LIGHT ABSORPTION 75 



The spectrophotometer (Figs. 13 and 14), however, reveals a fourth very 

 weak band in the blue-green and a high Soret band. The whole spectrum 

 in the visible is compressed; band 1 and particularly band 3 are increased, 

 but band 2 and 4 are greatly diminished in intensity. This interpretation 



TABLE VII 



Absorption Spectra" of Porphyrins in 0.1 iV Potassium Hydroxide 



" According to Schumm {2506) . 



differs from that of A. Stern, who assumed a correlation between the weak 

 absorption band la, lying between bands 1 and i of the neutral spectrum 

 and the first absorption band of similar position of the porphyrin spectra 

 in hydrochloric acid. It is, however, in agreement with the observations of 

 Pruckner {2188), who found that increase of the symmetry of the molecule 

 increased the absorption bands 1 and 3. The transformation of a porphyrin 

 into a cation or metal complex increases the symmetry of the molecule 

 (c/. Section 6). 



The absorption spectra of the porphyrins are described in detail 

 in many publications (36,130,861,1085,1313,2053,2189,3506,36^- 

 26Ii.3), the ultraviolet absorption is described particularly in some 

 papers (310,954,1183). The main results are summarized in Tables 

 VI, VII, and VIII. At the temperature of liquid air, when vibra- 

 tional energy changes are diminished, the bands become still sharper 

 and are partly split up and shifted toward shorter waves. It can be 

 seen from these tables that the replacement of hydrogen by saturated 

 side chains in porphin or deuteroporphyrin causes a slight shift of the 

 absorption curves toward longer wavelengths, while the introduction 

 of unsaturated vinyl side chains (protoporphyrin) causes a much 

 larger shift of more than 10 m^t. There is a difference of 9 to 10 m^ 

 in the position of the absorption bands of porphyrins with saturated 

 and unsaturated side chains, which is of considerable importance for 

 the study of porphyrin compounds, since a great variety of pyrrole 

 compounds show the same influence of unsaturated side chains on 

 the position of absorption bands. The absorption spectra of the iso- 

 meride types I and III are identical, nor can coproporphyrin be dis- 

 tinguished spectroscopically from mesoporphyrin. The introduction 



