Strongly underestimated dispersion energy in cryptophanes and their complexes

Abstract

Cryptophanes, composed of two bowl-shaped cyclotriveratrylene subunits linked by three aliphatic linker groups, are prototypal organic host molecules which bind reversibly neutral small guest compounds via London forces. The binding constants for these complexes are usually measured in tetrachloroethane and are in the range of 10(2)-10(3) M(-1). Here we show that tetrachloroethane is--in contrast to the scientific consensus--enclosed by the cryptophane-E cavity. By means of NMR spectroscopy we show that the binding constant for CHCl3@cryptophane-E is in larger solvents two orders of magnitudes higher than the one measured before. Ab initio calculations reveal that attractive dispersion energy is responsible for high binding constants and for the formation of imploded cryptophanes which seem to be more stable than cryptophanes with empty cavities.