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 isoto pically labeled derivatives and by calculations using ah initio method s. The calculated Raman spectra of the model and its isotopically labe led derivatives are in good agreement with the experimental data. Base d 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 bas e C=N configuration from Raman spectroscopy are developed. Two of thes e criteria have been previously proposed on the basis of empirical or semiempirical calculations. The agreement between experiment and calcu lations suggests that ah initio methods are reliable enough to interpr et the vibrational spectra of retinal protonated Schiff bases in vario us configurations and environments.