A 250-GHZ ESR STUDY OF HIGHLY DISTORTED MANGANESE COMPLEXES

ESR spectra at 250 GHz and magnetic fields ranging from 45 to 95 kG have been measured for the compounds Mn(gamma-picoline)4X2 and Mn(o-phenanthroline)2X2, where X = Cl, Br, and I. These spectra are in the high-field limit and well resolved; hence, they are inherently simple to analyze. We find these spectra to be very sensitive to the precise values of the zero-field splitting (zfs) parameters. Thus D and eta (E/D) can be estimated directly from the spectrum, with very accurate values obtained using third-order perturbation theory for the electron spin Hamiltonian of high-spin Mn2+. The contributions of all five allowed electron spin transitions are observed and successfully simulated. The gamma-picoline complexes have axial symmetry and D increases from 0.186 to 0.999 cm-1 with the size of the halogen. The o-phenanthroline complexes show a wide range of rhombic distortion and E increases linearly with the magnitude of D. The chloride compound is nearly axial (D = 0.124 cm-1, eta = 0.04), while the iodide compound is highly distorted (D = 0.590 cm-1, eta = 0.246). To demonstrate the applicability of the high-frequency ESR technique to biological samples, we have also measured the high-field spectrum of Mn(II) protoporphyrin IX and determined its zfs parameters to be D = 0.775 cm-1 and eta = 0.048.