Spectra ami Redox Potentials of Metalloporphyrins and Haemoproteins 59 



distorted from square (Robertson, 1936), probably because the hydrogens 

 bound on opposite nitrogen atoms each form hydrogen bonds with an 

 adjacent nitrogen atom. This distortion probably occurs in the porphyrins 

 also (Mason, 1958; Piatt, 1956), leading to the fairly constant difference of 

 6-7 kcal between the energy levels of corresponding absorption bands (I and 

 III, II and IV). This difference is removed in the di-anion, the di-cation and 

 the metal complexes, and in all these cases only two of these bands are found. 



By making some simplifying assumptions Longuet-Higgins, Rector and 

 Piatt (1950) and Seely (1957) have carried out molecular orbital calculations 

 to find the nature of the transitions giving rise to the observed porphyrin 

 spectra. Essentially the same conclusions are reached using an electron-gas 

 model (Kuhn, 1959). The visible bands all arise in a similar manner, as 

 transitions between filled orbitals of /IgM'typ^ symmetry and vacant ^'^-type 

 orbitals (Fig. 1). In all cases these bands are associated with an electronic 

 displacement towards the periphery and this may be either along (bands III 

 and IV) or perpendicular to (bands I and II) the axis tlirough the two H's 

 which are on opposite nitrogens (Piatt, 1956; Mason, 1958). Because bands 

 I and III are for a 0-0 vibrational transition which is classed as forbidden, 

 their intensities will depend very much more on any loss of symmetry in the 

 poi-phyrin molecule than will bands II and IV which are interpreted as 0-1 

 vibrational bands (Piatt, 1956). This symmetry, which is to be thought of in 

 terms of possible pathways for the mobile electrons, will not be much affected 

 by substituents such as alkyl groups, or carboxyl groups which are insulated 

 by at least two CHg's as in propionic acid side-chains. Much greater distor- 

 tion would be expected from substitution of a peripheral hydrogen by a 

 group such as — CHO, — COCH3, — COOCH3 and COCgHj, and it is among 

 such porphyrins that 'oxorhodo' (III > II > IV > I) (Lemberg and Falk, 

 1951) and 'rhodo' (III > IV > II > I) type spectra are found rather than 

 the 'actio' (IV > III > II > I) type which is the usual one with alkyl and 

 similar substituents (Stern and Wenderlein; for references see Lemberg and 

 Falk, 1951). Extension of tliis generalization to porphyrins containing more 

 of the symmetry-disturbing groups is difficult because of the necessity to 

 allow for the vector directions of the moments of the substituents, but if this 

 is done there is a reasonable correlation between observed and predicted 

 spectra (Piatt, 1956). 



The a- (long wavelengths) and /S-bands of metal-porphyrin complexes 

 seem to be related to bands I and III, II and IV, respectively. Thus bands 

 II, IV and /5 are little affected by substituents, while bands I, III and a vary 

 considerably. It has also been shown that the intensity of the a-band in 

 some copper-porphyrin complexes varies with the intensity of band III of 

 the corresponding porphyrins (Williams, 1956). 



The Soret band is attributed to the transition to an ^^-type orbital of an 

 electron in an y4i„-orbital (Fig. 1) in which it was confined to the carbon atoms 



H.E. — VOL. I — F 



