Conformational equilibrium and intramolecular hydrogen bond of 4′X and 4X substituted 2′(OH)chalcones

By using the AM1 method on 4X and 4'X substituted 2'(OH)chalcones (X = H, OH, F, Cl, CH3, CH3O, N(CH3)(2), and NO2), certain currently unclear structural characteristics, conformational equilibria, intramolecular hydrogen bonds and UV spectroscopic properties had been clarified. The compounds studied have non-planar structures. The 2'(OH),4'Xchalcones only present the trans-s-cis conformation. trans-s-cis and trans-s-trans conformers of 2'(OH),4X-chalcones have a comparable thermodynamic stability. For these chalcones, a conformational equilibrium at 298 K exists, with 91% of the trans-s-cis form. The proposal of analyzing the intramolecular hydrogen bond of 2'(OH),4X-chalcones as an intramolecular proton-transfer reaction, is acceptable because the results and theoretical prediction of this study agree well with available UV spectroscopic experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.