ANALYSIS OF THE EFFECT OF SOLVENT ON THE TAUTOMERIC EQUILIBRIUM IN NAPHTHALDEHYDE SCHIFF-BASES USING MULTIVARIATE-STATISTICS AND PATTERN-RECOGNITION

The UV-visible spectra of a model Schiff base formed from 2-hydroxy-1- naphthaldehyde and 4-methoxylaniline were recorded in 12 different sol vents, each characterised by a wide variation in physicochemical prope rties The spectra show three bands, one at < 400 nm assigned to the en ol tautomer and two at > 400 nm assigned to the keto form Pattern reco gnition methods and multivariate statistics have been used to relate t hese solvent properties to the propertions of keto and enol forms in t he various solvents. The results indicate that hydrogen bonding donati on character and polarisability of the solvent are the most important experimental properties affecting the tautomeric proportions. It is al so shown that the two bands at > 400 nm arise exclusively from the ket o form but that the absorption at < 400 nm, although principally from the enol form, must contain intensity from another source. Regression of the principal components of the experimental solvent parameters aga inst the UV extinction coefficients allowed the prediction of the UV i ntensities which were in good agreement with the observed values and c ould therefore be used to predict the UV spectrum in other solvents.