Anisotropic Hyperfine Interaction in the Manganese(III) Hexaaqua Ion

Abstract

The manganese(iii) ion is ubiquitous among chemical systems that exhibit intriguing biological and physical properties, ranging from photosystem II and superoxide dismutase to the celebrated single-molecule magnet Mn 12 . It is for this reason that complexes and clusters of the manganese(iii) ion are widely recognized as the delicicae of high-field electron paramagnetic resonance (EPR) spectroscopists, [1] the fact that they invariably yield good-quality spectra being but a secondary consideration. [2][3][4][5][6][7] Hitherto, the spatial property of the hyperfine interaction between the S = 2 electronic spin and the I = 5/2 nuclear spin in any chemically relevant manganese(iii) compound has escaped observation. There are two fundamental reasons why knowledge of the hyperfine interaction energies is important. First, the magnitude and sign of the anisotropy relate directly to the manganese(iii) coordination sphere. Secondly, the hyperfine interaction has been demonstrated to be instrumental in quantum tunneling of the magnetization in singlemolecule magnets. [8,9] In an EPR study of manganese(iii) superoxide dismutase, [2] the hyperfine splittings were resolved and compared to those documented for a photo-oxidation product of manganese(iii) in the high-affinity site of photosystem II. Consistency was found between the observed hyperfine splitting and the sign of the zero-field-splitting parameter in the two systems. However, these experiments were confined to X-band parallel-mode experiments, which can determine just one component of the hyperfine interaction matrix. Information concerning the spatial property of the hyperfine interaction matrix requires the use of higher frequencies, as shown in a study of manganese(iii)-doped rutile. [12] In this communication we report single-crystal and powder EPR spectra of the [Mn(H 2 O) 6 ] 3+ ion at high fields and multiple frequencies. To minimize line broadening due to spin-spin dipolar interactions, the [Mn(H 2 O) 6 ] 3+ ion was doped into the diamagnetic cesium gallium alum, Cs-[Ga(H 2 O) 6 ](SO 4 ) 2 ·6 H 2 O, thus enabling the precise determination of the metal hyperfine interaction parameters.Pale orange-red crystals of this sample were grown at 0 8C from a saturated 6 m sulfuric acid solution of the cesium gallium alum that contained 1 % cesium manganese alum. A trace of the corresponding chromium(iii) alum was also added. The EPR lines of [Cr(OH 2 ) 6 ] 3+ were readily identified and were useful for crystal alignment.The single-crystal EPR spectra exhibited several magnetically nonequivalent species, reflecting the space group of the host and the propensity of the [Mn(OH 2 ) 6 ] 3+ ion to distort along a Jahn-Teller active coordinate. There are four tervalent cations in the cubic unit cell, each occupying a site of S 6 symmetry on one of the four unique threefold axes of the unit cell. In addition, at low temperatures the [Mn(OH 2 ) 6 ] 3+ ion is expected to be locked into one of the three possible tetragonally elongated structures that results from the JahnTelle...

Characterization of Monomeric MnII/III/IV–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

“…In comparison with literature values for each respective oxidation state, the present values are within the observed ranges of isotropic hyperfine constants for Mn. 20,34,35,44–46 …”

“…This causes mixing of spin states and multiple overlapping transitions, and consequently, the interpretation of spectra has been qualitative. High-field EPR studies on monomeric Mn II and Mn II have been reported in literature, which can give more accurate determinations of g - and D -values 16–20 . We have developed a methodology for quantitative interpretation based on computer simulation of spectra, and here we demonstrate its use in the interpretation of X- and Q-band spectra for both parallel ( B 1 ‖ B ) and perpendicular ( B 1 ⊥ B ) orientations of the oscillating microwave field ( B 1 ) relative to the static field ( B ) 21 .…”

A five-coordinate manganese(iii) complex of a salen type ligand with a positive axial anisotropy parameter D

“…6 Among the most appropriate techniques for studying high-spin (HS) manganese(III) complexes is high-frequency and -field EPR spectroscopy (HFEPR), first of all because this spin system usually produces good-quality spectra. 7 In addition, although HS manganese(III) systems can also produce X-band EPR spectra, 8 extraction of spin Hamiltonian parameters such as the Landé g factor, the magnitude and sign of the axial anisotropy parameter, D, the presence of rhombic anisotropy, E, and fourth order spin Hamiltonian terms is not always possible. 9 The magnitude and sign of the anisotropy, however, can provide information on the coordination sphere of manganese (III).…”

Comparison of Two Yeast MnSODs: Mitochondrial Saccharomyces cerevisiae versus Cytosolic Candida albicans

“…Such a small effective 55 Mn hyperfine coupling parameter ( A ∥ ) has been observed previously for several 6-coordinate Mn(III) centers. 33,35,36 There has also been a preliminary report that showed a fraction of 5-coordinate Ec Mn 3+ SOD converts to a new species at pH 11.54 that is characterized by a parallel-mode EPR spectrum with A ∥ ( 55 Mn) = 3.3 mT. 31 …”