The Structure of Water in p-Sulfonatocalix[4]arene

Tetrasodium p-sulfonatocalix[4]arene exists as a hydrate with approximately 14 water molecules and has three polymorphic modifications, all of which contain a water molecule in the molecular cavity that is engaged in OH⋅⋅⋅π interactions. Single-crystal neutron structures are reported for two of these three forms and reveal a “compressed” water molecule with short OH bonds. Partial atomic charges and hardness analysis (PACHA) calculations based on the neutron coordinates give an OH⋅⋅⋅π interaction energy of 6.9–7.5 kJ mol−1. The PACHA analysis also reveals the dominance of the charge-assisted hydrogen bonds from the Na+-coordinated water molecules. The instability of the crystal towards dehydration can be traced to an uncoordinated lattice water site. The remarkable calixarene-Na+-hydrate motif is conserved almost unchanged across all three polymorphs. A single-crystal neutron structure is also reported for pentasodium p-sulfonatocalix[4]arene⋅12 H2O, which exhibits an intracavity water molecule that is engaged in both OH⋅⋅⋅π and OH⋅⋅⋅O hydrogen bonding. The shorter covalent bond to the hydrogen atom that forms the interaction with the aromatic ring is again apparent.