Polarized Raman dispersion spectroscopy probes planar and non-planar distortions of Ni(II) porphyrins with different peripheral substituents
✍ Scribed by Christina Lemke; Wolfgang Dreybrodt; John A. Shelnutt; J. Martin E. Quirke; Reinhard Schweitzer-Stenner
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 374 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0377-0486
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✦ Synopsis
Porphyrins exhibiting ideal symmetry show depolarization ratios of their Raman-active and D 4h A 1g , B 1g , B 2g A 2g type vibrations which are independent of the wavenumber of the exciting radiation, namely 0.125 for 0.75 for A 1g , and and inÐnite for modes. Symmetry-lowering distortions, which can be classiÐed in terms of the B 1g B 2g A 2g irreducible representations of the point group and expressed as linear combinations of normal coordinate D 4h deformations, i.e. static displacements along the normal modes, mix Raman tensors of di †erent symmetry. This can cause a dispersion of the depolarization ratios with respect to the excitation wavenumber. By expanding the vibronic coupling operator in the quantum mechanical expression of the Raman tensor to second order with respect to the normal coordinate deformations, one can show by group theoretical arguments that the dispersion of the depolarization ratio can be induced by in-plane and or by at least two or out-of-plane A 1g , B 1g , B 2g A 2g E u A 1u , and distortions. This is illustrated by experimentally determined Raman excitation proÐles obtained A 2u , B 1u B 2u from the polarized spectra of Ni(II)-octaethylporphyrin, the meso-carbons of which are substituted by at NO 2 position 5 or the positions 5 and 15 In both cases symmetry-lowering distortions of symmetry are (C 2v ) ( D 2 h ). B 2g induced. The depolarization ratio dispersion, however, reÑects additional out-of-plane distortions of (ruffling) B 1u and (saddling) symmetry. These data are compared with depolarization ratios observed for Raman lines of the B 2u planar Ni(II)-porphin and the non-planar Ni(II)-octaethyltetraphenylporphyrin. The latter exhibits sym-(D 4h ) S 4 metry reÑected by a strong depolarization ratio dispersion of and type modes. Finally, a theoretical A 1g
A 2g approach is discussed which formulates the Raman tensor in terms of vibronic coupling parameters depending on the symmetry classiÐed normal coordinate deformations. These parameters are determined by simultaneous Ðts to the depolarization ratio dispersion data and the corresponding resonance excitation proÐles. Hence the applicability of polarized Raman dispersion spectroscopy to identify symmetry-lowering distortions induced by peripheral substituents, and in proteins, by speciÐc heme-protein interactions is demonstrated.