EPR of Uranium. Ions

Actinides, the last known group of the Periodic Table are the subject of a special interest, both practical and scientific. Though their use is mainly based on their unique nuclear properties, the electronic structure is of no little interest because it is of paramount importance in determining the physical and chemical properties of the actinides and their compounds. Despite the common tendency to consider the actinides group as a new rare-earth group, similar to the lanthanides, numerous data show that this group has a definite identity, evidenced mainly in the first half of the group. At the beginning of the actinides group, a contraction of the 5f electronic shell takes place (actinide contraction), but this is not so sharply manifested as the contraction of the lanthanides 4f shell. In the actinide group the energy separation of the 5f, 6d, and 7s electronic levels is small, whereas for the lanthanides the 4f electrons are strongly localized. This leads to a strong competition between the 5f, 6d, and 7s electrons in determining the electronic properties of the actinides and their compounds. Another consequence of the weaker 5f contraction is that the ionic radius of the actinide ions is greater than that of the corresponding lanthanide ions. A determinant factor for actinides is the increased importance of the relativistic effects, mainly shown in the increase of the spin-orbit coupling (which is usually twice that of the lanthanides), which combined with a decrease in the Coulomb interelectronic repulsion, leads to a severe