BINDING OF NME4-IONS BY A BRIDGED CALIX[6]ARENE IN CHLOROFORM - A MOLECULAR-DYNAMICS INVESTIGATION( AND NH4+ AMMONIUM)

We report a theoretical study of the binding of NR4+ (R = Me; H) catio ns to a calix[6]arene-crown-6 host in chloroform solution in the prese nce of acetate counterion. The MD simulations show that the preferred structure results from interactions among the cation, its counterion, the host and the solvent. NMe4+ does not display a clear preference fo r the cone of the host, but displays multiple modes of binding, involv ing one at the lower rim of the cone ''on the top of the phenolic oxyg ens'', with different relationships with the counterion. The NH4+ cati on slightly prefers an ''exo'' binding to the crown ether moiety of th e host, reminiscent of its 18-crown-6 complex, rather than an encapsul ated position, A modified host without crown has been studied in order to assess the role of the crown moiety. Our study suggests that the c rown is not directly implicated in the binding of NMe4+, but Is import ant to ''preorganize'' and rigidify the host.