The interfaces of complex mixed-matrix materials are often the center of chemical activity. Such interfaces exist in great variety including solid-solid, solid-liquid, and solid-gaseous interfaces and more complex multi-component engineered materials that are used for a variety of end uses that include synthesis, separations, and catalysis. This general class of materials have in common at least one solid interface, and by the nature of the synthesis (wet chemistry, CVD, ) the materials are often amorphous or microcrystalline. In this circumstance, NMR spectroscopy is a premier tool for the investigation of these systems. The technique reveals local structure at a particular nuclear site and has the capability to identify unique sites or to determine the site distributions in complex phases. With the recent advent of high field NMR at the lab (H0 = 16.4 T), we have pursued NMR studies using quadrupolar nuclei (spin > ˝) as probes of local structure. The figure below shows 27Al multiple quantum magic angle spinning (MQMAS) spectra that demonstrate quenching of five-fold aluminum sites by exposure to benzoic acid.
Since many of the materials of interest in our catalysis studies are metal oxide and mixed metal oxides, and many of the catalytic reactions of interest are redox reactions, it follows that a key element present at the reaction interface is oxygen. We have chosen to develop the use of this element, in particular the 17O isotope, a spin 5/2 quadrupolar nucleus, to probe the structure at reaction sites. The common isotopes of oxygen 16O, 18O, have no magnetic moment and are invisible in the NMR experiment. 17O is just 0.037% naturally abundant, so experiments incorporate a synthetic step to introduce the 17O label. While labor intensive, this chemical dimension provides a very important site-selectivity advantage, depending on the 17O component that are introduced at high abundance.
Microcoils expand the important parameter space in NMR. Signal-to-noise is improved as the coil becomes small because the coupling of the sample magnetization improves as the coil diameter decreases.
Indeed, the historic driving force for the implementation of µcoils is the gain in sensitivity achieved by this technique. Our interest in µcoils is in optimizing quadrupole NMR. We have generated large excitation fields by employing coils with ca 100 um diameters. The nutation plots to the right demonstrate rf field strengths of 6.2, 12.4 and 24.8 MHz in (a), (b), and (c) respectively. Future developments will combine high rf excitation and MAS NMR, with the goal of making solid state quadrupolar NMR more efficient.
Physical Organic Chemistry R & D Projects