EXCITATION OF POLYENES AND PORPHYRINS 



93 



Kuhn found a number of other symmetrical ions with a reversed 

 arrangement, the /-level being even and the g-leve\ odd. In these the 

 shift on central aza substitution was consequently in the opposite direc- 

 tion, a blue shift, and of about the same amount, 1000 A, just as expected. 



For more complicated molecules with a central atom, the oddness or 

 evenness of the /-level may usually be found easily by Longuet-Higgins' 



IV, 



lA^H 



EFFECT OF REPLACING 

 CENTRAL CH BY N 



^V^ 



'^^^^W-^w^v^ 



I'za, 



/ 

 / 

 / 

 / 



I 



I 



6100 A 

 CH 



7100 A 

 N 

 — O 



-^^s<^-^^^v<^^^^y^^^^ 



^^'^^v^^^'^^^X^^-^^^^^ . 



-&e — e 



-©« 



— e* — e 



— e« — o 

 — ©e — e 



VINYL POLYENE 



TYPE TYPE 



ORBITALS ORBITALS 



Xi^i^' 



^1^77??^ . 



-^^-^^^^^^^"^^ 



-^<<^::'^^:^^^^^-^. 





^^^^^^ 



f 



.C — c. 



■c — c 



^c — c ^c — c-^ 



MICHLER'S HYDROL BLUE 



/^-'\ C-C 



N — c c — N— c:^ j;c— N 



BINDSCHEDELER'S GREEN 



Fig. 2-13. Orbitals and transition-s of symmetric phenylpolyonc ions, and effect of 

 aza substitution {Adapted from Kuhn, 1950.) 



very simple method (LCAO) for "zero-energy orbitals" (1950b), pro- 

 vided that no odd-membered ring systems are present. Dewar (1950) 

 has applied this method to a prediction of substitution effects on spectra 

 of odd-atom dye systems for any position of substitution. 



The Coulson- Rushbrooke Theorem. The shifts of the 7r-transitions with 

 aza substitution in most of the stable neutral even-ring hydrocarbons are 

 much smaller than in the ions discussed by Kuhn. This fact has its 

 explanation in the Coulson-Rushbrooke LCAO theorem (1940), which 

 states that any hydrocarbon that has no odd-membered rings and no 

 zero-energy orbitals must have the electron density the same in the 



