Visualization of an ADF 2004 frequency calculation on the transition state for oxidation of isobutane by mCPBA. All bonds to O3 are elongated, in particular the O3−O8 bond (not automatically detected anymore). The vectors of the imaginary frequency are shown, indicating a concerted mechanism of simultaneous O8−O3 bond dissociation, O3 insertion into the C1−H2 bond, and transfer of H7 to O10 of the carboxylate moiety.