136 research outputs found
Structural Properties and Aggregation Behavior of 1-Hexyl-3-methylimidazolium Iodide in Aqueous Solutions
No full textThe
structural properties of 1-hexyl-3-methylimidazolium iodide
([C<sub>6</sub>mim]I)/water mixtures with molar ratios ranging from
1:1 to 1:200 have been investigated using molecular dynamics (MD)
simulations with extended X-ray absorption fine structure (EXAFS)
experimental data. The presence of a complex network of interactions
among cations, anions, and water molecules has been highlighted from
the MD simulations, even if water molecules have been found to interact
preferentially with the I<sup>–</sup> anion. The EXAFS results
show that, also for the 1:1 [C<sub>6</sub>mim]I/water mixture, the
water molecules are placed next to the I<sup>–</sup> anion,
and the I<sup>–</sup> hydration shell becomes more and more
crowded with increasing water content. Tight ion pairs have been detected
in the [C<sub>6</sub>mim]I/water mixtures with molar ratios from 1:1
to 1:12, while no ionic pairs were found in the most diluted solutions.
The aggregation behavior has been determined from MD simulations with
the aid of <i>S</i>(<i>q</i>) functions. For the
most concentrated IL/water mixtures with molar ratios between 1:1
and 1:12 the existence of long-range structural correlations has been
evidenced, even if the apolar chains are not completely segregated
as expected for micelle-like structures. Conversely, for the 1:200
mixture, that is above the experimental critical aggregation concentration
value, the alkyl chains are completely separated from each other
Local structure and spin transition in Fe2O3 hematite at high pressure
No full textThe pressure evolution of the local structure of Fe2O3 hematite has been determined by extended x-ray
absorption fine structure up to ∼79 GPa. Below the phase-transition pressure at ∼50 GPa, no increasing of FeO6
octahedra distortion is observed as pressure is applied. Above the phase transition, an abrupt decrease of the
nearest-neighbor Fe-O distance is observed concomitantly with a strong reduction in the FeO6 distortion. This
information on the local structure, used as a test-bench for the different high-pressure forms proposed in the
literature, suggests that the orthorhombic structure with space group Aba2, recently proposed by Bykova et al.
[Nat. Commun. 7, 10661 (2016)], is the most probable, but puts into question the presence of the P21/n form in
the pressure range 54–67 GPa. Finally, the crossover from Fe high-spin to low-spin states with pressure increase
has been monitored from the pre-edge region of the Fe K-edge absorption spectra. Its “simultaneous” comparison
with the local structural changes allows us to conclude that it is the electronic transition that drives the structural
transition and not vice versa
Thermal and magnetic anomalies of α-iron: An exploration by extended x-ray absorption fine structure spectroscopy and synchrotron x-ray diffraction
No full textThe local structure and dynamics of α-iron have been investigated by extended x-ray
absorption fine structure (EXAFS) spectroscopy and x-ray diffraction (XRD) in order to
shed light on some thermal and magnetic anomalies observed in the last decades. The
quantitative EXAFS analysis of the first two coordination shells reveals a peculiar local
vibrational dynamics of α-iron: the second neighbor distance exhibits anharmonicity
and vibrational anisotropy larger than the first neighbor distance. We search for possible
distortions of the bcc structure to justify the unexplained magnetostriction anomalies of
α-iron and provide a value for the maximum dislocation of the central Fe atom. No thermal
anomalies have been detected from the current XRD data. On the contrary, an intriguing
thermal anomaly at about 150 K, ascribed to a stiffening of the Fe–Fe bonds, was found
by EXAFS
Coupled X-ray absorption/UV-vis monitoring of fast oxidation reactions involving a nonheme Iron-Oxo complex
No full textTime-resolved X-ray absorption (XAS) and UV-vis spectroscopies with millisecond resolution are used simultaneously to investigate oxidation reactions of organic substrates by nonheme iron activated species. In particular, the oxidation processes of arylsulfides and benzyl alcohols by a nonheme iron-oxo complex have been studied. We show for the first time that the pseudo-first-order rate constants of fast bimolecular processes in solution (milliseconds and above) can be determined by time-resolved XAS technique. By following the Fe K-edge energy shift, it is possible to detect the rate of iron oxidation state evolution that matches that of the bimolecular reaction in solution. The kinetic constant values obtained by XAS are in perfect agreement with those obtained by means of the concomitant UV-vis detection. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase that involve changes of the oxidation state of a metal center, and it is particularly useful in complex chemical systems where possible interferences from species present in solution could make it impossible to use other detection techniques. © Copyright © 2019 American Chemical Society
Local order and long range correlations in imidazolium halide ionic liquids: a combined molecular dynamics and XAS study
No full textA thorough characterization of the structural properties of alkylimidazolium halides ionic liquids (ILs), namely 1-alkyl-3-methylimidazolium bromide ([Cn mim]Br with n = 5, 6, 8, 10) and iodide ([C6 mim]I), has been carried out by combining Molecular Dynamics simulations and the EXAFS spectroscopy. The existence of a local order in [Cn mim]Br ILs has been evidenced, with anions and imidazolium head groups forming a local three-dimensional bonding pattern that is common to all the [Cn mim]Br IL family, regardless the length of the alkyl chain attached to the cation. On the other hand, upon alkyl chain elongation significant differences have been highlighted in the long-range structure of these ILs. Theoretical X-ray structure
factors have been calculated starting from the MD simulations and a low q peak has been found for all [Cn mim]Br ILs, indicating the existence of long-range structural correlations. The low q peak moves to longer distances, increases in intensity and sharpens with increasing alkyl chain length on the cation. Similarities and differences between the ion three-dimensional arrangements
in [C6 mim]Br and [C6 mim]I were highlighted and the structural arrangement of Br− and I− was found to be different in the
proximity of the most acidic hydrogen atom of the imidazolium ring: the I− ion is preferentially located above and below the
ring plane, while the Br− ion has a high probability also to be coplanar with the imidazolium ring. A quantitative analysis of the
Br and I K-edge EXAFS spectra of alkylimidazolium halides ILs has been carried out starting from the microscopic description
of the systems derived from MD simulations. A very good agreement between theoretical and experimental EXAFS signals has
been obtained, allowing us to assess the reliability of the MD structural result for all the alkylimidazolium halides ILs investigated
in this work
The dynamics of pseudocapacitive phenomena studied by Energy Dispersive X-Ray Absorption Spectroscopy on hydrous iridium oxide electrodes in alkaline media
No full textThe behavior of highly hydrated IrOx porous electrodes under potentiostatic conditions is studied by operando time-resolved Energy Dispersive X-Ray Absorption Spectroscopy (EDXAS) at the Ir LIII edge. The potential steps are selected in order to drive the electron and charge transfers into the Ir(III) ⇄ Ir(IV) and Ir(IV) ⇄ Ir(V) transition domains. The adoption of sodium hydroxide aqueous solutions provides the best conditions for the potentiostatic control of the iridium oxidation states, as evidenced by the two well-separated, symmetric peaks observed in cyclic voltammetry. The intrinsic time resolution of ca. 10 ms implies the acquisition of 10000–20000 spectra per run, prompting the introduction of an effective procedure for treating this large number of data for analysis and comparison with the chronoamperometric response. The combined treatment of electrochemical and XAS signals mutually supports the working hypothesis of full complementarity between the two techniques, that allow observing the pseudocapacitive reactions and its time evolution even in the presence of possible parasitic side-processes, and highlights the XAS features to be used as effective kinetic parameters
Structure and spin dynamics of a metal complex studied by synchrotron radiation
No full textCette thèse intitulé «structure et dynamique de spin d’un complexe métallique étudié par rayonnement synchrotron» décrit une étude expérimentale du complexe métallique [FeII(phen)3]]2+ en solution par la diffusion des rayons X résolue en temps et la spectroscopie d’émission, dont l’objectif est de surveiller les changements structurels et de spin au cours du photocycle du complexe. Dans l’état photo-excité du complexe, un électron de l'orbitale 3d est transféré au ligand pour une fraction de picoseconde. Après ce “transfert de charge métal - ligand” (MLCT), l’électron revient au métal dans un état haut spin (HS) quasi-stable. Le photocycle se termine par un retour à l’état bas spin (BS), l’état fondamental, qui s’effectue en 725 ps. La structure et la rotation de l’état HS ont été mesurées avec une résolution temporelle de 100 ps.Le 1er chapitre commence par décrire l’importance de visualiser à l’aide de courtes impulsions de rayons X la façon dont les atomes se déplacent au cours de réactions chimiques et transformations. L’utilisation de rayons X pour des études structurelles a permis de visualiser les réactions induites par photons avec une résolution de 100 ps.Dans le chapitre 2 «Sonder la structure moléculaire en solution avec des rayons X», la théorie de la diffusion des rayons X est mentionnée. La diffusion de Debye à partir de molécules orientées au hasard est présentée. Lorsque la structure moléculaire est connue, le modèle de diffusion est facilement calculé. La diffusion inélastique domine la diffusion totale à grands q et doit également être prise en compte.La position des atomes dans une solution est décrite par des fonctions inter-atomiques g_ab(r) qui peuvent être simulées par la dynamique moléculaire. S(q) est calculé pour l’eau basées sur le modèle TIP4P. La diffusion d’un film d’eau de 0,36 mm d’épaisseur est calculée pour une impulsion de rayons X avec 1E+9 ph/pulse à 18 keV.Les modèles DFT des structures à bas et haut spins de Fe(phen)3 ont été générés et les fonctions S(q) ont ensuite été calculées par la fonction de Debye. La conclusion est que les liaisons Fe-N dans l’état HS s’allongent de 0,19 Å.La diffusion des rayons X par les liquides sonde toutes les distributions atome-atome dans l’échantillon, y compris celle du solvant ‘bulk’. Dans la théorie hydrodynamique, le liquide est supposé être en équilibre thermique local. L’expression pour le refroidissement des points chauds est examinée. Pour une concentration de 2 mM de Fe(phen)3 excité l’équilibre est atteint en 100 ps.La dernière section présente l’émission Ka, Kb; et valence-à-noyau (VtC, Valence-to-Core). Les spectromètres Johann (JS) et Von Hamos (VH) sont présentés. Le taux de comptage de Kb; est faible, 0,01 ph/pulse/analyseur, et l’échantillon a été exposé pendant 1 heure par délai pour obtenir un bon rapport signal-sur-bruit.Les expériences WAXS et XES sont décrites au chapitre 4. Après la photo-excitation en MLCT, l'électron transféré retourne à l'état HS centré sur le métal en environ 320 fs pour ensuite revenir à l'état BS par désintégration non radiative en 725 ps. Le changement de structure dans la transition BS-HS a été mesuré par diffusion de rayons X aux grands angles (WAXS) avec une résolution temporelle de 100 ps. L’échantillon était excité avec des impulsions de 1,2 ps à 400 nm et sondé par des impulsions de rayons X à 18 keV. Les courbes dS(q,t) sont des instantanés de 100 ps du changement structurel moyen pour toutes les paires d’atomes dans l’échantillon au moment t. Les données WAXS sont compatibles avec les structures BS et HS simulées par DFT avec une durée de vie HS de 725 ps. Le changement du rayon de la cage est déduit des données à petits q. La cage se contracte de 0,3 Å dans l’état HS.La ligne spectrale Kb a été mesurée avec le spectromètre von Hamos et comparée aux simulations Crispy. Les simulations confirment que l’état 725 ps est le quintet HS.The thesis “Structure and Spin Dynamics of a Metal Complex Studied by Synchrotron Radiation” describes an experimental study of the metal complex [FeII(phen)3]2+ in solution by time-resolved X-ray scattering and emission spectroscopy aimed at monitoring changes in structure and spin during its photocycle. In the photoexcited state, a 3d electron is transferred to the ligand for a fraction of a picosecond. From this so-called metal-to-ligand charge transfer state (MLCT), the electron returns to the metal in an excited high spin state (HS) that in turn decays to the low spin (LS) ground state in 725 ps. The structure and spin of the HS state were measured by X-ray scattering (WAXS) and X-ray emission spectroscopy (XES) respectively with 100 picosecond resolution using single X-ray pulses from the synchrotron.Chapter 1 describes the importance of visualising atoms in chemical reactions and transformations. The use of X-rays to gain structural sensitivity is now allowing to visualise photoinduced reactions with 100 picosecond resolution at synchrotrons and lately at 100 femtosecond resolution at XFELs.In chapter two “Probing Molecular Structure in Solution with X-rays”, the theory of X-ray scattering is presented stressing that when the structure is known, the molecular scattering pattern is readily calculated. Compton scattering dominates the scattering at high q and has to be included in the scaling. The intensity of the scattering from a 0.36 mm water sheet is calculated for a 1E+9 photon pulse at 18 keV.When a solute is dissolved in a solvent, the atomic positions are described by statistical atom-atom functions g_ab(r) that can be calculated by MD. The scattering function S(q) is then calculated from g_ab(r) for Fe(phen)3 in water using the TIP4P model with LS and HS structures from DFT.Liquid X-ray scattering probes all atom-atom pairs in the sample including that of the solvent. In the hydrodynamic scattering theory, the liquid is assumed to be in local thermal equilibrium. The theory for the cooling of hot points is presented and the calculation shows that a solution with 2 mM excited Fe(phen)3 attains local thermal equilibrium in 100 ps.The end of chapter 2 gives a summary of X-ray emission spectroscopy (XES). Ka, Kb and valence-to-core (VtC) emission is discussed including their intensity, spin and ligand sensitivity. Kb is the most sensitive probe of the spin state.In chapter 3 the ESRF and ID09 are shortly described. The details of the Johann (JS) and Von Hamos (VH) spectrometers for XES are described with emphasis on the VH since it was used for the first time in this project. The count rate from Kb is extremely low, 0.01 ph/pulse/analyser and the sample has to be exposed for about 1 hour per time delay to get a Kb spectrum with a good S/N ratio.The WAXS and XES experiments are described in Chapter 4. After photoexcitation to the MLCT state, the electron returns to a metal centred HS state in < 100 fs for then to return to the GS in 725 ps. dS(q,t) are 100 ps snapshots of the average structural change for all pairs of atoms at time t. On short time scales t < 10 ns, the solvent is heated adiabatically at constant volume. The thermal response of water was measured in a dye/water mixture. The solvent corrected WAXS data show that the Fe-N distance increases by 0.19 Å in the HS state and that the HS population returns to the LS in 725 ps. The change in the water cage radius is inferred from the low-q data. It is found to contract by 0.3 Å in the HS state in spite of the 0.19 Å expansion of the Fe-N distance.The XES line shapes of the Kb lines were measured with the VH spectrometer and compared with Crispy simulations. The simulations confirm that the 725 ps state is the HS S=2 quintet. Very weak VtC emission, 100 times weaker than Kb, was also observed
Expanding the accessible P/T domain of the Paris-Edinburgh Press : a versatile tool to study liquid network structures at extreme P/T conditions
No full textLAUREA MAGISTRALELa conoscenza della distribuzione di temperatura in dispositivi per l’alta pressione e alta temperature come la pressa Parigi-Edimburgo (PEP) è importante per assicurare l’acquisizione in-situ di dati di alta qualità, e anche per identificare le parti che influenzano maggiormente le prestazioni termiche in questi dispositivi. La PEP è uno dei classici strumenti usati per studiare le proprietà dei materiali fino a 25 GPa e 2300K. Per esempio, è generalmente usata per determinare le proprietà di liquidi fortemente reattivi, come la loro struttura locale, densità e viscosità con la diffrazione e la spettroscopia di assorbimento di raggi X o la diffrazione di neutroni. Tuttavia, questi dati possono essere ottenuti solo da simulazioni e fino ad ora solo uno studio riporta le distribuzioni termiche su un tipo particolare di guarnizione. Questo è uno studio numerico della distribuzione termica nelle tre più diffuse guarnizioni a più componenti PEP. Nei primi due capitoli viene introdotta la PEP in molte delle sue molteplici varianti. Nel terzo è introdotto il modello numerico usato in questi calcoli e viene spiegato come questi risultati verranno confrontati con i dati sperimentali parzialmente acquisiti durante questo studio e presentati nei capitoli cinque e sei.
Un importante risultato dal quarto capitolo di questo studio è che la massima differenza di temperatura dei campioni dentro tutte e tre le guarnizioni studiate è molto piccola (60K a 2500K nella guarnizione più semplice). Questo mostra che le peculiarità della PEP sono uniche, risultando un dispositivo molto attraente agli occhi di tutti i geoscienziati. Inoltre, abbiamo mostrato che le differenze di temperatura tra i campioni e i calibranti sono molto minori delle incertezze intrinseche della tecnica di calibrazione incrociata. Le simulazioni effettuate hanno permesso di identificare l’importante ruolo che gli elettrodi di molibdeno e la fornace di grafite hanno sulle prestazioni termiche globali. L’utilizzo di materiali con una conducibilità termica selezionata può portare a un significante aumento delle prestazioni termiche. Infine abbiamo condotto delle simulazioni di nuove celle che permettono di estendere il dominio P/T della pressa Parigi-Edimburgo.The understanding of thermal profiles in high P/T devices such as the Paris-Edinburgh press (PEP) is important for ensuring the acquisition of high-quality in-situ data as well as for the identification of assembly parts that affect the overall heat performance. The PEP is one of the classical high P/T instruments used to study material properties in-situ up to 25 GPa and 2300 K. For example, it is commonly employed to determine properties of highly reactive liquids, such as their local structure, density and viscosity with X-ray absorption spectroscopy, X-ray or Neutron diffraction. However, the thermal distributions in the sample at such high T can only be obtained from calculations and so far only one study reported results on a very specific assembly type. This thesis was therefore dedicated to study the T distributions and heat performances in the three most employed PEP assemblies using finite element calculations. One of the main results of the present study are the rather small maximal of all PEP assemblies (60 K at 2500 K for the most basic cell assembly), indicating that the PEP is a very well suited device for high P/T experiments. Moreover, we could reveal that T offsets between sample and calibrant material positions are lower than intrinsic uncertainties of the cross-calibration technique. The present calculations also allowed us to identify the graphite furnace and the molybdenum electrode as key assembly parts that highly affect the overall heat performance. The employment of materials with well-chosen thermal conductivities can therefore significantly increase the T performance. Finally, we conducted calculations on new optimized cell assemblies that allow to extend the current P/T domain of the PEP
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