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...

Ligand Substituent Effects in Manganese Pyridinophane Complexes: Implications for Oxygen-Evolving Catalysis

“…Given that Mn(III) complexes in general are highly amenable to study by HFEPR, [32][33][34][35] Energy (cm -1 ) clearly show the spin quintet ground state with the "hole" in dxy in agreement with the earlier, DFT calculations on 5 ( Figure 5). 17 The reported sa-CASSCF(4,5) based calculations lead to D values ranging from −3.8 cm -1 to −4.0 cm -1 when taking into account the spin-spin coupling (SSC) interaction (using MRCI) and NEVPT2 dynamic electron correlation effects, where the SSC contribution is about 5 times larger (−0.5 cm -1 ) than dynamic electron correlation effects (−0.1 cm -1 ) in the case of the optimized geometries, see Tables S5 and S6 (Supporting Information).…”

Tetramer Compound of Manganese Ions with Mixed Valence [MnII MnIII MnIV] and Its Spatial, Electronic, Magnetic, and Theoretical Studies

“…It is important to note that the zero-field split is not observed in the ESR spectra of the solid sample at two temperatures, and we assume that this splitting at the zero-field is smaller than the ℎ = 0.31 at X-band. Nevertheless, the ESR spectrum and its simulation ( Figure 5) gave > 4 and < 2 values characteristic of Mn(IV) species, which is identifiable by the transition at higher field since the ~2 signal is weak, which has been observed in other works [78][79][80][81][82][83][84].…”

Superhyperfine Interaction in [MnF6]3−

“…The spectrum can be satisfactorily reproduced by the effective spin Hamiltonian for a non‐Kramers doublet [Eq. (2)] which is valid for the lowest energy M S =±2 electronic components 7 …”