| Autor: |
Lemke, C., Schweitzer-Stenner, R., Shelnutt, J. A., Quirke, J. M. E., Dreybrodt, W. |
| Zdroj: |
The Journal of Physical Chemistry - Part A; July 2001, Vol. 105 Issue: 27 p6668-6679, 12p |
| Abstrakt: |
The structure of metalloporphyrins is to a major extent determined by its peripheral substituents. To explore the influence of NO2 meso substituents, we have measured polarized resonance Raman spectra of Ni(II)(5-NO2−octaethylporphyrin) and Ni(II)(5,15-NO2−octaethylporphyrin) at various excitation wavelengths in the Soret and Q band region in CS2. We obtained very complex and overcrowded spectra that were self-consistently analyzed by a global fitting procedure. This revealed that numerous Raman lines in the Raman frequency region are composed of sublines. This includes all prominent structural marker lines. Thus, we found that for both substances at least three conformers coexist in solution. The resonance excitation profiles of the corresponding sublines indicate that these conformers exhibit different degrees of nonplanarity. Altogether, the data suggest that the nitro groups destabilize the porphyrin macrocycle and increase conformational heterogeneity. We also performed a normal-mode calculation with a spectroscopically determined molecular mechanics force field for a nonplanar structure of Ni(II)(5,15-NO2−octaethylporphyrin). This revealed that the NO2 substituents lower the symmetry of some vibrations by vibrational mixing (e.g., for ν4) and by changing the eigenvectors in particular of low-frequency modes (e.g., of ν8). |
| Databáze: |
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