Electron Spin Resonance Study of Metal Ion Separations in Dimeric Copper (II) and Vanadyl Porphyrins

Electron spin resonance measurements have been made on a number of copper (II) and vanadyl porphyrins. The spectra recorded at a sample temperature of 77°K have low field components attributed to ΔM=2 transitions which provided evidence for the existence of dimer species in solution. In one case, namely copper hematoporphyrin (IX) dimethylester, a resonance centered at 950 G is thought to be due to a ΔM=3 transition arising from a trimer species. The theory of the ESR spectra of dipolar coupled $\mathrm{S=}\frac{1}{2}$ transition metal ions with particular reference to systems involving copper (II) and vanadyl ions is outlined. The treatment has been developed to the point which makes a computer simulation of the ΔM=1 and ΔM=2 transitions possible in the systems studied. An important product of this is the evaluation of the distance between the metal ions in the dimeric species. The variation of this distance in the metalloporphyrins studied is related to subtle changes in structure brought about by the various substituents in the macrocylic system as well as to the nature of the transition metal ion.