Metal Dependence of the Contributions of Low-Frequency Normal Coordinates to the Sterically Induced Distortions of Meso-Dialkyl-Substituted Porphyrins

The influence of central metals of different sizes on the macrocyclic structure of a series of 5,15-disubstituted metalloporphyrins has been investigated with X-ray crystallography, molecular mechanics (MM) calculations, and resonance Raman spectroscopy. MM calculations indicate that the series of porphyrins are in a gabled (gab) conformation consisting of a linear combination of ruf (B{sub 1u}) and dom (A{sub 2u}) out-of-plane normal-coordinate deformation types. The MM-calculated gab structures have been structurally decomposed into equivalent displacements along the lowest-frequency normal coordinate of each symmetry type. The contributions of each normal coordinate to the total distortion agree well with the contributions obtained from normal-coordinate structural decomposition of the X-ray crystal structures. Symmetry considerations show that any relative proportion of the ruf and dom deformations is allowed in the gab distortion. Varying the size of the central metal causes a change in the ratio of the dom/ruf contributions. A large metal like zinc disfavors ruffling over doming, significantly reducing the ruffling contribution and slightly increasing the doming contribution. In addition, the total degree of nonplanar distortion is reduced for large metals. This is confirmed for the series of disubstituted metalloporphyrins by smaller downshifts of the structure-sensitive Raman lines for the larger metals.