EXCITATION OF POLYENES AND PORPHYRINS 101 



was settled by Erdman and Corwin (1946), who showed that replacing 

 one of these protons by a methyl group produced little change in the 

 spectrum. Such a N-methyl group would be expected to be rather tightly 

 bound to a single nitrogen and should not undergo very rapid shifts of 

 position to another nitrogen. Also, in the presence of the bulky methyl 

 group, the remaining proton would probably prefer the opposed position 

 for steric reasons. The adjacent configuration, the bridge configuration, 

 and the rapid-transformation hypothesis are therefore unnecessary in 

 accounting for the spectrum. It suffices to assume that the protons are 

 bound by normal covalent bonds to opposed nitrogen atoms. 



There are three ways of modifying porphin so that it can have strict 

 Dih symmetry: (1) we may remove the central hydrogens, as in the 

 disodium salt; (2) we may add two additional hydrogen atoms, as in 

 the dihydrochloride or in concentrated acid solution; or (3) we may 

 replace the central hydrogen atoms by a single central atom, as in the 

 copper or zinc complexes. Any of these changes produces a great simpli- 

 fication in the visible spectrum, as seen for the dihydrochloride in Fig. 

 2-15 (top), and, except for small wave-length shifts, all three changes 

 produce very similar absorption curves! 



Porphin itself in acid has an A-Q transition with only a single sharp 

 peak, and its A-B transition becomes extremely sharp. This must be 

 the spectrum which belongs to the square Dn, compound we treated 

 theoretically, with strict degeneracy. 



The Intevpretation of the Visible Bands. The observed single ^A-^Q 

 peak in acid can be interpreted as a 0-1 vibrational band, normally the 

 second peak in an absorption-band system. The first, or 0-0 band, is 

 completely absent, as it would be in a strictly forbidden transition, and it 

 appears only when the system is slightly perturbed, as by alkyl substi- 

 tution [etioporphyrin in acid (Fig. 2-15)]. The one observed ^A-^B peak 

 in acid is presumably the 0-0 band, for it does not acquire a longer- 

 wave-length companion with alkyl substitution. 



If this is the ^4^ spectrum, the complexities of the free-base porphin 

 spectrum are evidently due to the change to Dih, or rectangular, sym- 

 metry, with removal of the degenerac/, when two protons only are pres- 

 ent in the center. A change of symmetry from external substitution on 

 the ring gives less dramatic spectral changes than when the central sym- 

 metry is altered. 



The visible spectrum of neutral D^h porphin looks like a superposition 

 of two of the Dih spectra, shifted 800 A apart. This separation is what 

 would be expected when the degeneracy of the upper ^Q state is removed. 

 It therefore seems reasonable to assign the four visible bands I, II, III. 

 IV (numbered from the red end) as follows: 



Bands I and II: 0-0 and 0-1 vibrations of 'A 'Ql, 

 Bands III and IV: 0-0 and 0-1 vibrations of ^4-iQ°, 



