EXCITATION OF POLYENES AND PORPHYRINS 105 



rings in a slightly different compound. The vector directions will then 

 shift, as in going from Fig. 2-206 to c, and neighboring vectors that can- 

 celed in the one compound may add in the other, as at positions 2 and a 

 in the diagrams. But even this added complication can be resolved if 

 we stabilize the H-H direction by two very strong opposite substituents 

 and then examine the effect of weaker substituents at all other positions. 



In view of the theoretical and experimental limitations of provisos (1), 

 (2), and (3), this program is overambitious. Nevertheless, by examining 

 the data on the first 84 compounds in the comprehensive studies of Stern 

 and Wenderlein (1934, 1935, 1936; Stern, Wenderlein, and Molvig, 1936), 

 as summarized in Fig. 2-22, and by making some simplifying assump- 

 tions, a start can be made. 



These assumptions are as follows: All alkyls, cycloalkyls, and alkyls 

 bearing an insulated auxochrome on the outer end are treated as equal. 

 Acids and esters are treated as equal. It proves possible to treat hydro- 

 gen addition to the external rings, with formation of the di- and tetra- 

 hydroporphins, also as a perturbation like substitution. The effects on 

 intensity are like those found with strong positive (o,p-directing) sub- 

 stituents at each of the hydrogen positions. 



The central metal atom in complexes can be treated like a substituent 

 on the central nitrogen atoms. 



Polarizations of Bands I and III. The first four methyl groups on 

 opposite rings (as 1,2,5,6 substitution) enhance band III strongly but 

 not band I (Fig. 2-22, compound 2). The next four (compound 7) do not 

 enhance band III further but enhance band I to about the same intensity. 

 This proves that the 1,2,5,6 positions are not equivalent to 3,4,7,8 in 

 either band and that band I is polarized perpendicular to band III. 

 [After this chapter had been prepared, Stupp and Kuhn (1952) gave an 

 experimental proof that the corresponding bands in chlorophyll, which 

 we discuss later, are also polarized perpendicular to each other.] There- 

 fore the essential nodal lines of the wave functions in either band cannot 

 pass between the central nitrogen atoms (this would make all four rings 

 equivalent) ; they must pass through the central nitrogen atoms. This 

 confirms Erdman and Corwin's conclusion that the central hydrogen 

 atoms are not on adjacent nitrogen atoms and are not on opposed 

 N — H — N bridges. It is even possible from this argument and from 

 the intensity data to set an upper limit on the relative abundance of 

 that tautomer which has adjacent hydrogen atoms. Its abundance must 

 be less than 6 per cent under the conditions of Stern and Wenderlein's 

 measurements. 



The fact that the total increase of intensity in band I under octaalkyl 

 substitution is almost identical with that in band III shows that the 

 moments of substituents are essentially the same for both transitions. 

 This seems theoretically reasonable. 



