1,721,044 research outputs found
Symmetry-based recoupling of O-17-H-1 spin pairs in magic-angle spinning NMR
No full textWe have performed magic-angle-spinning solid-state NM R experiments in which protons are recoupled to oxygen-17 nuclei by applying a symmetry-based recoupling sequence at the proton Larmor frequency. Two-dimensional quadrupole-dipole correlation spectra are produced, in which the second-order quadrupolar shift of the oxygen-17 central transition is correlated with the recoupled heteronuclear dipole-dipole interaction. These spectra are sensitive to the relative orientation of the electric field gradient at the site of the oxygen-17 nucleus and the O-H internuclear vector. We also demonstrate experiments in which polarization is transferred from protons to oxygen-17, and show that oxygen-17 signals may be selected according to the protonation state of the oxygen site. We discuss the small observed value of the heteronuclear dipolar splitting in the central-transition oxygen-17 spectra. (c) 2005 Elsevier Inc. All rights reserved
Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 glasses : a multinuclear NMR study
No full textThe incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O–CaO aluminosilicate glasses with ∼65 mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In 19F MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2 mol% F the following F environments were identified: F–Ca(n) at ∼−113 ppm, Si–F–Na(n) or Al–F–Ca(n) at ∼−146 ppm, Al–F–Al at ∼−168 ppm, Al–F–Na(n) at ∼−188 ppm and F–Na(n) at ∼−225 ppm (“n” indicates that the number of atoms is variable or uncertain). F–Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F–Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2 mol%. The addition of up to 16.5 mol% (5.3 wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F–Al sites compared to F–Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH.
In the peraluminous glasses containing up to 18.3 mol% F only three F environments Si–F–Na(n) or Al–F–Ca(n) at ∼−149 ppm, Al–F–Al at ∼−170 ppm and Al–F–Na(n) at ∼−190 ppm are observed. Al–F–Na(n) is the most abundant site with a fraction of 54–61%. The F speciation also changes with the F concentration, with a minimum in Al–F–Na(n) sites between 3.5 and 9.7 mol% F.
Fluorine has only a small effect on the 23Na and 29Si MAS NMR spectra. 27Al MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses ∼5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F
Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin-7/2 nuclei
No full textWe report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-(7)/(2) systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90 pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the Sc-45 NMR of Sc-2(SO4)(3) (.) 5H(2)O and the La-139 NMR of LaAlO3. We also demonstrate the generation of FAM pulses by double-frequency irradiation
Constraints on the incorporation mechanism of chlorine in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses
No full textIncorporation mechanisms of Cl in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses as a model system for phonolitic melts were investigated using Cl-35, Na-23, Al-27, and Si-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The size and large distribution of electric field gradients for Cl-35 causes loss of signal in the MAS NMR experiment arid this, in combination with the low concentration of Cl and the large chemical shift dispersion, means that even at the highest available fields we are at the limits of MAS NMR. Nevertheless clear differences in the Cl environment in peralkaline and peraluminous glasses can readily be seen. In both glass types Cl exists in relatively symmetric Na-Ca-Cl environments. The Cl-35 chemical shift indicates that the Cl environment is dominated by the presence of Na cations, consistent with the Na/Ca ratio of 5/1 in the glasses. Cl-35 MAS NMR spectra of the peraluminous glasses show a larger chemical shift distribution and a more positive isotropic chemical shift, similar to-75 ppm, than the peralkaline glasses, similar to-100 ppm. They also have a larger quadrupole coupling constant with a larger distribution, indicating greater disorder in the peraluminous glasses. It is likely that there are more Ca cations present in the Cl environments in the peraluminous glasses than in the peralkaline glasses despite their having the same Na/Ca ratio. In the peralkaline glasses the formation of Na-Ca-Cl environments leads to a decrease in the number of network-modifying cations, which causes a polymerization of the glass network. No effect on the glass polymerization was observed in the peraluminous glasses. Some Cl-35 signal is also lost in the static spectra indicating that similar to 20% of Cl for a peralkaline glass and more than similar to 70% for a peraluminous glass must be in environments where there is a large enough electric field gradient that the resulting very broad line is unobservable. These environments could be simply Na-Ca-Cl with higher electric field gradients than those producing the observed Cl-35 signal or non-bridging Cl environments like for example Al-Cl. The Cl environment in the present mixed Na2O-CaO aluminosilicate glasses appears to be more disordered than was to be expected from previous NMR spectroscopic studies on simpler glass compositions
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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