Gotowa bibliografia na temat „Chirality density”

In this work, effect of chirality on gas adsorption property of semiconducting single-wall carbon nanotubes (SWCNTs) is reported for the first time. First principles simulation of the interaction of three different chirality SWCNTs with different gas molecules is performed maintaining equilibrium tube–molecule distance. Results are obtained employing density functional theory, using the Atomistic toolkit simulation package (ATK-DFT). Nanotube density of states is observed to vary significantly due to interaction with different types of gases as well as for same gas if chirality of SWCNTs varies. The most significant finding is, the change in DOS near Fermi level is highest in mod 2 type semiconducting SWCNT for different gas molecules irrespective of donor or acceptor. Thus, proper selection of chirality of SWCNT is important to make nanotube based gas sensor and mod 2 types semiconducting SWCNTs should get preference over mod 1 type as a sensing element so as to get better sensitivity.

The synthesis of high-quality chirality-pure single-walled carbon nanotubes (SWCNTs) is vital for their applications. It is of high importance to modernize the synthesis processes to decrease the synthesis temperature and improve the quality and yield of SWCNTs. This review is dedicated to the chirality-selective synthesis, sorting of SWCNTs, and applications of chirality-pure SWCNTs. The review begins with a description of growth mechanisms of carbon nanotubes. Then, we discuss the synthesis methods of semiconducting and metallic conductivity-type and single-chirality SWCNTs, such as the epitaxial growth method of SWCNT (“cloning”) using nanocarbon seeds, the growth method using nanocarbon segments obtained by organic synthesis, and the catalyst-mediated chemical vapor deposition synthesis. Then, we discuss the separation methods of SWCNTs by conductivity type, such as electrophoresis (dielectrophoresis), density gradient ultracentrifugation (DGC), low-speed DGC, ultrahigh DGC, chromatography, two-phase separation, selective solubilization, and selective reaction methods and techniques for single-chirality separation of SWCNTs, including density gradient centrifugation, two-phase separation, and chromatography methods. Finally, the applications of separated SWCNTs, such as field-effect transistors (FETs), sensors, light emitters and photodetectors, transparent electrodes, photovoltaics (solar cells), batteries, bioimaging, and other applications, are presented.

Based on the Density Functional Theory (DFT) calculations, we analyze the structural and electronic properties of boron phosphide nanotubes (BPNTs) as functions of chirality. The DFT calculations are performed using the M06-2X method in conjunction with the 6-31G(d) divided valence basis set. All nanostructures, (n,0) BPNT (n = 5–8, 10, 12, 14) and (n,n) BPNT (n = 3–11), were optimized minimizing the total energy, assuming a non-magnetic nature and a total charge neutrality. Results show that the BPNT diameter size increases linearly with the chiral index “n” for both chiralities. According to the global molecular descriptors, the (3,3) BPNT is the most stable structure provided that it shows the largest global hardness value. The low chirality (5,0) BPNT has a strong electrophilic character, and it is the most conductive system due to the small |HOMO-LUMO| energy gap. The chemical potential and electrophilicity index in the zigzag-type BPNTs show remarkable chirality-dependent behavior. The increase in diameter/chirality causes a gradual decrease in the |HOMO-LUMO| energy gap for the zigzag BPNTs; however, in the armchair-type BPNTs, a phase transition is generated from a semiconductor to a conductor system. Therefore, the nanostructures investigated in this work may be suggested for both electrical and biophysical applications.

Chirality depends on particular symmetries. For crystal structures it describes the absence of mirror planes and inversion centers, and in addition to translations, only rotations are allowed as symmetry elements. However, chiral space groups have additional restrictions on the allowed screw rotations as a symmetry element, because they always appear in enantiomorphous pairs. This study classifies and distinguishes the chiral structures and space groups. Chirality is quantified using Hausdorff distances and continuous chirality measures and selected crystal structures are reported. Chirality is discussed for bulk solids and their surfaces. Moreover, the band structure, and thus, the density of states, is found to be affected by the same crystal parameters as chirality. However, it is independent of handedness. The Berry curvature, as a topological measure of the electronic structure, depends on the handedness but is not proof of chirality because it responds to the inversion of a structure. For molecules, optical circular dichroism is one of the most important measures for chirality. Thus, it is proposed in this study that the circular dichroism in the angular distribution of photoelectrons in high symmetry configurations can be used to distinguish the handedness of chiral solids and their surfaces.

Ab initio molecular orbital and density function theory (DFT) calculations as used to calculate the structure optimisation and configurational features of cycloundeca-1,2,4,5,7,8,10-heptaene (2) showed that the combination of two allenic units of the same chirality and a unit of opposite chirality yields an enantiomeric Z-isomer pair ( RSR and SRS) of C2 symmetry, which is the most stable configuration.

Recent advancements have revealed the growing effectiveness of optical nanofibers in enabling the implementation of atom-photon hybrid quantum systems. These nanofibers serve as non-intrusive tools for probing cold atoms, offering a unique approach to circumvent the limitations imposed by the Rayleigh domain, thereby achieving increased intensities in a beam of light over long distances. This study investigates the interaction between the atom and light, focusing on the dipole transition in sodium atoms near a nanofiber. Notably, we uncover the influence of the direction of light propagation, known as the optical chirality effect, on the spatial distribution of the steady-state density matrix elements. Furthermore, we examine the optical forces acting on a two-level atom during the 32S1/2 →32P3/2 transition in sodium. Our findings demonstrate that optical chirality’s effect significantly impacts the magnitude of these optical forces. The concept of optical chirality holds great promise for advancing technology and enhancing our understanding of atomic behavior. The numerical results presented in this work are based on experimental parameters within a realistic range.

The distinct response of biological systems to the two enantiomers of a chiral chemical has led to a large market for enantiopure pharmaceuticals and raised fundamental issues about the origin of biological homochirality. It is therefore important to understand the interactions of chiral molecules with chiral environments. Chiral environments associated with solid surfaces could potentially play a useful role in chirally specific chemical processing. There are a variety of routes for creating chiral solid surfaces. Surfaces of materials whose bulk crystal structure is enantiomorphic can be used as one type of chiral solid surfaces. Metal surfaces that are intrinsically chiral due to the presence of kinked surface steps provide another route for creating chiral solid surfaces. Alternatively, we can impart chirality onto surfaces by attaching irreversibly adsorbing chiral organic species on otherwise achiral surfaces. Understanding and ultimately controlling enantiospecific interactions of molecules on this kind of surfaces requires detailed insight into the adsorption geometries and energies of these complex interfaces. To tackle these issues, we performed density functional theory (DFT) calculations that have proved to be a useful tool for quantitative prediction of these effects. Besides our main topic above, we theoretically examine the effects of K atoms as a promoter coadsorbed with small molecules on Mo2C surfaces, a promising catalyst for a range of chemicals applications. Our results in this thesis provide fundamental information about these systems and demonstrate that using DFT for this purpose can be a useful means of identifying the phenomena that control chiral surface chemistry.

Dans ce manuscrit, nous décrivons comment nous avons développé la chimie des arynes atropisomères ayant une triple liaison réactive en ortho de l’axe stéréogène. Ces intermédiaires réactionnels sont facilement générés in situ et réagissent de manières énantiospécifiques, et permettent de synthétiser des atropisomères originaux. En particulier, nous décrivons la génération et des applications d’un équivalent synthétique de bis(aryne) atropisomère dérivé du BINOL, ainsi que la mesure semi-expérimentale de la barrière de réaction de tels arynes en présence d’un arynophile standard. À la fin du manuscrit, nous décrivons nos efforts pour développer une approche synthétique de deux triple hélicènes régioisomères
In this manuscript, we describe how we have developed the chemistry of aryne atropisomers with an ortho reactive triple bond to the stereogenic axis. These reaction intermediates are easily generated in situ and react in enantiospecific manners, and allow the synthesis of novel atropisomers. In particular, we describe the generation and applications of a synthetic equivalent of a BINOL-derived bis(aryne) atropisomer, as well as the semi-experimental measurement of the reaction barrier of such arynes in the presence of a standard arynophile. At the end of the manuscript, we describe our efforts to develop a synthetic approach to two regioisomers triple helices

Cationâ Ï interaction is an important determinant in protein structure and function. Among the three proteinogenic aromatic amino acids, tryptophan (Trp) is the strongest cationâ Ï donor. We reported the asymmetric syntheses of tryptophan regioisomers in which the amino acid side chain is attached at different position of the indole moiety. These new tryptophan regioisomers can effect a different mode of cationâ Ï interaction. In nature, dramatic increases in binding affinity can be achieved through multivalent binding. Following a fragmentation-dimerization approach, we synthesized Taxol-based dimer in which the baccatin III core of Taxol is coupled with flexible PEG linker. However, microtubule assembly assay suggested that these new dimers are not capable of effecting bivalent binding to the Taxol binding sites in microtubules. Memory of chirality (MOC) is an emerging theme in asymmetric synthesis in which the dynamic chirality of the reactive intermediate "memorizes" the static chirality of the reactant. Using dynamic 1D and 2D NMR and density functional theory (DFT) methods, we studied the MOC effect of 1,4-benzodiazepin-2-ones. Reconstruction of the reaction pathway using DFT calculations supported our proposed contra steric, retention of configuration mechanism.
Ph. D.

La détection de molécules chirales à l'aide de résonateurs plasmoniques est un domaine de recherche prometteur pour améliorer la sensibilité et la flexibilité de la détection. Cette approche vise à surmonter les limitations des méthodes conventionnelles, telles que la méthode chiroptique, qui présente des limitations en termes de sensibilité. Les résonateurs plasmoniques sont capables d'interagir de manière résonante avec la lumière, ce qui permet d'augmenter le couplage entre les molécules chirales et la lumière, tout en offrant un contrôle sur les propriétés de polarisation de la lumière. Les avancées récentes dans ce domaine ont porté sur la création de surfaces nanostructurées chirales avec des résonateurs spécifiques, mais le mécanisme sous-jacent à la réponse différentielle des biomolécules à la lumière polarisée circulairement reste mal compris. Dans le cadre de ce projet de doctorat, l'approche novatrice consiste à utiliser des nanostructures achirales anisotropes, telles que des nanoslits, pour interagir avec des molécules chirales. Ces nanostructures achirales offrent l'avantage de pouvoir inverser le signe du dichroïsme circulaire en contrôlant la polarisation incidente ou le sens de propagation de la lumière. En manipulant les symétries du champ électromagnétique à proximité des résonateurs, il devient possible d'étudier plus en détail le couplage électromagnétique entre les biomolécules chirales et les nanorésonateurs. Le projet vise à développer des nanorésonateurs plasmoniques innovants, basés sur des nanoslits, qui seront fonctionnalisés pour détecter des biomolécules chirales. Contrairement aux résonateurs chiraux, les résonateurs achiraux peuvent générer des signes de champs chiraux, offrant ainsi une grande flexibilité dans la détection. Le travail comprend la caractérisation et la compréhension de l'origine des champs chiraux, ainsi que des méthodes pour les rendre homogènes. Une partie de la recherche se concentre sur la conception d'une source de lumière superchirale pure à l'aide de nanoslits, qui peut être accordée en longueur d'onde et en polarisation. Dans cette perspective, des méthodes expérimentales sont présentées, notamment l'utilisation de la fluorescence détectée par dichroïsme circulaire (FDCD) pour les molécules sensibles aux énantiomères. Pour la réalisation de ces expériences, des résonateurs plasmoniques avec une résonance à 680 nm ont été choisis, correspondant à la bande d'absorption chirale de LHCII. Une idée intéressante consiste à bloquer le faisceau d'excitation pour ne recueillir que l'émission des molécules chirales, en étudiant les résonances des ouvertures dans une couche d'or opaque. En résumé, ce projet de doctorat vise à exploiter les avantages des nanostructures plasmoniques achirales pour améliorer la détection des molécules chirales en offrant une plus grande flexibilité dans la manipulation de la polarisation de la lumière et en explorant de nouvelles méthodes expérimentales pour cette détection
The detection of molecules based on fluorescence or Raman scattering has been widely studied and is currently used in industry and laboratories. However, many organic molecules of interest are chiral, and their chemical and biological properties depend on their enantiomer as well as on the chirality of their secondary structure. The quantity and chirality of biomolecules are classically determined by measuring the differential absorption between the two opposite circular polarizations (chiroptic method). However, this method is limited by the low differential absorption of chiral molecules, which is of the order of 10-3 in the UV part of the spectrum. Plasmonic resonators have the ability to resonantly interact with light and are characterized by a moderate quality factor and a low effective volume. This resonant interaction allows (i) to increase the coupling between molecules and light and (ii) to control the polarization properties of light. So far, the latest advances concern the implementation of nanostructured chiral surfaces with gammadion-type resonators or stacked twisted resonators that interact preferentially with a given helicity of light. However, the mechanism behind the differential response of biomolecules coupled to chiral resonators to circularly polarized light is still unclear, preventing the optimization of such detection. Moreover, in the research published so far, two different chiral sensors are needed to interact with right- and left-handed circularly polarized light, which requires complex calibration procedures. During the course of my PhD, I have studied the use of anisotropic achiral nanostructures to interact with chiral molecules. Indeed, they have the significant advantage over chiral nanostructures of changing the sign of the circular dichroism by controlling the incident polarization or the direction of propagation. Indeed, the symmetries of the electromagnetic field in close proximity to the resonators can be manipulated at will by changing illumination conditions hence providing a unique tool for studying the origin of the electromagnetic coupling between chiral biomolecule and nanoresonators. Consequently, in my PhD project I propose to use plasmonic nanoresonators to increase the light - “chiral matter” interactions in order to detect and study chiral molecules. I will use the concept of achiral plasmonic nanostructures (nanoslits) to develop innovative nanoresonators that will be used, once functionalized, to detect chiral biomolecules with enantiomer sensitivity. Indeed, achiral resonators can generate both signs of chiral fields as opposed to chiral resonators which would make their use very flexible. This work implies characterizing, describing and understanding the origins of chiral fields and how to make them homogeneous. Through the study of nanoslits, I demonstrate numerically and theoretically how to design a nanosource of pure superchiral light, free of any background and for which the sign of the chirality is tunable on-demand in wavelength and polarization. In the perspective, I will present experimental methods that could monitor the CD via fluorescence emission (FDCD for Fluorescence Detected Circular Dichroism) in the case of light harvesting molecules for molecules that need to be excited in the UV, autofluorescence may be used in conjunction with aluminum resonators. Without loss of generality, these considerations lead to the decision of investigating plasmonic resonators with resonance at 680 nm which correspond to the chiral absorption band of LHCII. The idea of blocking the excitation beam to collect only the emission of the chiral molecules leaded to the idea of investigating the resonances of openings in an opaque layer of gold

Doctor of Philosophy
Department of Chemistry
Christine M. Aikens
Gold and silver particles with dimensions less than a nanometer possess unique characteristics and properties that are different from the properties of the bulk. They demonstrate a non–zero HOMO–LUMO gap that can reach up to 3.0 eV. These differences arise from size quantization effects in the metal core due to the small number of atoms. These nanoparticles have attracted great interest for decades both in fundamental and applied research. Small gold clusters protected by various types of ligands are of interest because ligands allow obtaining gold nanoclusters with given sizes, shapes and properties. Three main families of organic ligands are usually used for stabilization of gold nanoclusters: phosphine ligands, thiolate ligands and DNA. Usually, optical properties of these NPs are studied using optical absorption spectroscopy. Unfortunately, sometimes this type of spectrum is poorly resolved and tends to appear very similar for different complexes. In these cases, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopy can be applied. However, the interpretation of experimental CD and MCD spectra is a complicated process. In this thesis, theoretically simulated CD and MCD spectra were combined with optical absorption spectra to study optical activity for octa– and nona– and undecanuclear gold clusters protected by mono– and bidentate phosphine ligands. Additionally, optical properties of bare and DNA protected silver NPs were studied. Theoretical CD spectra were examined to learn more about the origin of chirality in chiral organometallic complexes, and to contribute to the understanding of the difference in chiroptical activity of gold clusters stabilized by different phosphine ligands and DNA–stabilized silver clusters. Furthermore, optical properties of the small centered gold clusters Au₈(PPh₃)₈²⁺ and Au₉(PPh₃)₈³⁺ were examined by optical absorption and MCD spectra using TDDFT. Theoretical MCD spectra were also used to identify the plasmonic behavior of silver nanoparticles. These results showed that CD and MCD spectroscopy yield more detailed information about optical properties and electronic structure of the different chemical systems than optical absorption spectroscopy alone. Theoretical simulation of the CD and MCD spectra together with optical absorption spectra can be used to assist in the understanding of empirically measured CD and MCD and provide useful information about optical properties and electronic structure.

Memory of chirality (MOC) is an emerging strategy for asymmetric synthesis which relies upon the intermediacy of transiently non-racemic reactive species. In these reactions the configuration of the sole stereogenic center of the enantiopure starting material is "memorized" by a chiral non-racemic conformation in the intermediate; trapping then captures the stereochemical information, and generates a new stereogenic center with high fidelity. We experimentally and computationally studied the highly retentive deprotonation/alkylations of 1,4-benzodiazepin-2-ones (BZDs) that rely upon this strategy. We captured a transiently non-racemic BZD enolate intermediate in enantiopure form, then released the enolate and observed its subsequent reaction. This approach allowed the first ever step-wise observation of the stereochemical course of such a MOC process. Approximately 2 million deaths are caused by malaria every year in the world. In total roughly 3.2 billion people are living under the risk of malaria transmission. Current use of anticholinesterase insecticides has been limited by their toxicity to human beings. A major African malaria insect vector, Anopheles gambiae (Ag), was targeted. Based on sequence alignment and homology models of AgAChE, a strategy of dual-site binding was adopted that targets Trp84 in the active site and Cys286 at the peripheral site. Selective AChE inhibitors have been designed and synthesized.
Ph. D.

Bivalent drug design is an efficient strategy for increasing potency and selectivity of many drugs. We devised a strategy to prepare agonist-benzodiazepine heterodimers that could simultaneously bind to agonist and BZD sites of the GABAAR. We synthesized a benzodiazepine-MPEG model compound that relied on physiological GABA to elicit flux. We established that a tether at the N1 position of the BZD would not prevent binding to the receptor. However, coupling of GABA amides with long chain PEG tethers studied by another group member resulted in complete loss of agonist activity. We therefore ceased research in this particular area. 1,4-Benzodiazepin-2,5-diones display a wide range of pharmacological activities. Compounds containing the tricyclic proline-derived subtype have received attention as potent anxiolytic agents and as starting materials for anthramycin-inspired anticancer agents. More recently enantiopure (S)-proline-derived 1,4-benzodiazepin-2,5-diones have been recognized as selective α5 GABAA receptor ligands. Despite the impressive diversity of 1,4-benzodiazepine-2,5-diones prepared to date, enantiopure examples possessing a quaternary stereogenic center have been largely unexplored. "Memory of chirality" (MOC) is an emerging strategy for asymmetric synthesis. This technique enables the memory of a sole chiral center in the substrate to be retained in a process that destroys that center. We have used this technique to prepare a library of quaternary proline-derived, thioproline-derived and hydroxyproline-derived 1,4-benzodiazepin-2,5-diones, in high ee. We have developed an efficient synthetic method for preparing oxaproline-derived 1,4-benzodiazepin-2,5-diones in high yields, and by applying the MOC strategy we have prepared quaternary derivatives in acceptable %ee. We envision oxaproline-derived 1,4-benzodiazepin-2,5-diones may exhibit similar or more potent pharmacological properties than proline-derived 1,4-benzodiazepin-2,5-diones. Using density functional theory (DFT) methods, we modeled the formation of an enantiopure, dynamically chiral enolate intermediate and the slow racemization of the enolate on the alkylation reaction time scale.
Ph. D.

Over the past 20 years’ researchers have tried to utilize the remarkable properties of single-walled carbon nanotubes (SWCNTs) to create new high-tech materials and devices, such as strong light-weight composites, efficient electrical wires and super-fast transistors. But the mass production of these materials and devices are still hampered by the poor uniformity of the produced SWCNTs. These are hollow cylindrical tubes of carbon where the atomic structure of the tube wall consists of just a single atomic layer of carbon atoms arranged in a hexagonal grid. For a SWCNT the orientation of the hexagonal grid making up the tube wall is what determines its properties, this orientation is known as the chirality of a SWCNT. As an example, tubes with certain chiralities will be electrically conductive while others having different chiralities will be semiconducting. Today’s large scale methods for producing SWCNTs, commonly known as growth of SWCNTs, gives products with a large spread of different chiralities. A mixture of chiralities will give products with a mixture of different properties. This is one of the major problems holding back the use of SWCNTs in future materials and devices. The ultimate goal is to achieve growth where the resulting product is uniform, meaning that all of the SWCNTs have the same chirality, a process termed chirality-specific growth. To achieve chirality-specific growth of SWCNTs requires us to obtain a better fundamental understanding about how they grow, both from an experimental and a theoretical point of view. This work focuses on theoretical studies of SWCNT properties and how they relate to the growth process, thereby giving us vital new information about how SWCNTs grow and taking us ever closer to achieving the ultimate goal of chirality-specific growth. In this thesis, an introduction to the field is given and the current state of the art experiments focusing on chirality-specific growth of SWCNTs are presented. A brief review of the current theoretical works and computer simulations related to growth of SWCNTs is also presented. The results presented in this thesis are obtained using first principle density functional theory. The first study shows a correlation between the stability of SWCNT-fragments and the observed products from experiments. Calculations confirm that in 84% of the investigated cases the chirality of experimental products matches the chirality of the most stable SWCNT-fragments (within 0.2 eV). Further theoretical calculations also reveal a previously unknown link between the stability of SWCNT-fragments and their length. The calculations show that at specific SWCNT-fragment lengths the most stable chirality changes. Thus, introducing the concept of a switching length for SWCNT stability. How these new results link to the existing understanding of SWCNT growth is discussed at the end of the thesis.

Ge₂Sb₂Te₅ (GST) is an established phase-change material that undergoes fast reversible transitions between amorphous and crystalline states with a high electro-optical contrast, enabling applications in non-volatile optical and electronic memories and optically-switchable structured metamaterials. This work demonstrates that optical anisotropy can be induced and recorded in pure and doped GST thin films using circularly polarised light (CPL), opening up the possibility of controlled induction of anisotropic phase transition in these and related materials for optoelectronic and photonic applications. While the amorphous-to-crystalline phase transition in GST has generally been understood to proceed via a thermal mechanism, significant optical anisotropy (measured by circular dichroism (CD) spectroscopy in this work) strongly suggests that there is an electronic athermal component of the phase change induced by the handedness of circularly polarised nanosecond laser pulses and implies the existence of chiral structures or motifs. Optically active and inactive regions in the films have also been studied using X-ray and electron diffraction and spectroscopic techniques in order to obtain a structural picture that can be correlated to the optical changes observed and the findings offer surprising evidence of the nature of the phase transition. Regions exhibiting higher CD signal intensities were found to be mostly amorphous with elemental phase separation observed within modified surface features. Several mechanisms are proposed for the observed phenomena, including the retention of chiral crystalline fragments in laser- irradiated and melt-quenched amorphous regions, which could explain the results of CD spectroscopy. This may be extended to other material systems and harnessed in potential metamaterials, plasmonics, photonics or chiroptical applications.

Le succès d’un catalyseur asymétrique n’est possible que si son ligand chiral est judicieusement conçu. La combinaison d’un carbène N-hétérocyclique avec un cycle oxazoline s’avère particulièrement prometteuse. Une nouvelle famille de sels d’imidazolium, où ces deux hétérocycles sont liés par un pont (diméthyl)méthylène a été générée. Les précurseurs imidazoliums sont obtenus par réaction d’un dérivé bromé avec un imidazole portant un cycle oxazoline. Sept différents précurseurs de carbènes ont pu être générés (rendements compris entre 60 et 90%), et les carbènes libres 1-(1-méthyl-1-((4S)-iso-propyl- et tert-butyl-4,5-dihydrooxazol-2-yl)éthyl)-3-(di(-naphtyl)méthyl) imidazole-2-ylidène (11a, 11b) ont pu être isolés. La réaction du sel d’imidazolium 9d avec le catalyseur de Karstedt en présence de KOtBu génère le complexe monodentate de platine(0) 13. Ce dernier peut être oxydé par CsBr3 pour former le complexe bidentate de platine(II) 15. Le complexe neutre de rhodium(I) 18 est quant à lui obtenu par déprotonation du sel d’imidazolium 9g puis réaction avec [Rh(nbd)Cl]2, et a pu être converti en son analogue cationique 19 par abstraction de bromure. La présence de groupements méthyles sur le pont (dimethyl)méthylène apporte une plus grande stabilité au complexe par rapport à l’air et renforce le pouvoir chélatant du ligand. De plus, pour la famille de ligands précédemment développée au sein du groupe où les deux hétérocycles sont directement liés, les limites de sa capacité à chélater un centre métallique ont été illustrées par la synthèse des complexes de cuivre(I) 12a et 12b, qui cristallisent sous la forme de dimère et d’oligomère de coordination. Les analogues cationiques des complexes de rhodium 16b, 18 et 20 ont été testés comme catalyseurs de la réaction d’hydrosilylation asymétrique des cétones. Les observations expérimentales ne pouvant pas être expliquées par le mécanisme communément accepté pour cette catalyse, une étude théorique fut menée par le biais de calculs DFT. Trois voies mécanistiques ont pu être établies : elles diffèrent par le mode d’insertion de la cétone, soit dans la liaison Rh-Si (mécanisme d’Ojima), soit dans la liaison Si-H (mécanisme de Chan) ou via la formation d’un intermédiaire silylène (nouveau mécanisme proposé). Ce dernier est énergétiquement favorisé par rapport aux mécanismes postulés jusqu’à maintenant (55 kJ. Mol-1 pour le mécanisme d’Ojima, 120 kJ. Mol-1 pour le mécanisme de Chan). De plus, n’étant accessible que lorsqu’un dihydrosilane est utilisé, ce nouveau mécanisme rend compte de l’accélération de la vitesse de réaction lorsqu’un silane secondaire est utilisé à la place d’un silane tertiaire. Enfin, seul ce mécanisme offre une interprétation possible de l’effet isotopique cinétique inverse observé
The efficiency of an asymmetric organometallic catalyst is determined by the appropriate design of the chiral ligand. The combination of the kinetic robustness of N-heterocyclic carbenes with oxazolines as stereodirecting elements appears to be very promising. A novel family of imidazolium salts, where both heterocycles are connected by a (dimethyl)methylene bridge was generated. Reacting various bromide derivatives with imidazoles bearing an oxazoline unit has afforded a family of seven different ligand precursors (60-90% yield). The free carbenes 1-(1-methyl-1-((4S)-iso-propyl- and tert-butyl-4,5-dihydrooxazol-2-yl)ethyl)-3-(di(-naphtyl)methyl)imidazole-2-ylidene (11a, 11b) could be isolated by simple deprotonation of the imidazolium salt. Reaction of the imidazolium salt 9d with the Karstedt catalyst in the presence of potassium tertbutoxide led to the formation of the monodendate trigonal planar Pt(0) complex 13, which could be oxidised to the bidendate Pt(II) complex 15 by reaction with CsBr3. Subsequent deprotonation of the imidazolium salt 9g and reaction with [Rh(nbd)Cl]2 gave the neutral square-planar Rh(I) complex 18 that is converted into the cationic bidendate complex 19 by bromide abstraction. The methyl substituents on the methylene bridge proved to provide more stability to complexes towards air and to enhance the chelating capacity of the ligand in solution. At the same time, the limit of the chelating capacity of the ligands previously developed in the group, where both heterocycles are directly connected, was emphasised by the generation of the Cu(I) complexes 12a and 12b, which crystallised as dimer and coordination oligomers. The cationic analogues of the rhodium complexes 16b, 18 and 20 were tested in the catalytic asymmetric hydrosilylation of ketones and afforded different activities and selectivities. The commonly accepted Ojima mechanism not accounting for some of the experimental observations, a comprehensive theoretical investigation of the catalysis mechanism via DFT calculations was carried out. Three viable mechanistic pathways could be established. They all involve a first oxidative addition step and differ in the mode of insertion of the ketone, either into the Rh-Si bond (Ojima mechanism), into the Si-H bond (Chan mechanism) or via the formation of a silylene intermediate (new mechanistic pathway). The latter is energetically favoured with regard to the postulated ones, by respectively 55 kJ. Mol-1 (Ojima mechanism) and 120 kJ. Mol-1 (Chan mechanism). Being only accessible when a dihydrosilane is used, this explains the observed rate enhancement when secondary silanes are used instead of tertiary silanes. Moreover, it accounts for the inverse kinetic isotope effect

This chapter argues that control of magnetic domains and domain wall structures is one of the most important issues from the viewpoint of both applied and basic research in magnetism. Its discussion is however limited to static and dynamic properties of magnetic vortex structures. It has been revealed both theoretically and experimentally that for particular ranges of dimensions of cylindrical and other magnetic elements, a curling in-plane spin configuration is energetically favored, with a small region of the out-of-plane magnetization appearing at the core of the vortex. Such a system, which is sometimes referred to as a magnetic soliton, is characterized by two binary properties: A chirality and a polarity, each of which suggests an independent bit of information in future high-density nonvolatile recording media.

Thermal transport processes in graphene nanoribbons (GNRs) within and beyond the linear response regime has been studied using classical molecular dynamics simulations. Zigzag-edged GNRs have higher thermal conductivities than armchair-edged ones, and the difference diminishes with increasing width. Analysis on the cross-sectional distribution of heat flux reveals that edge atoms cannot transport thermal energy as efficiently as interior ones. Edge localization of phonon modes reduces thermal transport through edge carbon atoms, especially on armchair edges, which results in a lower thermal conductivity. Isotope (13C) doping can reduce the thermal conductivity of GNRs by 30%–40% by an addition of only ∼20% isotope atoms. The significant reduction in thermal conductivity is partially attributed to phonon localization induced by isotope defects, which is confirmed by phonon mode participation ratio analysis. We also demonstrate that a GNR asymmetric in edge chirality or mass density can generate considerable thermal rectification, which is essential for developing GNR-based thermal management devices.