On the encapsulation of halide anions by bambus[6]uril

The relative affinity of the recently synthesized receptor bambus[6]uril (BU[6]) towards halides in encapsulation processes was theoretically studied by means of M06-2X and B3LYP calculations. The gas-phase results do not follow the affinity experimentally observed in solution, as the largest encapsulation energy was obtained for fluoride, and the smallest one for iodide. Nevertheless, this finding is in agreement with the gas phase hydrogen bond energies determined for the CH4⋯X– complexes, X = F–, Cl–, Br–, I–. The structural changes experienced by the receptor when the halide is inside decrease as we move from fluoride to iodide. Thus, the relative affinity is due to a balance between the hydrogen-bond energies between the methine groups inside the cavity and the anions (CH⋯X–) and the deformation of the receptor induced by the anion. When solvents effects are considered, the affinity of BU[6] for halides is in line with experimental evidence. In effect, the difference between the Δ G 298 ∘ computed for iodide and bromide is 1 kcal/mol, just 0.5 kcal/mol smaller than the experimental value. The effect of the counter ion was evaluated using Na+, but inclusion of the latter worsens the agreement with experiment. Finally, we discuss methodological problems observed for the BU[6]·X– complexes.