Theoretical investigation of the conformations of p-t-butylcalix(4)arene and its toluene complex were carried out. The hydrogen bonding potential function suitable for the cyclic hydrogen bonds of calix(4)arene was obtained by least-squares fitting to the ab initio MO calculation results. It was found that the calix(4)arene has an enough flexibility for the complexation with molecules a little larger than the pore composed of the t-butyl groups; the complex formation does not need any activation energy. During the complex formation, the conformational changes of the calix(4)arene are small. Contribution of the dispersion interaction to the complexation energy is predominant. The cyclic hydrogen bond may not induce the allosteric effect at the binding site.
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