INVESTIGATION OF THE RELATIONSHIP BETWEEN DIPOLAR PROPERTIES AND CIS-TRANS CONFIGURATION IN RETINAL POLYENES - A COMPARATIVE-STUDY USING STARK SPECTROSCOPY AND SEMIEMPIRICAL CALCULATIONS

Three biologically active conformers of retinal, all-trans, 9- and 13- cis, and their Sckiff bases (SB), are studied using a combination of e lectroabsorption (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 (\Delta mu\ greater than 8 D) and a large change in polarizability between the ground and excit ed states (<(Delta alpha)over bar> greater than 300 Angstrom(3)). The experimental \Delta mu\ values in the aldehydes are well predicted by semiempirical calculations. However, the calculated Delta mu values fo r 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 prob ed by our calculational method. For both the aldehydes and SB, Delta a lpha(calc) values are 2-8 times lower than experimental values. Possib le reasons for the deviation between theory and experiment are discuss ed.