EPR Spectra from “EPR-Silent” Species:  High-Field EPR Spectroscopy of Aqueous Chromium(II)

High-field (up to 14.5 T) multifrequency (∼90−440 GHz) electron paramagnetic resonance (EPR) spectroscopy has been used to probe the non-Kramers, S = 2, Cr2+ ion in frozen aqueous solution, in the presence and absence of the glassing agent, glycerol. Solutions with both chloride and sulfate counterions were investigated. Detailed analysis based on a combination of analytical and full-matrix solutions to the spin Hamiltonian for an S = 2 system gave zero-field splitting parameters D = −2.20(5) cm-1, E = 0.0(1) cm-1, and g⊥ ≈ g∥ = 1.98(2), independent of solvent system and counterion. These results are in agreement with an early single-crystal EPR study of CrSO4·5H2O; however, the present study allows unequivocal determination of the sign of D and shows that in solution [Cr(H2O)6]2+ is a perfectly axial system (tetragonally elongated), as opposed to solid CrSO4·5H2O, which showed a measurable E value, indicative of slight orthorhombic distortion as seen in its crystal structure. HF-EPR data is combined with earlier electronic absorption data to provide a complete picture of the electronic structure of Cr2+ in this chemical environment. The results are also compared to a recent HF-EPR study of a Mn3+ complex, which showed that electronic excited states affect the ground state of the complex, but these effects are less pronounced for Cr2+. The present study also shows the applicability of high-field EPR to aqueous solutions of integer-spin (“EPR-silent”) transition metal complexes, as previous studies have employed only solid (single-crystal or polycrystalline) samples.