Computational protocols capable of modelling supramolecular complexes have been evaluated. The complexation of cations by crown ethers and quaternary ammonium ions by an oxacalix[3]arene are presented as examples. In the latter case reliable qualitative results were obtained using the semi-empirical PM3 method where guest LUMO and electrostatic potential energies have been shown to correlate with experimental binding data. The optimal method for more accurate results combines semi-empirical equilibrium geometry and property calculations with single point energy calculations at the HF/6–31G* or BP/6–31G* quantum mechanical level.
Oxacalix[3]arenes exhibit transient binding to Na+, K+ and Ca2+, with a strong preference for Na+, as determined by a quartz crystal resonant sensor technique. Computational models for the behaviour indicate that the preference for Na+ is due to the depth to which that cation is drawn into the macrocycle's central cavity.
Abstract Computational techniques can be used to simulate molecular and atomic behavior based on fundamental descriptions of atomic and molecular orbitals ( ab initio quantum mechanics), experimental data ( a priori molecular mechanics), or a combination of both (semiempirical methods). The choice of method depends on the task in hand and the computational resources available. Low‐level molecular mechanics, molecular dynamics, conformational analysis, and periodic boundary simulations are described along with quantum mechanical approaches (Hartree‐Fock, density functional theory, and other post‐Hartree‐Fock methods) and intermediate semiempirical methods. Keys to the success of any computational method are the adoption of an appropriate model and the methods used to generate that model. By constructing models that reflect both the correct structures of the complex and its components together with changes in experimental parameters, supramolecular phenomena, such as host–guest binding, can be studied in detail using theoretical models. Applications of these methods in the field of supramolecular chemistry are illustrated with examples drawn from crown ether, cyclodextrin, and calixarene complexes.
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