Knowledge Bank

“The visible and near UV porphyrin bands are assumed to be due to transitions between a filled pair of MO’s which have angular momentum 4 (in a $D\infty h$ group) to an empty pair with angular momentum 5. Some plausibility arguments can be given to justify the application of this interpretation to porphyrin, where the actual symmetry is $D4h$. One then considers the effects on these states of metal substitutions, which are good substitutions to consider as they preserve the symmetry of the molecule. With some simplifying assumptions as to how the metal affects the Hamiltonian, it is possible to explain for a number of metals the observed relations of: (a) the oscillator strength to the energy for the visible band of the tetraphenyl porphins; (b) the same for the octaalkyl porphins; (c) the visible band energy of the octaalkyl to that of the tetraphenyl porphins; (d) the UV band energy to the visible band energy for the tetraphenyl porphins.”