Theory of the Optical Absorption of Retinylidene Iminium Salt in Solution

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Abstract
We assume such a model for retinylidene iminium salt in solution that retinylidene iminium cation interacts not only with its counter anion X - but also with the local field E due to solvent molecules. On the basis of the model, the π-electronic states of the cation are calculated by our previous LCAO-ASMO-SCF-CI method, assuming that X - is a negative point-charge in the plane of the π-system and E is a reaction field acting on the salt dipole moment in a spherical cavity. It is thus shown that the observed wavelength λ max of absorption maximum of the salt in solution can be explained with a reasonable choice of both the distance R of X - from N + 16 and the radius a of the cavity. It is also shown that the λ max decreases with decreasing R while it steeply increases to reach a saturated value as the dielectric constant ε of solvent increases.

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