  High-frequency electron paramagnetic resonance (EPR) studies of the
antiferromagnetic Mn-$[3\times 3]$ molecular grid clearly reveal a breaking of
the $\Delta S = 0$ selection rule, providing direct evidence for the mixing of
spin wavefunctions ($S$-mixing) induced by the comparable exchange and
magneto-anisotropy energy scales within the grid. This finding highlights the
potential utility of EPR for studies of exchange splittings in molecular
nanomagnets, which is normally considered the sole domain of inelastic neutron
scattering, thereby offering improved sensitivity and energy resolution.
