X-Band Pulsed ENDOR Study of57Fe-Substituted Sodalite— The Effect of the Zero-Field Splitting

Abstract

X-band (∼9.3 GHz) pulsed ENDOR measurements were carried out on57Fe-substituted sodalite (FeSOD) which contains only one type of Fe(III) (S=52) located at a framework site. The ENDOR spectrum recorded atg= 2 shows three doublets corresponding to the sixMSmanifolds. The assignment of these signals was confirmed by hyperfine-selective and triple ENDOR experiments. The components of each of the doublets had different intensities, reflecting the different populations of the EPR energy levels at the measurement temperature, 1.8 K. ENDOR spectra were recorded at magnetic fields within the EPR powder pattern, and the field dependence observed showed an anisotropic behavior, unexpected from the isotropic character of the57Fe(III) hyperfine coupling. This dependence was attributed to the high-order effects of the zero-field splitting (ZFS) interaction on the ENDOR frequencies. Three different theoretical approaches were used to account for the dependence of the ENDOR spectrum on the ZFS interaction. The first involves the exact diagonalization of the total spin Hamiltonian, the second uses third-order perturbation approximations, and the third employs an effective nuclear Hamiltonian for each of theMSmanifolds. The simulations showed that the ENDOR signals of theMS= ±5/2 (ν±5/2) manifold are the least sensitive to the magnitude of the ZFS parameterDand are therefore the most appropriate for the determination ofaiso. It is shown that at X band andaisovalues of about 30 MHz, the perturbation approach is valid up toDvalues of 500 MHz if all three doublets are concerned. However, if only the ν±5/2doublet is considered, then this approach is valid forD< 1000 MHz. The third approach was found inappropriate foraisovalues of ∼30 MHz. Using the method of exact diagonalization together with orientation selectivity, the trends observed in the experimental spectra could be reproduced. The ENDOR spectra of the57Fe-substituted zeolites ZSM5, L, and mazzite showed broad and ill-defined peaks since the ZFS of Fe(III) in these zeolites is significantly larger than that of FeSOD. Because this broadening is a high-order effect, it can be significantly reduced at higher spectrometer frequencies.