Magnetic circular dichroism of UCl₆⁻ in the ligand-to-metal charge-transfer spectral region

We present a combined ab initio theoretical and experimental study of the magnetic circular dichroism (MCD) spectrum of the octahedral UCl₆⁻ complex ion in the UV-Vis spectral region. The ground state is an orbitally non-degenerate doublet E_5/2u and the MCD is a -term spectrum caused by spin–orbit coupling. Calculations of the electronic spectrum at various levels of theory indicate that differential dynamic electron correlation has a strong influence on the energies of the dipole-allowed transitions and the envelope of the MCD spectrum. The experimentally observed bands are assigned to dipole-allowed ligand-to-metal charge transfer into the 5f shell, and 5f to 6d transitions. Charge transfer excitations into the U 6d shell appear at much higher energies. The MCD-allowed transitions can be assigned via their signs of the -terms: Under O_h double group symmetry, E_5/2u → E_5/2g transitions have negative -terms whereas E_5/2u → F_3/2g transitions have positive -terms if the ground state g-factor is negative, as it is the case for UCl₆⁻.