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Electron spin resonance of some vanadyl, copper and cobalt porphyrins Lau, Pui-Wah

Abstract

Electron Spin Resonance study was carried out on a series of metalloporphyrins (where metal = VO²⁺, ⁶³Cu²⁺ , and CO²⁺, porphyrin = (4-pyridyl)porphin, (p-carboxylphenyl)porphin, tetrabenzporphyrin, and octamethyltetrabenzporphyrin), magnetically diluted in the free base porphyrins, or organic solvents, aiming at the understanding of the electronic structures of these biological important compounds and the factors affected them. In the case of vanadyl porphyrin, the unpaired electron was found to be in the b₂ orbital. The changes in ligand field energies as the function of porphyrins were found to correlate with the sizes of the porphyrin holes. Some solvent effects were noted. In the case of copper porphyrins, the unpaired electron was in b[sub lg] orbital. The changes in ligand field energies as the function of porphyrins showed the same trend as the vanadyl counter-parts. The solvent effect was also noted. However, the most interesting results were those of cobalt porphyrins, which gave dramatically different ESR spectra in different environments. When a cobalt porphyrin was magnetically diluted in the free base porphyrin, g[sub ║]< 2 .0 , 2 .0 < g[ sub ┴] < 3.5, |A[ sup CO]|~ 100-200 x 10⁻⁴cm⁻¹, |B[sup CO]|~ 200-400 x 10⁻⁴cm⁻¹. The spin Hamil-tonian parameters correlated very well with the crystal packings. Upon dissolving in pyridine under vacuum, the ESR spectrum showed superhyperfine structures at both g[sub ║] and g[ sub ┴] parts due to two solvent molecules co-ordinated at the axial positions, and changes in other spin Hamiltonian parameters as well. These results gave an insight to the electronic structures of cobalt porphyrins. When the cobalt porphyrin-pyridine complex was exposed to air, an oxygen molecule replaced a solvent molecule. Since g[sub ≈] and A[sub ≈][sub CO] tensors were not coincident, a method for treating the experimental results was given.

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