Electron paramagnetic resonance (EPR) remains one of the most direct probes for assigning adsorbate-derived radical states on ionic-oxide surfaces, yet the connection between measured magnetic parameters and local adsorption motifs on MgO(100) is often not unique. In this work, we use an embedded-cluster density functional framework to assess \(\text{NO}_2\) - and \(\text{CO}_2\) -derived paramagnetic motifs on MgO(100) by directly comparing calculated g tensors, hyperfine tensors, and powder-spectrum simulations with benchmark experimental spectra. For \(\text{NO}_2\) , a weakly bound terrace \(\text{O}^{2-}\) motif reproduces the experimental EPR response most closely, whereas vacancy-bound structures show stronger binding but substantial spin redistribution and a clearly less consistent magnetic signature. For \(\text{CO}_2\) , electron transfer at regular \(\text{O}^{2-}\) sites does not by itself guarantee an EPR-active \(\text{CO}_2^-\) state; instead, these motifs commonly relax toward carbonate-like species with strongly reduced \(^{13}\text{C}\) hyperfine couplings. In contrast, hydroxylated low-coordinated environments preserve stronger adsorbate-centered spin density, and the experimental \(\text{CO}_2^-\) signature is most consistently represented by a family of related OH-stabilized low-coordinated motifs, especially edge and corner environments. The results provide a unified EPR-based comparison of competing \(\text{NO}_2\) and \(\text{CO}_2^-\) adsorption motifs on MgO(100) and identify the local environments that are most consistent with the available benchmark spectra.
