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Keywords:

Science & Technology, Physical Sciences, Chemistry, Multidisciplinary, Chemistry, 03 Chemical Sciences, General Chemistry, 34 Chemical sciences, 40 Engineering

Abstract:

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 mechanisms, 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 Q° 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 ir-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 Gaussian fitting procedure. Parameter shifts caused by a change of the axial ligand are shown to follow the theoretical predictions. © 1986, American Chemical Society. All rights reserved.