INTERACTION OF THE T-BUTYLCALIX[14]ARENE ANION WITH AMMONIUM CATIONS IN ACETONITRILE - HOST-GUEST COMPLEXES OR EXO COUNTERIONS - A MOLECULAR-DYNAMICS INVESTIGATION

We report a theoretical study of the NR4+ salts (R = H; Me; Et) of the t-butylcalix[4]arene monooxyanion L- in acetonitrile solution, to com pare the endo complexes (NR4+ inside the cone of the host) with the ex o ones. For a given cation, we find that the complexes display structu res of similar type in the gas phase and in acetonitrile solution. :In trinsically, the endo forms are more stable than exo ones, but they ar e less well solvated. As a result, exo complexes are predicted to be m ore stable than the endo ones in acetonitrile. In the gas phase and in solution, the exo complexes of NMe4+ and NEt4+ display interesting ex amples of fluctional intimate ion pairs, where the cation oscillates b etween the oxygen lower rim region of L- and exo pi stacking with the phenolic rings of L-. Based on free energy perturbation calculations, we compare endo NH4+/NMe4+ complexes and find that the hypothetical NH 4+ complex is more stable in acetonitrile solution.