As a result of work in our and other laboratories over the past twenty years, magnetic circular dichroism (MCD) has become virtually an essential spectroscopic technique for definitive studies of porphyrins and related macrocycles both in and outside protein matrices since MCD can provide information that is either not available or is not as readily accessible by other methods. The occurrence of sign variation in the MCD of porphyrins provides a further important--but as yet largely unexplored--focus of applications of MCD, particularly to structural problems. Our proposition is that the MCD and, concomitantly, the absorption spectrum of any substituted (vinyl, alkoxycarbonyl, formyl, acetyl, cyano) porphyrin regardless of the number, kind, or relative orientations of the substituents, can be predicted provided only that the spectra of a limited number of symmetry related disubstituted porphyrins are known. We propose to synthesize the required sets of disubstituted porphyrins and other particular porphyrins through the use of total synthesis procedures and to measure their spectra as free-bases and as complexed with a limited number of metal ions. We also propose to elaborate and to develop theoretical models for the interpretation and prediction of spectra that will be accessible to a broad range of porphyrin chemists--not just to those with expertise in spectroscopic and theoretical problems. Finally, since MCD is a complex phenomenon (being governed by the sum of scalar triple products of electric and magnetic moments over all states coupled by the magnetic field), we hope to use MCD as a focus for explicit numerical calculations aimed at providing a more complete description of the electronic states of substituted porphyrins.
