𝓢 𝟑 𝓑 𝟐 systems Building on our earlier example of tautomerism in free-base subporphyrin monoanion, an additional bonder (H + ) gives neutral free-base subporphyrin partitioned as 𝒮 3 ℬ 2 as shown in Fig. . Here, there are two mobile bonders (both H) free to move inside the inner cavity of the macrocycle. The second chemical system, also shown in Fig. , corresponds to the general formula H2[MO3] where the two mobile bonders (both H) are free to move around the periphery. The central atom M could be one of many chemical elements, for example, carbon.