X-band electron paramagnetic resonance (EPR) spectroscopy was used to probe the ground-state electronic structures of mononuclear Mn-IV complexes [Mn-IV(OH)(2)(Me2EBC)](2+) and [Mn-IV(O)(OH)(Me2EBC)](+). These compounds are known to effect C-H bond oxidation reactions by a hydrogen-atom transfer mechanism. They provide an ideal system for comparing Mn-IV-hydroxo versus Mn-IV-oxo motifs, as they differ by only a proton. Simulations of 5 K EPR data, along with analysis of variable-temperature EPR signal intensities, allowed for the estimation of ground state zero-field splitting (ZFS) and Mn-55 hyperfine parameters for both complexes. From this analysis, it was concluded that the Mn-IV-oxo complex [Mn-IV(O)(OH)(Me2EBC)](+) has an axial ZFS parameter D (D = +1.2(0.4) cm(-1)) and rhombicity (E/D = 0.22(1)) perturbed relative to the Mn-IV-hydroxo analogue [Mn-IV(OH)(2)(Me2EBC)](2+) (vertical bar D vertical bar = 0.75(0.25) cm(-1); E/D = 0.15(2)), although the complexes have similar 55Mn values (a = 7.7 and 7.5 mT, respectively). The ZFS parameters for [Mn-IV(OH)(2)(Me2EBC)](2+) were compared with values obtained previously through variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) experiments. While the VTVH MCD analysis can provide a reasonable estimate of the magnitude of D, the E/D values were poorly defined. Using the ZFS parameters reported for these complexes and five other mononuclear Mn-IV complexes, we employed coupled-perturbed density functional theory (CP-DFT) and complete active space self-consistent field (CASSCF) calculations with second-order n-electron valence state perturbation theory (NEVPT2) correction, to compare the ability of these two quantum chemical methods for reproducing experimental ZFS parameters for Mn-IV centers. The CP-DFT approach was found to provide reasonably acceptable values for D, whereas the CASSCF/NEVPT2 method fared worse, considerably overestimating the magnitude of D in several cases. Both methods were poor in reproducing experimental E/D values. Overall, this work adds to the limited investigations of Mn-IV ground-state properties and provides an initial assessment for calculating Mn-IV ZFS parameters with quantum chemical methods.