Resonance Raman (RR) spectra have been analyzed for porphyrin radical cations prepared electrochemically from the Ni²⁺, Cu²⁺, VO²⁺, and (Cl^-)Fe³⁺complexes of octaethylporphyrin (OEP) and meso-tetraphenylporphyrin (TPP). Band assignments have been secured via polarization measurements and isotope frequency shifts associated with ¹⁵N and meso-d₄ (OEP) or pyrrole-d₈and phenyl-d₂₀(TPP) substitution. These isotope shifts are similar to those of the neutral metalloporphyrins, establishing that the normal mode compositions for high-frequency skeletal modes are not significantly altered by radical formation. There are, however, appreciable changes in the mode frequencies between neutral porphyrin and radical cation, which are different for the OEP's and TPP's. In particular, the mode v₂, which is largely pyrrole C_βC_βstretching in character, shifts up and down upon radical formation in OEP's and TPP's, respectively. These are the directions expected if the OEP and TPP radicals have a_1uand a_2ucharacter, respectively, since the a_lu and a_2uorbitals are antibonding and bonding, respectively, with respect to the C_βC_βbonds. The pyrrole half-ring stretch, v₄, shifts down on OEP radical formation, consistent with the C_αC_β bonding character of the a_1uorbital. The a_2uorbital is slightly antibonding with respect to the C_αN and C_αC_βbonds, and the TPP radical v₄shifts are smaller and variable. A novel feature of the radical cation RR spectra is the appearance of anomalously polarized bands in the-1000-cm^-1region with excitation near the Soret electronic transitions. These bands are sensitive to meso-d₄ (OEP) or pyrrole-d₈ (TPP) substitution, but neither their frequencies nor their isotope shifts correlate with A_2g modes of the neutral porphyrins. These bands are suggested to reflect pseudo-Jahn--Teller mixing between the A_luand A_2u electronic states via the A_2g vibrational modes (A_1u x A_2u = A_2g). The resulting decrease in the A_2g force constants greatly lowers the frequencies and remixes the mode compositions. It is further suggested that the saddle distortion observed in several porphyrin radical cation crystal structures so far reported might also be a pseudo-Jahn—Teller manifestation. The normal mode associated with the saddle distortion is B_2u in D_4h symmetry, but A₂ in the D_2d symmetry induced by the saddle distortion itself. The dynamic and/or static pseudo-Jahn--Teller distortions can account for the A_lu/A_2umixing which is evident from EPR spectra; they may also play a role in the electron-transfer dynamics of porphyrins and related aromatic macrocycles. Striking departures for the radical cations from the usual resonance enhancement pattern of metalloporphyrins (B-band resonant A_2g mode enhancement, Q-as well as B-band resonant A_1gmode domination) are discussed in terms of the altered electronic character of the excited states and their effects on A, B, and C term RR scattering mechanisms.

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