A UV-Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl 4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl 3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

Electronic transitions appear in the spectrum as individual bands, described by three basic parameters (position, intensity and width), which can be used for estimation of the fundamental transition characteristics as well as for quantitative analysis. The main problems concerning the mathematical resolution of overlapped individual bands in a complicated spectrum (estimation of the number of overlapping bands, the noise problem, artificial baseline and efficiency of the computing procedure) are discussed and a principal scheme and some solutions are suggested. The use of the resolution of overlapping bands technique for quantitative analysis is given step-by-step in its historical development on real spectral problems. Although the described methods for quantitative analysis are discussed only in terms of UV-Vis spectroscopy they can be easily used in all branches of molecular spectroscopy.

The enol imine a enaminone tautomerization constants, K T , and thermodynamic parameters, ∆H T and ∆S T , of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, K T ) 0.20 (cyclohexane) and K T ) 1.49 (ethanol)). Both donor (MeO, NMe 2 ) and acceptor (CN, NO 2 ) substituents lead to a decreased K T independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine a enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ∆G T to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ∆G T values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O‚‚‚N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a-1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2-4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7-9, and the N-alkyl derivative 10.