Infrared Spectra of ?-Cation Radicals of Magnesium, Zinc, and Cobalt Octaethylporphyrins

Article abstract of DOI:10.1021/j100317a020

Infrared spectra in the 1700-250-cm_-1 region were measured for the bromide salts of the ?-cation radicals of MOEP (M=Mg(II) and Zn(II), OEP=octaethylporphyrin) and for the bromide and perchlorate salts of Co^IIIOEP⁺, all of which were prepared by chemical oxidation.By comparing the infrared spectra of the ?-cation radicals with those of the parent molecules, assignments were made for the in-plane stretching and bending modes as well as the out-of-plane bending modes of the porphyrin ring of the radicals and the frequency shifts of these modes relative to the unoxidized molecules were determined.Common shift patterns were observed for the Mg(II) and Zn(II) radicals, which are known to be in a ²A_1u state from the ESR study.These patterns were, however, quite different from the corresponding shift patterns observed for the perchlorate salt of the Co(III) radical, which is in a ²A_2u state, suggesting that the measurement of the infrared spectra is one of the straightforward methods for determining the ground electronic structure of the ?-cation radicals of metalloporphyrins.The infrared spectrum of the bromide salt of the ?-cation radical of the Co(III) complex, whose ground electronic structure is also in a ²A_1u state, however, shows features much simpler than those observed for the Mg(II) and Zn(II) radicals.This result suggests the existence of a vibronic coupling effect in the Co(III) radical, which reduces the intensities of some infrared bands due to porphyrin ring modes.