EXCITATION OF POLYENES AND PORPHYRINS 



89 



NH2, SH. The effects of the last two are so large that they are almost 

 equivalent to the addition of two more x-electrons to the conjugated sys- 

 tem, i.e., to a vinyl substitution; and frequencies and intensities may be 

 estimated from those of the vinylog of the original hydrocarbon, as seen 

 in Fig. 2-10. 



STATES 



SINGLE COUPLED 



MOLECULAR ORBITALS 



,w^ 



/> (. 



/Wl,v 



.^ — 



o 



Uj 



■ I- 

 cc <. 

 UJ o 

 0-3 

 >- -5 

 X2 

 O 

 U 



SINGLE COUPLED 



/o-'s 



ANTISYMMETRICAL 

 TO PLANE 



(T S SYMMETRICAL 

 TO PLANE 



INCREASING COUPLING >- INCREASING LENGTH — >- 



Fig. 2-11. Behavior of hydrocarbon orbitals and states with substitution by saturated 

 auxochromes, and with replacement by conjugated hetero atoms. 



Any substituent, even another conjugated system, which is separated 

 from the original conjugated system by a methylene group, or especially 

 by a longer alkyl chain, is insulated from the original system and has 

 scarcely more effect on its spectrum than if it were in another molecule. 

 The absorptions of two conjugated wvstems which are in the same mole- 

 cule but which are separated in this way are simply additive. 



Auxochromes add no additional bands to the long-wave-length spectra 

 of a conjugated system; at least, none has ever been identified. 



