Experimental and Ab Initio Characterization of Mononuclear Molybdenum Dithiocarbamates in Lubricant Mixtures

Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures

Magnetic hexamers interacting in layers in the (Na,K)2_2Cu3_3O(SO4)3_4)_3 minerals

Magnetic properties and underlying magnetic models of the synthetic A$_2$Cu$_3$O(SO$_4)_3$ fedotovite (A = K) and puninite (A = Na) minerals, as well as the mixed euchlorine-type NaKCu$_3$O(SO$_4)_3$ are reported. We show that all these compounds contain magnetic Cu$_6$ hexamer units, which at temperatures below about 100 K act as single spin-1 entities. Weak interactions between these magnetic molecules lead to long-range order below $T_N$ = 3.4 K (A = Na), 4.7 K (A = NaK), and about 3.0 K (A = K). The formation of the magnetic order is elucidated by ab initio calculations that reveal two-dimensional inter-hexamer interactions within crystallographic $bc$ planes. This model indicates the presence of a weakly distorted square lattice of $S=1$ magnetic ions and challenges the earlier description of the A$_2$Cu$_3$O(SO$_4)_3$ minerals in terms of Haldane spin chains.Comment: published version, re-worked compared to the initial submissio

Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13C and 29Si NMR Enhanced by Dynamic Nuclear Polarization

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of13C and 29Si nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(N-phenylureido)propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via 1H–1H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements εon/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to εon/off ≈ 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated

Special Issue: Electron Paramagnetic Resonance

International audienceElectron paramagnetic resonance (EPR) is the tool of choice to probe and understand the dynamics of electron spin. Recent achievements in both theory and instrumentation have made EPR central in order to study new interdisciplinary topics. Such achievements are found in the collection of papers from the Special Issue, devoted to EPR, with contributions in technical improvement [1,2], theory in physics [3], biology [2,4,5], and biochemistry [6]. This issue is articulated around three original articles and three specific reviews. First of all, Daniella Goldfarb and co-workers [1] presented a modified sequence for distance measurements. Using a high field pulsed spectrometer and an arbitrary wave generator, they managed to implement this sequence of shaped pulses. The goal was to improve the signal noise ratio and so the accuracy of the distance between two Gd(III) rare earth ions was labeled on ubiquitin. They demonstrated a challenging low concentration protein labeled by Gd(III) distance measurements. Staying in high field EPR, Klaus Möbius and Anton Savitsky and co-workers [6] presented an impressive review on complementary high field EPR and NMR (nuclear magnetic resonance) in advance biomolecular spectroscopy. More than 600 papers were presented and classified, showing the entire history as well as recent improvements and state of the art in the domain. Pedro Schlottmann [3] presented the theory of EPR in ferromagnetically correlated heavy fermion, and more specifically, the role of the Kondo effect (screening of magnetism of impurity by conduction electrons) on EPR signals. In the field of biology and medicine, EPR has attracted a lot of interest. Dariush Hinderberger and co-workers [4] presented an original study on the effect of pH on human serum albumin. Using both the continuous and pulse waves, they showed the improvement of using EPR, compared to other technics, to study the pH-induced functional phase space. Fabien Peyrot and Nikola Babic [5] presented a review on EPR spectroscopy and imaging of evaluation of oxidative stress in vivo. They presented a recent achievement, state of the art, as well as the limitations in the field. Finally, Hirotada Fujii and co-workers [2] presented a review of brain redox imaging using EPR and Nitroxide probe. They showed how EPR imaging could be used to study brain disorders like Parkinson's and Alzheimer's diseases. They showed the superiority of EPR imaging compared to MRI in such studies. We hope that this first Special Issue of Magnetochemistry, devoted top electron paramagnetic resonance, will be pleasant and useful to specialist and non-specialist readers. We thank the authors for providing such impressive and interesting papers and the referees for their time and valuable comments

Propriétés magnétiques et optiques de cristaux dopés terres rares pour l'information quantique

La maitrise de l information représente un avantage concurrentiel de nos jours. Malgré une intensification des moyens développés pour protéger les flux de données, il n est actuellement pas possible d échanger à distance et de façon complètement sure, une information entre deux interlocuteurs. Néanmoins, des travaux menés par Bennett et Brassard ont montré qu il est possible d atteindre un niveau de sécurité maximum en utilisant un protocole quantique de transmission de l information. Ce protocole se base sur l utilisation de réseaux télécom utilisant des répéteurs quantiques à la place des répéteurs classiques. La voie étudiée dans cette thèse, réalisée en partie dans le cadre du projet européen QuRep, a pour but l amélioration des connaissances sur les monocristaux dopés aux ions de terre rare qui sont des candidats de choix pour la mise au point de répéteurs quantiques. Deux grands axes ont émergés: dans un premier temps nous avons essayé de comprendre quels sont les facteurs de succès et limitatifs dans l utilisation du cristal de Nd: YSO en tant qu hôte pour les mémoires quantiques avec pour objectif le transfert de la cohérence électronique vers des niveaux hyperfins. Dans un second temps, nous avons étudié un cristal présentant une structure hyperfine directement accessible en optique, Er: YLF afin de vérifier sa potentielle utilisation pour les mémoires quantiques. Ces travaux ont permis, entre autre, de réaliser un transfert de cohérence d un niveau Zeeman électronique vers un niveau hyperfin avec un temps de stockage de plus de 300 s, ce qui permet d envisager une mémoire quantique dans Nd: YSO permettant de réémettre un photon à la demande.The control of information is a competitive advantage today. Despite an intensification of the means developed to protect the data stream, it is currently not possible to exchange remotely and in a completely safe way information between two parties. However, the work of Bennett and Brassard have shown that it is possible to achieve a maximum level of security using a protocol for transmitting quantum information. This protocol is based on the use of telecom networks using quantum repeaters in place of conventional repeaters.The route studied in this thesis, carried out partly in the framework of the European Project QuRep, aims to improve knowledge on single crystals doped with rare earth ions that are good candidates for the development of quantum repeaters. Two main areas emerged: on the one hand, we tried to understand the success and limiting factors regarding the use of Nd : YSO single crystal as host for quantum memories. The objective was also to transfer the coherence from an electronic Zeeman level to the hyperfine levels. In a second step, we studied a crystal with a hyperfine structure directly accessible in optics, Er : YLF to assess its potential use for quantum memories. Among other things, we achieved the transfer of coherence from a Zeeman level to an hyperfine level with a storage time of over 300 microseconds, which allows to consider the development an on demand readout quantum memory in Nd : YSO.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Investigation of resonance-stabilized radicals associated with soot particle inception using advanced electron paramagnetic resonance techniques

Abstract In order to tackle the climate emergency, it is imperative to advance cleaner technologies to reduce pollutant emission as soot particles. However, there is still a lack of complete understanding of the mechanisms responsible for their formation. In this work, we performed an investigation devoted to the study of persistent radicals potentially involved in the formation of soot particles, by continuous wave and pulsed electron paramagnetic resonance. This work provides experimental evidence of the presence in nascent soot of highly branched, resonance-stabilized aromatic radicals bearing aliphatic groups, linked together by short carbon chains, and reinforced by non-covalent π-π interactions. These radicals appear to be highly specific of nascent soot and quickly disappear with the increasing soot maturity. Their presence in nascent soot could represent an underestimated health risk factor in addition to the already well documented effect of the high specific surface and the presence of harmful adsorbates

The inhibition of mild steel corrosion in acidic solutions by 2,5-bis(4-pyridyl)-1,3,4-thiadiazole: Structure–activity correlation

International audienceThe effect of 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (4-PTH) on the corrosion of mild steel in acidic media (1 M HCl, 0.5 M H2SO4, 1 M HClO4) has been investigated using weight loss measurements, electrochemical impedance spectroscopy and potentiodynamic polarisation. These studies have shown that 2,5-bis(4-pyridyl)-1,3,4-thiadiazole is good inhibitor for mild steel in 1 M HCl, 0.5 M H2SO4 and 1 M HClO4 solutions, the better performances are seen in the case of 1 M HCl solutions. But in 1 M HClO4, the 4-PTH stimulates corrosion at low concentrations. Polarisation curves indicate that the 4-PTH is a mixed-type inhibitor in all acidic media and E (%) is temperature-dependent. Adsorption on the mild steel surface follows the Langmuir isotherm model in all acidic media. The electronic properties obtained using the Hartree–Fock AB initio 3-21G quantum chemical approach, were correlated with the experimental efficiencies