1,720,977 research outputs found
Study of the spectral and relaxation properties of nitronylnitroxide monoradicals: an experimental and computational approach
No full textThe present study deals with a thorough characterization of the ESR-related properties of nitronylnitroxide radicals; more in detail we determined the mutual orientation between the eigenframes of the magnetic interaction tensors and the spin relaxation properties in fluid solution. These latter measurements show that the transverse spin relaxation rates of nitronylnitroxides are slower with respect to nitroxide radicals; the analysis of the relaxation rate in terms of contributions, which relies on the application of the Redfield spin relaxation theory, allows to understand the different behaviour.
All these considerations promote the routine use of nitronylnitroxide radicals in spin labeling studies, even though the analysis of the spectra is more involved due to the delocalization of the unpaired electron spin density over two nitrogen nuclei rather than one. We showed, however, that the introduction of new computational tools for the calculation of the spectral profiles relying on the a priori determination of the parameters affecting the lineshape allowed the reproduction of a series of experimental spectra recorded in a partially oriented fluid with minimal resort to best fitting procedures; this result further confirms the feasibility of a routine use of nitronylnitroxides as paramagnetic probes in ESR studies.Il presente studio affronta una valutazione dettagliata delle proprietà dei radicali nitronilnitrossido inerenti alla spettroscopia ESR; nello specifico vengono determinate le mutue orientazioni tra i sistemi di riferimento principali dei tensori magnetici e le proprietà di rilassamento di spin in fluido. Queste ultime misure, in particolare, mettono in luce un più lento rilassamento trasversale da parte dei radicali nitronilnitrossido, e la separazione dei contributi secondo la teoria di rilassamento di Redfield porta a determinare la causa della differenza di comportamento.
Tutte queste valutazioni vanno nella direzione di favorire l’utilizzo di radicali nitronilnitrossido a fianco dei radicali nitrossido negli studi di spin labeling, sebbene i primi siano più complessi da un punto di vista dell’analisi degli spettri a causa della delocalizzazione della densità di spin elettronico su due nuclei d’azoto anziché su uno. Nondimeno l’introduzione di nuove metodologie di calcolo dei profili spettrali, le quali fanno affidamento su una valutazione a priori dei parametri che influenzano lo spettro, ha reso possibile la riproduzione di una serie di tracciati sperimentali acquisiti in un fluido parzialmente orientato con una introduzione solo marginale di parametri da sottoporre ad ottimizzazione, confermando dunque la possibilità di un uso comune dei radicali nitronilnitrossido nella spettroscopia ESR in qualità di sonde paramagnetiche
First determination of the spin relaxation properties of a nitronyl nitroxide in solution by Electron Spin Echoes at X-band. A comparison with Tempone
No full textWe studied by electron spin echo pulse methods the spin relaxation properties of a phenyl nitronyl nitroxide radical (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, PTIO) at X-band in fluid toluene solution in a wide temperature range, and in a water/glycerol 1:1 mixture near room temperature. The relaxation properties of PTIO have been compared with that of Tempone, as a widely used nitroxide. By a new procedure, based on experimental results on the temperature dependences of the relaxation times T1 and T2, and on the approximation of an isotropic Brownian rotational diffusion, we separated non-secular, spin rotational and residual terms from the transverse relaxation rate to isolate secular and pseudosecular contributions. By comparing the results for the two radicals we found the differences in the magnetic properties that give rise to slower transverse (T2) and longitudinal (T1) electron spin relaxation for PTIO in the whole temperature range explored in this work
An advanced approach to the evaluation of the spin-rotational term for a nitronyl nitroxide in fluid solution
No full textIn this work we focus on the spin-rotational contribution to transverse relaxation rate 1/T_2 for a nitronyl nitroxide radical
(PTIO) in fluid solution of toluene. We recorded the X-band continuous-wave electron paramagnetic resonance spectra of
the radical in a wide temperature range and compared them with the calculated spectra. The calculation was afforded in
two steps: first, we calculated the spectral profiles in the same range of temperatures taking into account the hyperfine
interaction with the two ^14 N nuclei, by integration of the stochastic Liouville equation with the E-SpiReS package in the
presence of the tumbling motion of the molecule, which modulates the hyperfine and the g-tensors. In the second step, we included the proton hyperfine structure by convoluting the spectrum with the pattern due to the 17 PTIO-coupled protons. A further Lorentzian broadening was added by a best fitting procedure to reproduce the experimental linewidths. The additional broadening is associated with a relaxation rate,
Wγ, which varies linearly on k_B T/η; this trend is expected for the spin-rotational relaxation term modelled by the well-known Atkins–Kivelson expression. We found a discrepancy between thetwo parameters associated with the radii of the radical, obtained either from the rotational diffusion tensor in the framework of the Debye–Stokes–Einstein model or from the spin-rotational contribution. We discuss this issue in relation to the intrinsic approximations of the spin-rotational model and, in particular, the isotropic Brownian rotation
A slow relaxing species for molecular spin devices: EPR characterization of static and dynamic magnetic properties of a nitronyl nitroxide radical
No full textNitronyl nitroxides (NitR) are a family of persistent radicals widely used in molecular magnetism and recently suggested as potential candidates for spintronic applications. In this paper we characterize by X- and W- band Electron Paramagnetic resonance (EPR) spectroscopy the new radical S-4-(nitronyl
nitroxide) benzyl ethanethioate (NitSAc) designed for assembling on Au surfaces. We determined the radical magnetic tensors and studied by X-band pulse EPR its spin relaxation behaviour in fluid and glassy solutions of toluene. A comparison with the well known nitroxide 3-carbamoyl- 2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl (CTPO) is afforded. The advantages of using NitSAc in
technological applications are discussed on the basis of the slow spin relaxation demonstrated by this study
Interpretation of cw-ESR spectra of p-methyl-thio-phenyl-nitronyl nitroxide in a nematic liquid crystalline phase
No full textIn this paper we report on the characterization by continuous wave electron spin resonance spectroscopy (cw-ESR) of a nitronyl nitroxide radical in a nematic phase. A detailed analysis is performed by exploiting an innovative modeling strategy alternative to the usual spectral simulation approach: most of the molecular parameters needed to calculate the spectrum are evaluated a priori
and the ESR spectrum is obtained by direct application of the stochastic Liouville equation. Allowing a limited set of fitting parameters it is possible to reproduce satisfactorily ESR spectra in
the temperature range 260 K–340 K including the nematic-to-isotropic phase transition (325.1 K). Our results open the way to a more quantitative understanding of the ordering and mobility of
nitronyl nitroxide radicals in nanostructured environments
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
Electron Paramagnetic Resonance and petrographic analysis for dating Mesolithic and Neolithic pottery from Al Khiday (Sudan)
No full textElectron Paramagnetic Resonance (EPR) dating, like luminescence techniques, is based on the timedependent
accumulation of trapped charges at mineral defect centres. However, Fe(III) ions prevent
the common Continuous Wave (CW-EPR) approach for dating pottery, which always contains iron. The
Pulsed method (ED-EPR) allowed this limitation to be overcome, with recording of radiation-induced
defect signals, as shown by increased signal intensity after artificial irradiation of samples. The
method was applied to studying Mesolithic and Neolithic pottery from Al Khiday (Central Sudan),
characterized by quartz-rich tempers and coming from dated contexts. As the occurrence of a natural EDEPR
signal was found to be related to the quartz grain size of the temper, a petrographic study was
carried out. This first attempt at age determination of pottery by ED-EPR meant that experimental
conditions and important parameters could be taken into account in developing a new dating procedure
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