PLANAR NONPLANAR CONFORMATIONAL EQUILIBRIUM IN METAL DERIVATIVES OF OCTAETHYLPORPHYRIN AND MESO-NITROOCTAETHYLPORPHYRIN

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-oct aethylporphyrin (NO2-OEP) are investigated using resonance Raman spect roscopy. The structural heterogeneity is assessed by analysis of the l ine shapes of the structure-sensitive Raman lines. First, heterogeneit y 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 the se 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-non planar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the p orphyrin core (Ni(II) < Co(II) < Cu(II) < Zn(II)) displace the equilib rium toward the nonplanar conformer. Finally, the frequencies of sever al Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray c rystallographic studies). The calculations predict and the marker line frequencies verify that a small expansion of the core results from th e steric repulsion between the nitro and the ethyl groups. Core size d ependence of the intensities and frequencies of the NO2 stretching vib rations suggests that the NO2 stretches are coupled to nearby vibratio nal modes of the porphyrin macrocycle.