COMPUTATIONAL STUDY OF THE STRUCTURAL, ENERGETICAL, AND ACID-BASE PROPERTIES OF CALIX[4]ARENES

In this study some of the properties of calix[4]arenes were assessed by various computational methods. The conformational properties of substituted calixarenes were studied by using molecular mechanics. The preferred conformation of a calix[4]arene depends on the number and the positions of the substituents on the oxygen atoms and is mainly determined by electrostatic interactions. Para substituents have only a minor influence on the relative energies of the conformations. In a number of cases the conformation with the lowest calculated energy is different from the conformation found in solution and in the solid state. The calculated geometries of the conformations found in the X-ray structures and in solution show good agreement with the corresponding X-ray structures. Calculations on calix[4]arene li using different force fields give quite different results, which are mainly caused by differences in the electrostatic energies calculated by these force fields. Some pathways for conformational transitions were studied by molecular dynamics, indicating that the partial cone conformation is a key intermediate. In contrast to previous suggestions, no indications for so-called flip-flop hydrogen bonding were found. The difference in acidity between calixarene Ih and its acyclic model 2a was predicted by free energy perturbation methods to be 9-11 pKtunits, in qualitative agreement with experimental data from similar systems. Finally, the energetical and hydrogen-bonding properties of the various calix[4]arene (poly)anions in the gas phase and in aqueous solution were studied by semiempirical molecular orbital and free energy perturbation methods. © 1990, American Chemical Society. All rights reserved.