Conformations of calix[5]arenes—a molecular mechanics study

Abstract

The conformational possibilities of p-methylcalix[5]arene 1 and its pentamethyl ether 2 have been assessed by means of molecular mechanics calculations using the TRIPOS and MM3 force fields. Starting from a comprehensive search of the energy hypersurface numerous low energy structures have been identified and analysed. The customary way of describing calixarene conformers by the ‘up’ and ‘down’ arrangement of the aromatic moieties relative to the reference plane through the methylene carbon atoms proved to be inappropriate to assign all computationally generated conformers. Instead, a simple algorithm for the classification of calix[5]arene conformers has been developed. The results of the molecular mechanics calculations are in reasonable agreement with the available experimental data in the case of MM3 but the TRIPOS force field fails to reproduce the stability of the cone conformation of 1. Molecular dynamics simulations indicate that the dynamic behaviour of 1 is characterised by the pseudorotation of the pentagon formed by the methylene carbon atoms on the one hand and by the well-known ring inversion process of calixarenes on the other.