Band structure of even and ions of odd polyenes

The analysis of experimental data for singlet transitions (E_n) of even polyenes (I), cations (II) and anions (III) of odd polyenes show that for infinite chains E(I)/E(II)=E(I)/E(III) = 2:1. It is shown that the energy gap is equal for the three systems. In cases (II) and (III) there is a level (NBMO) in the gap which is vacant in (II) and occupied in (III). That is why the first optical transition in (II) and (III) depends on the semiwidth of the gap.