Ab Initio Study of the Complexation Behavior of Calix[6](aza)cryptand with Alkyl Ammonium Cations

The structures and complexation energies of p-tert-butylcalix[6](aza)cryptand 1 with a series of alkylammonium cations have been calculated by using ab initio HF/6-31G method. After geometry optimizations, B3LYP/6-31G(d) single point calculations of the final structures are carried out including the effect of an electron correlation and the basis set with a polarization function. The calculated complexation efficiencies of 1 for alkylammonium guests are better than those of the previously reported calix[5]arenes, and much better than those of the calix[4]crown-6-ether. The calculation results show that calix[6](aza)cryptand also has much better complexation ability with smaller ammonium cations than with bulky alkylammonium guests. The structural characterizations of the calculated complexes are described as the function of the nature of the alkyl substituents of the ammonium guests.