The structure of water in p-sulfonatocalixarene
Tetrasodium p-sulfonatocalixarene 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-sulfonatocalixarene·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. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.