Rotational Flexibility in NH4+−Encapsulated 18-Crown-6 Studied by Molecular Dynamics and Positron Annihilation: Where Does the Positron Localize?

Conformational flexibility of the ammonium complex of macrocyclic 18-crown-6 at 100 K � T � 300 K is studied using a positron microprobe in conjunction with molecular dynamics simulations. The ammonium cation, encapsulated in a three-pointed hydrogen-bonding perching arrangement, undergoes unhindered facile rotation inside the crown cavity, and the ether backbone adopts itself to the dynamic cation conformation. Preferential localization of the positron in the vicinity of the encapsulated cation results in the positron sensing changes in the local electron density distribution arising from the unrestricted rotational motion of the cation as well as from the slow motion of the crown backbone and hence sensing only an average conformation of the complex. ? 2005 American Chemical Society.