Effet Overhauser nucléaire | EPFL Graph Search

En spectroscopie RMN, l'effet Overhauser nucléaire décrit une interaction entre deux spins à travers l'espace et non pas à travers les liaisons chimiques comme le couplage scalaire. Cette interaction est limitée à environ 5-6 Å. En anglais, cet effet s'appelle "Nuclear Overhauser Effect", soit NOE. Cet acronyme est souvent utilisé en français sous l'expression "effet NOE". Origine Une des conséquences de la résonance magnétique nucléaire est l'interaction dipôle-dipôle à travers l'espace. Si cette interaction disparaît en solution, il reste son effet sur la relaxation nucléaire qui correspond à l'effet Overhauser nucléaire. Dans les faits, lorsque l'on observe un noyau X en le découplant d'un autre noyau Y, on observe une légère variation positive ou négative de l'intégration du signal X (environ 10% au maximum). Ceci reflète un transfert d'énergie entre les deux noyaux excités. Ceci peut poser un problème lorsque l'on veut pouvoir intégrer les signaux RMN pour éva

Soft matter probed by nonlinear scattering: self-assembly, interfaces, hydration, and long-range order

Jan Dedic

All organisms are made to a large extent of soft matter - macromolecules such as proteins and polysaccharides, or assemblies of small molecules such as lipids embedded in an aqueous environment. Understanding the role of order in soft matter presents a challenge for experimentalists because such systems are neither completely disordered nor perfectly crystalline. This is especially the case for the aqueous environment itself whose order fluctuates on femto- and pico-second time scales. In this thesis, we attempt to elucidate the presence, extent, changes, and implications of orientational order in several soft matter systems: from polyelectrolyte solutions to lipid membrane interfaces and supramolecular structures. To perform this task, we employ nonlinear optical techniques: second-harmonic and sum-frequency scattering. In the first two chapters, we explore the structure of water in aqueous polyelectrolyte solutions. Polyelectrolytes of high molecular mass are shown to induce orientational order in water that spans hundreds of nanometers. Using an adapted model of second-harmonic scattering from charged particles, we can explain the observed ordering in terms of weakly screened long-range electrostatic interactions that perturb the structure of water via charge-dipole interactions. Upon exchanging ordinary (light) water for heavy water, we observe a significant nuclear quantum effect in the induced order, indicating that bulk water-water correlations are enhanced as well by polyelectrolytes. In the following chapter, we exploit the same nuclear quantum effect in polyelectrolyte solutions to show that the polyelectrolyte-induced orientational order correlates with an anomalous increase in the reduced viscosity. We propose that the viscosity anomaly arises from enhanced order in a solvent which hinders the viscous flow. This observation represents a rare direct link between a microscopic property and a macroscopic observable. In the last two chapters, we change focus from homogeneous solutions to dispersions of liposomes which serve as models of biological membranes. In Chapter 5 we explore the binding of the protein alpha-synuclein to anionic vesicles from the perspective of interfacial water. We show that the adsorption of the protein to a vesicle decreases the order in the interfacial water by 30% but does not affect the surface nor the zeta potential. We propose that the changes in the structure of interfacial water can serve as a label-free probe of protein binding. In the last chapter, we investigate the supramolecular chemistry of inclusion complexes of methyl-Î²-cyclodextrin with various lipids. By combining multiple nonlinear scattering techniques, we show that the complexes self-assemble into micrometer-long directed fibers. Based on measurements of different lipids, we propose that the self-assembly process is driven by hydrogen bonding between the lipid headgroup and cyclodextrin. Lastly, we show that the long-range order of the self-assembled structure is transmitted into the structure of the hydrating water.

EPFL2020

Adsorption Sites of Metal Atoms on MgO Thin Films and Rotational Quantum State Spectroscopy of Physisorbed H₂

Edgar Fernandes

The work presented in this thesis covers two different topics of surface science, both investigated with the scanning tunneling miscroscopy and spectroscopy at low temperatures (5~K). First, we are interested in the spectroscopic properties of physisorbed H2 (and D2) layers on metallic samples. The energy of the first rotational quantum state transition for H2 (D2) is characterized on a variety of metallic substrates. For homonuclear molecules, these rotational quantum states are directly dependent on the nuclear spin arrangement. Thus the rotational spectroscopy of molecular H2 (D2) allows for unprecedented spatial resolution of nuclear spin configurations. To isolate one physisorbed molecule from the surface, we grow Xe islands before exposing the sample to H2 (D2). Reducing the Xe islands size to support only one molecule would allow us to probe the rotational quantum state at the individual molecule level. We show that rotational spectroscopy of H2 (D2) on Xe islands is feasible. We find that this method is not viable to isolate a H2 (D2) molecule as small Xe islands become unstable in presence of them. The second topic deals with the adsorption of metallic atoms on thin MgO(100) films grown on a Ag(100) substrate. We use Ca-doping of the MgO layer to identify the Mg lattice positions. Subsequent adsorption of Ho allows the determination of their adsorption site, on top of the O and on the bridge site, halfway between two O (Mg) lattice positions. The abundance of the population over these sites differs with the MgO thickness. On the MgO monolayer, the Ho adatoms are distributed following the abundance of these sites, i.e., 1/3 and 2/3, respectively. On the MgO bilayer, exclusively the O site is populated. Atomic manipulation of the Ho adatoms with the STM tip permits the creation of a new species, adsorbing on the Mg site, which was correctly predicted by DFT. Adsorption sites for other metal atoms have further been measured. Dy and Er adsorb on the bridge and O site, with abundances depending upon the atomic species and the MgO thickness. Au adsorbs exclusively on bridge sites. Fe and Co populate uniquely the O site, independent of MgO thickness, while Tb adsorbs on all the three sites, i.e., O, bridge, and Mg. We finally investigated the Ho adsorption sites as a function of deposition and annealing temperature. Ho atoms on the O site become unstable at a temperature between 58 K and 77 K, while Ho atoms on the bridge site start to diffuse only at 125 K. This results in a hopping to the bridge site of all the Ho adatoms adsorbing on O between 58 K and 77 K. Additionally, we show that evaporation of Ho on the MgO bilayer held at 30 K gives rise to adsorption on almost uniquely bridge sites, in neat contrast to what is found for lower temperature (approximatively 10 K). We interpret this result as the action of entropy which changes the free energy of the two types of adsorption site as the temperature increases. Ideas for the source of entropy are given, but the amount of entropy needed to explain our observations is unrealistically large.

EPFL2017

An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

Arnaud Comment, Tim Rolf Eichhorn, Martin Haag, Sami Jannin, Jacques Van Der Klink

In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1 K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient He-4 flow cryostat that cools the sample to temperatures between 4 K and 300 K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7 mm inner diameter that can accommodate cubic samples of 5 mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3 T. (C) 2013 Elsevier Inc. All rights reserved.