The characterization of the solution structures of transition metals remains an area of active investigation. Manganese (II) complexes, in particular, present special challenges since there are so few spectroscopic tools available that are sensitive to solution speciation and potentiometric titrations often do not yield unique models. The longitudinal (T1) and transverse (T2) 1H relaxation rates as a function of pH at 20 MHz and 37°C for solutions containing a variety of manganese (II) complexes have been measured. As the pH decreases, the measured T1/T2 ratio tracks the predicted concentration of [Mn(H2O)6]2+ as determined from the stability constants for the complex and its protonated forms. These changes in T1 and T2 have been previously attributed to, for example, long electronic relaxation times and/or large 1H contact hyperfine coupling contributions to the relaxation of [Mn(H2O)6]2+. An analysis of the data in terms of the Solomon-Bloembergen-Morgan model of paramagnetic relaxation provides insights into the fundamental chemical and physical parameters that influence the T1/T2 ratio in these complexes. A protocol for relating the measured T1/T2 ratio to the chemical speciation of manganese (II) complexes in solution will be presented. The possibility for the extension of this analysis to other metal ions will also be discussed.

1H relaxation rate ratios as a probe of solution speciation for labile manganese (II) complexes

251st National Meeting (2016)

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