Vibrational analysis of a retinal protonated Schiff base analog

A vibrational analysis of a retinal protonated Schiff base analog has been conducted by Raman measurements on this Schiff base and its isotopically labeled derivatives and by calculations using ab initio methods. The calculated Raman spectra of the model and its isotopically labeled derivatives are in good agreement with the experimental data. Based on comparisons between the calculated Raman spectra of C=N syn and C=N anti Schiff bases, a number of criteria to determine the Schiff base C=N configuration from Raman spectroscopy are developed. Two of these criteria have been previously proposed on the basis of empirical or semiempirical calculations. The agreement between experiment and calculations suggests that ab initio methods are reliable enough to interpret the vibrational spectra of retinal protonated Schiff bases in various configurations and environments.