Density-functional computation of (99)Tc NMR chemical shifts

99Tc chemical shifts of TcO4(-), TcH9(2-), TcOF5, TcO2F4(-), TcOCl4(-), Tc2(CO)10, and Tc(CO)3L3(+) (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(99Tc) 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 99Tc 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, UO2(2+), is predicted to affect delta(99Tc) of TcO4(-) only slightly; somewhat larger complexation shifts are obtained for the oxygen nuclei of pertechnetate, suggesting that 17O NMR could be a useful probe for the extent of association between both radionuclides in solution.