QUINONIC TAUTOMER PRESENT IN A CRYSTAL OF N-(2,3-DIHYDROXYBENZYLIDENE)ISOPROPYLAMINE - STRUCTURAL AND VIBRATIONAL DATA AND AM1 CALCULATIONS

The experimental structural data of the crystalline title compound are compared with geometrical parameters calculated for the phenolic (P) and quinonic (Q) tautomers; evidences are provided for an equilibrium largely shifted toward the Q tautomer. This conclusion is also support ed by the IR spectral analysis. Involved in intra-and in intermolecula r hydrogen bonds, the OH group located in position 3 appears as a prob e of the tautomeric equilibrium. The corresponding stretching IR absor ption band presents several components; the more intense one is assign ed to QQ dimers and others to PQ and PP dimers which are also present in the crystal. Two other components of that broad IR absorption band belong to QQ and PQ N-H stretching vibrations. To account for qualitat ive informations about the tautomeric equilibrium (P dominates, but Q is present in CCl4 solution) and about the salicylidenamine derivative s (Q is a minor component in crystal and is absent in CCl4 solution), semiempirical AM1 calculations were performed on the methylamine deriv atives; the H...O hydrogen bond involved in the five-membered pseudocy cle appears stronger in Q tautomers. Intermolecular hydrogen bonds con tribute to increase the Q tautomer stability as proved by its dominanc e in H2O solution.