An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids

The crystal structures of three porphyrin diacid species, [H4OEP](ClO4)2, [H4TPP](ClO4)2, and [H4TMP](ClO4)2, have been determined from low-temperature X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. [H4OEP](ClO4)2 (|Cb| = 0.46 A), [H4TMP](ClO4)2 (|Cb| = 0.67 A, molecule 1), and [H4TPP](ClO4)2 (|Cb| = 0.93 A) show increasingly saddled core conformations with effective D2d symmetry. The mean porphyrin−aryl group dihedral angles in [H4TPP](ClO4)2 and [H4TMP](ClO4)2 (molecule 1) are 27(2)° and 63(13)°, respectively. The steric bulk of the mesityl substituents in [H4TMP]2+ limits the range of observed porphyrin−aryl group dihedral angles to >50° and, consequently, the magnitude of the core distortion. [H4TMP]2+ is therefore less flexible than [H4TPP]2+. Molecular mechanics calculations, using a modified version of MM2(87) and a newly developed force field for porphyrin diacids, correctly predict that the flexibility of meso-tetraar...