Further insight to selectivity issues in halide binding in a tiny octaazacryptand

The crystal structure of the hexahydrochloride salt of the tiny azacryptand, 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane, reveals a chloride ion inside the tiny bicyclic cavity; H-1 NMR studies indicate that while only fluoride binds strongly in the pH range 5.5-2.5, a dramatic increase in the affinity for chloride occurs at pH less than or similar to 2.5.