1,720,971 research outputs found
Probing Polymer Colloids by 129Xe NMR
No full textModel aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The Xe-129 NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the Xe-129 spectral time-scale and a single Xe-129 Signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the Xe-129 spectral time-scale and two Xe-129 NMR signals can be observed. If the signal of sorbed Xe-129 is observed, its chemical shift, line width and integral relative to the integral of free Xe-129 can be used for the characterization of the particle core. The line width of free Xe-129 provides the residence time of xenon Outside the Particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle Surface
On the Fluxional Behavior of Dess-Martin Periodinane: A DFT and (17)O NMR Study
No full textThe structure and dynamics of the Dess-Martin periodinane, a I(V) iodobenzene compound widely used in organic synthesis as a mild oxidant, were studied by a combined (17)O NMR and DFT calculations approach. The results show that a degenerate [1,3] sigmatropic shift of iodine between the two oxygen atoms of each of the three acetoxy groups occurs in solution. The energy barrier of this process depends on the position of the acetoxy group with respect to the iodoxolone ring and is much lower than the energy barrier observed for similar I(III) compounds
Highlighting cavities in proteins by NMR using sulfur hexafluoride as a spy molecule
No full textSCOPUS: ar.jinfo:eu-repo/semantics/publishe
Probing systems in solution by NMR using sulfur hexafluoride as a spy molecule
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Dynamic NMR of low-sensitivity fast-relaxing nuclei: (17) O NMR and DFT study of acetoxysilanes.
No full text(17) O NMR is not routinely used for structure characterization, and kinetic studies of fluxional organic compounds are seldom undertaken because poor sensitivity and fast quadrupole relaxation are frequently regarded as intractable issues. This work shows how, nowadays, quantitative (17) O dynamic NMR studies on small organic molecules are feasible without enrichment being needed. It reports on acetoxysilanes, a class of fluxional compounds whose structure and dynamics were to be clarified. Natural abundance (17) O NMR spectra were recorded over a wide range of temperatures using standard instrumentation. The analysis relies on simple linewidth measurements and directly provides the activation parameters. The activation enthalpy is found to decrease with increasing number of acetoxy groups bound to silicon. Density functional theory calculations properly predict this trend and show that a single oxygen atom of the acetoxy group is bound to silicon, excluding chelation as binding mode, and that the dynamic process involves the shift of the silicon atom between the two oxygen atoms of the acetoxy group. Copyright © 2012 John Wiley & Sons, Ltd.JOURNAL ARTICLESCOPUS: ar.jFLWINinfo:eu-repo/semantics/publishe
NMR study of the reversible trapping of SF6 by cucurbit[6]uril in aqueous solution
No full textSCOPUS: ar.jinfo:eu-repo/semantics/publishe
Insight into the dynamics of lanthanide-DTPA complexes as revealed by oxygen-17 NMR.
No full textDTPA chelates of various diamagnetic and paramagnetic lanthanide(III) metal ions, as well as the chemically similar DTPA chelate of Y(3+), were studied in aqueous solution by variable temperature (17)O NMR with the aim of characterizing their internal dynamics. As a consequence of poor chemical shift dispersion and fast quadrupole relaxation, no dynamic exchange process could be detected for the diamagnetic complexes nor for the Sm-DTPA complex. In contrast, the spectra recorded for the Eu-DTPA complex show chemical exchange due to the well-known racemization process and, at high temperature, feature signal broadening that reveals a fluxional process involving the interchange of the coordinated and noncoordinated oxygen atoms of the carboxylate groups. The spectra recorded for the Pr-DTPA complex feature coalescence events due to such a fluxional process, which is ascribable to the rotation of the carboxylate groups. The activation free energy barriers determined experimentally are remarkably lower than the calculated activation barriers recently reported for the rotation of the carboxylate groups of various Ln-DOTA complexes. Furthermore, the smallest activation free energy measured for the Pr-DTPA complex, about 45 kJ mol(-1), is significantly lower than the activation free energy characterizing the racemization process. The fluxional behavior of the carboxylate groups is, however, not expected to significantly affect the residence time of the water molecule coordinated to the metal ion.Journal ArticleSCOPUS: ar.jinfo:eu-repo/semantics/publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Stereochemical studies by molecular palpation
No full textThe conformational equilibrium of cyclohexanol was investigated by 129X NMR spectroscopy. While the classical NMR approach focuses on a carbon or proton atom belonging to the molecule under investigation, in our 129Xe NMR methodology we use the xenon atom as an external spy. Xenon is able to monitor the interconversion between the: axial and the equatorial isomers of cyclohexanol. The conformational equilibrium constant was estimated and is in excellent agreement with the values obtained by 1H and 13C NMR. ©2004 John Wiley & Sons, Ltd.SCOPUS: cp.jFLWINinfo:eu-repo/semantics/publishe
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