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Three biologically active conformers of retinal, all-trans, 9- and 13-cis, and their Schiff bases (SB), are studied using a combination of electroabsorption (Stark) spectroscopy and semiempirical calculations. All of the retinal isomers studied show both a large change in dipole moment between the ground and excited states (|Δ μ | greater than 8 D) and a large change in polarizability between the ground and excited states ( greater than 300 Å 3 ). The experimental |Δ μ | values in the aldehydes are well predicted by semiempirical calculations. However, the calculated Δμ values for the SB are more than 2 times smaller than experimental values. This discrepancy suggests mixing of the 1B u state with another state with a large dipole moment, most likely the 2A g state, which is not probed by our calculational method. For both the aldehydes and SB, values are 2−8 times lower than experimental values. Possible reasons for the deviation between theory and experiment are discussed.
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Locknar et al. (1998) studied this question.
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