16O/18O Oxygen Isotope Exchange Kinetics in MnO4- as Probed by 55Mn NMR

The effect of the O-16 <-> O-18 substitution in the coordination sphere of permanganate anion on MnO4- on the chemical shift of Mn-55 nuclei have been studied by O-17 and Mn-55 NMR. Time constants tau(n,k) of oxygen exchange in the water-permanganate anion system have been estimated. In nearly neutral solutions (pH approximate to 6.8-7.2), the oxygen exchange time is on the order of tens of hours. Bubbling gaseous HCl through this solution for a few seconds leads to the equilibrium distribution of oxygen isotopes in the manganese coordination sphere. The observed temperature dependences of isotope-induced Mn-55 NMR shifts in Mn (O4-nOn-)-O-16-O-18 (n - 0-4) have been treated as a result of rovibrational averaging of Mn-O bond lengths. The change in the Mn-O bond length in MnO4- caused by the O-16 -> O-18 isotope substitution is on the order of 10(-4) angstrom.