CONFORMATION AND DYNAMICS OF 18-CROWN-6, CRYPTAND-222, AND THEIR CATION COMPLEXES IN ACETONITRILE STUDIED BY MOLECULAR-DYNAMICS SIMULATIONS

We have performed MD simulations on 18-crown-6 and cryptand 222 and their metal cation complexes in a solution of acetonitrile in order to gain insight into the conformational preferences of the free ligands and into the solvation patterns of the host-guest complexes in a nonaqueous solvent. In contrast to what was observed in water, the interactions between the free hosts and acetonitrile are not markedly dependent on the conformation of the solute. For uncomplexed 18C6, conformational sampling is performed for 2 ns. The D-3d form is the most populated, followed by the C-1 form observed in the solid state for the free crown. All structures are compared with those previously reported in the solid state and with those most populated in vacuo or in water. Upon complexation of cationic guests (Li+-Cs+, Ca2+, and Eu3+), the first salvation shells reorganize and some MeCN molecules are coordinated to the encapsulated ion. The coordination numbers of the free and complexed cations are computed. Computer experiments also give insights into the nature of ion pairs in acetonitrile. 18C6/K+,Cl- and 18C6/Li+,Cl- remain as intimate ion pairs for at least 50 ps, while 222/K+,Cl- dissociates rapidly. The complexes of 18C6 with Na+, Ca2+, and Eu3+ dissociate in acetonitrile but have lifetimes larger than 150 ps when Cl- counterions forming intimate ion pairs are included in the simulation. Finally, we report the first free energy simulations for the complexation of alkali cations in acetonitrile. A qualitative agreement is found with experiment concerning the relative orders of stabilities and the ''recognition'' by 222 of K+ among Li+, Na+, Rb+, and Cs+ cations.