A Computational Study of the Effects of Different Solvents on the Characteristics of the Intramolecular Hydrogen Bond in Acylphloroglucinols

Acylphloroglucinols are a broad class of compounds, derivatives from 1,3,5-trihydroxybenzene, and exhibiting a variety of biological activities. They are characterized by the presence of at least one COR group, whose sp² O can form an intramolecular hydrogen bond with a neighboring phenolic OH. This H-bond plays dominant roles in determining conformational preferences and energy, and is expected to play significant roles in biological activity mechanisms, which strongly motivates the study of its characteristics in solution. A computational study of a representative number of actual and model structures with different R was carried out in three solvents with different polarities and different types of interactions with solute molecules: water, acetonitrile, and chloroform, utilizing the PCM model. Calculations were mostly performed at the HF/6-31G(d,p) level because of affordability reasons in view of the size and number of the structures considered (the smallest structures were also calculated at MP2/6-31+G(d,p) level). Comparison with the results of a previous study in vacuo shows similar patterns within each medium, pointing to similarities in the influence of relevant geometry factors on the characteristics of the H-bond. The medium appears to have little influence on the parameters of the H-bond. Comparison across media of the energy increase on H-bond removal (an indication of the H-bond strength) is complicated by the greater solvent stabilization of the conformer resulting from H-bond removal, with respect to the one in which the H-bond is present. Several factors, however, would point to a strength not too different from that observed in vacuo.