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 cluster models, including all six surrounding Si or Al tetrahedra terminated by either H or OH groups. Structures were optimized by means of density functional theory (DFT), using a nonlocal (BP86) approach and keeping the orientation of all dangling bonds frozen at the X-ray diffraction (XRD) positions. Electronic spectra were calculated using multiconfigurational perturbation 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 calculated 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 general assignment of the spectrum is proposed, different from earlier assignments based on ligand field theory. The calculated excitation energies of the optimized structures are in excellent agreement with the experimental band positions, thus proving the strength of the present combined DFT-CASPT2 approach. Our results further suggest that the experimentally observed splitting 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 coupling. The latter may, however, at least partly be responsible for the splitting of the weak feature at 25 000 cm(-1).(()