Theory of Porphyrin Spectra

All recent theoretical treatments of porphyrin spectra regard the lowest-lying absorption bands as intra-porphyrin re re transitions. Simpson (29) proposed a model in which the highest-energy filled re-orbitals had an angular momentum about the fourfold axis of L — 4. The lowest empty re -orbitals had L = 5, so that the lowest-lying re—re transitions corresponded to AL = 1 and 9. The former is strongly allowed and 

Band positions are given in kK, with intensities in 1. moIe 1cm 1 X 10-3 in parentheses  

The effects of substituents at the pyrrole carbon atoms on the spectrum can be explained in terms of the four-orbital model. However, in all these calculations the metal is simply regarded as a positive charge whose magnitude affects the charges on the pyrrole nitrogen atoms only the inductive effect of the metal is considered. The mesomeric effect of -interaction between metal and porphyrin on the spectrum has been discussed qualitatively (44), and it was noted that the transitions 

Calculations in which the metal orbitals are explicitly considered will be discussed in Section HID. The molecular orbital theory as outlined here has been quite successful in dealing with the spectra of non-ferric porphyrins, with some exceptions. Ferrous porphyrins and haemopro-teins with small unsaturated ligands such as NO and O2 have additional bands (45) and Mn(III) porphyrins exhibit some quite unusual features (46). Distortion of the haem group may also give rise to anomalous effects. We shall discuss these further in Section V. 

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This article is not intended to present an exhaustive review of the extensive literature on haemoprotein spectra, which dates back almost 90 years. Our aim is to discuss the origin of the absorption bands of ferric haems and haemoproteins in the light of the most recent physico-chemical and theoretical studies. We consider that the spectra of ferrous haems and haemoproteins are reasonably well understood in terms of the theory of porphyrin spectra those of the ferric compounds are much more complex, and the number of distinct assignments and interpretations which have been offered in the literature in the last twelve years is approaching double figures. A critical summary of the relevant experimental facts and theoretical interpretations seems timely. 

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