Metodi sperimentali e teorici per la spettroscopia EPR di nitronilnitrossidi

Lo scopo del presente lavoro consiste nell’analizzare gli spettri CW-EPR di un radicale nitronilnitrossido (MTPNN) in cristalli liquidi (ZLI-1083) al variare della temperatura tramite un approccio computazionale integrato quantomeccanico – idrodinamico; tale metodologia di calcolo si contraddistingue rispetto al più classico procedimento di simulazione per l’esiguo numero dei parametri richiesti al fine di ottenere lo spettro. ================================================================================= The aim of the present work is to analyze the temperature-dependent CW-EPR spectra of a nitronylnitroxide monoradical (MTPNN) in a nematic phase (ZLI-1083) by a quantomechanical-hydrodynamic integrated computational approach; as just a few free phænomenological parameters are needed in order to calculate the spectrum, this method differs significantly from the “classical” simulation approach to the interpretation of EPR spectra

Study of the spectral and relaxation properties of nitronylnitroxide monoradicals: an experimental and computational approach

The 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

We 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

In 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, W\u3b3, which varies linearly on k_B T/\u3b7; this trend is expected for the spin-rotational relaxation term modelled by the well-known Atkins\u2013Kivelson 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\u2013Stokes\u2013Einstein 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

Nitronyl 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

none6In 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.noneAlberto Collauto; Mirco Zerbetto; Marina Brustolon; Antonino Polimeno; Andrea Caneschi; Dante GatteschiCollauto, Alberto; Zerbetto, Mirco; Brustolon, MARINA ROSA; Polimeno, Antonino; Andrea, Caneschi; Dante, Gattesch

Graphite anodes for Li-ion batteries – an electron paramagnetic resonance investigation

Graphite is the most commercially successful anode material for lithium (Li) ion batteries: its low cost, low toxicity and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterised. However, questions remain regarding its electronic structure and whether the electrons occupy localised states on Li or delocalised states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterising the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphite. In this work, ex situ variable-temperature (10-300 K), variable frequency (9-441 GHz) EPR was carried out to extract the g-tensors and linewidths, and understand the effect of metallicity on the observed EPR spectra of charged graphite at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to 7Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.T.I. and C.P.G. were supported by an ERC Advanced Investigator Grant for C.P.G. (EC H2020 835073). E.N.B. was supported by the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018. K.M. was supported by the Faraday Institution Degradation Project (FIRG001 and FIRG024). The Pulsed EPR measurements were performed at the Centre for Pulse EPR at Imperial College London (PEPR), supported by the EPSRC grant EP/T031425/1

Graphite Anodes for Li-Ion Batteries: An Electron Paramagnetic Resonance Investigation.

Graphite is the most commercially successful anode material for lithium (Li)-ion batteries: its low cost, low toxicity, and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation, and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterized. However, questions remain regarding their electronic structures and whether the electrons occupy localized states on Li, delocalized states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterizing the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphites. In this work, ex situ variable-temperature (10-300 K), variable-frequency (9-441 GHz) EPR was carried out to extract the g tensors and line widths and understand the effect of metallicity on the observed EPR spectra of electrochemically lithiated graphites at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.T.I. and C.P.G. were supported by an ERC Advanced Investigator Grant for C.P.G. (EC H2020 835073). E.N.B. was supported by the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018. K.M. was supported by the Faraday Institution Degradation Project (FIRG001 and FIRG024). The Pulsed EPR measurements were performed at the Centre for Pulse EPR at Imperial College London (PEPR), supported by the EPSRC grant EP/T031425/1