Knowledge Bank

“The various intensity and energy changes in the visible (6500A to 5500A) and near UV (4000A to 3300A) bands on substitution are discussed in terms of perturbations among states that derive from the transitions $a2u$ to $eg$ and $a1u$ to $cg$ of symmetrical porphin. The states derived from these have combined in unsubstituted porphin to give a weak visible and strong UV band, which are mixed by the substituent perturbations. The perturbations are expanded as irreducible representations of the $Dth$ group. This expansion when applied to the empirical data leads to: (1) another proof that the protons in free base porphin must be opposite; and (2) quantitative predictions of intensity changes with substitution. The theory is not limited to small changes in the intensity and energy. The spectra of the azaporphins can also be understood. Although, in this case, the mixing turns out to be extensive, quantitative predietions on intensity and energy changes check reasonably well with experiment.”