Selectivity of calixtubes towards metal ions: A molecular dynamics study
Molecular dynamics simulations have been carried out on the calixtube, created from two calixarenes joined at their lower rims, with a series of different metal ions. The simulations show that for potassium, for which the tube is highly selective, the ion enters the tube at the upper rim of one calixarene and forms an intermediate complex interacting with the aromatic rings and then proceeds to the centre of the tube, concomitant with a conformational change in the calixtube, where it forms bonds to the 8 oxygen atoms. Simulations have been carried out with six other ions. For ions smaller than potassium, e.g. silver, sodium and lithium, the ion proceeds more readily to the centre of the tube while for ions larger than potassium, e.g. thallium, rubidium and caesium, the ion remains in the intermediate position close to the aromatic rings and does not proceed to the centre unless the temperature is raised significantly. However even at 1000 K, the large caesium ion does not proceed to the centre of the tube. Simulations also show how different alkyl substituents at the upper rim of the calixarenes affect the movement of the ions. The results of the simulations are consistent with the experimental observations on the selectivity of the calixtube with these different metal ions.