Geometric, electronic and NMR properties of hemicucurbit[n]urils and their anionic complexes

Substantial DFT calculations are presented for the geometric, electronic and NMR properties of hemicucurbit[n]urils and their anionic complexes. Hemicucurbit[n]urils prefer an alternate conformation with antiparallel imidazolidin-2-one units. The inner hydrogens of these units form a positively charged cavity which is able to accommodate anions like halide ions. The imidazolidin-2-one units act as flexible response elements attached to rigid methylene bridges in the complex formation process to adapt to the needs of a ligand. All molecular species studied do not show VIS but only UV absorption. Anionic guests lead to a characteristic splitting-up pattern of the proton NMR shifts of the coordinating and non-coordinating protons of the host that can be used for experimental detection. Hemicucurbit[n]urils with an odd number of imidazolidin-2-one units can easily be detected by characteristic proton NMR absorption. Larger hemicucurbit[n]urils tend to twisted conformations up to the twisted hemicucurbit[12]uril that can be regarded as a twofold hemicucurbit[6]uril. Whereas dispersion effects are negligible for the smaller hemicucurbit[n]urils and their complexes they do play a significant role for the clearly twisted geometry of hemicucurbit[12]uril.