Experimental and theoretical study on vibrational spectra of nickel and zinc complexes of 5,10-diphenylporphyrin

The vibrational spectra of nickel and zinc 5,10-diphenylporphyrin (NiDaPP and ZnDaPP) have been studied by density functional theory (B3LYP/6-31G(d)) and experimental measurement. The assignment of observed Raman and IR bands are proposed based on theoretical calculation. Theoretical study indicates that 5,10-diphenyl substitution together with out-of-plane distortion diversify the structure of four pyrrole rings and the environment around C_m atoms, which lowers the symmetry of porphyrin skeleton and brings about some variation to vibrational spectra. The first is the activation of all normal modes both in Raman and IR spectra. The second is the lifting of original degenerate E_u representation and its splitting into two A modes in NiDaPP (C₁ group point) and A′/A″ presentation in ZnDaPP (C_s group point), respectively. Furthermore, one or both of the two split components, especially for some vibration involving the motion of C_m atom, undergoes further mixing with original non-degenerate mode (A_1g, A_2g, B_1g, or B_2g) of same vibration. This produces new modes involving only partial motion of original mode. Besides general increasing frequency of NiDaPP with respect to ZnDaPP, different vibrational structure is also indicated by calculation for some structure-sensitive bands, especially ν₈ mode that couples with γ₆ mode and split into two modes for NiDaPP but not for ZnDaPP.