NMR Paramagnetic Relaxation of the Spin 2 Complex Mn^IIITSPP:  A Unique Mechanism

The S = 2 complex, manganese(III) meso-tetra(4-sulfonatophenyl)porphine chloride (Mn^IIITSPP) is a highly efficient relaxation agent with respect to water protons and has been studied extensively as a possible MRI contrast agent. The NMR relaxation mechanism has several unique aspects, key among which is the unusual role of zero-field splitting (zfs) interactions and the effect of these interactions on the electron spin dynamics. The principal determinant of the shape of the R₁ magnetic relaxation dispersion (MRD) profile is the tetragonal 4th-order zfs tensor component, , which splits the levels of the m_S = ±2 non-Kramers doublet. When the splitting due to exceeds the Zeeman splitting, the matrix elements of 〈S_z〉 are driven into coherent oscillation, with the result that the NMR paramagnetic relaxation enhancement is suppressed. To confirm the fundamental aspects of this mechanism, proton R₁ MRD data have been collected on polyacrylamide gel samples in which Mn^IIITSPP is reorientationally immobilized. Solute immobilization suppresses time-dependence in the electron spin Hamiltonian that is caused by Brownian motion, simplifying the theoretical analysis. Simultaneous fits of both gel and solution data were achieved using a single set of parameters, all of which were known or tightly constrained from prior experiments except the 4th-order zfs parameter, , and the electron spin relaxation times, which were found to differ in the m_S = ±1 and m_S = ±2 doublet manifolds. In liquid samples, but not in the gels, the -induced splitting of the m_S = ±2 non-Kramers doublet is partially collapsed due to Brownian motion. This phenomenon affects the magnitudes of both and electron spin relaxation times in the liquid samples.