16O/18O oxygen isotope exchange kinetics in MnO 4 − as probed by 55Mn NMR
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The effect of the 16O ↔ 18O substitution in the coordination sphere of permanganate anion MnO 4 − on the chemical shift of 55Mn nuclei have been studied by 17O and 55Mn NMR. Time constants τ n,k of oxygen exchange in the water–permanganate anion system have been estimated. In nearly neutral solutions (pH ≈ 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 55Mn NMR shifts in Mn16 O 44-n 18 O n – (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 caused by the 16O → 18O isotope substitution is on the order of 10–4 Å.
Keywords55Mn and 17O NMR permanganate anion isotope shifts equilibrium constants isotope exchange
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