Polarized resonance Raman spectroscopy reveals two different conformers ofmetallo(II)octamethylchlorins in CS2

We have for the first time measured and analyzed the Raman spectra of the m odel hydroporphyrins nickel(II) and copper(II) 2,2,7,8,12,13,17,18-octameth ylchlorin 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, an d 2,2-dimethylpropane revealed that the respective normal mode is comparabl e with the porphyrin mode nu(10) despite significant localization effects d ue to the reduction of a pyrrole ring. The resonance excitation profile of the low-frequency subline of nu(10) 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 nu(10) band of the copper(II) octamethylchlorin reveal ed that it is also composed of two sublines. The frequencies obtained are 1 639 and 1645 cm(-1). Thus, evidence is provided that copper(II) chlorins ca n exist in a nonplanar conformation. These results underscore the notion th at even in the absence of any steric interactions between substituents and the presence of metals with an optimal ionic radius pyrrole reduction signi ficantly destabilizes the pi-electron system of the porphyrin macrocycle.