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151.
Tancini F Yebeutchou RM Pirondini L De Zorzi R Geremia S Scherman OA Dalcanale E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(48):14313-14321
The outstanding complexing properties of tetraphosphonate cavitands towards N‐methylpyridinium salts were exploited to realise a new class of linear and cyclic AABB supramolecular polymers through host–guest interactions. The effectiveness of the selected self‐association processes was tested by 1H NMR studies, whereas microcalorimetric analyses clarified the binding thermodynamics and revealed the possibility of tuning entropic contributions by acting on the flexibility of the guest linker. Although the formation of linear polymeric chains for a rigid system was demonstrated by X‐ray analysis, the presence of a concentration‐dependent ring–chain equilibrium was indicated by solution viscosity measurements in the case of a very flexible ditopic BB guest co‐monomer. 相似文献
152.
153.
Rita C. Almeida‐Lafet Marcelo J. P. Ferreira Vicente P. Emerenciano Maria A. C. Kaplan 《Helvetica chimica acta》2010,93(12):2478-2487
In the first phytochemical study of the Aureliana genus (Solanaceae), two new withanolides, 1 and 2 , together with two known sterols, were isolated from the MeOH extract of the leaves of Aureliana fasciculata var. fasciculata. The structures were established as (4S,22R)‐16α‐acetoxy‐5β,6β‐epoxy‐4β,17α‐dihydroxy‐1‐oxowitha‐2,24‐dienolide (aurelianolide A) and (4S,22R)‐16α‐acetoxy‐4β,17α‐dihydroxy‐1‐oxowitha‐2,5,24‐trienolide (aurelianolide B). The new compounds possessed the unusual 16α,17α‐dioxygenated group and were fully characterized by spectroscopic techniques, including 1H‐ and 13C‐NMR (DEPT), as well as 2D‐NMR (HMBC, HMQC, 1H,1H‐COSY, NOESY) experiments, and HR‐MS. 相似文献
154.
The rigid [6]ferrocenophane, L1, was synthesised by condensation of 1,1′-ferrocene dicarbaldehyde with trans-1,2-diaminocyclohexane in high dilution at r.t. followed by reduction. When other experimental conditions were employed, the [6,6,6]ferrocenephane (L2) was also obtained. Both compounds were characterised by single crystal X-ray crystallography. The protonation of L1 and its metal complexation were evaluated by the effect on the electron-transfer process of the ferrocene (fc) unit of L1 using cyclic voltammetry (CV) and square wave voltammetry (SWV) in anhydrous CH3CN solution and in 0.1 M nBu4NPF6 as the supporting electrolyte. The electrochemical process of L1 between −300 and 900 mV is complicated by amine oxidation. On the other hand, an anodic shift from the fc/fc+ wave of L1 of 249, 225, 81 and 61 mV was observed by formation of Zn2+, Ni2+, Pd2+ and Cu2+ complexes, respectively. Whereas Mg2+ and Ca2+ only have with L1 weak interactions and they promote the acid-base equilibrium of L1. This reveals that L1 is an interesting molecular redox sensor for detection of Zn2+ and Ni2+, although the kinetics of the Zn2+ complex formation is much faster than that of the Ni2+ one. The X-ray crystal structure of [PdL1Cl2] was determined and showed a square–planar environment with Pd(II) and Fe(II) centres separated by 3.781(1) Å. The experimental anodic shifts were elucidated by DFT calculations on the [ML1Cl2] series and they are related to the nature of the HOMO of these complexes and a four-electron, two-orbital interaction. 相似文献
155.
Novel 2,3-dihydro-4-pyridinones were synthesized via the reaction of curcumin and primary amines or amine acetates under microwave irradiation. Montmorillonite K-10 was used as a catalyst. Reaction times did not exceed 120 s. The structures of the compounds were established by elemental analysis and from their mass, 1H- and 13C-NMR spectra. 相似文献
156.
The merits and limitations of gas-phase electron diffraction as a technique for determining accurately the structures of small
organic molecules are demonstrated by comparing the molecular structures of five benzene derivatives studied in two different
laboratories. The existence of many possible representations of the structure of a molecule (r
a, r
g, r
α, r
α0, r
e, …), implying differences in the physical meaning of geometrical parameters, is discussed in some detail as it may cause
difficulties in a comparison. Also illustrated are different ways of estimating and expressing uncertainties in the geometrical
parameters from a gas-phase electron diffraction study. 相似文献
157.
Novice students have difficulty with the topic of limits in calculus. We believe this is in part because of the multiple perspectives and shifting metaphors available to solve items correctly. We investigated college calculus instructors' personal concepts of limits. Based upon previous research investigating introductory calculus student metaphorical reasoning, we examined 11 college instructors' metaphorical reasoning on limit concepts. This paper focused on previous research of metaphor clusters observed among students to answer the following: (a) Do college instructors use metaphorical reasoning to conceptualize the meaning of a limit? (b) Can we characterize instructor metaphorical reasoning similar to those observed among students? (c) Will an instructor's self‐identification of metaphor clusters be consistent with our metaphor coding? We found that college instructors' perspectives vary, either graphical or algebraic, in their explanations of limit items. All the instructors used metaphors, and instructor metaphorical reasoning aligned with student metaphor clusters. Instructors tended to change their metaphors with respect to the limit item. Instructors were not aware of their use of metaphors, nor were they aware of their inconsistency in their choice of metaphor. We believe that instructor awareness of their own distinct perspectives and metaphors would assist students' understanding of limit concepts. 相似文献
158.
Rita Walczyna Zygfryd Smiatacz Zbigniew Ciunik 《Journal of carbohydrate chemistry》2013,32(8):1161-1171
Abstract The title compound 3 has been synthesized from 3,4,6-tri-O-acetyl-2-deoxy-2-nitroso-α-D-glucopyranosyl chloride (1) via compound 2. Azide reduction of 3 is accompanied by O→N-acetyl migration to afford N-acetyl-N-(3,4,6-tri-O-acetyl-2-deoxy-2-hydroxyimino-β-D-arabino-hexopyranosyl) amine (4), also characterized as its Z and E peracetates. On the basis of IR, 1H NMR and X-ray structural data from compound 4, its β-NHAc configuration, (Z) 2-hydroxyimino, and °S2 conformation, were established. 相似文献
159.
160.
Matthias Koch Wolfram Bremser Robert K?ppen Rita Kr��ger Tatjana Rasenko David Siegel Irene Nehls 《Accreditation and quality assurance》2011,16(8-9):429-437
Mycotoxins are important non-anthropogenic food and feed contaminants, which can be present on almost every agricultural commodity. Effective consumer protection therefore essentially depends on food surveillance by reliable quantitative analysis enabled by appropriate quality control. Certified (matrix) reference materials (CRMs) are versatile tools to support quality assurance. However, in the case of ochratoxin A (OTA), a hepato- and nephrotoxic mycotoxin, which is regulated in various foods, there is a lack of suitable CRMs. This lack has now been overcome by the development of two European Reference Materials (ERM?) for the determination of OTA in roasted coffee (ERM?-BD475) and red wine (ERM?-BD476). This article discusses the material preparation process as well as the results of homogeneity and stability testing. Furthermore, the results of the in-house certification studies carried out at BAM Federal Institute for Materials Research and Testing are presented and discussed. Interlaboratory comparison studies involving selected expert laboratories with documented expertise in the field of mycotoxin analysis were conducted to confirm the certified values determined by BAM. The certified ochratoxin A values and their corresponding expanded uncertainties (k?=?2) were assigned in full compliance with the requirements of ISO Guide 35 and are as follows: (6.0?±?0.6)???g?kg?1 for roasted coffee, ERM?-BD475, and (0.52?±?0.11)???g?L?1 for red wine, ERM?-BD476. 相似文献