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.
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