PNNL

Abstract

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 for three different theoretical models, which include both an unsolvated ion approximation and treatments of the solvated ion based on the COSMO approach. According to these analyses, the temperature- and isotope-dependent shifts in energy observed in the 99Tc NMR spectra can be quantitatively explained by changes in the average Tc-O bond length of the order of 1 x 10¯4 Å, which are induced by a redistribution of vibrational state populations and solvent-ion interactions. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental determinations.