Resonance Raman spectra of metallooctaethylporphyrin cation radicals with a_1u and a_2u orbital character

Resonance Raman (RR) spectra are reported for radical cations of M^IIOEP (OEP = octaethylporphyrin; M = Zn, Mg, Ni, Cu) and for the perchlorate and bromide salts of [Co^IIIOEP]⁺, produced by electrochemical and/or chemical oxidation. The enhancement patterns obtained with B (4067 Å) and Q (5682 Å) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal modes above 1000 cm^-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged. Those radicals, magnesium(II), zinc(II), and cobalt(III) bromides, whose EPR spectra have been assigned on the basis of electron removal from an a_1u molecular oribtal show a common pattern of shifts, while those with a_2u-type EPR spectra, nickel(II) and cobalt(III) perchlorates, show a different pattern. The Cu^II radical, whose visible absorption spectrum resembles that of other a_2u radicals, likewise has an a_2u RR frequency shift pattern. The largest shifts are shown by the most prominent RR bands in the B-band-excited spectra, v₄ (C_aN breathing, primarily) and v₂ (C_bC_b breathing, primarily). These shift up (+14 to +21 cm^-1) and down (-19 to -38 cm^-1), respectively, for the a_1u radicals, but down (-12 to -22 cm^-1) and up (+20 to +23 cm^-1) for the a_2u radicals. Thus an identification of the radical type is quite straightforward in B-band-excited RR spectra. The directions of the shifts, however, are opposite to those expected from simple orbital symmetry arguments, since the a_2u molecular orbital is bonding with respect to the C_b-C_b bonds and antibonding with respect to the C_a-N bonds, and vice versa for the a_1u orbital. This discrepancy invites further examination via vibrational and electronic calculations.