High‐Field EPR Study of Frozen Aqueous Solutions of Iron(III) Citrate Complexes

High‐field electron paramagnetic resonance spectra have been measured for frozen aqueous solutions of Fe III citrate at acidic and neutral pH values. At high fields (up to 12 T) and high frequencies (95, 190 and 285 GHz), fine‐structure peaks due to resonance between Kramers doublets (±1/2, ±3/2, ±5/2) of an S = 5/2 species can be observed when the experiments are performed at high temperatures. A full‐matrix diagonalisation approach has been used to derive the spin‐Hamiltonian parameters for the S = 5/2 spin state. In acidic media (pH = 2), the spacing of the fine structure reveals that the value of the axial zero‐field splitting (ZFS) parameter D is −0.12 cm −1 without rhombicity. A small but significant g anisotropy can be observed which depends slightly on the frequency and the temperature ( g x = 2.014, g y = 2.014 and g z = 1.983, at 60 K and 285 GHz). In neutral media (pH = 6), complete rhombic HF‐EPR spectra were observed with a fivefold lower | D | value (0.024 cm −1 ) and E = 0.008 cm −1 . A satisfactory simulation of the experimental data was obtained with a g value equal to 2.00. The different ZFS parameters obtained under neutral and acidic conditions are discussed in terms of the symmetry of the complex. At low temperatures, a spin transition from 5/2 to 1/2 occurs at all frequencies. The transition temperature is pH‐ and field‐dependent. A model including magnetoelasticity has been suggested in order to interpret the field dependency of the spin transition observed for the two systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)