Hydrogen bonding and conformational equilibrium in p-tert-butyldihomooxacalix[4] arene

Here we report theoretical calculations on the structure and conformational equilibrium of p-tert-butyldihomooxacalix[4]-arene which belongs to a family of model systems in host-guest chemistry. The calculations were based on AM1 geometry optimisations followed by ab initio and density functional single point calculations. The most stable conformer is a cone conformer stabilised by the formation of a cyclic structure of four intramolecular hydrogen bonds. The second most stable structure is a distorted cone related to the first by a rotation of one phenol ring that favors hydrogen bonding involving the oxygen atom on the main ring. Our best prediction (BLYP/6-31G*) for the energy barrier between the two most stable conformers is 7.7 kcal/mol. The calculated molecular structure for the cone conformer is in good agreement with recent data from X-ray measurements. (C) 1998 Elsevier Science B.V. All rights reserved.