Resonance Raman Spectra of Metalloctaethylporphyrin Cation Radicals with a1u and a2u Orbital Character

Article abstract of DOI:10.1021/j100406a005

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 IIIOEP>⁺, produced by electrochemical and/or chemical oxidation.The enhancement patterns obtained with B (4067 angstroem) band excitation (except M = Zn and Mg, for which fluorescence overwhelms the Q band RR spectra) permitted assignment of most of the porphyrin skeletal models above 1000 cm^-1, and frequency shifts relative to the unoxidized porphyrins have been cataloged.Those radicals, magnesium(II), zink(II), and cobalt(III) bromides, whose EPR spectra, have beenassigned on the basis of electron removal from an a_1u molecular oribtal show a cammon 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 resebles that of other a_2u radicals, likewise has an a_2u RR frequency shift pattern.The largest shifts are showen 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 straighforvard in B-band-excited RR spectra.The directions of the shifts, however, are opposite to those expected from simple orbital simmetry 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 vive versa for the a_1u orbital.This discrepancy invites further examination via vibrational and electronic calculations.