Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy

David Lyndon Emsley, Aaron James Rossini

Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these materials at an atomic level can often slow their further development Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one Intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of similar to 658 for H-1 NMR. In this Account, we discuss the application of DPIP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI) a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal-organic frameworks.

AMER CHEMICAL SOC2013

Scanning Tunneling Microscopy using Superconducting Tips to Probe Absolute Spin Polarization

Matthias Alexander Eltschka

The interplay of superconductivity and magnetism is investigated for systems with dimensions ranging from the mesoscopic to the atomic scale by scanning tunneling microscopy (STM) at millikelvin temperatures and by numerical calculations. Based on geometrically confined superconductors in magnetic fields, a novel STM approach is introduced to quantitatively probe the spin polarization of tunneling electrons. In the first part of this work, the effects of magnetic fields and geometrical confinement are probed for superconducting vanadium STM tips. Due to the unique confinement ranging from the atomic to the mesoscopic scale, the superconducting properties of the STM tips vary considerably from their bulk counterparts. To analyze the experimentally determined magnetic field dependence of several V tips, the superconductivity is numerically calculated for modeled cone geometries with various opening angles. The numerical approach based on a one-dimensional Usadel equation leads to a direct correlation between the opening angle ¿ and the order of the superconducting phase transition. First order phase transitions occur when the opening angle is smaller than a critical value (¿ < ¿c), whilst larger opening angles (¿ > ¿c) result in second order phase transitions. The comparison of experimental findings and numerical results reveals the existence of first and second order quantum phase transitions in the V STM tips. In addition, the numerical calculations also explain experimentally observed broadening effects of the superconducting spectra by the specific tip geometry. In the second part, the superconducting V tips are employed in a novel approach to quantitatively probe the spin polarization of tunneling electrons on the nanoscale. For this purpose, the Meservey-Tedrow-Fulde technique is transferred to STM in order to combine their virtues, such as the quantitative probing capability of the spin polarization, the precise control at the atomic scale and the well-defined vacuum tunnel barrier. To demonstrate the capabilities of the new technique, the local spin structure is resolved for a magnetic Co nanoisland, where spin polarizations ranging from -56% up to 65% were found, depending on the local position. Furthermore, the spin polarization P strongly varies with the tip-to-sample distance z (dP/dz ¿ 10%/Å), which is described by the different decays of the spin-up and spin-down wave functions into the vacuum tunnel barrier. The final part describes the local interaction between isolated magnetic moments and the superconducting ground state. Copper phthalocyanine molecules on the superconducting V(100) surface induce bound states within the superconducting gap due to the magnetic coupling and the Coulomb potentials. Spatially resolved measurements reveal the non-isotropic structure of the spectral weights that is explained by the adsorption site on the 5x1 reconstruction of the V(100) surface. The quasi-particle excitations are not only observed on the magnetic molecule but also occur in its close vicinity. With increasing distance from the molecular structure, the intensities of the bound states decay within the distance x ¿ ±30Å and show periodic oscillations at the same time. Comparing the experimental findings to a one-dimensional model suggests the presence of a complicated scattering potential, which can be simplified by assuming two point scatterers within the molecular structure.

EPFL2015

Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production

David Lyndon Emsley, Jeremie Pelletier, Aaron James Rossini

Recent discoveries highlighted the activity, and the intrig-uing, mechanistic features of NIDCl5 as a molecular catalyst for the cycloaddffion, of CO2 and epoxides under ambient cooditions. This hag inSpired the preparation, of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCf5 center dot OEt2], with Silica dehydroxylated at 700 degrees C (SiO2-700) or at 200 degrees C (SiO2-200) to generate diversetrface complexes. The product of the reaction between SiO2-700 and [NbCl5 center dot OEt2] was identified as a monopodal supported surface species, [equivalent to SiONbCl4 center dot OBt(2)] (1a). The reactions, of SiO2-200 with the niobium precursor, according to two different protocols, generated surface complexes 2a. and 3a, presenting significant, but different,, populations of the monopodal surface complex along with bipodal [(equivalent to SiO)(2)NbCl3 center dot OEt2]. Nb-93 solid-state NMR spectra of 1a-3a and P-31 solid-state NMR On their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [equivalent to SiONbCl4 center dot OEt2] and 3a being mostly bipodal [(equivalent to SiO)(2)NbCl3 center dot OEt2]. A double-quantum/single-quantum 3113 NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface- for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO, and propylene Oxide under mild catalytic conditions,: the performance of 2a was found to significantly surpass that of la and 3a. With the support of a syStetnatic DFT study Carried' out model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previbils discoveries regarding the mechanism Of NbCl5-catalyzed cycloaddition in the homogeneous phase.

American Chemical Society2015