SELF-CONSISTENT MOLECULAR ORBITAL CALCULATIONS OF PORPHYRIN AND RELATED RING SYSTEMS.

Author Institution: Conant Chemical Laboratory, Harvard UniversitySelf-consistent molecular orbital calculations on the porphyrin ring by the method of Pariser, Parr, and Pople (SCMO-PPP) confirm earlier conclusions of Gouterman and Platt that the visible and Soret bands of the porphyrin absorption spectrum can be explained by a four-orbital model. According to this model, the lowest two pairs of excited configurations are accidentally degenerate, and the extensive interaction between them accounts for the great intensity of the Soret and the relative weakness of the visible bands. Interaction with higher configurations is negligible. The SCMO-PPP calculations reproduce the difference between the metal and free base porphyrin. They give a reasonably good interpretation of reduced porphyrins and lead to a reassignment of the spectra of azaporphyrins and phthalocyanine. There is a consistent overestimation of configuration interaction