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

Abstract

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 k_ex²⁹⁸ = 28.2 × 10⁶ s⁻¹, and A_O/ħ and A_H/ħ 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⁺·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 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.