Karl T. Mueller

High-Resolution Solid-State NMR Spectroscopy of Complex Materials

Professor Mueller's group studies complex solid-state materials with high-resolution nuclear magnetic resonance (NMR) spectroscopy. Continuing studies of zeolites, aluminophosphates, and oxide glasses have been joined by recent experiments focusing on polypeptides and other complex biomolecular species. Both theoretical and experimental aspects of NMR are under development, with the goal of increasing the utility and scope of solid-state NMR as a tool for materials and molecular research.

The spin energy levels of a nucleus are perturbed by interactions with the local electromagnetic environment, and the chemical shift (observed in liquids and solids) is one useful manifestation of these interactions. NMR spectra of solids are also governed by interactions that may be directly related to local bond-order, internuclear distances, through-bond interactions, or proximity to other nuclei. Unfortunately, many materials are microcrystalline or amorphous in nature, and the distribution of orientations or environments within a solid sample is often random or nonperiodic, causing a distribution of both the nuclear spin resonance frequencies and the strength of local spin interactions. These anisotropic interactions give rise to NMR line broadening, and much useful information is lost or obscured.

Quick reorientation (or spinning of a powdered sample in a cylindrical rotor) eliminates anisotropic broadening in the NMR spectra of many spin-1/2 nuclei (e.g., 13C, 29Si, and 31P). The technique of magic-angle spinning (MAS) NMR has helped to make high-resolution solid-state NMR an indispensable tool in the study of complex solids. However, many other important nuclei (including 11B, 17O, 23Na, and 27Al) have nuclear spin quantum numbers greater than one-half, and simple MAS is often incapable of completely narrowing and separating the resonance lines. More general line-narrowing techniques such as dynamic-angle spinning (DAS) NMR are needed to gain higher resolution from quadrupolar species. MAS, DAS, and related multiple-quantum MAS techniques are routinely used in the Mueller lab for characterization of materials and molecules.

Double- and triple-resonance heteronuclear correlation NMR experiments between nuclei aid in the determination of internuclear connectivities and bonding information. These methods allow the study of bonding networks or nuclei close to one another in space by simultaneously performing NMR experiments on two or three different types of nuclei (for example, 27Al/31P or 1H/13C/15N). Extensions to two-dimensional NMR spectroscopy provide spectral editing depending on the local connectivities of nuclei, and the Mueller group has successfully performed new high-resolution heteronuclear correlation NMR experiments on both polycrystalline and amorphous materials. Figure 1 demonstrates the results of a two-dimensional 1H/31P correlation experiment performed upon a phosphate glass after aqueous attack.

Further studies in the Mueller group focus on the dynamics of spin magnetization in experiments such as Rotational-Echo Double-Resonance (REDOR) and Transferred-Echo Double-Resonance (TEDOR), where dipolar couplings are retained under MAS conditions using double-resonance methods. The evolution of observed magnetization in these experiments is not periodic, so that Fourier analysis is incapable of providing useful information from complex systems. In solving equations for the time-evolution of spin magnetization in these experiments, simple analytic solutions have been found for the observed signals. Importantly, it is possible to find inverse functions so that an analytic transform can be performed and pure dipolar spectra may be obtained. This is most important in systems with many different dipolar coupling constants (and therefore a number of different internuclear distances), or when a distribution of couplings is present. Figure 2 shows the difference between Fourier transform spectra of REDOR signals (top) and pure dipolar spectra obtained using this new transform method (bottom). These techniques are now in use for the simultaneous measurement of multiple internuclear distances in a number of complex systems.