Electron paramagnetic resonance at high magnetic fields (up to 25 T) and high frequencies (up to near-THz: 95 – 980 GHz), known as HFEPR, has been developed over the past two decades. There are many applications of HFEPR, such as in the precise determination of g values in systems with multiple radical species (e.g., photosynthesis). Another application is in transition metal coordination chemistry: there are many paramagnetic complexes for which conventional EPR (fields below 1.5 T, frequencies up to 35 GHz) is less than ideal. Primarily, such systems are high-spin, wherein the effects of zero-field splitting can make the complex either “EPR-silent” using conventional EPR, or the EPR spectrum is not particularly informative, such as for the “g = 4.3” signal in Fe(III). We will describe here the use of HFEPR to study several high spin first row transition metal complexes, e.g., integer spin (non-Kramers) ions such as Mn(III) (3d4, S = 1 or 2) and half-integer spin (Kramers) ions such as Co(II) (3d7, S = 3/2) in conjunction with other techniques such as MCD. We will discuss how ligand-field theory (LFT) can be used to provide chemical information, based on analysis of the spin Hamiltonian parameters determined by HFEPR.
