Figure presents geometrical highlights and energetics of the optimized cis tautomers and trans-to-cis transition states of H2Cor -. In terms of electronic energy, the cis tautomers BC and AD are remarkably stable, about 0.15-0.25 eV (3.5-6.0 kcal/mol) above the ground state (Figure ). Their stability undoubtedly reflects the fact that they both preserve the ultra-short hydrogen bonds between pyrroles A and B and between pyrroles C and D. For comparison, the cis tautomer of a simple porphyrin is about 7-8 kcal/mol above the trans ground state. 9 (It may be mentioned in passing that substituent and environmental effects may result in the stabilization and isolation of a cis tautomer for certain porphyrins. ) In contrast, cis tautomers AB and CD exhibit much higher energies, about 2/3 of an eV (16 kcal.mol) relative to the trans ground state, reflecting loss of preferred hydrogen bond pathways.