The Bruker lecture. Single-crystal 13C, 14N and 23Na ENDOR on Fe(CN) 3–6 doped into NaCl

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, I_a 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 D_2h symmetry and the positions of the negatively charged cation vacancies assumed for the centre I_a. 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.