A reinterpretation of the EPR spectra of Cu(II) in zeolites A, Y and ZK4, based on ab initio cluster model calculations

A theoretical study is presented of the EPR spectra of the dehydrated Cu(ii )-A, Cu(ii)-Y and Cu(ii)-ZK4 zeolites. B3LYP-DFT geometry optimizations wer e performed on cluster models representing six-ring sites with different Al contents, as observed for the different zeolites. All calculated structure s indicated a strong preference of the Cu(ii) ion for coordination to oxyge ns bound to Al rather than Si, together with a striving for a planar four-f old oxygen coordination in the six-rings. Depending on the number and relat ive positions of the aluminiums in the ring two distinct four-fold coordina tion modes were distinguished, containing either only one or several alumin iums "competing'' for a position of both their oxygens in the first Cu(ii) coordination sphere. The electronic spectra and EPR g-tensors of all optimi zed cluster models were calculated by means of the CASPT2 method (multiconf igurational perturbation theory based on a complete-active-space reference wavefunction), with inclusion of spin-orbit coupling. These calculations po inted to the appearance of two distinct EPR-signals in connection with the two different four-fold coordination modes. Based on the close corresponden ce between the calculated g-factors and the experimental EPR-signals of the three zeolites under investigation, a new interpretation of the latter sig nals is suggested. According to this new interpretation the occurrence of t wo EPR signals in zeolite Y as opposed to only one signal in zeolite A is c onnected to the higher Si/Al ratio in the former zeolite, rather than to a different topology (as was suggested in earlier assignments of the spectra) . Our new interpretation is corroborated by the experimental EPR signals ob tained for Cu-ZK4: with the same topology as zeolite A, but containing a Si /Al ratio closer to zeolite Y, two rather than one Cu(ii) EPR signals were indeed observed. Finally, our calculations also indicate that, in six-rings containing more than one aluminium, Cu(ii) is likely to undergo a hopping process at room temperature.