X-Band Pulsed ENDOR Study of ⁵⁷Fe-Substituted Sodalite -the Effect of the Zero-Field Splitting

X-band (∼9.3 GHz) pulsed ENDOR measurements were carried out on ⁵⁷Fe-substituted sodalite (FeSOD) which contains only one type of Fe(III) (S = 5/2) located at a framework site. The ENDOR spectrum recorded at g = 2 shows three doublets corresponding to the six M_S manifolds. 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 the ⁵⁷Fe(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 the M_S manifolds. The simulations showed that the ENDOR signals of the M_S = ±5/2 (ν_±5/2) manifold are the least sensitive to the magnitude of the ZFS parameter D and are therefore the most appropriate for the determination of a_iso. It is shown that at X band and a_iso values of about 30 MHz, the perturbation approach is valid up to D values of 500 MHz if all three doublets are concerned. However, if only the ν_±5/2 doublet is considered, then this approach is valid for D < 1000 MHz. The third approach was found inappropriate for a_iso values 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 the ⁵⁷Fe-substituted zeolites ZSMS, 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.