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The receptor is a triazolophane with a relatively rigid central cavity. Its inward-facing triazole and phenylene C–H groups act cooperatively as hydrogen bond donors to an anion. The supplied preference is $\mathrm{C}{\mathrm{l}}^{-}>\mathrm{B}{\mathrm{r}}^{-}\gg {\mathrm{F}}^{-}>{\mathrm{I}}^{-}$, so it is not a simple ranking by charge density or basicity.

Chloride has the best fit for several favourable C–H···anion contacts simultaneously; bromide is close enough in size to retain strong binding. Fluoride is too small to engage the whole donor array optimally, and its desolvation penalty can further oppose binding in solution. Iodide is too large for the cavity and cannot sit in the same favourable geometry. Molecular preorganization reduces the conformational penalty for binding a matched guest, but makes the cavity less adaptable to mismatched ones.

The exact solvent is not specified in this question, so a quantitative hydration argument would be unwarranted. The structure and observed nonmonotonic order support size complementarity plus cooperative C–H hydrogen bonding, with solvation modifying the measured constants. The original receptor study experimentally demonstrated halide size selectivity and tunability through the phenylene substituents.^[ref]

A macrocycle containing triazole and other linking units. Appropriate inward-directed C–H donors can bind anions cooperatively; cavity size and molecular preorganization can produce strong halide selectivity.