Energetics of the inclusion of organic molecules by rigidified cone calix[4]arenes in carbon tetrachloride

The complexation of acetonitrile and nitromethane by the calix[4]biscrowns 1, 2, 3 and the upper rim pyridine brigded calix[4]arene 4, all fixed in a C-4 nu, structure, was studied by H-1 NMR and calorimetry in CCl4. The spec troscopic data show that the inclusion of the two organic guests occurs via their - CH3 moiety in all cases. For all the investigated hosts the bindin g constants with both guests determined by both H-1 NMR and calorimetric ti trations, show that the inclusion is favoured by the presence of functional izing groups at the upper rim. Furthermore log K values indicate that no st ability difference was found between the two guests. Calorimetric data show that the inclusion is enthalpically driven in all cases. The favourable en thalpic contribution is to be attributed mainly to the CH - pi interactions between the pi - donor aromatic moieties of the hosts and the activated me thyl residue of the guests. The combination of 1H NMR and calorimetric info rmation allows for a more detailed description of the inclusion process.