Bowl-shaped structures from acylphloroglucinols: an ab initio and DFT study

Molecular structures containing bowl-shaped cavities are interesting for purposes such as hosting metal ions or a variety of molecules, including drug molecules. This study considers bowl-shaped structures formed from phloroglucinol (1,3,5-trihydroxybenzene) or acylphloroglucinols (its derivatives containing a COR group) and containing 3 or 4 identical monomeric units. The monomeric units are linked in the same way as in naturally occurring trimeric and tetrameric acylphloroglucinols, i.e. by a methylene bridge and two intermonomer hydrogen bonds. The representativeness of the structures considered is pursued by selecting different R chains. Two conformers were identified for each structure, differing by the orientation of the OH in para to the COR group. Calculations were performed at the HF/6-31G(d,p) and DFT/B3LYP/6-31+G(d,p) levels with fully relaxed geometry, and were complemented by single point MP2/HF/6-31G(d,p) calculations. Corresponding open structures were also calculated for comparison sake. The results indicate the presence of steric stress in trimeric bowl-shaped structures, whereas the geometry of tetrameric bowl-shaped structures is close to one of the local-minima geometries of corresponding open structures and appears free of steric stress.