Porphyrin Isomers:  Geometry, Tautomerism, Geometrical Isomerism, and Stability

Density functional calculations have been carried out on free-base porphyrin (1) and its seven possible isomers (2-8) with an N₄-metal coordination core. A total of 27 structures resulting from geometrical isomerism ((E/Z)-configurations) and NH tautomerism were studied. Geometries were fully optimized with the nonlocal density functional approximation (BLYP) using the 3-21G and 6-31G** basis sets. The calculated geometries compare favorably with the available X-ray crystal structures. Porphycene (2) is predicted to be the most stable among the eight isomers and is about 2 kcal/mol more stable than porphyrin due to its exceptionally strong hydrogen bonding. Compounds 5-8 are much less stable than porphyrin due to severe ring strain in these compounds. When a -(CH)n- linker is in a (Z)-configuration, each compound is planar or nearly planar with significant π-delocalization; the corresponding (E)-configured structures are predicted to be somewhat distorted into bowl-like geometries in order to avoid severe steric interactions involving the inner hydrogens.