Density-functional computation of Tc-99 NMR chemical shifts

Tc-99 chemical shifts of TcO4-, TcH92-, TcOF5, TcO2F4-, TcOCl4-, Tc-2(CO)(10), and Tc(CO)(3)L-3(+) (L = CO, MeCN, H2O) are computed using geometries optimized with the gradient-corrected BP86 and hybrid B3P86 density functionals, at the gauge-including atomic orbitals (GIAO), -BPW91 and -B3LYP levels. For this set of compounds, substituent effects on delta(Tc-99) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition-metal chemical shifts studied so far. A rough, qualitative correlation is found between computed electric-field gradients at the Tc nuclei and the corresponding Tc-99 NMR line widths. Thermal and solvation effects on magnetic shielding constants of aqueous TcO4-, as assessed by averaging these properties over trajectories from Car-Parrinello molecular dynamics simulations, are indicated to be small and comparable to those of MnO4-. Complexation to aqueous uranyl, UO22+, is predicted to affect delta(Tc-99) of TCO4- only slightly; somewhat larger complexation shifts are obtained for the oxygen nuclei of pertechnetate, suggesting that O-17 NMR could be a useful probe for the extent of association between both radionuclides in solution. Copyright (C) 2008 John Wiley & Sons, Ltd.