Characterization of low frequency resonance Raman bands of metallo-protoporphyrin ix. observation of isotope shifts and normal coordinate treatments

Resonance Raman spectra have been observed for six specifically-deuterated derivatives of protoporphyrinato-Ni(II) [Ni(PP)] in CH₂Cl₂, CCl₄ and benzene solutions using 406.7, 441.6 and 514.5 nm excitation lines. Normal coordinate calculations are carried out for approriate model porphyrins and on the basis of the calculated deuteration shifts the observed Raman bands of Ni(PP) are assigned. The assignments of Raman bands previously established for symmetric octaethylporphyrinato-Ni (II) [Ni(OEP)] are generally applicable to the high frequency skeletal modes of Ni(PP), although the vinyl CH₂ rocking mode is considerably mixed with the porphyrin modes near 1000-1200 cm^-1. For the modes below 450 cm^-1, asymmetric disposition of the peripheral groups makes the corresponding modes of the four pyrrole rings non-equivalent and it appears that each pyrrole ring has specific intensity and frequencies in the Raman spectra.