Systems with Charged Perimeters

The spectra of (4N-i-2)-electron charged perimeter systems are appreciably different from uncharged perimeter n systems, irrespective of whether the perimeter is even or odd—that is, of whether or not the ion is derived 

so only two transitions are to be expected. The difference between the benzene spectrum and the spectra of the ions is therefore due to different symmetry of the excited-state configurations. This was first pointed out by Heilbronner (1966) in the case of the tropylium ion. Symmetry also affects the spectra of derivatives of charged perimeters with 4A( + 2 electrons. (Cf. Section 2.2.5.) 

Similar arguments apply to perimeters with AN n electrons, but the situation is more complicated and a different classification of excited states applies. (Cf. Section 2.2.7.) 

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The fact that due to symmetry there is no mixing between the L states in charged perimeters with 4N-I-2 electrons also has consequences for systems derived from these charged perimeters. Even after introducing the perturbation the polarization directions of the L and Lj transitions as well as those of the B, and B2 transitions are mutually perpendicular, and the splitting of the L bands is only small. Important examples are indole derivatives... 

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