ESR Studies on 205T1 Hyperfine Couplings in the Radical Cations of T1(III)Porphyrins

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The isotropic 205T1 hyperfine couplings obtained from the ESR spectra of the radical cations of Tl (III) meso tetraphenyl porphyrin (TPP), octaethyl porphyrin (OEP) and octaethyl chlorin (OEC) are reported. The radical cations were generated by electrooxidation in dichloromethane as solvent. A karplus-Fraenkel type bilinear relation is used to interpret the 205T1 couplings, taking into account the sigma-pi spin polarization of Tl —N bonds by the spin density at the nitrogen atoms in the ligand as well as the direct pi interaction of the orbitals of Tl with the pi orbitals of the ligand. It is shown that for the cation radicals of Tl porphyrins, both these mechanisms contribute to the Tl couplings whereas for the cation radicals of Co- and Zn-porphyrins the sigmaoi polarization alone is sufficient to account for the metal hyperfine couplings. It is suggested that Tl-hyperfine couplings can be used to estimate the nitrogen spin densities of porphyrin radical systems when the nitrogen splittings are not resolved in the ESR spectra. © 1974, Walter de Gruyter. All rights reserved.

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Zeitschrift fur Naturforsch.ung - Section A Journal of Physical Sciences

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