Water exchange on magnesium(II) in aqueous solution: a variable temperature and pressure17O NMR study

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Fast water exchange rate constants on diamagnetic metal ions can be measured using either a paramagnetic chemical shift agent or a paramagnetic relaxation agent. An approximate analytical equation was developed for the chemical shift agent method, valid in the limit of rapid exchange. The limits of validity of these methods are discussed. These were applied to the study of water exchange on [Mg(H2O)]62+ over a large temperature range (252–350 K) and up to a pressure of 200 MPa by 17O NMR, leading to the following kinetic parameters: kMg298=6.7±0.2×105 s-1; ΔHMg‡=49.1±0.7 kJ mol-1; ΔSMg‡=+31.1±2.2 J K-1; ΔVMg‡=+6.7±0.7 cm3 mol-1. From these data, it is concluded that this process has an Id mechanism with strong d character, without excluding a limiting D mechanism.