The Bruker lecture. Single-crystal13C,14N and23Na ENDOR on Fe(CN)3–6 doped into NaCl

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Abstract
A single-crystal ENDOR study on the 3d5(S= 1/2) low-spin complex Fe(CN)3– 6 doped into NaCl is reported. Among the various centres caused by different configurations of two charge-compensating cation vacancies the orthorhombic centre, Ia which is characterized by a large g anisotropy has been selected for this investigation. ENDOR transitions due to 13C, 14N and 23Na are observed; nearly all hyperfine and nuclear quadrupole coupling tensors were determined. The 14N spectra are strongly influenced by the nuclear quadrupole interaction which reaches a magnitude comparable to that of the hyperfine and nuclear Zeeman interaction. The orientation of the principal values of all tensors confirm the D2h symmetry and the positions of the negatively charged cation vacancies assumed for the centre Ia. The 13C couplings are quite large, indicating a noticeable covalency of the Fe–C bonds. The hyperfine and nuclear quadrupole data are used to discuss qualitatively the spin density distribution in the Fe(CN)3– 6 unit. Comparison with other experimental studies on the spin distribution reveal that the effect of the vacancies on the spin distribution is large. For the interpretation of the 14N quadrupole data the Townes–Dailey model has been applied.