Ab initio and density functional studies of cooperative hydrogen bonding in calix[4]-and calix[6]arenes

The cone conformation of C ₄ symmetry is shown by the Hartree-Fock method (3-21G basis) to be the predominant conformer of calix4]arene; the compressed cone of C ₂ symmetry is the major conformer of calix6]arene. Using quantum chemical methods we calculated hydrogen bond cleavage energies for calix4]-(ab initio and density functional methods) and calix6]arene (ab initio), and also for the complex of calix4]arene with carbon disulfide. These energies along with structural data point to the cooperative effect of hydrogen bonds. The results of these studies provided an explanation to the greater conformational lability of calix6]arene compared with calix4]arene molecules. It is also predicted that the nucleophilic substitution reaction involving calix6]arene in the presence of weak bases and in aprotic solvents, as well as in the gas phase, will occur via diastereomeric transition states.