A theoretical study of porphyrin isomers and their core-modified analogues: cis-trans isomerism, tautomerism and relative stabilities

Semiempirical (AM1 and PM3) and density functional theory (DFT) calculations were performed on about 50 porphyrin isomers with 25 each of 1,2 (syn) and 1,3 (anti) tautomeric forms. The corresponding oxa- and thia-core-modified analogues were also computed. The variations of relative energies and stabilities of the core-modified analogues were compared with parent porphyrin 1 and the corresponding oxa- and thia-analogues. The trends in relative energies are not significantly changed while going from parent system to oxa- and thia-core-modified porphyrins in case of both syn and anti tautomers. Isomers of types [2,2,0], [3.0.1.0], [3,1,0,0], and [4,0,0,0] are destabilized due to the absence of methine bridge, which results in ,ngle strain for tetrapyrroles. Isomers having [2,1,1,0], [2,1,0,1], [2,0,2,0] and [2,2,0] connectivity, the Z isomers, are more stable compared to the corresponding E isomers in both syn and anti forms of parent and core-modified analogues.