SPECTRUM AND STRUCTURE OF PORPHIN DERIVATIVES 



625 



to the proljlem of acid-base color (;hanges (concerning the latter, see 



Epstein, Kariish and Rabinowitch 1941, and Lewis and Bigeleisen 1943). 



According to Pruckner (1942), imidoporphyrins (which differ from por- 



phin derivatives by the substitution of an XH gi-oiip for a bridge carbon) 



0.800 



0.700 



0.600 



0.500 



3 



0.400 



0.300 



0.200 



0.100 



500 



550 



650 



700 



600 

 WAVE LENGTH, m/x 



Fig. 21.14. Effect of increased acidity on spectrum of a tetraphenylporphin (after 

 Aronoff and Calvin 1943). 10 ml. alcohol, containing 5 ml. 3 X 10~' ^^ solution of free 

 base and variable amounts of hydrochloric acid. (1) no acid, (2) 0.0204 A'' HCl, 0.5 ml., 

 (5) same, 1 ml, (,/,) same, 2 ml, (5) same, 3.50 ml, (6) 6.3 A HCl, 5 ml. 



also have a strong absorption band in the red. She suggested tliat the 

 appearance of this band is generally associated with increased molecular 

 symmetry; but it is not quite clear why hydrogenation of one or two 

 pyrrole nuclei, or substitution of NIT groups for (' atoms, should lead to 

 higher symmetry. 



