Six resorcin[4]arene monomers, when dissolved in water-saturated chloroform, combine with eight water molecules to spontaneously form a supramolecular assembly held together by sixty hydrogen bonds. This assembly can host molecules in its ~1400 Å3 interior volume and represents an exciting approach to controlling chemistry. Indeed, it has previously been demonstrated that encapsulating a catalyst inside the resorcin[4]arene assembly significantly affects the reaction selectivity. Despite extensive experimental studies, there are many open questions about the molecular-level properties and thermodynamic driving forces that govern the host-guest dynamics of these assemblies. Key examples are: How does a guest molecule enter or leave the assembly? and How can the host-guest dynamics be modified to enhance the rates of intra-assembly reactions? Molecular dynamics simulations have been used to address these questions. The approach is to modify the assembly, by removal of water molecules, to probe the effect on the dynamics. Simulations were run at multiple temperatures to estimate the rate-constant of assembly dissociation and obtain the corresponding activation energy. For a resorcin[4]arene hexameric assembly in pure chloroform (no additional waters), the activation energy was found to be the same for an assembly with eight waters and one with no waters. In water-saturated chloroform, where waters in the assembly can be replaced by those in the solvent, the activation energy was also independent of the initial number of waters in the assembly. These surprising results suggest that the favorable enthalpy of the water hydrogen bonds in the assembly may not be as important as previously thought in preventing dissociation. Instead it indicates that entropic effects may be key or that reassembly dynamics may be more strongly affected by the presence of water.

Dynamics study of the resorcin[4]arene supramolecular assembly

249th National Meeting (2015)

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