The planar and nonplanar conformers of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and 5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (N02-OEP) are investigated using resonance Raman spectroscopy. The structural heterogeneity is assessed by analysis of the line shapes of the structure-sensitive Raman lines. First, heterogeneity in the conformation of the macrocycle has been detected in solutions of the nickel and cobalt derivatives of OEP, that is, both planar and nonplanar conformers are found to coexist at room temperature for these metal porphyrins but not for the Cu and Zn derivatives. The latter metals expand the porphryin core, shifting the equilibrium entirely to the planar conformer. Second, we find that substitution with a single NO2 group at one of the methine-bridge carbons shifts this planar-nonplanar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the porphyrin core (Ni(I1) < Co(I1) < Cu(1I) < Zn(I1)) displace the equilibrium toward the nonplanar conformer. Finally, the frequencies of several Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray crystallographic studies). The calculations predict and the marker line frequencies verify that a small expansion of the core results from the steric repulsion between the nitro and the ethyl groups. Core size dependence of the intensities and frequencies of the NO2 stretching vibrations suggests that the NO2 stretches are coupled to nearby vibrational modes of the porphyrin macrocycle.

The infrared and visible spectra of divalent metal complexes of OEP were obtained. The variations in precise absorbances for five metal-sensitive infrared bands at ca. 3050, 1230, 990, 920, and 750 cm-1 are examined as a function of macrocyclic conformation and metal-pyrrolic nitrogen (M-Np) bond distances. The exact positions of the bands at ca. 920,990,1230, and 3050 cm-1 are dependent on the M-Np bond distances. Additionally, the bands at 920,990, and 1230 cnr* 1 are dependent on the size of the lowest unoccupied pz orbital. The 1230-and 3050-cnr1 bands are sensitive to the macrocyclic conformation. In the visible region the variations in Xma, of the a and ß bands are also examined as a function of M-Np bond distance. These appear to be a function of the M-Np bond distance and size of the lowest unoccupied pz orbital. These findings are discussed in terms of stabilization of the a2u orbital on the porphyrin ligand. Implications for porphyrin reactivity are considered. 1988, 110, 3919.

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