Five- and six-member bowl-shaped structures from acylphloroglucinols: an ab initio and DFT study.

Molecular structures containing bowl-shaped cavities are interesting for purposes such as hosting molecules or metal ions. Acylphloroglucinols are derivatives of phloroglucinol (1,3,5-trihydroxybenzene) containing a CRO group. A previous study had considered bowl-shaped structures consisting of 3 or 4 identical acylphloroglucinol units linked by methylene bridges, selecting representative R chains and including also a structure with phloroglucinol units. The presence of three 'binding' levels between neighbouring units (consecutive hydrogen bonds in the lower rim, the methylene bridges at intermediate level, and another set of hydrogen bonds in the upper rim) makes these bowls 'deeper' than bowls from other hydroxybenzenes. The current study considers larger bowls, consisting of 5 and 6 identical units, to investigate how the molecular properties change with the increase in the size of the bowl. The monomeric units are the same as in the previous study, and the levels of theory are the same as in the previous study, to enable meaningful comparisons. Like in the previous study, two conformers are considered for each bowl, differing by the orientation of the OH groups in the lower rim. Calculations were performed at the HF/6-31G(d,p) and DFT/B3LYP/6-31+G(d,p) levels with fully relaxed geometry, complemented by single-point MP2/HF/6-31G(d,p) calculations. The results show that the C_nv symmetry (with n being the number of constituting monomers) is maintained for 5-member bowls, while 6-member bowls do not show C_6v symmetry but only C_2v symmetry. The molecular properties of the calculated bowls are analysed in detail and also compared with the properties of the previously calculated 4- and 5-member bowls. Graphical abstract Bowls built from acylphloroglucinol units have three levels of intermonomer linkages: hydrogen bonds in the bottom rim, methylene bridges at intermediate level, and other hydrogen bonds in the upper rim. They are the deepest bowls that can be built from hydroxybenzene units.