Theoretical study of the structure and spectroscopic properties of cobalt(II) coordinated to six-rings in zeolites

The structure of the local Co(II) six-ring oxygen environment in zeolite A and the corresponding ligand field spectrum have been studied using large c luster models, including all six surrounding Si or Al tetrahedra terminated by either H or OH groups. Structures were optimized by means of density fu nctional theory (DFT), using a nonlocal (BP86) approach and keeping the ori entation of all dangling bonds frozen at the X-ray diffraction (XRD) positi ons. Electronic spectra were calculated using multiconfigurational perturba tion theory based on a CASSCF wave function (CASPT2). It is shown that, in all cases, the presence of the Co(II) ion induces a local distortion of the zeolite surface, resulting in an oxygen coordination number of 3, 4, or 5, depending on the Si/Al ratio. This distortion is reflected in the calculat ed electronic spectra, showing an increased splitting of the Co2+ free-ion, F-4 and P-4 states as compared to the (average) XRD structures. A new I ge neral assignment of the spectrum is proposed, different from earlier assign ments based on ligand field theory. The calculated excitation energies of t he optimized structures are in excellent agreement with the experimental ba nd positions, thus proving the strength of the present combined DFT-CASPT2 approach. Our results further suggest that the experimentally observed spli tting of the main band in the spectrum is due to the presence of asymmetric coordination sites, rather than to Jahn-Teller effects or spin-orbit coupl ing. The latter may, however, at least partly be responsible for the splitt ing of the weak feature at 25 000 cm(-1).(()