The Cyclobutanocucurbit[5–8]uril Family: Electronegative Cavities in Contrast to Classical Cucurbituril while the Electropositive Outer Surface Acts as a Crystal Packing Driver

The structural parameters for the cyclobutanoQ[5–8] family were determined through single crystal X-ray diffraction. It was found that the electropositive cyclobutano methylene protons (CH₂) are important in forming interlinking crystal packing arrangements driven by the dipole–dipole interactions between these protons and the portal carbonyl O of a near neighbor. This type of interaction was observed across the whole family. Electrostatic potential maps also confirmed the electropositive nature of the cyclobutano CH₂ but, more importantly, it was established that the cavities are electronegative in contrast to classical Q[5–8], which are near neutral.