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191.
Dr. Julian Brunner Britta Maier Rose Rosenberg Sebastian Sturm Prof. Dr. Helmut Cölfen Dr. Elena V. Sturm 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(66):15242-15248
Applications in the fields of materials science and nanotechnology increasingly demand monodisperse nanoparticles in size and shape. Up to now, no general purification procedure exists to thoroughly narrow the size and shape distributions of nanoparticles. Here, we show by analytical ultracentrifugation (AUC) as an absolute and quantitative high-resolution method that multiple recrystallizations of nanocrystals to mesocrystals is a very efficient tool to generate nanocrystals with an excellent and so-far unsurpassed size-distribution (PDIc=1.0001) and shape. Similar to the crystallization of molecular building blocks, nonclassical recrystallization removes “colloidal” impurities (i.e., nanoparticles, which are different in shape and size from the majority) by assembling them into a mesocrystal. In the case of nanocrystals, this assembly can be size- and shape-selective, since mesocrystals show both long-range packing ordering and preferable crystallographic orientation of nanocrystals. Besides the generation of highly monodisperse nanoparticles, these findings provide highly relevant insights into the crystallization of mesocrystals. 相似文献
192.
Dr. Mathias Paul Eric Detmar Dr. Maria Schlangen Dr. Martin Breugst Dr. Jörg-Martin Neudörfl Prof. Dr. Dr. Helmut Schwarz Prof. Dr. Albrecht Berkessel Prof. Dr. Mathias Schäfer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(10):2511-2518
N-Heterocyclic carbenes (NHCs, :C ) can interact with azolium salts ( C−H+ ) by either forming a hydrogen-bonded aggregate ( CHC+ ) or a covalent C−C bond ( CCH+ ). In this study, the intramolecular NHC–azolium salt interactions of aromatic imidazolin-2-ylidenes and saturated imidazolidin-2-ylidenes have been investigated in the gas phase by traveling wave ion mobility mass spectrometry (TW IMS) and DFT calculations. The TW IMS experiments provided evidence for the formation of these important intermediates in the gas phase, and they identified the predominant aggregation mode (hydrogen bond vs. covalent C−C) as a function of the nature of the interacting carbene–azolium pairs. 相似文献
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Maurits Kuiper Manfred Speldrich Helmut Schilder Prof. Dr. Heiko Lueken 《无机化学与普通化学杂志》2012,638(10):1432-1436
Abstract. The magnetic behavior of the mononuclear nd1 systems MCp2Cl2 (M = V4+[3d1], Nb4+[4d1], Ta4+[5d1], space group P21/c, pseudosymmetry of the molecules C2v) deviates from pure single ion spin magnetism on account of ligand field effect (Hlf), spin‐orbit coupling (Hso), and intermolecular spin‐spin exchange interactions (Hex). For both VCp2Cl2 and NbCp2Cl2 excellent adaptations to the measured susceptibility data were obtained (2 K ≤ T ≤ 300 K) on the basis of spectroscopic data (lf, so) and cooperative metal–metal interactions (ex) of antiferromagnetic nature [molecular field model (mf)]. For TaCp2Cl2 experimental term structure data are not available. Therefore, Jørgensen's spectroscopical series (g‐factor of the central ion) was applied to extrapolate the data set for TaCp2Cl2. Hlf, Hso, and Hex (antiferromagnetic) increase in the order 3d1 → 4d1 → 5d1 leading, with rising atomic number of the metals, to a distinct enhancement of the magnetic anisotropy. At 4 K the μeff components μeff,y (oriented perpendicular to the cg–M–cg plane; “cg” = center of gravity of the Cp ring), μeff,z (oriented along the twofold pseudoaxis), and μeff,x are 1.73, 1.69, 1.68 (V), 1.73, 1.62, 1.59 (Nb), and 1.71, 1.59, 1.49 (Ta). While μeff,y is independent of T, both μeff,z and μeff,x decrease with decreasing T. 相似文献
196.
Differences and Commonalities in the Gas‐Phase Reactions of Closed‐Shell Metal Dioxide Clusters [MO2]+ (M=V,Nb, and Ta) with Methane
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Dr. Shaodong Zhou Dr. Jilai Li Dr. Maria Schlangen Prof. Dr. Helmut Schwarz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(21):7225-7228
High‐level electronic structure calculations, in combination with Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometric studies, permit the mechanism by which closed‐shell, “naked” [TaO2]+ brings about C?H bond activation of methane to be revealed. These studies also help to understand why the lighter congeners of [MO2]+ (M=V, Nb) are unreactive under ambient conditions. 相似文献
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Marcos Costa Souza William Pires Macedo Marcelo Campos M. Silva Guilherme Costa de O. Ramos Helmut G. Alt 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6):1047-1054
This article describes an attractive and one-pot synthesis of the title compound by phosphorylation of just prepared N-substituted guanidines from cyanamide and the desired amine. The method allows a variety of N-substituents to hang on the final phosphoryl guanidine as a function of the wider availability of commercial simple amines. 相似文献
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A rapid ultra-high performance liquid chromatography (UHPLC) protocol for the determination of amino acids as their respective 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatives was successfully applied for assessing free amino acid levels in commercial cheese samples representing typical product groups (ripening protocols) in cheesemaking. Based on the Waters AccQ.Tag? method as a high performance liquid chromatography (HPLC) amino acid solution designed for hydrolyzate analyses, method adaptation onto UHPLC was performed, and detection of AQC derivatives was changed from former fluorescence (λ Ex 250 nm/λ Em 395 nm) to UV (254 nm). Compared to the original HPLC method, UHPLC proved to be superior by facilitating excellent separations of 18 amino acids within 12 min only, thus demonstrating significantly shortened runtimes (>35 min for HPLC) while retaining the original separation chemistry and amino acid elution pattern. Free amino acid levels of the analyzed cheese samples showed a high extent of variability depending on the cheese type, with highest total amounts found for original Italian extra-hard cheeses (up to 9,000 mg/100 g) and lowest for surface mold- or bacterial smear-ripened soft cheeses (200–600 mg/100 g). Despite the intrinsic variability in both total and specific concentrations, the established UHPLC method enabled reliable and interference-free amino acid profiling throughout all cheese types, thus demonstrating a valuable tool to generate high quality data for the characterization of cheese ripening. 相似文献