Vibrational analysis of metalloporphyrins with electron-withdrawing NO2 substituents at different meso positions

The structure of metalloporphyrins is to a major extent determined by its p eripheral substituents: To explore the influence of NO2 meso substituents, we have measured polarized resonance Raman spectra of Ni(II)(5-NO2-octaethy lporphyrin) and NI(II)(5,15-NO2-octaethylporphyrin) at various excitation w avelengths in the Soret and Q band region in CS2. We obtained very complex and overcrowded spectra that were self-consistently analyzed by a global fi tting procedure. This revealed that numerous Raman Lines in the Raman frequ ency region are composed of sublines. This includes all prominent structura l marker lines. Thus, we found that for both substances at least three conf ormers coexist in solution. The resonance excitation profiles of the corres ponding sublines indicate that these conformers exhibit different degrees o f nonplanarity. Altogether, the data suggest that the nitro groups destabil ize the porphyrin macrocycle and increase conformational heterogeneity. We also performed a normal-mode calculation with a spectroscopically determine d molecular mechanics force field for a nonplanar structure of Ni(II)(5,(5- NO2-octaethylporphyrin). This revealed that the NO2 substituents lower the symmetry of some vibrations by vibrational mixing (e.g., for v(4)) and by c hanging the eigenvectors in particular of low-frequency modes (e.g., of v(8 )).