THE EVIDENCE AGAINST RESONANCE 83 



porphyrin cations and metal complex salts on the other, Erciman 

 and Corwin {697a) have recently concluded that there is no hydrogen 

 bonding in porphyrins. For the reasons adduced in Sections 4.3. 

 and 6.4., their evidence cannot be considered as conclusive. 



6.3. The Evidence against Resonance 



A great number of porphyrin syntheses have failed to indicate the existence 

 of nuclear isomerism (as distinct from isomerism caused by different order 

 of side chains relative to themselves). There are, however, some claims for the 

 existence of such isomerism. Conant and Bailey (473) found an isorhodopor- 

 phyrin which was further studied by Dietz and Werner (591). It differed 

 from rhodoporphyrin in absorption spectrum and HCl number. Conant 

 had suggested a prototropic isomerism (assuming in one porphyrin attach- 

 ment of the hydrogen atoms to adjacent, in the other, to opposite, pyrrole 

 rings). Since rhodoporphyrin is a highly unsymmetrically substituted por- 

 phyrin {cf. Table III), such a difference may conceivably exist, even with 

 hydrogen-bonded nitrogen (Fig. '20) {1361). Dietz and Werner showed. 



Fig. 20. Prototropic isomerism. 



however, that the zinc, magnesium, and iron complex salts were also different 

 and reconvertible into the isomeric porphyrins. Moreover, they observed 

 that on catalytic hydrogenation isorhodoporphyrin took up one more mole- 

 cule of hydrogen than rhodoporphyrin. It appears very likely that the "iso- 

 rhodoporphyrin" is identical with the "pseudoverdoporphyrin" of Fischer, 

 which was shown {cf. 861, p. 537) to have a vinyl group instead of one ethyl 

 group of rhodoporphyrin. The conversion of isorhodoporphyrin into rhodo- 

 porphyrin by 50% sulfuric acid, claimed by Dietz and Werner, should be 

 reinvestigated, since it is possible that not rhodoporphyrin but a spectro- 

 scopically similar porphyrin with a hydroxyethyl group may have resulted 

 from a conversion of the vinyl into the hydroxyethyl group. 



A nuclear isomerism would appear to be still less likely for porphin and 

 porphins substituted symmetrically on all four methene groups between the 

 pyrrole nuclei. The existence of such an isomerism has been claimed by 

 Rothemund {2352,2353) and Knorr and Albers {1555,1556). Rothemund 

 obtained an "isoporphin" when pyrrole and formaldehyde were condensed 

 at 150° C. inistead of at 90° C. as in the synthesis of porphin. Tetraphe- 



