Magnetic Field (g-Value) Dependence of Proton Hyperfine Couplings Obtained from ESEEM Measurements: Determination of the Orientation of the Magnetic Axes of Model Heme Complexes in Glassy Media

Abstract

Electron spin echo envelope modulation (ESEEM) studies were utilized to characterize the coupling between protons of axially bound pyrazole ligands (PzH) and the unpaired electron of low-spin tetraphenylporphyrinatoiron(III) chloride. Samples were prepared in mixed-solvent glasses to maximize the resolution of the electron paramagnetic resonance (EPR) signals. X-band two-pulse ESEEM experiments at 4.2 K in deuterated solvent glasses demonstrated that this coupling results in 0.2−0.7 MHz shifts of the να + νβ proton sum combination peak from twice the Larmor frequency. These shifts have been investigated across most of the EPR absorption spectrum of [TPPFe(PzH)2]+Cl-. Two-pulse ESEEM spectra were simulated at different magnetic field positions. Combination peaks were observed from both distant protons (DP) (β-pyrrole, ortho-phenyl, and β-pyrazole) and near protons (NP) (α-H of the pyrazole ligands). For the simulations, the orientation of the nearest four protons of the pyrazole ligands with respect to the...