17O and 1H Relaxometric and DFT Study of Hyperfine Coupling Constants in [Mn(H2O)6]2+

Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O NMR chemical shifts and transverse relaxation rates of aqueous solutions of Mn(H2O)6]2+ were recorded to determine the parameters governing the relaxivity in this complex and the 17O and 1H hyperfine coupling constants (HFCCs). The analysis of the NMRD and 17O NMR data provided a water exchange rate of kex(298) = 28.2x10^6 s-1, and AO/ħ and AH/ħ hyperfine coupling constants of 34.6 and 5.4 rad s1, respectively. DFT calculations (TPSSh model) performed on the [Mn(H2O)6]2+ and [Mn(H2O)6]2+ 12H2O systems were used to evaluate theoretically the 17O and 1H HFCCs responsible for the 17O NMR chemical shifts and the scalar contributions to 17O and 1H NMR relaxation rates. The use of a mixed cluster–continuum approach with the explicit inclusion of second-sphere water molecules is critical for an accurate calculation of HFCCs of coordinated water molecules. The impact of complex dynamics on the calculated HFCCs was evaluated with the use of molecular dynamics simulations within the atom-centered density matrix propagation (ADMP) approach. These molecular dynamics simulations show that the Aiso values are critically affected by the distance between the oxygen atom of the coordinated water molecule and the MnII ion, as well as by the orientation of the water molecule plane with respect to the Mn–O vector. The substantial scalar contribution to relaxivity observed for [Mn(H2O)6]2+ is related to a combination of a slow water exchange rate and a slow electron spin relaxation.