IR spectroscopic study of dipotassium and dicesium uranyl tetrabromides

The IR spectra of dipotassium and dicesium uranyl tetrabromides (K2UO2Br4, Cs2UO2Br4) have been measured in the region from 4000 down to 30 cm-1. A normal coordinate analysis of the (UO2Br4)2- ion apart from the K+ and Cs+ ions has been made using Wilson's FG matrix method. The force constants associated with the U-O and U-Br bonds have been obtained on the basis of the two force models of modified Urey-Bradley and valence force fields. An effort has been directed towards the understanding of a negative force constant of the U-O bond-bond interaction which plays an important role in the calculations of symmetric and asymmetric U-O vibrations, and detailed discussion has been made on it from the point of view of the mutual polarizability between the two U-O bonds. (author)