82 III. PORPHYRIN CHEMISTRY 



hydrogen bond. This deviation from tetragonal symmetryis decreased 

 by complex salt formation. 



These results can only be understood by assuming a continuous 

 resonance system. Even a random distribution of dijfferent molecules 

 in the crystals could not explain the degree of symmetry found. The 

 resonance also explains why the three valencies of the pyrrole nitrogen 

 atoms lie all in the same plane, which is of great importance for the 

 formation of complex salts. 



While the resonance system of porphyrins differs from that of 

 phthalocyanin on account of the replacement of the isoindole rings 

 by unsymmetrically substituted pyrrole rings, the sixteen-membered 

 internal ring structure is essentially the same and there is no reason 

 to doubt resonance in porphyrins. There is, therefore, no difference 

 between the four pyrrole rings. The Kiister-Fischer formula repre- 

 sents, like the phthalocyanin formula of Figure 19, only one of 

 several possible "resonating" structures. Complete x-ray studies of 

 porphyrin crystals have so far not yet been carried out. Octamethyl- 

 porphyrin, which would be expected to be fully planar, is not avail- 

 able in good crystals. O'Daniel and Damaschke {2063) have sub- 

 jected the crystals of one modification of tetramethylhematopor- 

 phyrin to x-ray analysis. Although this porphyrin contains side 

 chains which cannot be expected to lie in the plane of the porphin 

 ring, nothing has been found to contradict an essentially planar 

 structure of the molecule. There can be little doubt that the porphin 

 ring and the first eight atoms of the substituting side chains of por- 

 phyrins lie in one plane and that this planar part of the molecule is at 

 least 10 A in diameter (c/. 1125a). An attempt to prove the existence 

 of a hydrogen bond between the pyrrole nitrogen atoms of porphyrins 

 by infrared spectrophotometry {281 li) has not given conclusive 

 evidence. 



An iV-methylporphyrin has been prepared by EUingson and Corwin 

 (6'66) and by McEwen (1809). In this substance the methyl group 

 is too large to be accommodated in the plane of the molecule between 

 the four pyrrole nitrogens, and the N-CH3 bond must be distorted 

 out of this plane. It is of interest that the .V-methylporphyrin does 

 not form a normal zinc complex salt, but that the zinc compound 

 contains chlorine. The ZnCl group bound to one pyrrole nitrogen 

 probably lies on the side of the porphyrin plate opposite to the 

 methyl group. From the similarity of the absorption spectra of 

 neutral porphyrins and iV-methylporphyrins on the one hand, of 



