17O and 1H relaxometric and DFT study of hyperfine coupling constants in [Mn(H2O)6]2+

Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and O-17 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 O-17 and H-1 hyperfine coupling constants (HFCCs). The analysis of the NMRD and O-17 NMR data provided a water exchange rate of k(ex)(298) = 28.2 x 10(6) s(-1), and A(O)/(h) over bar and A(H)/(h) over bar hyperfine coupling constants of -34.6 and 5.4 rad s(-1), respectively. DFT calculations (TPSSh model) performed on the [Mn(H2O)(6)](2+) and [Mn(H2O)(6)](2+)center dot 12H(2)O systems were used to evaluate theoretically the O-17 and H-1 HFCCs responsible for the O-17 NMR chemical shifts and the scalar contributions to O-17 and H-1 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 A(iso) values are critically affected by the distance between the oxygen atom of the coordinated water molecule and the Mn-II 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.