We report here the first non-Kramers (NK) ESEEM and ENDOR study of a mononuclear NK center, presenting extensive parallel-mode ESEEM and ENDOR measurements on the St = 2 ferrous center of [Fe(II)ethylenediamine-N,N,N′,N′-tetraacetato]2-; [Fe(II)EDTA)]2-. The results disclose an anomalous equivalence of the experimental patterns produced by the two techniques. A simple theoretical treatment of the frequency-domain patterns expected for NK-ESEEM and NK-ENDOR rationalizes this correspondence and further suggests that the very observation of NK-ENDOR is the result of an unprecedentedly large hyperfine enhancement effect. The mixed nitrogen-carboxylato oxygen coordination of [Fe(II)EDTA]2- models that of the protein-bound diiron centers, although with a higher coordination number. Analysis of the NK-ESEEM measurements yields the quadrupole parameters for the 14N ligands of [Fe(II)EDTA]2-, K = 1.16(1) MHz, 0 ≤ η ≤ 0.05, and the analysis indicates that the electronic zero-field splitting tetragonal axis lies along the N-N direction.
- Electron spin-echo envelope modulation
- Electron-nuclear double resonance
ASJC Scopus subject areas
- Nuclear and High Energy Physics
- Condensed Matter Physics