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

Rotem Vardi, Marcelino Bernardo, Hans Thomann, Karl G. Strohmaier, David E.W. Vaughan, Daniella Goldfarb

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25 Scopus citations


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 MS 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 MS manifolds. The simulations showed that the ENDOR signals of the MS = ±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 aiso. It is shown that at X band and aiso 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 aiso 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.

Original languageEnglish
Pages (from-to)229-241
Number of pages13
JournalJournal of Magnetic Resonance
Issue number2
StatePublished - 1 Jan 1997
Externally publishedYes

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Nuclear and High Energy Physics
  • Condensed Matter Physics


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