Previous calculations on the π-electron system in retinal are refined in a more rigorous way. Firstly, the σ-π interaction indispensable to derive the bond alternation is semiempirically determined from the first principle. Secondly, the effect of a sufficient number of singly- and doubly-excited configurations are examined in CI calculation. It is thus confirmed again that the wavelength of optical absorption maximum of retinal forming protonated Schiff-base binding is about 200 nm longer than that of retinal forming normal Schiff-base binding. The concept of partially-protonated Schiff-base binding of retinal, which is a phenomenological description of the previous theory for the interaction of retinal with opsin, is also formulated by introducing the degree ( x ) of protonation, and the x -dependences of the wavelengths and oscillator strengths of optical absorption are calculated.
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Suzuki et al. (1973) studied this question.
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