The atom-connectivity configuration differences for a pair of species indicates whether they are "adjacent" on their reaction graph, that is, a graph edge exists between those species. For free-base subporphyrin monoanion, only first-order and secondorder relationships are possible for unimolecular (nondissociative) H-tautomerism. Fig. shows the combined fistorder and second-order motions graphs. This corresponds to the traditional chemical-structure depiction of the tautomerism shown in Fig. with a layout corresponding to Fig. . In Fig. is the corresponding DFT-calculated qualitative PES for the tautomeric H-atom using a real-space polar coordinate system. On this PES are shown the locations of the seven species discussed and the minimum-energy pathways (MEPs). Note that the graph from Fig. maps onto the realspace PES, species, and MEPs. The white dashed paths map to the first-order motions and the light-red dashed paths map to the second-order motionsthey are isomorphic.