Polarized Resonance Raman Spectroscopy Reveals Two Different Conformers of Metallo(II)octamethylchlorins in CS2

• Published in: The journal of physical chemistry. B Vol. 103; no. 44; pp. 9777 - 9781
• Main Authors: Lipski, Robert J, Unger, Esko, Schweitzer-Stenner, Reinhard
• Format: Journal Article
• Language: English
• Published: American Chemical Society 04.11.1999
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp9918858

We have for the first time measured and analyzed the Raman spectra of the model hydroporphyrins nickel(II) and copper(II) 2,2,7,8,12,13,17,18-octamethylchlorin in CS2. A detailed spectral analysis of the fingerprint region of nickel(II) chlorin revealed that a band at 1654 cm-1 is composed of two sublines at 1654 and 1662 cm-1. A novel normal coordinate analysis based on a transferrable force field derived from nickel(II) porphin, propane, and 2,2-dimethylpropane revealed that the respective normal mode is comparable with the porphyrin mode ν10 despite significant localization effects due to the reduction of a pyrrole ring. The resonance excitation profile of the low-frequency subline of ν10 is red-shifted with respect to that at higher frequencies. Hence, the two sublines can be interpreted as resulting from the coexistence of a nonplanar (ruffled) and a more planar conformer. The analysis of the ν10 band of the copper(II) octamethylchlorin revealed that it is also composed of two sublines. The frequencies obtained are 1639 and 1645 cm-1. Thus, evidence is provided that copper(II) chlorins can exist in a nonplanar conformation. These results underscore the notion that even in the absence of any steric interactions between substituents and the presence of metals with an optimal ionic radius pyrrole reduction significantly destabilizes the π-electron system of the porphyrin macrocycle.