An EPR Study of Some Highly Distorted Tetrahedral Manganese(II) Complexes at High Magnetic Fields

Abstract

EPR spectra at 94.5 GHz (3 mm) and 249.9 GHz (1 mm) have been measured for the dichloro-, dibromo-, and diiodobis(triphenylphosphine oxide)manganese(II) complexes. These compounds have a distorted tetrahedral geometry, and the highest possible symmetry obtainable is C 2 v . Because the spectra are taken in the high-field limit, they are simple and much easier to interpret than at lower magnetic fields. Accurate values of the zero-field splitting parameters, D and η (E/D), are determined by least-squares fitting and homotopy using matrix diagonalization. The compounds have g-values near 2 and large values of D and η. Across the series, D increases from 0.165 to 0.906 cm-1, while η decreases slightly from 0.273 to 0.246.

Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

“…The ZFS parameters derived from spectral simulations are significantly different from those previously determined [110] and are noticeably larger than those for any other system studied in this repon. Usually such large values imply that the Mn(II) center is no longer in a pseudo-octahedral ligand field and is likely pentacoordinate [42,44,87] or some highly distorted tetrahedron [111]. However, NMR and X-ray crystallographic results clearly indicate that Mn(II)EDTA is seven-coordinate with a water ligand occupying an apical position [112,113].…”

New Methods of Simulation of Mn(II) EPR Spectra: Single Crystals, Polycrystalline and Amorphous (Biological) Materials

“…Figures 14 and 15 represent 250-GHz spectra for two ratios of E 0 /D 0 = 0 and E 0 /D 0 = 0.23. It is seen from these figures that when E 0 /D 0 is large, there is an apparent splitting of the -2D and -4D fine-structure lines (Wood et al, 1999). Computer fitting to the experimental spectra enables determination of the absolute value of ratio E 0 /D 0 along with other spinHamiltonian parameters uniquely for Mn(II) in glasses with sufficient accuracy.…”

“…The eigenvalues and eigenvectors required for calculation of line positions and line intensities are obtained by a numerical diagonalization of the spin-Hamiltonian matrix on a computer using the JACOBI subroutine (Press et al, 1992). The following description is based on details given by Misra (1999).…”

“…They are computed by the use of JACOBI subroutine (Misra, 1999), which diagonalizes real symmetric matrices, and is particularly efficient when the off-diagonal elements in the SH matrix are infinitesimally small as is naturally the case in homotopy. (Briefly, the diagonalization in the JACOBI algorithm is accomplished by successive rotations to annihilate the off-diagonal elements of the 2 × 2 submatrix constituted by the largest off-diagonal element and corresponding diagonal elements of the SH matrix at any stage of successive rotations.…”

“…In powders the interpretation of EPR spectra becomes difficult due to the orientational disorder of paramagnetic ions. There are three types of glasses in which Mn(II) spectra have been mainly studied: borate (Misra, 1996(Misra, , 1999, phosphate and silicate (Kliava & Purans, 1980;see Misra, 1996see Misra, , 1999de Wijn & Van Balderin, 1967, for more examples). Some materials pass from glassy to disordered state with variation of temperature as illustrated in Figure 6 (Allen, 1965).…”