1,720,965 research outputs found
Accurate molecular weight determination of small molecules via DOSY-NMR by using external calibration curves with normalized diffusion coefficients
No full textDetermination of the aggregation and solvation numbers of organometallic complexes in solution is an important task to increase insight in reaction mechanisms. Thus knowing which aggregates are formed during a reaction is of high interest to develop better selectivity and higher yields. Diffusion-ordered spectroscopy (DOSY), which separates NMR signals according to the diffusion coefficient, finds increasing use to identify species in solution. However, there still is no simple relationship between diffusion coefficient and molecular weight (MW). Some methods have been developed to estimate the MW but still with a significant error of 30%. Here we describe a novel development of MWdetermination by using an external calibration curve (ECC) approach with normalized diffusion coefficients. Taking the shape of the molecules into account enables accurate MW-predictions with a maximum error of smaller than 9%. Moreover we show that the addition of multiple internal references is dispensable. One internal reference (that also can be the solvent) is sufficient. If the solvent signal is not accessible, 16 other internal standards (aliphatics and aromatics) are available to avoid signal overlapping problems and provide flexible choice of analytes. This method is independent of NMRdevice properties and diversities in temperature or viscosity and offers an easy and robust method to determine accurate MWs in solution
Solution Structure of Turbo-Hauser Base TMPMgCl center dot LiCl in [D-8]THF
No full textTurbo-Hauser bases are very useful and highly reactive organometallic reagents in synthesis. Especially TMPMgCl center dot LiCl 1 (TMP = 2,2,6,6-tetramethylpiperidide) is an excellent base for converting a wide range of (hetero) aromatic substrates into highly functionalized compounds with a broad application in organic synthesis. The knowledge of its structure in solution is of essential importance to understand the extraordinary reactivity and selectivity. However, very little is known about the aggregation of this prominent reagent in solution. Herein, we present the THF-solution structure of 1 by employing our newly elaborated DOSY NMR method based on external calibration curves (ECC) with normalized diffusion coefficients
The Donor-Base-Free Aggregation of Lithium Diisopropyl Amide in Hydrocarbons Revealed by a DOSY Method
No full textLithium diisopropyl amide (LDA) is a very prominent reagent that plays a key role in organic synthesis, serving as a base par excellence for a broad range of deprotonation reactions. However, the state of aggregation in solution in the absence of donor bases was unclear. In this paper we solved this problem by employing DOSY NMR experiments based on a newly elaborated external calibration curve (ECC) approach with normalized diffusion coefficients
New External Calibration Curves (ECCs) for the Estimation of Molecular Weights in Various Common NMR Solvents
No full textNew external calibration curves (ECCs) for the estimation of aggregation states of small molecules in solution by DOSY NMR spectroscopy for a range of different common NMR solvents ([D-6]DMSO, C6D12, C6D6, CDCl3, and CD2Cl2) are introduced and applied. ECCs are of avail to estimate molecular weights (MWs) from diffusion coefficients of previously unknown aggregates. This enables a straightforward and elaborate examination of (de)aggregation phenomena in solution.DNRF [DNRF93
A Pd-Catalyzed Approach to (1→6)-Linked <i>C</i>-Glycosides
No full textA flexible and robust method for the assembly of (1 -> 6)-linked C-glycosidic disaccharides is presented. The key reaction is a Pd-catalyzed coupling of 1-iodo- or 1-triflato-glycals with alkynyl glycosides. Reinstallation of the native hydroxyl group pattern is achieved after selective hydrogenation of the triple bond using Raney-nickel. Epoxidation with DMDO and reductive epoxide opening gives access to either the alpha- or the beta-derivative, depending on the hydride source
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The Layered Structure of [Na(NH3)(4)][Indenide] Containing a Square-Planar Na(NH3)(4)(+) Cation
No full textDFG [1178]; DNR
Solution Structures of Hauser Base (Pr2NMgCI)-Pr-i and Turbo-Hauser Base (Pr2NMgCl)-Pr-i center dot LiCI in THE and the Influence of LiCI on the Schlenk-Equilibrium
No full textGrignard reagents that are at the simplest level described as "RMgX" (where R is an organic substituent and X a halide) are one of the most widely utilized classes of synthetic reagents. Lately, especially Grignard reagents with amido ligands of the type R1R2NMgX, so-called Hauser bases, and their Turbo analogue R1R2NMgX center dot LiCI play an outranging role in modern synthetic chemistry. However, because of their complex solution behavior, where Schlenk-type equilibria are involved, very little is known about their structure in solution. Especially the impact of LiCl on the Schlenk-equilibrium was still obscured by complexity and limited analytical access. Herein, we present unprecedented insights into the solution structure of the Hauser base (Pr2NMgCl)-Pr-i 1 and the Turbo -Hauser base (Pr2NMgCl)-Pr-i center dot LiCl 2 at various temperatures in THE-d(8) solution by employing a newly elaborated diffusion ordered spectroscopy (DOSY) NMR method hand-in-hand with theoretical calculations.DNRF [DNRF93
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