16O/18O oxygen isotope exchange kinetics in MnO4− as probed by 55Mn NMR

Resumo

The effect of the ¹⁶O ↔ ¹⁸O substitution in the coordination sphere of permanganate anion MnO₄⁻ 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-n¹⁸O_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.