Our research is directed toward understanding paramagnetic phenomena in NMR spectroscopy. Current work involves the development of NMR as a tool for measuring electron spin energy level diagrams, spin wavefunctions, and electron spin relaxation properties of transition metal ions with spins S³1. This work is motivated by the fact that fundamental properties of S³1 spin systems are currently very difficult to study (in practice, frequently impossible) using available techniques. Most integer spin systems are "ESR-silent", and for this reason fundamental information about the spin physics of these systems is not generally available. This kind of information is present in a powerful and direct way, however, in variable-field NMR relaxation measurements.
Our laboratory is addressing these problems from theoretical and experimental standpoints. Current experiments focus on the use of "stretched" polyacrylic gels as anisotropic matrices in which a paramagnetic solute complex can be immobilized and oriented macroscopically with respect to the NMR magnetic field. In isotropic solutions, spatial averaging of the solute acts to average (and thus obscure) all physical phenomena that are associated with spin level structure. In oriented gels, in contrast, the electron spin levels are well defined and can be manipulated experimentally in ways that are not possible in isotropic liquids. Because they provide experimental control of the spin level diagram, oriented gels provide a direct spectroscopic probe of the electron spin system. We are currently mapping out these phenomena in model chemical systems as well as developing associated experimental techniques. Theoretical work is directed toward interpreting experimental work, as well as, in a broader context, toward understanding NMR relaxation phenomena in paramagnetic chemical systems.
Electron spin relaxation due to reorientation of a permanent zero field splitting tensor. N. Schaefle and R. Sharp, J. Chem. Phys., 121, 5387-5394, (2004).
NMR Paramagnetic Relaxation of the Spin 2 Complex MnIIITSPP: a Unique Mechanism N. Schaefle and R. Sharp, J. Phys. Chem. A, 109, 3267-3275 (2005).
NMR-Paramagnetic Relaxation due to the High-Spin d3 Electron Configuration: CrIII-TSPP N. Schaefle and R. Sharp, J. Phys. Chem. A, 109, 3276-3284 (2005).
NMR Paramagnetic Relaxation due to the S=5/2 Complex, Fe(III)-TSPP: Central Role of the Tetragonal 4th-order ZFS Interaction. N. Schaefle and R. Sharp, J. Chem. Phys. 122, 184501-184511 (2005).
Four complementary theoretical approaches for the analysis of NMR paramagnetic relaxation. N. Schaefle and R. Sharp, J. Magn. Reson. 176, 160-170 (2005).