Temperature and Isotope Substitution Effects on the Structure and NMR Properties of the Pertechnetate Ion in Water

The uniquely well-resolved 99Tc NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the 99Tc chemical shift as a function of temperature has been measured over the range 10−45 °C for the three isotopomers Tc(16O)4 -, Tc(16O)3(18O)-, and Tc(16O)3(17O)- at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc−O scalar coupling was determined for the Tc(16O)3(17O)- isotopomer. Values for these parameters were computed using relativistic spin−orbit density functional theory with an unsolvated ion approximation and with treatments of the solvated ion based on the COnductor-like Screening MOdel (COSMO) approach. The temperature and isotope dependence of 99Tc NMR parameters inferred by these methods were in good quantitative agreement with experimental observations. The change in the Tc−O bond length associated with the changes in temperatures considered here was determined to be of the order of 10-4 Å. Vibrational energies and Tc−O bond lengths derived from these models also compare favorably with previous experimental studies.