Oxygen isotope effect on NMR parameters of pertechnetate anion TcO 4 −

Abstract

The effects of oxygen isotope substitution 16O ↔ 17O ↔ 18O in the coordination sphere of the pertechnetate anion (TcO4−) on the NMR 99Tc chemical shifts and 99Tc-17O and 17O-99Tc spin coupling constants have been studied by 17O and 99Tc NMR. The isotope shifts 16/17Δ and 16/18Δ in 99Tc NMR and the spin coupling constants of the Tc16O217O2−, Tc 16O318O−, Tc 16O317O−, Tc 16O217O18O−, and Tc16O218O2− isotopomers have been measured. For the Tc 16O318O− and Tc 16O317O− anions in an ammonium pertechnetate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are described by linear relationships 16/18Δ = −0.616 + 6.45 × 10−4T (ppm) and 16/17Δ = −0.302 + 2.67 × 10−4T (ppm), respectively. For the Tc16O317O− anion in a sodium pertechnetate solution, the magnitude of the 1Δ(16/17O) isotope shift nonlinearly decreases with increasing temperature. The nonlinear temperature dependence of the J(99Tc-17O) spin coupling constant and the extreme point on the curve of the 1Δ(16/18O) isotope shift versus temperature for the isotopomers in an NaTcO4 solution are presumably related to equilibrium between contact and water-separated ion pairs.