SCIENTIFIC FRONTIER II:
New Materials

New materials are being sought for optoelectronics, for coatings, as semiconductors and catalysts, and as light-weight, yet strong, composites for anything from airplanes to golf clubs. Sample characterization is a prerequisite for further design and development of new materials. Whereas, crystalline materials have been primarily characterized by X-ray diffraction, surface studies have been revolutionized by the development of modern microscopic techniques, such as scanning tunneling microscopy and atomic force microscopy. These techniques are, however, limited to special sample types, single crystals for X-ray diffraction studies and accessible surfaces for microscopy, and neither technique provides information about molecular dynamics, chemical kinetics and reactivity. NMR can characterize samples in most types of condensed matter, be it liquid or solid or any of the mesophases like liquid crystals and biological membranes. Given adequate sensitivity NMR can provide such truly unique information.

Current limits to NMR sensitivity have restricted its application to materials available in relatively large quantities (e.g. tens of milligrams). Several novel approaches for gaining sensitivity are on the horizon, but the most generally applicable one will be higher NMR field strengths. Signal strength is proportional to the square of the magnetic field strength (S B_o²), and the time required to achieve an adequate signal-to-noise (S/N) ratio through time-averaging is proportional to the cube of the field strength (S/N B_o³). This has been and continues to be one of the primary driving forces to push NMR spectroscopy to higher field strengths. Another approach for improving sensitivity is to transfer optically pumped polarization in He or Xe to protons. This provides a way to achieve very high nuclear polarization of surface sites. Such polarization sources represent non-invasive extrinsic probes that are highly complementary to intrinsic probes. In Dynamic Nuclear Polarization (DNP) microwave radiation induces electron-nuclear spin flips resulting in high nuclear polarization. In frozen aqueous solution signal enhancements approaching 200 have been demonstrated in static samples and enhancements of 50 have been achieved in magic angle spinning (MAS) spectra. Moreover, exciting developments in high temperature superconducting probes and in cold coils and cold preamplifiers have resulted in S/N improvements by factors as large as five fold. For a spectroscopy that has been aggressively developed for more than half a century an improvement by a factor of five represents nearly a decade's worth of improvement. Such factors will have a profound impact on the scientific applications of NMR.

For materials research, the inherently low sensitivity of NMR is compounded by the frequent need to observe less sensitive nuclei and the common need to characterize surfaces rather than bulk samples. To date only materials with high surface areas have been characterized by NMR. What often restricts the use of spins with high magnetic moments is the presence of strong magnetic dipole-dipole interactions that promote spin-diffusion. While spin-diffusion can be exploited as a means of elucidating the structural properties of materials, it gives rise to homogeneously broadened spectra in the slow motional regime characteristic of most solids, results in loss of spectral resolution, and obscures the relaxation processes induced by molecular dynamics. Zeolites are important in chromatographic separation, air purification and hydrocarbon processing, therefore characterization of the activation energies for mass diffusion of guest molecules are sought. Spin diffusion effects, especially at low temperature, complicates the interpretation of relaxation data. One promising approach is to make use of the high sensitivity of ¹H NMR for detection and to use partial deuteration to dilute the spins and hence reduce spin diffusion. Alternatively, lower sensitivity nuclei could be used. Higher fields and advanced NMR technology are necessary to compensate for this loss of sensitivity in either approach.

Materials research has been spurred on by a growing appreciation of the importance of interfacial phenomena in catalysis and in coatings. Surface science extends to the problems of biocompatible coatings used to prevent prosthetic device rejection by the surrounding tissue. Strategies are being developed to coat these devices with a biologically acceptable layer, such as a protein-like material compatible with the organism. NMR can characterize such polypeptide coatings, but the task is challenging because the biopolymer concentration in such samples is low, and because highly restricted, low frequency molecular motions cause severe line broadening of the NMR signals.

Chemists are laboring to imitate the precise control exerted by living cells, the masters of tailoring large, complex structures from simpler building blocks. Enormous advances stand to be gained by mimicking nature on a technologically practical scale, and unprecedented opportunities exist for manipulating the architecture of materials at the atomic level. For example, control of the surface of polypeptide chain folded lamellar crystals can be achieved through molecular biology. Non- native amino acids such as fluorinated leucine, can be incorporated biosynthetically into such polypeptides to generate a Teflon-like surface. Thus a whole new area of materials research has appeared under labels such as "supramolecular materials" and "self-assembled nanostructures."

Perhaps one of the most ingenious developments in materials research aims at imitating the highly specific synthetic processes taking place in living organisms. The pharmaceutical industry, in particular, is developing supramolecular assemblies, which imitate the properties of enzymes on a very large scale suitable for drug manufacturing. Such assemblies may be three- dimensional, designed for selected processes, or polypeptide films grown or deposited on special substrates, where the goal is to keep reactants close together and arranged in orientations favorable for reaction. Both solid-state and solution NMR provide unique information about molecular structure and dynamics as well as the interactions among reactants and products involved in reactions catalyzed by molecular assemblies. The success of developments in this area depends on structural and dynamic characterizations of such self-organizing assemblies. The characterization of heterogeneous preparations typical of both this synthetic world and the biosynthetic world is a daunting task, but NMR is virtually the only high resolution tool that has promise in this arena.

Another exciting field of research involves mimicking natural fibers, such as spider silk to generate synthetic polymers with very high tensile strength. Recent NMR research revealed how the organization of different domains in spider silk are responsible for its unique mechanical properties.

While spectral resolution increases linearly with the strength of the magnetic field, undesirable second order effects in the spectra of half-integral quadrupolar nuclei are also reduced substantially at higher fields. These nuclei are prevalent among metal ions (²³Na, ²⁴Mg, ²⁷Al, ³⁹K and ⁴⁰Ca), as well as 17 O used to probe the properties of both inorganic and organic materials. Computer simulations clearly show simplification of these spectra at high fields. The aluminosilicates are important catalysts and detailed structural characterization is essential for the design of more efficient catalysts. Both S/N and resolution improve dramatically with field strength as a result of reduced second order interactions. Consequently the number of different Al-sites and their coordination numbers in the sites can be readily determined at high field.

Again new challenges are posed by research requirements for sensitivity, selectivity, spatial and spectral resolution. To meet such demands, non- traditional, inter-disciplinary areas of chemistry have developed, as an understanding of the underlying physical and chemical properties of old and new materials is sought. This reinforces the need for a very closely linked set of facilities, a collaboratorium, where a broad range of scientists would have access to the highest field NMR instruments in the country. With higher fields and novel sensitivity-enhancing techniques, NMR will be able to contribute to a great many areas in materials research where characterizations are needed for structure and dynamics. The range of applications to lower fractional surface areas, to natural abundance samples, to very small samples will be increased dramatically with the large sensitivity enhancements possible through this National Collaboratorium.