GOUTERMAN'S FOUR-ORBITAL MODEL AND THE MCD SPECTRA OF HIGH-SYMMETRY METALLOPORPHYRINS

Article abstract of DOI:10.1021/ja00266a007

This paper offers a description of the magnetooptical properties of highly symmetric metalloporphyrins within the framework of the Gouterman four-orbital model.Expressions are derived for the Faraday parameters of the Q and B (Soret) transitions of the porphyrin system as a function of the energy and intensity parameters and of the angular momenta of the relevant excited states.A discussion of these results highlights the distinct roles of two competing mechanism, which procure the Q band intensity.On the one hand this transition may steal intensity from the Soret band as a result of interstate interactions.On the other hand the zeroth order Q⁰ state may also carry a non-zero transition dipole moment as a consequence of the tetragonal symmetry of the porphyrin skeleton.The latter feature is a typical characteristic of the Gouterman model and does not occur in the algebraic solution of the perimeter model for cyclic ?-electron systems.In a final section MCD spectra are reported for meso- and exo-substituted zinc porphyrins and the corresponding cyanide and pyridine complexes.From these data Faraday parameters were extracted by using a Gausian fitting procedure.Parameter shifts caused by a change of the axial ligand are shown to follow the theoretical prediction.