We consider retinal and its related compounds in planar configuration provided they have no ionic structure, and we extend the ordinary INDO-CI method to calculate their bond lengths in consistence with their electronic states. One of the important features of our extended INDO-CI method is that each bond length is calculated as a function of bond order which is similar to the Coulson type of relation derived by the π-electron approximation. The extended INDO-CI calculations of the bond lengths, electronic structure and optical absorptions of the molecules indicate that the previous results, which were obtained by Suzuki et al. using the π-electron approximation, are reasonable.
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