The Challenges and Opportunities of High-Spin Mn(II) EPR and ENDOR

We examine the electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy of three quite distinct high-spin Mn(II) systems and describe experimental techniques and methods of analysis that are useful in their study. We demonstrate that this S = 5/2 metal center provides useful orientation-selection through the Zero-Field Splitting (ZFS) tensor that enables determination of a ¹³C hyperfine-coupling tensor with extremely small hyperfine interaction. We also demonstrate that Mims suppression effects can be used in concert with orientation-selection to edit complex ^[1,2]H ENDOR patterns that can be produced by even a ‘simple’ center with a single Mn(II). We develop a perturbation-based approach to understanding second-order shifts in Mn(II) ENDOR responses that occur in systems with intermediate ZFS values, and show that these shifts can be used to estimate the values of the ZFS tensors.