Competitive Hydrogen Bonds and Conformational Equilibria in Polysubstituted 3-Formyl-2-hydroxy-benzoyl Compounds

Abstract

Stability sequences of rotational isomers were investigated for a series of polysubstituted 3-formyl-2-hydroxy-benzoyl compounds (acylresorcinols and acylphloroglucinols) at the B3LYP/6-31G(d,p) level of theory. Comparison with IR spectroscopically determined data revealed excellent agreement between theoretical prediction and experiment. The theoretical stability sequences were obtained by full geometry optimizations and, additionally, by an isodesmic reaction approach. In the latter case, the total interaction energy between the various substituents is determined from the sum of single interaction energies between each two adjacent substituents, which are calculated from corresponding interaction-forming reactions. It is shown that the isodesmic reaction approach provides a quick and easy, but nevertheless reliable means for estimating relative stabilization energies from a simple increment system. It also provides a valuable tool for a discussion of the importance of the single contributions. Expectedly, the number and kind of hydrogen bonds is clearly the first and most decisive factor that governs the conformational stabilities.