WATER EXCHANGE ON MAGNESIUM(II) IN AQUEOUS-SOLUTION - A VARIABLE-TEMPERATURE AND PRESSURE O-17 NMR-STUDY

Fast water exchange rate constants on diamagnetic metal ions can be me asured using either a paramagnetic chemical shift agent or a paramagne tic relaxation agent. An approximate analytical equation was developed for the chemical shift agent method, valid in the limit of rapid exch ange. The limits of validity of these methods are discussed, These wer e applied to the study of water exchange on [Mg(H2O)](6)(2+) over a la rge temperature range (252-350 K) and up to a pressure of 200 MPa by O -17 NMR, leading to the following kinetic parameters: k(Mg)(298) = 6.7 +/- 0.2 x 10(5) s(-1); Delta H-Mg(double dagger) = 49 +/- 0.7 kJ mol( -1);Delta S-Mg(double dagger) = +31.1 +/- 2.2 J K-1; Delta V-Mg(double dagger) = +6.7 +/- 0.7 cm(3) mol(-1). From these data, it is conclude d that this process has an I-d mechanism with strong d character, with out excluding a limiting D mechanism. (C) 1997 John Wiley & Sons, Ltd.