The Spectra and Electronic Structure of the Tetrahedral Ions MnO4-, CrO4--, and ClO4-

We have made use of a semiempirical treatment to calculate the energies of the molecular orbitals for the ground state and the first few excited states of permanganate, chromate, and perchlorate ions. The calculation of the excitation energies is in agreement with the qualitative features of the observed spectra, i.e., absorption in the far ultraviolet for ClO₄^-, two strong maxima in the visible or near ultraviolet for MnO₄^- and CrO₄⁼ with the chromate spectrum displaced toward higher energies. An approximate calculation of the relative f-values for the first transitions in CrO₄⁼ and MnO₄^- is also in agreement with experiment.

The data on the absorption spectra of permanganate ion in different crystalline fields is interpreted in terms of the symmetries of the excited states predicted by our calculations.