Nuclear magnetic resonance paramagnetic relaxation: Influence of the electron spin energy level structure.

Paramagnetic species in solution considerably enhance nuclear spin relaxation rates in nuclear magnetic resonance (NMR) experiments, a phenomenon described as NMR-paramagnetic relaxation enhancement (NMR-PRE). For electron spins S ≥ 1, the NMR-PRE depends sensitively on the competing effects of the electronic Zeeman interaction and the zero field splitting (ZFS) interaction. Recently developed theory that includes the effects on electron spin level structure due to the ZFS interaction with the ligand field has shown that small energy splittings induced by symmetry can profoundly affect the NMR-PRE. Here, the role of higher order symmetry terms within this theory are tested experimentally. Part of the theory was recast in a molecular frame formulation allowing for the elucidation of eigenstate specific contributions to relaxation. Magnetic relaxation dispersion profiles (plots of relaxation rate versus Zeeman field) of three metalloporphyrins (Me-TSPP, where Me = CrIII , MnIII, and FeIII, and TSPP is meso-tetra(4-sulfonatophenyl)porphine chloride), comprising a well-characterized series of water-soluble paramagnetic complexes with electron spins S = 3/2, 2, and 5/2, were studied. The primary focuses of this study are the effects of higher order low-symmetry terms of the ZFS tensor, principally the tetragonal fourth order, on NMR relaxation. These effects are quite different in the two spin system types. For non-Kramers spin systems, doublet manifold levels differing by Delta ms = +/-4 are split, principally ms = +/-2, strongly suppressing the NMR-PRE at the lowest fields due to the removal of the contribution of this doublet manifold from the NMR-PRE. As the Zeeman field increases, the NMR-PRE relaxation efficiency also sharply increases as the Zeeman energy becomes comparable to the level splitting, as is demonstrated in MnIIITSPP. For Kramers spin systems, the fourth order coupling, for S = 5/2, of eigenstates with Deltams = +/-4 mixes the wavefunctions of different Kramers doublet manifolds. This enhances (as opposed to suppresses, which was the case with non-Kramers spins) the NMR-PRE at zero field, as is demonstrated in FeIIITSPP. These results suggest that inclusion of higher order ZFS terms in the electron spin Hamiltonian is critical to interpretation of MRD data.Ph.D.Physical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124925/2/3163921.pd