Knowledge Bank

MCD spectra are presented for $Eu+3$ and $Gd+3$ in aqueous and several non-aqueous solvents. $Eu+3$ and $Gd+3$ have simple ground state levels and relatively narrow transitions in the visible and ultraviolet. Under moderately high resolution, ligand field splittings can be resolved in the MCD of certain transitions. In $Eu+3$, $(4fb)0$ with an even number of electrons, the $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}F0,1\to 5D1,2$, bands are resolved into a series of B terms indicating complete removal of degeneracy by the ligand field. In $Gd+3$ ($4f7$) all levels are at least twofold degenerate and the MCD shows a series of A terms consistent with maximum removal of degeneracy of the excited levels by the ligand field. In non-aqueous solvents additional lines in excess of the number allowed by degeneracy are observed. The observed A terms are shown to be consistent with theoretical predictions and the exceptions discussed. The $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}F1,2\to 5D2,1$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}F1\to 5D1$ transitions are allowed by both magnetic-dipole and ``forced electric-dipole” selection rules. The former are confirmed as primarily magnetic-dipole In character, whereas the latter are predominantly “forced electric-dipole”.