Distortion of the tetrahedral coordination of Fe(III) ions stabilized in ZSM-5 zeolite framework

A simple qualitative method to analyze d-d-electronic transitions in cations of the transition elements in oxide matrices is proposed. In the particular case, all the excited states of interest differ only in the electronic configuration of d-orbitals, and the energies of transitions can be computed via the configuration interaction (CI) method restricted by the active space of five cation d-orbitals. An ordinary cluster model that takes into account the first coordination sphere of transition metal ion consisting of the framework of oxygen ions is sufficient for this purpose. The systematic overestimation error of transition energies can be corrected through the empirical factor calculated to fit experimental UV-VIS spectra. The physical meaning of the scaling factor proposed is the dynamic part of electron correlation that remains unaccounted for in the chosen active CI space. The observed d-d-transitions of Fe3+ ions in MFI zeolites are analyzed in detail. It is suggested that the specifics of the observed electronic spectra are caused by the distortion of the tetrahedron of oxygen atoms around Fe3+. The latter can be easily taken into account when selecting an appropriate Fe3+ cluster model in the framework. It is shown that the occurrence of the weak low-frequency band below 21,000 cm-1 indicates the distortion of the tetrahedral environment around Fe3+.