Abstract
Encapsulation of small (bio-)organic molecules within water cages is governed by a subtle equilibrium between water–water and water–solute interactions. The competition between the formation of exohedral and endohedral complexes is investigated. The first step prior to a theoretical characterization of interactions involved in such complexes lies in the judicious choice of a level of theory. The β-propiolactone (BPL), a solute for which the micro-hydration was recently characterized by means of high resolution microwave spectroscopy (Angew. Chem. 2015, 127, 993), was selected for the present study, and a calibration step is carried out. It is shown that the dispersion-corrected density functional theory (DFT-D) suitably reproduce the geometric, energetic and spectroscopic features of the BPL:(H2O)1–5 complexes. The experimentally deduced structures of the BPL:(H2O)4,5 species are fully understood in terms of the maximization of interactions between complementary sites in the MESPs. DFT-D calculations followed by the topological analysis within the Quantum Theory of Atoms in Molecules framework have shown that the solute could efficiently interact with (H2O)6,10 clusters in a similar manner that the (H2O)4,5 clusters do. The interaction of the solute with two larger water clusters is further investigated. The exohedral and endohedral BPL:(H2O)20 isomers are close in energy with each other, whereas the formation of an inclusion complex is energetically more favored than the facial interaction in the case of the BPL:(H2O)24 cluster. The topological analysis suggests that the substantial energetic stability is due to interactions between the solute and almost all oxygen atoms of the water cage.
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