Polarized Raman Dispersion Spectroscopy Probes Planar and Non-Planar Distortions of Ni(II) Porphyrins with Different Peripheral Substituents Article

Lemke, C, Dreybrodt, W, Shelnutt, JA et al. (1998). Polarized Raman Dispersion Spectroscopy Probes Planar and Non-Planar Distortions of Ni(II) Porphyrins with Different Peripheral Substituents . JOURNAL OF RAMAN SPECTROSCOPY, 29(10-11), 945-953. 10.1002/(sici)1097-4555(199810/11)29:10/11<945::aid-jrs321>3.0.co;2-4

cited authors

  • Lemke, C; Dreybrodt, W; Shelnutt, JA; Quirke, JME; Schweitzer-Stenner, R



  • Porphyrins exhibiting ideal D4h symmetry show depolarization ratios of their Raman-active A1g, B1g , B2g and A2g type vibrations which are independent of the wavenumber of the exciting radiation, namely 0.125 for A1g , 0.75 for B1g and B2g and infinite for A2g modes. Symmetry-lowering distortions, which can be classified in terms of the irreducible representations of the D4h point group and expressed as linear combinations of normal coordinate deformations, i.e. static displacements along the normal modes, mix Raman tensors of different 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 A1g , B1g , B2g and A2g or by at least two Eu or out-of-plane A1u , A2u , B1u and B2u distortions. This is illustrated by experimentally determined Raman excitation profiles obtained from the polarized spectra of Ni(II)-octaethylporphyrin, the meso-carbons of which are substituted by NO2 at position 5 (C2v) or the positions 5 and 15 (D2h). In both cases symmetry-lowering distortions of B2g symmetry are induced. The depolarization ratio dispersion, however, reflects additional out-of-plane distortions of B1u (ruffling) and B2u (saddling) symmetry. These data are compared with depolarization ratios observed for Raman lines of the planar Ni(II)-porphin (D4h) and the non-planar Ni(II)-octaethyltetraphenylporphyrin. The latter exhibits S4 symmetry reflected by a strong depolarization ratio dispersion of A1g and A2g type modes. Finally, a theoretical approach is discussed which formulates the Raman tensor in terms of vibronic coupling parameters depending on the symmetry classified normal coordinate deformations. These parameters are determined by simultaneous fits to the depolarization ratio dispersion data and the corresponding resonance excitation profiles. Hence the applicability of polarized Raman dispersion spectroscopy to identify symmetry-lowering distortions induced by peripheral substituents, and in proteins, by specific heme - protein interactions is demonstrated. © 1998 John Wiley & Sons, Ltd.

publication date

  • January 1, 1998

published in

start page

  • 945

end page

  • 953


  • 29


  • 10-11