthesis of C‐terminal thioesters is integral to native chemical ligation (NCL) strategies for chemical protein synthesis. We introduce a new method of mild peptide activation, which leverages solid‐phase peptide synthesis (SPPS) on an established resin linker and classical heterocyclic chemistry to convert C‐terminal peptide hydrazides into their corresponding thioesters via an acyl pyrazole intermediate. Peptide hydrazides, synthesized on established trityl chloride resins, can be activated in solution with stoichiometric acetyl acetone (acac), readily proceed to the peptide acyl pyrazoles. Acyl pyrazoles are mild acylating agents and are efficiently exchanged with an aryl thiol, which can then be directly utilized in NCL. The mild, chemoselective, and stoichiometric activating conditions allow this method to be utilized through multiple sequential ligations without intermediate purification steps.