Abstract
The isoelectronic species UO/sub 2//sup 2 +/ and ThO/sub 2/ possess very different geometries, namely, UO/sub 2//sup 2 +/ is linear while ThO/sub 2/ is strongly bent (theta/sub exptl/ = 122 +- 2/sup 0/). Relativistic effective core potential (RECP) calculations using Hartree-Fock wave functions and double-zeta-plus-polarization quality basis sets were performed to determine the origin of this difference. The RECP calculations correctly predict the linear and bent geometries of UO/sub 2//sup 2 +/ and ThO/sub 2/ (theta/calcd/ = 118/sup 0/). The Th-O bond length, which is not known experimentally, is calculated to be 1.91 A. Analysis of the results shows that the difference in geometries for UO/sub 2//sup 2 +/ and ThO/sub 2/ has its origin in the relative ordering of the 5f and 6d levels. For uranium the 5f levels are lower and dominathe te back-bonding from the oxygen in UO/sub 2//sup 2 +/, while for thorium the 6d levels are lower and dominate the back-bonding in ThO/sub 2/. Finally, the 5f levels prefer linear geometries, while the 6d prefer bent geometries, hence, the difference between UO/sub 2//sup 2 +/ and ThO/sub 2/. The relative ordering of the 5f and 6d levels has a profound effect.
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